Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on October 12, 2022, 10:13:16 am
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I notice some amps (not many though) have one of more preamp stages where a 12AX7 triode has both sides paralleled with each other. What is the reasoning behind this? Tone i assume, but in what way does it affect tone?
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Do you have a schematic example to post? I'm interested. The first reason I could see is doubling the max current. Why i don't know. Maybe to drive something downstream that needs it. And you're not talking about a phase inverter right?
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I recently installed a tremonator trem circuit into a blackface reverb clone. This leaves an unused half of the trem 12AX7. I was thinking of trying out "paralleling" the two 12AX7 sections to see what the effect would be on the trem. I've paralleled preamp stages before and it yields a fatter fuller sound. Of course we all know Fender used a paralleled 12AT7 to get enough ooomph to drive the signal through the reverb tank.
Maybe it would stop oscillating.
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Heres one... https://elektrotanya.com/PREVIEWS/63463243/23432455/egyeb/matchless_chieftain_reverb.pdf_2.png (https://elektrotanya.com/PREVIEWS/63463243/23432455/egyeb/matchless_chieftain_reverb.pdf_2.png)
Also, off topic but i notice the loop in that amp is passive. Seems odd for what if i recall was a $2500 back when $2500 was probably like $3500 or more today.
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When used for input / early stages, it supposedly reduces a noise type, though with a 68k series input resistor, that may not have been a goal for the example in the last post.
Also reduces output impedance of the stage, so beneficial for driving tonestacks etc.
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My understanding is that paralleling the triodes gives around 30% more gain with no increase in noise, IIRC. The information came from Kevin O'Connor in the design of his Soma 84 in the Ultimate Tone books.
With respect, Tubenit
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Interesting.Maybe i'll try it, as i have a spare socket left over from when i tried and failed to make a transparent loop. Pretty easy to just run a couple heated extensions wires and then just parallel wires from grid to grid, cathode to cathode and plate to plate. Would i also want to half the cathode R tho? And what about the bypass cap?
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... a 12AX7 triode has both sides paralleled with each other. What is the reasoning behind this? ...
All of what pdf64 and tubenit already said.
Gain of a typical single 12AX7 is around 60.
Putting two 12AX7s in parallel typically resulting in individual gains adding: 60 + 60 = 120
Gain of cascaded 12AX7 stages multiply: 60 x 60 = 3600
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Matchless has a few designs with that style of preamp. Merlin has a good chapter on this in the preamp book, but I do not think that chapter is posted online. I built a Matchless Spitfire years ago that has a parallel 12AX7 in the V1 position. It is low noise, takes pedals well. Its not my favorite, but neither is any EL-84 amp. As others have said, it seems to simply behave as a somewhat boosted preamp tube.
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You can use different value cathode caps and resistors on the two sections of tube for more tonal variance also. I did this on the input of an amp I made for myself, and I have a switch to have either one, or the other, or both. The tone is fatter and even with one stage or another there is a loss of high end as compared to cascaded stages, but it is a nice sound. I tend to use both stages together most of the time though as it sounds the best.
As mentioned previously, Merlin has some great stuff in his book on how to use the parallel stage and all the options. Highly recommended.
Greg
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...parallel typically resulting in individual gains adding: 60 + 60 = 120....
I doubt it. (I think you wrote faster than you thought.)
But we must also define if we are paralleling the resistors (including load), or as in the Matchless example, keeping them at the 1-triode values.
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The difference in gain is less than half a dB, very subtle.
We have the stock Fender one-triode plan ("uno"), two triodes with stock resistors shared ("dos"), and two triodes with half-value resistors ("halfZ"). (Yes, Mexican lunch today and Mexican battery charger this afternoon.)
Voltage gains (above bass droop):
uno = 53
dos = 59
halfZ = 57