Conference napalm::guitar

>    Price of the kit is $69 minus the front panel (another $20)
                             ^^^^^^^^^^^^^^^^^^^^^
    
    Probably a blessing, did you see the photo of the front panel with the
    article?  Totally cheezy looking!
    
>    But there *is* an XLR output in addition to the 1/4" out,
    
    I just skimmed over that article, is this a *balanced* XLR out?
    
>    For an extra few bucks, you could buy a Real Tube or Blue Tube pedal
>    which would be more robust but you then lose the speaker emulator and
>    XLR outs. 
    
    Actually, one of those would be almost twice as much, unless you're
    talking about getting a used one, but it'd get you tone controls too. 
    And the rack mount version would be more flexable.
    
>    The circuit provides more distortion by keeping the tube power supply
>    low (42 volts) so the tube goes non-linear sooner than in most tube
>    amps.  
    
    I've heard people say that while the low voltage tube effects distort
    more, that they don't have the warmth or tone of a more normal tube
    circuit.  Do you believe that to be true?  (Where do they expect you to
    get a 42v power supply???)
    
>    Also in the same issue is an article on hot-rodding black and silver
>    face Bassman heads.  Great, now the price on those is going to shoot
>    up, too!  It's about time someone pointed out that these make great
>    (and cheap) guitar amps.
    
    Yeah, I was thinking about that too.  It kinda bummed me out 'cause
    I've been wanting to get a Bassman for quite awhile (to use as a guitar
    amp most of the time) and have been real pleased at the low prices I've
    seen them available for.  I just haven't had money that I could
    prioritize high enough to actually get one.  Maybe I should now, before
    the prices skyrocket...  ;^(
                     
    Greg
                                

    
    Re: .1
    
    The XLR is balanced.  It uses complementary op amp drive rather than
    a transformer.  
    
    Price?  The kit plus front panel and power supply is about $100, I see
    the Tube pedals starting at about $125.  Sure, if you buy the raw kit,
    it's twice as much.  If you really wanna be cheap, send PAIA $20 and
    you can have the bare board, or better yet send them a SASE and they
    will send you the drawings so you can etch your own board, then
    scavenge broken gear you own for the parts, and built it for about
    $7.50.
    
    There is a voltage stepup in the circuit that generates 42V from the
    12V in.  Only the tube gets the higher voltage, so it's a trivial power
    supply circuit, this isn't an SVT!  I don't see where the tube voltage
    is going to affect the "warmth".  If it goes non-linear faster, you'll
    get more harmonic distortion, which is what you want, right?  The
    higher the voltage, the cleaner you can make the sound as it places the
    non-linear operating region farther out (see the drawings in the
    article).
    
    						Poindexter

Also, even if you buy the (admittedly ugly) front panel from PAIA, it
*still* doesn't include an enclosure.  I don't know about you, but I'm
not real wild about using a piece of equipment that's just a front panel
with the electronics sticking out the back with no protection.

Me, I'd save the front-panel money to put towards a real rack-mountable
enclosure.  Only problem is, I don't know a good source for such 
enclosures, especially for reasonably priced ones.

-(clever name omitted)

>    I don't see where the tube voltage is going to affect the "warmth".  If
>    it goes non-linear faster, you'll get more harmonic distortion, which
>    is what you want, right? 

    I donno, I'm no electronics whiz, that's why I asked.  I was just
    regurgitating something I thought I remembered from a long time ago,
    when the first 12v tube stomps came out.  People said they were harsh
    and that the 110v models were a lot better.

    For me, this one would probably be worth it for the speaker emulation
    and XLR out if it sounded good.  Maybe I'll try building it (in my
    *copious* spare time...)

    Greg

Folks,


	HELP !!!!!

I have just read the SIAB article and it looks real neat, now I'm an analog
design engineer, but I know little about tubes.

It looks like the 12v ac transformer is used for two things:

1. To provide drive for the voltage multiplier that gives the 42 volts
   and a source to rectify and smooth for the + and - 15v for the IC's

2. The heater voltage for the tube...HERE IS MY PROBLEM!!

I thought heater voltages were 6.3 volts at 300 mA (so assume a nominal 
resistance of about 20 ohms). Is my memory getting dim??

I assume that the 12v ac means 12 rms (this would be the case in order to
provide + and - 15 volts for the IC's). 

This suggests the heaters are getting double the voltage, and assuming about
20 ohms = 600 mA which doesn't make sense for the 12v @500 mA transformer
described in the article.

So, where have I gone wrong ??

Dave

Folks,

	By the way, (it's me again, re: the heater voltage question).

I have re-drawn the schematic and offer the following thoughts.....

1. For those, like me, who intend to use the SIAB feeding my amp only( ie
   I don't need a speaker simulator), the schematic breakes down (power supply
   apart, into just the tube and two op-amps...the op-amps would be in one 
   package only). This would lower the cost and the front panel complexity. 

   I am considering re-housing the thing in a much smaller package...a foot
   operated stomp box.

2. The absence of tone controls could be very simply fixed.  If for example
   you wished to use a Fender style bass/treble/middle control set-up, then
   all the components would be built away (off) the pcb.  Taking this further,
   if you wished to purchase the pcb as offered in the article, then.....

   The second tube could drive these components separately before it goes into
   the op-amp, or if you prefer, you could use the speaker simulation op-amps
   as a tone control drive.  This would have the advantage of being able to
   use the speaker simulation on/off switch as a tone defeat switch.

   I have the tone scematics as used in the Marshall range of pedals including
   the gov'nor if any are interested.


Nothing mind blowing here, but if some of you are put off by the lack of any
tone controls.......don't !!

If enough folks were interested, I would be willing to post a suitable 
schematic for the mods.

Dave

    How to add a tone control to your SIAB
    --------------------------------------
    
    The following is quite long but well worth a read if you intend to add
    a tone control to the SIAB schematic.  I have designed several options
    so you would be advised to read my brief introduction to each to enable
    you to decide which you prefer.
    
                                  
    On page 109 is the schematic of the tone control mod to the Bassman
    amp. This looks a real killer, so rather than post other tone
    schematics, use this one!!!
    
    By the way, the excellent article on modding a bassman could be seen as
    one way to beef up you old warhorse, but, if you feel that the mods are
    too expensive and tricky for you to perform, then adding the new
    bassman tone controls (and the benefits therein!) to the SIAB AND using
    it to drive a Bassman, could be a second, and more prefered option!!
    
    Note that the tone control schematic "input" is the circuit to the
    right of V3 cathode (but does not include the 100K cathode resistor).
    
    The ground reference (o volts) for the tone schematic is the wiper of
    the 10K MID potentiometer.
    
    The tone control schematic "output" is the point marked  'to "A" '
    
    I will use this input and output description in quotes throughout the 
    following.
    
    
    What I am suggesting is that you build the whole tone control network
    on and around the back of the potentiometers.  THIS IS SO THAT IF YOU
    WISH TO USE THE SIAB PCB, YOU WILL NOT NEED ANY MAJOR REDESIGNS. If you
    intend to design and build the whole SIAB and tone controls on a new
    board, then you could obviously design to make room for the new tone
    controls. Alternatively you could mount the few tone control components
    on a small piece of "veroboard" The following options are assuming you 
    wish to build using the current pcb design.
    
    Now, refering to the SIAB schematic on page 94.
    
    You have four choices for "inserting" the tone control within the
    schematic...none of them entails moving or cutting tracks on the pcb
    (only modifying components).
    
    OPTION 1.
    ---------
    Placing the tone control before the tube. 
    
    a. Do not use R15, the tone circuit will have its "input" at point P on
       the schematic. 
     
    b. The other end (the hole in the pcb) of where R15 would have been
       (junction of R15 and C13) is the "output" of the tone control.
    
    c. Replace R22 with a 1M resistor.
    
    d. Due to the insertion loss of the tone controls, the gain of IC1:A
       will need to be increased. Do this by replacing the "CRUNCH" control
       with a higher value. Something in the order of 22K or 47K as a maximum
       value.
    
    OPTION 2
    --------
    
    Fitting the tone control after the tube, but leaving all other SIAB
    functions as before.
    
    a.  The tone controls "input" will be point R on the schematic.
        The tone controls "output" will be the S1A switch conection itself
        ie. that marked as 'OUT S1A' on the schematic. In effect, you are
        "cutting" the "wire" shound as point R and inserting the tone control
        in series with it.
    
    b.  Replace the R39 resistor with a 1M resistor.
    
    OPTION 3
    --------
    
    Fitting the tone controls after the tube by utilising the inverting
    output of the SIAB (and thereby removing the functionality of having an
    inverting output). The reason I have given this option is so that you
    can use the inverting output as a tone-modified output, and use the
    non-inverting output as a "straight" output....neat huh!!
    
    The polarity switch is now not needed, and the socket J7 will need to
    be hard wired for a fixed polarity.
    
    a.  The wiper of the output pot (R23) is off the board and currently
        joins at point M. This will need to be modified by the following:
    
    		-  connect point M to 0 volts (the best place is to solder
                   a short wire from it to the bottom of R36, although any 
                   close point would be okay).
    
                -  replace  R36 with a 1 M resistor
    
    
                -  The "input" to the tone control will now be the wiper of
                   pot R23.
    
                -  The "output" of the tone control will now go to the top
                   of the new value (1 M ) of R36...ie where this resistor 
                   goes into the + input of IC2:B
    
    
    OPTION 4
    --------
    
    This option is for those who do not want the cabinet simulator.  It has
    the advantage of using the now redundant, two op-amps provided for the
    simulator. There will be some changes to the front panel ie.
    
    1. There will be no SIM 1 and SIM 2 switch
    
    2. The Simulation IN/OUT switch will now be a tone control IN/OUT
       switch.
    
    3. All other functions, reverse polarity etc. are retained apart from
       the FAT switch.
    
    The follwoing seems complicated, but all I am doing for the most part
    is replacing components (that you would have needed to buy) with links.
    Links are just suitable diameter tinned copper wire for shorting out
    where pcb components would have sit.
    
    a. replace the following components with links:
    
       R10, C4, R6, R7,
    
    b. Do not fit the following components:
    
       R13, C8, C10, R20, C15, R27, R28, C17, C18, 
    
    c. Replace the following components:
    
       R39......replace with 1M
    
       join point S to point T (you do not need switch S1A).  
    
       connect switch S1 B "IN" to point J (you do not need S4)
    
    d. The "input" to the tone control is now Point J and the "output" of
       the tone control is point S or T.
    
    Okay, schematic followers will note I have converted IC1:B and IC2:A to 
    non-inverting op-amps with a gain of about unity and inserted the tone
    control between them.
    
    
    There you have it........take your choice from the above!!!!
    
    regards,
    
    Dave
     
                 
    
    
    
    
    
    
    
    
    
    
    

In the case of a tube, the heater voltage is often the 1st value (but, not
always). In the case of 6L6GC, this uses 6.3 volts to drive the heater
element. The 12AX7 (7025 is roughly the same tube), which I think is what 
the kit uses, requires a 12.6 volt supply for the heater. Tubes work in
the same fashion as an FET, so there are some similarities there. 

The tube heater voltages can be either AC (RMS voltage) or DC.

							Jens

Jens,

	I have contacted my local tube stockist (here in the UK).

The 12AX7 (=7025) is called I believe, an ECC83 over here.

The local shop says the ECC83 requires 6.3 volts for the heater.

Do you USA guys have different heater elements fitted inside standard
types of tubes?????

Can anyone help resolve this....I'm starting to lose sleep !!!

Dave

 The 12AX7/7025/ECC83 requires 12.6V OR 6.3V for the heater.

 The pin-out is :

   4 : Heater
   5 : Heater
   9 : tap point heater.

     
      |-----|-----|
      |     |     |
      |     |     |
      4     9     5


 The wiring for 6.3V heater is : solder 4 and 5. Apply 6.3V/0.3A between 4+5 and 9

 The wiring for 12.6V heater is : Apply 12.6V/0.15A between 4 and 5. 
 Pin 9 is unused.

 I highly recommand to use DC voltage for the heater. This gives a much better
 S/N ratio and cancels heater hum.
 If you want to used DC voltage, use an integrated regulator (LM350K for example)
 and adjust the output voltage to the required value.
 NEVER add rectifier and cap to the stock supply transformer without voltage
 regulator. The result is 6.3*1.414 = 8.9V or 12.6*1.414=17.8V and RIP.....

 Regards
 J.P.

>
>
> The 12AX7/7025/ECC83 requires 12.6V OR 6.3V for the heater.
>
> The pin-out is :
>
>   4 : Heater
>   5 : Heater
>   9 : tap point heater.
>
>     
>      |-----|-----|
>      |     |     |
>      |     |     |
>      4     9     5
>
>
 
J.P.

	Thanks a bunch!!! so the old ECC83 has a dual heater circuit!!

This could explain why my local "expert" thinks it is 6.3 volts.

I will assume that the USA and UK variants are identical in this respect.

Yes, I quite agree about using a dc voltage for the heater...it is just 
cost reduction practices that lead to Fender etc using ac for their heater
circuits. It is particulaly true for high voltage gain front - end tubes to
need dc voltages.

Thanks again for clearing this up, I can now go ahead and build knowing that
the schematic is correct!

Dave

    Re DC for heaters,
    
    I have been using bridge rec/smoothing cap direct on 6.3 volt heater
    supply without problems for several years.  As far as I can tell I
    don't get reduced tube life.
    
    Only do this for pre-amp tubes though.
    
    By the time I take away 2 X .7 volts for the bridge rec, and then some
    resistive losses in the rects I am pretty much back at 6.3 volts
    anyway.
    
    Still, if you really want to be sure, use a reg.
    
    I like DC for heaters.
    
    Nick

To All,

	Since this topic has got my juices going on design thoughts around
the tube schematic in GP..........

Could any of you tube designers out there recommend a good Design Book for
tube amps.                                                 -----------

I have one or two old text books that show the principles of operation of
a typical tube voltage amplifier, and so I am aware of the *basic* priciples,
but I need a comprehensive book (or books) along (hopefuly) with design
steps/tips/formulae/ transformer selection/ etc.  It can come simple or with 
heavy maths.

Any good ideas you tube guru's ?????

regards,

Dave

    
    	A month late or so, but I find the best way to design using
    tubes is leveraging off the experience of others.
    
    	I've got a book that covers many different tube amps schematic
    wise, but, alas, it's out of print. It was a SAMS publication...
    
    	One thing you could do would be to subscribe and get all the
    back issues from a magazine called GLASS AUDIO. ($20/yr, PO Box
    176, Peterborough NH 03458). That should give you enough basic info
    to play around with for a long time...
    
    	Joe