Dual triode - can both sides be run in parallel ?

[HaroldBrooks]

I think I've seen someone do this before, my question is, can you just parallel connect two sides of a dual triode tube, say a 6SN7 or a 12AX7 and increase gain at that point ?

The reason I ask is I just received a pretty cool National amp in an equipment trade, and someone years ago it was fitted with a 6SN7 in place of the original 6J7 for the V1 preamp tube.

I also believe they rewired the socket to split the two halves of the tube to operate from two separate input jacks (three inputs in total), with two separate volume controls. Wondering if I could make the both halves work together in parallel easily ? I would rather have the boost from both halves in this case, as the amplification factor on the 6SN7 is 20, and I've already jumped the channels (there are three inputs with 2 on one volume control and the other on another pot) so I like the way the more gain from the preamp sounds up front.

Thanks for your help !

http://www.oldfrets.com/Valco/Schematics/valco_1212.pdf

[Dave H]

Yes, the two halves could be wired in parallel but the gain increase won't be dramatic (only a few dB)

[Helmholtz]

Paralleling 2 triodes does not increase the (resulting) amplification factor µ, but decreases the plate/output impedance of the circuit. This allows actual circuit gain to be closer to the theoretical µ value, using the same plate resistor as with the single tube. The latter condition may not provide good center-biasing, though.

In any case, the upper limit for circuit gain is always the µ of the single triode.

Parallel tubes make sense in input stages to reduce noise - and in circuits where a single tube could not deliver enough power, e.g. reverb driver, output stage.

[mozz]

6j7 is a pentode, so the circuit had to be rewired for a dual triode. Could it have been factory done?

[HaroldBrooks]

Yes, the two halves could be wired in parallel but the gain increase won't be dramatic (only a few dB)

So do you think I am better off running one triode into the other ? I have this arrangement on another amp of mine :

http://www.oldfrets.com/Valco/Schematics/valco_1212.pdf

[HaroldBrooks]

6j7 is a pentode, so the circuit had to be rewired for a dual triode. Could it have been factory done?

Perhaps... But there was a mysterious circuit addition to the amp using one of those six flat prong plugs. The circuit has an old Long Plate RCA 12AX7 and a few caps and resistors in it, and it looks as old as the amp. It may be a Tremolo, or a type of boost, or perhaps a negative feedback circuit, but those are just guesses. I will see if I can figure it out, and if I can't I will call upon you guys for help !

The Valco factory did make a change I believe to later models of the same amp, as I saw one with a 6SQ7 and no hole in the chassis for the grid cap wire, so I believe that one to be a factory change. My "new" amp has the hole for the 6J7 grid wire, and the base of the 6SN7 is too big for the rivets of the sockets, so unless that is a newer tube, I have a feeling it was a 6J7 from the start, and then modified (quite nicely I might add) to use the two sides of the 6SN7, along with the "Mystery" add on circuit.

My temptation is to not change it any further until I can figure out the mystery circuit, because it might end up being something cool. I have very little use for Tremolo these days, but I am fascinated by filtered Negative feedback in the form of a "Presence" or "Resonance" circuit. I also would like a tube boost built right into the amp that could be called upon as needed.

When I get a bit better at this, I may be able to build one, or use what I have with this amp to accomplish either of those goals.

[Helmholtz]

So do you think I am better off running one triode into the other ?

That would be the only way to achieve a total gain like a pentode (or more as tube gains multiply). Or use a pentode .

[HaroldBrooks]

That would be the only way to achieve a total gain like a pentode (or more as tube gains multiply). Or use a pentode .

I might switch the amp back to a 6J7 preamp tube (pentode). But first I have to see if that added circuit is of any use, and if it helps the amp then I will leave well enough alone.

I'm a bit obsessive sometimes about maximizing the gain of my amps, as if I was tuning a race motor (used to do that in my youth), but I realize it can be a dead end.

Better perhas to figure out how to modify for the best tone, and perhaps added flexibility built into the circuit with switches and nobs to control values on the fly and morph the amp as needed.

Just some thoughts.

[the fatch]

In the usual Fender input circuit, the single 12AX7 triode has a gain of around 60. According to the Valve Wizard, the two 12AX7 triodes wired in parallel in the Matchless Lightning input stage achieve a signal swing of 295V p-p and gain of 76, an improvement of just+2dB over the single triode. https://el34world.com/charts/Schemat..._lightning.pdf

[Mick Bailey]

Cascading the triodes and using a dual volume pot works well. My main amp has this arrangement and will go from really clean to more than enough gain off a single knob.

[HaroldBrooks]

Wow, crazy how little you gain, but that's why you don't see it that often, and that's why they came up with more powerful tubes like the 6J7, 6SQ7, and the newer noval 12AX7.

I think I am going to back off on doing the parallel trick because the net change is minimal. I may in the near future get an adapter to mate a 12ax7 to the 6sn7 socket, and I am sure one is out there. Then I can test the effect of a higher preamp gain on the circuit without committing to rewiring everything, and changing the plate and cathode resistors to match the new tube better.

[HaroldBrooks]

Cascading the triodes and using a dual volume pot works well. My main amp has this arrangement and will go from really clean to more than enough gain off a single knob.

Not sure if it's the same thing, but someone in the distant past took the three inputs and assigned one to one side of the 6SN7, and the other two to the other side of the tube. They also have two volume knobs on the amp, so I can jump between two inputs and get both channels working, it just requires that extra plug. It sounds beefier, but not necessarily better for this amp and what I need, so there you go.

I will try the 12AX7 adapter and I will order one and let you guys know how it worked out. I can tell you I did that very thing with a Zenith Hi-Fi amp I converted, that is replace the 6J5 week tube in that amp with a 12AX7 using an adapter, and I actually liked the 6J5 better ! running a boost in front sounded better with the older and weaker tube, who would have guessed. Just trying and exploring a lot of things, so then I know how it worked out rather than speculating based on the pure maths of the change. That's useful, but your ear is the final judge of the outcome.

[Helmholtz]

In the usual Fender input circuit, the single 12AX7 triode has a gain of around 60. According to the Valve Wizard, the two 12AX7 triodes wired in parallel in the Matchless Lightning input stage achieve a signal swing of 295V p-p and gain of 76, an improvement of just+2dB over the single triode. https://el34world.com/charts/Schemat..._lightning.pdf

True and the extra dB gain with the 6SN7 triodes in parallel would be even less as this is already a very low impedance (Ra) tube.

OTOH using a single 12AX7 would increase gain from 14 to 60 corresponding to +13dB.

[Justin Thomas]

I thought the parallel triode input stage was more useful for "reducing" noise than boosting gain, or at least that was the primary intended effect. In my experiences there were times when a single triode wasn't enough, an ef86 was too much, & a parallel 12AX7 was juuuuuuuust right. The two stages cascaded resulted in a great RAWK amp, but with no clean headroom.

Note: I build simple amps. I usually don't have room to add more controls, so switching from a one-knob tone stack or adding extra volume controls aren't an option.

Wouldn't hurt anytning to try; just twiddle cathode & plate resistors to match.

Justin

[Helmholtz]

a parallel 12AX7 was juuuuuuuust right. The two stages cascaded resulted in a great RAWK amp, but with no clean headroom.

But he is using a very low µ 6SN7, resulting in less than 1/4 the gain of a 12AX7. Gain values with Ra = 100k and bypassed cathode resistor 6SN7: G=14, 12AX7: G=60.
With the two 6SN7 triodes in parallel gain will increase from 14 to 16 or by 1.16dB.