Conference napalm::guitar

2692.0. "Re-use old tube amp for electric guitar" by ELESYS::JASNIEWSKI (Why not ask why?) Tue Mar 16 1993 12:13

	The following has been submitted to Doug Wellington USGS, to be 
included as an article in his mailing list. Please feel free to comment 
constructively -

	Joe Jasniewski


         "Relacing Old Tube Amps for Use with Electric Guitar"

    1. Introduction: 

            - Why modifiy an old integrated or PA amp?

            - How difficult to do?

            - What can I expect for results?

            - What can I learn from doing this?

            - What will I need to accomplish this?

    2. Finding "Old Iron"

            - What should I be looking for?

            - What to avoid?

            - Where would this stuff be found?

            - How much to pay?


    3. Assessing the situation

            - PA or Integrated amplifier

            - Schematic or no schematic?

            - Visual Inspection

            - Operational Checkout


    4. Goin in for the kill

            - Functional Section Identification

            - Amplification stage ordering

            - Location of current volume control circuitry

            - Location of current tone control circuitry

            - Preliminary Modification Archetecture decisions

            - Destructive Surgery

    
    5. Constructive Surgery - Relacing the New Signal Flow

            - Installation of Input Jack

            - Connecting to 1st Amplification Stage

            - Installing pre-volume control

            - Interstage gain reduction and coupling circuit

            - Installing additional pre-volumes

            - Marshall / Fender tone control circuit
    
            - Location of tone control circuit in amplification string

            - Installing post-volume control

    6. Additional Nice-ities

            - Output Jack

            - Three - wire line cord.

            - Standby Switch

            - Power Supply Modifications


    7. Conclusion



            - What can I expect for results?

    
    Introduction.

        Whether you're a beginner or seasoned Guitar electronics enthusiast,
you can re-build an existing generic tube amplifier which will rival the
"name-brand" units in performance when used with an electric guitar. An old 
Integrated tube amplifier, or an old PA tube amplifier will make a great
platform for this project. Often, these units can be found in the used
marketplace at virtually "giveaway" prices, when compared to amplification
intended for use with musical instruments. Just about anyone knows the
collectable value of a Fender Bassman amp, but that old DuKane or Bogen amp?
You can often pick them up for a song!

        The modifications might look intimidating at first, but are actually
suprisingly easy to accomplish. Most Integrated or PA tube amplifiers follow
the same basic layout; once you've "relaced" one, you can pretty much do them
all. The good news is that most of what you need is already there; it just
needs a little re-arranging to sound good with an electric guitar. This consists
of the removal of some stuff, de-soldering of wires and resistors from tube
sockets and the like, and adding in the new connections in the right places.
No different than, say, that "triode" modification you did one time on an
amplifier's output stage. Or perhaps the "Heathkit" you built once upon a
time...

        The results often turn out to be quite good, especially in the area
of getting that "tube sound". This of course depends on the initial piece of
"iron" you have to work on; its operational condition at the time you start
working on it - the condition of its tubes and other components. However, a
lot of how well things turn out - and this is the fun part - depends on the
choices you make in installing the modifications. There's ample room for
experimentation, and, maybe *you'll* come up with the "golden" arrangement
of circuit stages and connections that just kicks Marshalls right off the stage!
"Kick butt on a Marshall amplifier?" you ask? Yes, because all a "Marshall"
amplifier has over your amplifier is the way the circuits are arranged - 
they're both basically "tubes" and "iron". If this interests you, by all means, 
read on!

        As I alluded to above, this all can be quite a learning experience.
This happens in several contexts, from the theory of amplifier operation, to
how a typical amplifier is hooked up, to the practical experience of doing the 
physical changes to the circuitry. But most of what you'll learn is from 
making the necessary decisions and choices in the archetecture of your new
amplifier. In this respect, it'll be *your* amplifier, quite possibly with 
your own unique "signature sound" to it. Not to mention it's appearance, 
arrangement of controls, and special functions - all of which you can choose.
This can be extended to modifying amplifiers professionally, building guitar
amplifiers completely from scratch or having a better idea of what to do in the
area of repairs.

        To accomplish this, the usual battery of electronic hobby equipment
is essential - things like a soldering iron, voltmeter, electric drill with
bits and center punch, tools for undoing screws, nuts and bolts, cutting
and stripping wire - and a fair stock of common electrical components is
very helpful. (Of course, you can determine exactly what you need at the time
you assess the amplifier you intend to relace and order them from an electronic
part source like Digi-Key or Mouser Electronics.) A notebook to jot down your
findings and plans is also most helpful.


    Finding "Old Iron"

        The first step is to find a suitable amplifier to make the modifications
on. Perhaps you already have an amplifier in mind, like that old tube Fender
PA amp your band no longer uses. Perhaps after reading this section, you'll
change your mind about considering a certain amplifier to consider a different 
one.

        In this article, I wont be considering transistor amplifier types at
all. Although the general principles might apply, it's not my intent to suggest
that a transistor amplifier can be modified in the same ways as described here.
What you want to look for is exclusively TUBE electronics.

        Most of the "iron" that's suitable for doing these modifications was
built in the 1950's or 1960's, during the heyday of tube amplification. Amps
that work best for this are monaural and are either an "Integrated" type or
a "PA" type. The PA types offer a special advantage that I'll get into in a
bit. An Integrated amplifier means that the amplifier contains at least the
following; a volume control, some sort of tone controls (usually bass and
treble) and an input selector switch. When considering an Integrated amplifier,
check the input selector switch to see that it has a "microphone" or a magnetic
phonograph input. This will ensure that the unit already has enough gain stages
to serve you well as a guitar amplifier. Note: if the unit does not have these
markings, or if it clearly can handle only a crystal phono input, pass it by
- or you'll possibly be dissappointed with the results after all your hard work.
    
        Of the two types, the PA type amplifier is the one to get. This is
usually characterized by having multiple microphone inputs, a phono input,
some sort of tone controls and a master volume. The advantages of this type
are that the master volume is already present (it has a spot on the control
panel) and that there's lots of gain stages. This gives two advantages; one,
there's lots of tube stages already in this type of amplifier; two, there's
lots of control panel positions for potentiometers, which will give you the 
most flexability for controlling the sound.

        Of course, there's some specific stuff to avoid - besides transistor
amplifiers. One thing to definitely avoid is tube stereo amplifiers. There's
two very basic reasons for this. One, is that a stereo amplifier would require
you to do twice the amount of circuitry relacing - once for the left channel
and once for the right channel. The second reason is a bit less obvious. There's
quite a market for stereo tube electronics these days. You could easily, being
"unenlightened", hack up a very valuable piece of gear - something you could
sell or trade for perhaps a bunch of amplifiers more suitable for this type of
work. Certainly, if you run across a McIntosh, a Marantz or a Dynaco amplifier,
just set that one aside from this type of endevour.

        When looking for a suitable unit, a judicious application of the "silk
purse from a sow's ear" theory is beneficial. In other words, the better the
amp you start out with, in terms of size, quality and condition, the better 
your project will turn out in the end. I've made some real screamers out of
a simple monaural 15 watt Heathkit amp, but...practically speaking, that little
amp just would not cut it playing in the band. Look for at least a pair of 6L6
or 6CA7 types for output tubes; possibly a quad of 6BQ5s in an amplifier before 
you buy. If you come up with an integrated or PA amp that has a quad of 6L6's - 
Oooo baby - you'll be able to create quite a machine!

        There's numerous place to check for amplifiers of this genre. One of
the most fruitful sources I've found is the common town fleamarket. Often what
happens is the church finially ends up replacing their old tube PA with some
newer equipment, and that old amplifier ends up on someone's table for $25.
There's also the electronic-specific oriented fleamarkets - these are good
because the sellers recognize that "amplifiers" are part of the market - but
maybe not as good because sellers also recognize the possible value of "the 
old beast" and generally ask more. Other sources might be the local Want-ads, 
Salvation Army stores or other used goods shops. One specific place I might
mention is the for sale ads in antique radio hobbiest magazines, such as "The 
Antique Radio Collector" (PO BOX XXX Carlisle MA 0XXXX - $25 yr)

        What to pay depends entirely upon a number of factors, some of which
have been mentioned already, like the output tube compliment. In my opinion,
that and the condition of the amplifier are chief considerations in deciding
how much I'm willing to spend. People's asking prices seem to be fairly 
random from my experience; I've seen sellers want $100 for an amp that was
just absolutely trashed, while others have virtually given away amps that were 
very nice units. I've recently acquired a 30W DuKane PA amplifier, in pretty 
much "as new" condition; shiny paint, dust free and "unweathered" - I paid 
$40 for it, which I feel was a fair deal for both the seller and myself. 

    Assessing the situation

        Let's say you've now found your amplifier - and it's a "beauty" like 
my recently acquired DuKane. Whether it's a PA or Integrated type, it's time to 
open it up and assess the situation; find out just what you've got on hand!
To do this will generally require you to remove the top cover and bottom plate
to expose the electronics or "guts" of the amplifier. Often, if the manufacturer
was generous, there will be a schematic diagram glued onto the bottom plate
of the amplifier. This can be most helpful - especially if you're well versed
at reading this type of diagram. A suggestion that I can make at this point
is to make several copies of this schematic on a copy machine; these can later
be used as a "scratch-pad" to sketch out circuit changes you're considering.

        The first thing you'll want to note is the number of gain stages that
are available in the amplifier you've chosen. Generally, this can be assessed
by counting the number of triode sections in 12AX7 type tubes in the amplifier;
each 12AX7 tube has two sections. A typical Integrated type has one 12AX7
providing initial gain, another 12AX7 associated with the tone control
circuitry, and perhaps another 12AX7 or 12AU7 used to drive the output tubes. 
That last tube wont change much in the modifications described here; it's part 
of the stuff "already taken care of" by the original amplifier's circuitry.
Therefore, I count 4 stages available to "play" with, in this particular
arrangement.

        A typical PA type amplifier is arranged differently. Most often, 
there is a 12AX7 type tube for each pair of microphone inputs; each section
amplifies one channel. If this is the case in your PA type amplifier, it sure
makes things easy to relace a multiple section "pre" gain arrangement. So,
if it was a PA amp with 4 microphone inputs, that's 4 stages of pre-gain
already. Likely, the amplifier has another 12AX7 to handle mixing the original
4 inputs and the tone controls, so, we're up to 6 stages to play with before
we hit the output driver tube. Makes your ears ring just thinking about it,
right?

        Other things that would be useful at this point would be a general
visual inspection - check the condition of the wiring, look for burned or
overheated resistors - especially the cathode and screen grid resistors
of the output tubes, and those of the power supply - most often located around 
the filter capacitor can. One thing that often breaks on an old amplifier is
the power switch; now's the time to check its operation and think about ordering
a replacement if need be. 

        The last thing to do before you "tear into it" is an operational
checkout. You'll want to connect a speaker or suitable load resistor to the
amplifier output because tube type amplifiers require an output load to
work properly. You can ruin the output transformer by running a loud signal 
through the amplifier with its output unloaded. Remember, since this is an old 
used amp whose operation you're probably unfamiliar with, it's best to take this
simple precaution the first time you turn it on. Plug it in, turn it on and
observe the filaments glowing in the tubes and any abberant operation like
the fuse blowing. If all's well after a few minutes, I'd recommend that you 
make some measurements with your meter around the power supply and output 
tubes, verifying the voltage values called out in the amplifier's schematic 
(if you have one).

        Another good reason for doing the operational checkout is that you'll
be assured your amp worked before you make your modifications to it! If repairs
are needed, its best to do them at this point and verify that your amp works
in its original form before proceeding. For a PA amp, simply plug in your
guitar into the 1/4" jacks for each microphone channel and check that the sound
comes through. For an Integrated type amplifier, you may need an adapter cable
to connect your guitar's 1/4" jack to the RCA type input connector. Select
"Microphone" or "Mag Phono" on the selector switch and verify the operation
of the volume and tone controls. Note if the controls are "scratchy" or
intermittant, a shot of "tuner cleaner" or "Cramolin" might be in order. Check
for an undue amount of AC "hum" from the loudspeaker; with the volume control
turned all the way down, this is an indication that the filter caps need to be
replaced.
    Going in for the kill

         From the notes you made when you inspected the amplifier, (and perhaps
by looking into it again) it's now time to "map out" functional sections. Doing 
so will invariably help with your understanding of what you've got, where you 
are and where you're going with your modified design.

        For an Integrated amplifier, your map might look something like the
following: (crudely drawn using the ascii characters...)

  |---------------|
Input           First       Second      Tone            Third       Volume
Selector ------ 12AX7   --- 12AX7 ----- Control ------- 12AX7 ----- Contol --->
Switch          Stage   |   Stage       Circuitry       Stage       
  |---------------------|

        Phase    -----  Push Output Tube ------ 
>------ Splitter                                Output Transformer ----> Speaker
        Driver   -----  Pull Output Tube ------

        What's important at this point is to note the locations of the various
functional components, like the tone control circuits and the volume control.
Secondly, note how intimate the circuitry is between the 1st amplification 
stage and the input selector switch. This may take some looking at to figure
out, but the good news is that you dont have to figure it out, because in the
modifications you'll make, the entire input selector switch is removed; only
the 1st 12AX7 amplification stage needs to be identified.

        If you have a schematic with your amplifier, you should mark down
which tube and section you'll call the "1st" and "2nd" amplification stages, 
etc. If you dont have a schematic, decide which section of the tube is 
physically closest to the input selector switch and pick that one as section
"1". Of course, the corresponding section of that tube will be section "2"
and so on. If you're having trouble finding the circuit sections at all in the 
tangle of wires, the best thing to look for is the plate resistors connected to 
the 12AX7 sockets, which typically range from 100K to 1Meg. (One idea to keep
things in order would be to physically mark these resistors with a tape label.)

        For an PA type amplifier, your map might look something like the
following: 

    First    Volume
    12AX7 -- Control --|
    Stage              |
                       |
    Second   Volume    |
    12AX7 -- Control --|
    Stage              |    Fifth       Tone            Sixth       Master
                       |--- 12AX7 ----- Control ------- 12AX7 ----- Volume --->
    Third    Volume    |    Stage       Circuitry       Stage       Control
    12AX7 -- Control --|
    Stage              |
                       |
    Fourth   Volume    |
    12AX7 -- Control --|    
    Stage

        Phase    -----  Push Output Tube ------ 
>------ Splitter                                Output Transformer ----> Speaker
        Driver   -----  Pull Output Tube ------
        
        Again, if you have a schematic, you should mark down which tube and 
section you'll call the "1st" and "2nd", etc. If you dont have a schematic, 
use the channel 1 section as section "1". Of course, the next channel will be 
section "2" and so on...

        With your amplifier sufficiently and hopefully, accurately mapped out,
you can make some preliminary modification archetecture decisions. It's best
to do this before going into it with the wire cutters and de-soldering tools.
The first and most basic choice is where in the circuit to place the pre-volume
control. In most amplifiers, including those built by both Fender and Marshall,
this is placed between the first and second 12AX7 gain stages. Other choices
might be prior to the first stage, but realistically, this would simply be
duplicating the control already on your guitar. After the second stage could
give you a degree of distortion controllable from your guitar's volume control,
because two stages of 12AX7 gain is quite a bit. Generally, you can assume
the best spot for the pre volume is wired in between stage 1 and stage 2.

        Of course in the PA type amplifier, we have an entirely different
situation and hence, options. There's a number of gain stages with associated
volume controls already connected. One idea that I like is to wire all of them
in a cascade arrangement; the output of volume control 1 feeding into the
input of stage 2 and so on. It's what I call a "multi-variate" arrangement,
because by adjusting the 4 pre-volumes in various ways, different tonal
characters can result. 

        The second most important choice is the location of the tone control
circuitry. If your amplifier was originally a HiFi Integrated amplifier, the
tone controls are basically useless for electric guitar. This is because the
turnover frequencies are in all the wrong places in the frequency spectrum,
considering the appx 80 to 10,000 Hz frequency range of an electric guitar.
Even with the PA amplifier type (unless the unit you're working on happens to 
be a Fender or a Traynor or something...) I've found that the tone controls
are just all wrong. I've found that the best solution is to replace the entire 
circuit with the classic Fender / Marshall Bass-Mid-Treble tone control circuit
which will be described later.

        The location of the tone controls in a classic Fender or Marshall amp
is just after the second stage, feeding into the third. You can experiment with
the locations in both the PA and Integrated amplifiers; where you choose to
place this circuit in the amplification string will undoubtably determine the
whole sound of the amplifier.

        Now, the whole reason to think about this and perhaps make some
decisions around where you want to place things, is because you're now at the
point where you're going to go in and start cutting out things you dont need.
In an integrated amplifier, you definitely dont need the input selector switch;
in fact, a good place to locate your input jack is right in the hole left by
its removal! When you start cutting, it makes sense to do so in a way that
most helps you relace the existing circuitry into your new arrangement. This
means leave certain wires intact, cut component leads in such a way that they 
are still useful in that they can be re-wired according to your new diagram.

    Constructive Surgery - Relacing the New Signal Flow

        The first item to install is, of course, the 1/4" Input Jack. There's
a trick to doing this I'd like to share, which will result in a better overall
performance of your project. If you simply bolt the jack to the chassis, you'll
make a ground connection different from where the cathode resistor of the 1st
amplification stage is. Digi-key among others sell special washers which will
allow you to mount the input jack insulated from the chassis. This way, you
can connect both signal and ground directly to the 1st amp stage, with much 
better noise performance. Another thing to do here is to purchase a jack with
an integral switch which contacts the signal prong when the plug is removed.
by connecting this contactor to ground, the input to the amp shorts out when
you unplug your guitar - and things stay nice and quiet and well behaved.

        I'd like to make a comment here before I go on with the wiring details.
The quality of your wiring job will directly effect the performance of your
amplifier! I suggest that you use a coaxial cable wherever possible; a good
source of this cable is from an old defunct Japanese cassette tape deck - 
which you could possibly pick up at the same fleamarket you find your amplifier 
at. This type of wire is small, easily stripped and easy to solder. A typical 
deck contains enough of it to easily do all the wiring needed for your amp.
Of course, you can probably order some of this stuff from the same electronic
supply house you get the necessary potentiometers, capacitors, jacks and
resistors from.
        
        To connect the input jack to the 1st amplification stage, simply wire
it with coaxial cable (a must at this point) to the grid pin of the tube socket
and the ground to the circuit ground connection of the cathode or cathode
resistor. Be sure to keep any exposed inner conductor as short as possible,
so this highly sensitive connection is well shielded from noise pickup. Make
sure you wire up the contactor lug to ground also, if you've obtained a jack
of this type.

        Likewise, install the pre-volume control by wiring it with coaxial 
cable. Here, the high side of the potentiometer (corresponding to "full blast")
connects to the output decoupling capacitor of the first stage via the inner
conductor of the coax, while the wiper of the potentiometer connects to the
grid pin of 2nd amplification stage. The ground of both coaxs connects to the
low side of the potentiometer and the cathode resistor ground connection of
the 2nd amp stage only.

        Another suggestion I'd like to share is the consideration of the circuit
loaded by the potentiometer used for the pre-volume. Usually, it's most
convienent to use the same units that came with the amplifier wherever possible,
to be able to use the same or similar matching knobs on the control panel. This
is of course provided that they're in good shape and restorable with a contact
cleaner as mentioned earlier. If this is all true, you'll want to consider
changing the value of the decoupling capacitor from the 1st stage. The reason
for this is because the value typically used in a PA or Integrated amp is too
large for use with electric guitar (Ignore this advice if you're building an amp
intended for Bass guitar use).

        A good choice for the low frequency rolloff point of the RC circuit
formed by the decoupling capacitor feeding into the impedance of the pre-volume
control is 60 Hz. It's a good choice because at this point, the circuit has
a lot of gain and lower frequency signals coming from the guitar pickups when
you strike the strings can modulate the sound, knocking the stability of the
amplifier's sound around as you play. This ultimately produces an annoying
"pumping" effect which is undesirable. The following table shows, for various
common values of potentiometers, appropriate values of decoupling capacitors.
If you're ordering these caps, you'll want to order 400V values, or the same
voltage values as is already in your amplifier.

                 Potentiometer Value     Capacitor Value
                        100K                    .025
                        250K                    .01
                        500K                    .005
                        1Meg                    .0025
                        2Meg                    .001

        For the case of a PA amplifier, another problem presents itself, if
you're going to use the multi-variate circuit arrangement that I like. All
those gain stages presents too much gain to simply connect up directly in a
cascade. Yet, if you introduce a series resistor to attenuate the signal by
making a voltage divider where the lower leg is the potentiometer impedance,
the total impedance seen by the decoupling cap of the preceeding stage is
really huge, therefore the turnover frequency is really low. A better solution
is to load the decoupling cap of each preceeding stage with a value which
will set the crossover frequency to around 60 hz - and then tap across that
resistor with the voltage divider. If this sounds confusing, the following
diagram of the Load / Attenuator will hopefully clear this up some.

  From Plate             0.005       500K
  of previous          >---||---.---/\/\/\----> (ground)
  12AX7 stage                   |
                                |
                                |    5Meg       500K Pre-Volume Pot
                                |---/\/\/\-----/\/\/\---> (ground)
                                                 |
                                                 |------->  To Grid
                                                            of Next
                                                            Stage

        This circuit may, of course, be used in the Integrated amplifier 
modification should you find that the overall gain is too much to be usable.
That sometimes happens, because the type of tone controls used in a typical
Integrated amplifier have a lot of attenuation and when you replace them with
the Marshall / Fender tone control circuit (which I highly reccommend), the
overall gain of the amplifier is very high.

        The following diagram shows the classic Marshall / Fender tone control
circuit. In a Marshall or Fender amplifier, this is usually installed between
the 2nd and 3rd 12AX7 amplification stage. You might choose to place this
somewhere in the "multi-variate" pre-volume arrangement, or just before the
phase splitter stage. Experiment! Probably the only completely inappropriate
place to install this circuit is just after the input jack, because the gain
there is so high that you'd never be able to wire it in a way that wouldnt
pick up a lot of interferance.
    
        To install the post-volume control, there's basically two choices for 
its location in the circuit topology. One is just before the phase splitter 
tube, and this requires only a single potentiometer metering the level into the
grid input of that stage. An arguably better choice is to place this control
just before the two output tubes. However, you'd need a dual potentiometer in
that case, in order to meter the voltage to both tubes. This arrangement is
considered better because it includes the sound of the phase splitter tube.
There's no reason you couldnt do both, ending up with a multi-variate
arrangement of two pre-volumes. In an Integrated amplifier, you'd only be
limited by the number of control locations on the front panel.

        Let's take a look at a couple of the circuit topologies possible. The
first one I'll present assumes you're working on an Integrated type amplifier:

    ---------   ---------   ---------   ---------   ---------   ---------
    | 1/4"  |   | 1st   |   | Pre 1 |   | 2nd   |   | M/F   |   | Pre 2 |
 >--| Input |---| 12AX7 |---| Volume|---| 12AX7 |---| Tone  |---| Volume|-->
    | Jack  |   | Stage |   | Contr |   | Stage |   | Contr |   | Contr |
    ---------   ---------   ---------   ---------   ---------   ---------

    ---------   ---------   ---------   ---------
    | Phase |---| Master|---| Power |---| Output|
 >--| Split |   | Volume|   | Output|   | Trans |--->
    | Tube  |---| Contr |---| Tubes |---| former|
    ---------   ---------   ---------   ---------

        This one shows the multi-variate arrangement made up from cascading
4 microphone inputs that were perhaps found on a PA type amplifier.

    ---------   ---------   ---------   ---------   ---------   ---------
    | 1/4"  |   | 1st   |   | Load  |   | Pre 1 |   | 2nd   |   | Load  |
 >--| Input |---| 12AX7 |---| Atten |---| Volume|---| 12XA7 |---| Atten |-->
    | Jack  |   | Stage |   | Circ  |   | Contr |   | Stage |   | Circ  |
    ---------   ---------   ---------   ---------   ---------   ---------
    ---------   ---------   ---------   ---------   ---------   ---------
    | Pre 2 |   | 3nd   |   | Load  |   | Pre 3 |   | 4th   |   | M/F   |
 >--| Volume|---| 12XA7 |---| Atten |---| Volume|---| 12AX7 |---| Tone  |-->
    | Contr |   | Stage |   | Circ  |   | Contr |   | Stage |   | Contr |
    ---------   ---------   ---------   ---------   ---------   ---------

    ---------   ---------   ---------   ---------
    | Phase |---| Master|---| Power |---| Output|
 >--| Split |   | Volume|   | Output|   | Trans |--->
    | Tube  |---| Contr |---| Tubes |---| former|
    ---------   ---------   ---------   ---------

        As you can see, the variations in the circuitry are only limited by your
imagination and experience with experimenting in the placement of the different
functions. As always, it makes sense to plan things out first, arranging how the
sections will be connected, obtaining the parts you'll need and deciding how
you'll secure things physically. One of the limitations is, of course, the
original control panel of the amplifier you're building from. I've eliminated
indicator lamps and relocated power switches to the rear panel in an effort
to get just one more spot up front for another control, or for the input jack.

        For example, say your front panel has an input selector switch, a volume
control, and bass and treble controls. The AC power is part of the volume
control. That leaves you with 4 available positions, but, considering the above
diagram for the Integrated amplifier, 7 positions are required!! This shows
another "logistical" reason why the PA amps are the ones to look for, because
generally they have more open front panel positions to work with. This is not to
say that with some effort and creativity, you couldnt retrofit your amplifier
with an entirely new front panel, cut and drilled to accomodate your design.

        Another option is that there's nothing hard and fast saying you have to 
use the full implementation of the multi-variate control system. You could 
easily replace the panel potentiometers with trimmer potentiometers which are
mounted internally, and adjusted "to taste" such that all you really need up
from is a "pre" and a "post" volume control. Another trick would be to find a
4 or 5 gang deep potentiometer and turn all of the sections of the multi-variate
pre volume control simultaneously. There's lots of options and possibilities
to try, any one specific combination of choices could lead to your own signature
sound.

        Additional Nice-ities and Conclusion.

        I've found that most often, a generic Integrated or PA amplifier does 
not have the same style 1/4" output jacks as do your typical Guitar amp head.
The addition of these is a nice touch for compatability with speaker cabinets
you or another musician might already have. At times, you can find a spot for
the jack on the backpanel of the amplifier, other times it's necessary to drill
a 3/8" hole to accomodate it. In this case, it's okay to allow contact with
chassis ground, given that the common of the output transformer secondary is
also connected to chassis ground.

        Most amplifiers of this nature offer several impedance taps on the
output transformer secondary. An easy way to handle the inpedance selection is
just to have a flying lead, which is connected internally to the output jack,
come through the chassis so it can be connected to the proper impedance tap.
By keeping this wire short, dressing the chassis hole with a grommet and tying
it off behind, will give this setup a "well attended to" appearance. A crimped
on terminal lug at the end of the wire finishes it off nicely.

        In all these type of amplifiers I've built, a three wire cord; line,
neutral and ground is a must. I dont bother with the "polarity" switches found
on the older tube guitar amps. Adding a three wire line cord will reduce the
noise pickup and interferance when operating the new amplifier at high levels
of gain. The best place to connect the ground is via a bolt directly to the
amplifier chassis.

        A Standby Switch is also a nice addition if you have room for it,
preferably next to the power switch. This allows the amplifier to remain
warmed up, but with no B+ current flowing, which can help to save the output
tubes. The place in the circuit to put this switch is just after the rectifier
tube and just before the filter capacitors. If you insert it downstream of this
point, the filter capacitors stand to possibly charge up beyond their rated
voltage values when in standby mode, which could stress them to the point of
failure. That's no good, especially when considering that Standby mode is
supposed to help things last longer!

        While we're talking about the amplifier's power supply, another useful
addition is to beef up the total supply capacitance. You can order 400 uf
450V caps (which should be a sufficient voltage rating for most applications)
from Mouser electronics. They're expensive, about $10 a pop, but placing one 
of these across the main B+ decoupling capacitor (the one the center tap of the
output transformer is connected to) gives a marked improvement in the dynamics
of the amplifier and the amount of hum and noise coming from the power supply.

        In conclusion, I hope this article helped you significantly in going
about restoring and relacing an old tube PA or Intergated amplifier for use with
electric guitar. Following the same guidlines and process I've described here,
I've successfully modified tube PA amplifiers and sold them in the used
marketplace, to the complete satisfaction of the guitarist making the purchase.
While we might not make a living doing this, it is a satisfying and rewarding
endevour to pursue, at very little risk. If you mess up on your first try, no
big deal, because it wasnt some vintage Fender Twin you accidently drilled in
the wrong spot! After doing just a few generic amplifier relacings, it becomes
"as easy as tying your shoes" (couldnt resist) and you can easily apply what
you've learned to any tube amplifier, be it a Grommes or a Gibson.

    Yo, Joe, 
    
    You left out the diagram of the tone controls!
    
    						Brian

    
    JJ,
    
    Interesting......
    
    You might follow up with 6L6 power upgrades/additions..
    
    Question (unrelated) Do you remember the "Ice Cube" plug unit
    for Fenders?  I would like to build one but can't remember
    the components??????
    
    Seems like it was passive connect between reverb patches on
    rear panel.  This was for 70s vintage panels with or without
    master volume.  The Reverb control became a gain pot
    which overdrove the 6L6's I think?  Maybe it drove an input 
    stage I can't remember and dont have the schematics.
    
    Thanks for any assistance
    
    Mike

    
    	Re .1 - I know! I remembered "Oh yeah" on my way out to lunch
    this morning...I never typed it in because my books were showbound
    out in the garage this weekend. I'll get to it.
    
    	re .2 - Thankx for the 6L6 input. I could add some to the
    additional nice-ities part. I've found that doing this can be
    unfruitful, because when you upgrade the tubes, then you're limited
    by the output transformer or if not that, the power supply. It
    could be something to suggest though and I appreciate the input.
    
    	What the Ice Cube might have been/done, was to dispense with
    the reverb tank, but still load the reverb driver and couple that
    to the reverb input. A single resistor to load, a cap to block DC,
    and possibly another resitor to ground or a voltage divider to set
    the input level to the reverb control not too wildly. Probably a
    4 part device; I could guess the schematic at some later point if
    you'd like.
    
    	Thanks again,
    
    	Joe

    Joe,
    
    I Apologize for the sidetrack from your article!!
    
    Thanks for the thoughts.  Now that I have had a few cups of 
    coffee I believe the "Cube" was a RC filter connecting the 
    reverb input and ouput.  This allowed you to use the Reverb
    driver as a seperate gain with great CRUNCH/sustain.  
    
    Great for old Fender's with no master.  Increased tube decay though.
    
    I'll dust the old DC circuits textbook off and follow up. 
    
    Any input appreciated. 
    
    Mike
      

    	Here's the M/F tone circuit -
    
    Note that the Bass control is wired counter- clockwise from the schematic.

                          |------> to next pre-volume control (1 meg)
                          |           _________    ________
                          |           |       |    |      |
    From Plate  >-------/\/\/\------/\/\/\-------/\/\/\-----------> (ground)
    of 12AX7        |   R1       |    R2      |    R3
                   ---  Treble  ---   Bass   ---   Midrange
    Fender:        ---          ---          ---            Marshall:
                    | C1         | C2         | C3
    R1 = 250K       ----/\/\/\-----------------             R1 = 250K
    R2 = 250K             R4                                R2 = 1M, log
    R3 = 10K                                                R3 = 25K
    R4 = 100K                                               R4 = 56K
    C1 = 250 pf                                             C1 = 250 Pf
    C2 = 0.1 uf                                             C2 = 0.022 uf
    C3 = 0.047 uf                                           C3 = 0.022 uf

    I think J.J. is right about the "Ice Cube". It may have had an R/C in
    the input to tweak the tone. You could make one up with a couple of
    pots & fiddle around until it sounds right. I've "hot wired" many a
    reverb unit this way. If you don't put in a hi Z input, you'll end up
    dissipating the reverb drive power in the kludge. 
    	You also may want to run the output of the "kludge" to the non reverb 
    channel instead of the back into the reverb effects loop. This way you get
    free tone controls, a mixture of clean & crunch, & a ton of gain !
    
    Paul