Using 12AU7 instead of 12AX7 to lower the gain

[Emetal]

Read that some people use 12AU7 instead of 12AX7 in V4 position to lower the gain, of monstrously high gain third channel of 5150 III 6l6 amp. That should not be a problem for an amp right, it is safe, right? Schematic attached below.

[Helmholtz]

Replacing the V4 12AX7 with a 12AU7 is safe for amp and tube.
Nevertheless the 12AU7 will not be properly biased, so comparison with the 12AX7 is not fair.
Do it if you like the results.

[Delta362]

Try a 5751 tube in there instead. It's a lower gain version of the 12AX7 and the circuit will still be happy.

[loudthud]

V4a has inverting feedback for a gain of -1 and V4b is a cathode follower. According to the numbers in ovals the signal goes from 16 VAC on V3b to 13.4VAC on V4a plate and down to 3.6VAC on V4b cathode. That's already a loss of about 13dB with a 12AX7.

Shove a 12AU7 in there and you're likely to get big grid current on the V4a side because of really low plate Voltage, but that won't do much because there are no pots between V3b and V4a. The major effect of the 12AU7 will be a weak signal with asymmetrical distortion to the CH3 tone stack. Can't you just turn the Three Volume control down ?

[mozz]

12AU7 work much better than a 12AX7 when plate voltage is low. Try it and see, i doubt very much it will damage anything. Also try 12AT7.

[g1]

Also up for consideration, 12DW7 (ECC832) or reverse 12DW7 (JJ ECC823). These tubes are half 12AX7, other half 12AU7.

[Emetal]

Can't you just turn the Three Volume control down ?

With the drums, bass, other guitar player, singer... no I can not turn the volume of the CH3 down. I need minimum at 10 o'clock. That means that Gain pot can not go over 9 o'clock without getting feedback. With Gain pot set so low, I do have enough gain, but the sound is flabby, loose, it's not tight. Read that some people used 12AU7 and that gave them use of full range of the Gain pot. I am just worrying about possible damage to the amp or tube..

[Mick Bailey]

Also worth considering is the 12AY7.

[Emetal]

Also worth considering is the 12AY7.

I'we read somewhere that somebody from EVH warned against using 12AY7 in this amp. Did not offer specifics.. as why exactly
Looking on the net :
The gain factors for each tube are (approximately):

12AX7 = 100
5751 = 70
12AT7 = 60
12AY7 = 40
12AU7 = 17

Don't know why would anybody single out 12AY7 but that's the info I stumbled upon.

[Helmholtz]

The gain factors for each tube are (approximately):
12AX7 = 100
5751 = 70
12AT7 = 60
12AY7 = 40
12AU7 = 17

These are theoretical max gains. Actual gain is determined by the circuit.

As loudthud explained, both V4 triodes are wired for a gain just below 1.
Because of the NFB employed a different tube will not much change the gain (probably less than 10%).

[Mick Bailey]

I'we read somewhere that somebody from EVH warned against using 12AY7 in this amp. Did not offer specifics.. as why exactly

Don't know why would anybody single out 12AY7 but that's the info I stumbled upon.

Maybe because V4b is wired as a cathode follower and the cathode to heater voltage for a 12AY7 is given as 90v. However the Fender Bassman 5E6A is similarly configured and uses higher voltages than the Peavey, and those tubes don't seem to suffer as a result. Maybe new production tubes aren't as robust in this position.

[Emetal]

Just tried it at a practice pad, nothing special happend, very little difference, a bit less volume and flabines but nothing really worth it..

[Helmholtz]

Just tried it at a practice pad, nothing special happend, very little difference, a bit less volume and flabines but nothing really worth it..

See post #10.

[Emetal]

Would it make more difference putting 12AU7 at some other position V3 or V2?

[Helmholtz]

Would it make more difference putting 12AU7 at some other position V3 or V2?

Yes, but the 12AY7 might be a better candidate here.