Preamp tubes in parallel

stoo · Post by **stoo** » Wed Nov 08, 2006 12:34 am

I've tried a few searchs to no avail. What happens when you hookup a second triode(12ax7) in parallel with another. ie.grid to grid,cathode to cathode.plate to plate. Twice the voltage gain?. What about the cathode and plate resistors ...halved or doubled?
Thanx
stew'

rockstah · Post by **rockstah** » Wed Nov 08, 2006 12:40 am

stoo wrote:I've tried a few searchs to no avail. What happens when you hookup a second triode(12ax7) in parallel with another. ie.grid to grid,cathode to cathode.plate to plate. Twice the voltage gain?. What about the cathode and plate resistors ...halved or doubled?
Thanx
stew'

stoo,
if you paralled v1 to itself,in a plexi, you would remove the wires on pins 1,2,3( plate, grid and cathode of normal channel - for the time being just have them sticking up in the air and not touching anything while you try the mod ) and now jump a wire from pin 1 to 6 - from pin 2 to 7 and from 3 to 8 - this would remove the use of the normal channel but you would now have a "super tube" so to speak for v1 -
it would give more gain, sustain, touch sensitivity.
try it.

Mark

SteadyEddie · Post by **SteadyEddie** » Wed Aug 11, 2010 10:53 pm

I'm considering doing this to a 50w plexi clone. Any drawbacks? Will it still sound like a plexi, just gainier? This is one of the few posts I found referring to this mod, I know other "boutique" amps run parallel triodes but I haven't found much discussion on this. It seems that if it'd be so great, everyone would be all over it.

I know the answer ... try it. But I'm out of town on work right now, so I was just wondering if it's even worth getting the soldering iron warmed up for it or not.

flemingmras · Post by **flemingmras** » Thu Aug 12, 2010 12:34 am

Theoretically, doubling the plate resistor value would yield the same effect as paralleling two halves of a 12AX7 and keeping the same plate resistor (i.e. achieving a bigger overall swing with less input signal swing). Again another one of those areas where it's all relative and one thing offsets another.

The only time it's really useful to parallel a tube stage with another in this fashion is when you're trying to get double the current/twice the power. But we're dealing with a VOLTAGE amplifier here...not a power amp.

Not saying you can't do it...just saying that in a voltage amp application you may find it to be a rather pointless mod when you can just simply double the plate resistor value.

PCollen · Post by **PCollen** » Thu Aug 12, 2010 10:24 am

stoo wrote:I've tried a few searchs to no avail. What happens when you hookup a second triode(12ax7) in parallel with another. ie.grid to grid,cathode to cathode.plate to plate. Twice the voltage gain?. What about the cathode and plate resistors ...halved or doubled?
Thanx
stew'

Effective value of now-common Rp and Rk will be reduced, and value of now-common Ck (if Rk bypassed) increased; do the math.

flemingmras · Post by **flemingmras** » Thu Aug 12, 2010 10:54 am

PCollen wrote:
stoo wrote:I've tried a few searchs to no avail. What happens when you hookup a second triode(12ax7) in parallel with another. ie.grid to grid,cathode to cathode.plate to plate. Twice the voltage gain?. What about the cathode and plate resistors ...halved or doubled?
Thanx
stew'
Effective value of now-common Rp and Rk will be reduced, and value of Ck increased; do the math.

If you're wanting to keep the voltage gain and the bias point the same and/or you were concerned with maximum power transfer (again only REALLY useful in a power amp configuration) you would be correct.

However, we're dealing with VOLTAGE amplifiers here so we're not really concerned with current. Furthermore, these guys are relying on the "one change at a time" method of altering tone because they don't realize that it's not about "silver bullet" changes but rather different COMBINATIONS of things that alter tone, so they're all gonna do it without changing any resistors, then go tell their friends "Oh yeah...run your tubes in parallel and you'll get more gain" when all they REALLY had to do was just increase the plate resistor value to increase the voltage gain.

joey · Post by **joey** » Thu Aug 12, 2010 12:29 pm

Doing the way it was suggested originally really isn't giving parallel valves justice. They do certainly have a distinct sound to them all things kept equal, and a slightly different harmonic content, not the same as just increasing the plate load, which has a different sound as well. The problem is that this effect isn't given full justice if you share the cathode TBH. The best thing to do if you are going to do it is to split the cathode/cathode bypass, and bias each half differently, maybe one side cold, and one side warm, and play with the cathode bypass. you can also bias them the same and just play around with frequency response instead if you wish, but you do have to put in the effort, and listen to make it work for you. If you are just looking for gain however there are tons of ways to do that, the methods of which seem to come up here very often.

HTH · Post by **HTH** » Thu Aug 12, 2010 4:45 pm

afaik, another reason for paralleling the two triodes is to reduce the plate resistance (Ri), thereby being able to drive the next stage better (would be more useful in a Fender type amp imo where the first stage is driving the tonestack directly).

from what I can remember of doing this, I didn't really get any more gain, just a slightly thicker tone.

I'd agree with an earlier post saying that using different Rk and Ck values will give more interest to the tone, but I can't say I'd ever played with this myself.

SteadyEddie · Post by **SteadyEddie** » Fri Aug 13, 2010 9:56 pm

Excuse my ignorance on the subject, but I'm having a hard time believing that running the tubes in parallel will give the exact same effect as increasing the plate resistor. It seems that two triodes in parallel would give more gain AND headroom? More clean gain? Am I mistaken here?

P.S. I'm home now and have a few hours to myself tomorrow to tinker with the amp. So I'll report my personal findings here.

flemingmras · Post by **flemingmras** » Fri Aug 13, 2010 10:07 pm

SteadyEddie wrote:Excuse my ignorance on the subject, but I'm having a hard time believing that running the tubes in parallel will give the exact same effect as increasing the plate resistor. It seems that two triodes in parallel would give more gain AND headroom? More clean gain? Am I mistaken here?

P.S. I'm home now and have a few hours to myself tomorrow to tinker with the amp. So I'll report my personal findings here.

"Headroom" = maximum available peak-peak output signal swing is preset by the available B+ voltage at the plate resistor. Maximum peak-peak swing of the output signal cannot exceed the value of the B+ voltage.

"Gain" is the ratio of output signal as compared to input signal. More "gain" = less input signal required to obtain maximum peak-peak swing.

When a grid is hit with a positive going input signal, the "valve opens up" (i.e. the cathode-plate resistance drops) to allow more current to flow through the plate resistor. The more current flow increases through the plate resistor, the more the voltage drop across the plate resistor increases.

Conversely, when the grid signal is going negative it does the opposite and the voltage drop across the plate resistor does the same.

With a bigger plate resistor, less current flow is needed to obtain the same voltage drop across it (simple Ohm's Law). This means that for a given input signal voltage you will have a bigger voltage drop across the plate resistor with a bigger value plate resistor than you will with a smaller value plate resistor and the same input signal amplitude.

It is this fluctuating voltage drop across the plate resistor that becomes your output signal once the coupling cap filters out the DC offset of the idle plate voltage.

The max peak-peak swing of the output signal CANNOT exceed the available B+ voltage. This is your "headroom".

With paralleled stages, less input signal is required to reach the amount of plate current required to achieve maximum peak-peak swing, hence more gain.

With a bigger plate resistor value, less current flow through the resistor is required to reach maximum peak-peak swing, hence less input signal is required to do the same.

It's a simple voltage controlled voltage divider...it just depends on which end of the datum you want to be on to accomplish the same end result.

demonufo · Post by **demonufo** » Sat Aug 14, 2010 5:40 am

SteadyEddie wrote:It seems that two triodes in parallel would give more gain AND headroom? More clean gain?

I don't see why that would be the case, since they are still going into another stage, and pushing that following stage hard.

rip · Post by **rip** » Wed Aug 25, 2010 1:20 pm

When you parallel two triode stages, you are effectively halving the rp, but mu stays constant, and gm is doubled. Since you are doubling gm, the noise can theoretically be 3dB lower... 6db lower for 4 paralleled triodes, but that is all debatable since in real life you can't get there because of other factors.

The small increase in gain in triodes has more to do with the decrease in rp, reducing the upstream loading effect, so depending on the circuit, you will get varying results.

As far as the "fatness" is concerned, if you parallel devices, they won't be absolutely identical, so you will have all kinds of different freq bouncing around as they react differently, but to the same input signal. That will be even more pronounced if you split the source resistor and bias them up differently. how that translates into sound... i dunno. If it sounds fatter then it is fatter, but you may also get phase cancellation, which may make it thinner depending on the frequency. Try it and see.

Has anyone heard of this? Using parallel stage to lower noise floor? I've seen this a few times while searching, but frankly I trust you guys more. I'm going to continue searching, but wanted to see if I'm at least on the right track, this guys post related to what Jon was saying.

Thanks
Rip

rip · Post by **rip** » Wed Aug 25, 2010 1:53 pm

Found some good info here http://ampgarage.com/forum/viewtopic.ph ... 847106afc5 if anyone is interested.

Rip

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