parallel triode question

[Rick]

Whether or not a parallel triode stage has more gain than a single triode stage depends on the rest of the circuit.

And don't worry about losing any 2nd harmonics - the signal just doesn't get big enough on an input stage to see that effect.

If you have a typical input stage, want to keep the same frequency response and voltage swing on the output, and just get the advantage of lower noise, you'd need to:

1) Cut value of grid stopper in half (since input capacitance doubles with dual triodes, halving resistance keeps same freq response).
2) Cut values of plate resistor and cathode resistor in half.
3) Double value of cathode bypass cap.

So a center-biased 12AX7 parallel input stage (in typical range of preamp B+ values) might use a 10k grid stopper, 51k plate resistor and 750R cathode resistor, and would be fully bypassed with a 44uF cathode bypass cap.

The lower noise (higher SNR) comes from using smaller resistor values in the circuit.

This also has the benefit of producing lower output impedance, which will mean less loss if a tone stack immediately follows the parallel triode stage.

Thanks!

[stretch2011]

Something I'm experimenting with right now is paralleling not the first stage, but the tone stack driver. That way even though the gain its self isn't all that much, the drive current is a lot more, therefore more gain.

[gingertube]

Search this forum using the search term Junkbox to find my Junkbox Trainwreck schematics.
I run a parallel triode input BUT with the two triodes at different operating points for some harmonic emphasis.

The use of 2 triodes paralled give you a root 2 improvement in signal to noise.

Cheers,
Ian

[SamRE]

Search this forum using the search term Junkbox to find my Junkbox Trainwreck schematics.
I run a parallel triode input BUT with the two triodes at different operating points for some harmonic emphasis.

The use of 2 triodes paralled give you a root 2 improvement in signal to noise.

Cheers,
Ian

Hi Ian, I know this is an incredibly old post, but is that a 0.707 SNR improvement in terms of dB?

[LOUDthud]

Checkout the input stage to an 18 Watt Marshall. The low gain input jack drives one triode, the high gain input jack drives both triodes in parallel.

[didit]

Hello —

Different Ian, but gingertube makes rare stops at TAG these days.

Hi Ian, I know this is an incredibly old post, but is that a 0.707 SNR improvement in terms of dB?

Assuming my recollections are correct that’d be a voltage ratio for noise of 1:0.707 — e.g., assume 1mV for a single triode then expect 0.707mV with a pair. And again trusting sketchy recollections it’d theoretically give 3dB down.

..

[SamRE]

Hello —

Different Ian, but gingertube makes rare stops at TAG these days.

Hi Ian, I know this is an incredibly old post, but is that a 0.707 SNR improvement in terms of dB?

Assuming my recollections are correct that’d be a voltage ratio for noise of 1:0.707 — e.g., assume 1mV for a single triode then expect 0.707mV with a pair. And again trusting sketchy recollections it’d theoretically give 3dB down.

..

Ah yes, ofc, I forgot about dBv. -3dB of voltage noise seems pretty good. I wonder if that’s a general aspect of parallel triodes and what concessions there are?

[soundmasterg]

One of the concessions with running a parallel stage is that the tone has a bit of high end loss. I noticed this in an amp I built that is using a parallel input stage, but every time to tried to get more high end the amp would squeal so I ended up living with it.

Greg

[Helmholtz]

Depending on exact circuit, parallel input triodes can result in more than doubled input capacitance.
Might mean something like 300pF.
The increased low pass filtering and lowering of the PU resonance are likely to be audible.

[soundmasterg]

Depending on exact circuit, parallel input triodes can result in more than doubled input capacitance.
Might mean something like 300pF.
The increased low pass filtering and lowering of the PU resonance are likely to be audible.

It was audible in my case. The crisp highs that might otherwise be there with a single triode just aren't there with the parallel stage, and I tried to add back in with some tricks but it would oscillate, so I left it as it was. It still sounds good.

Greg