Using 12AU7 instead of 12AX7 to lower the gain

[mozz]

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

Why do people keep posting this? In a "actual" circuit, a 12AX7 usually has a voltage gain of 50 or less, check out the RCA tube manual resistance coupled amplifier charts.. You can get that gain in a 12AT7 if you change the circuit. People swap the tubes out without understanding what is going on. Now optimize a circuit for max gain on a 12AY7 and the 12AX7 will sequel and howl and not work properly.

[Helmholtz]

If a tube howls it's microphonic.

[Chuck H]

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.

Helmholtz has already mentioned that tubes can howl due to microphonics. I'd like to mention that pickups can do that too relative to gain.

But you've already mentioned that the tone when the amp is used as above is too flabby. There may be other changes that can be made different from tube swaps that could remedy that. ie: smaller coupling cap or preamp stage cathode bypass cap for the channel in question.

Is it possible your pickups are squealing due to gain? If you're sure they're not then you have to suspect a microphonic tube.

[Emetal]

No problem with tube, tried several 12ax7. No problem with guitar or pick up (shilded cavity, EMG) it's just that CH3 simply have too much gain..

[Emetal]

There may be other changes that can be made different from tube swaps that could remedy that. ie: smaller coupling cap or preamp stage cathode bypass cap for the channel in question.

Since I do not have much experience with design if you could be so kind of suggesting what values to try and which exact capacitors - I can definitely try it out.

[Chuck H]

That means that Gain pot can not go over 9 o'clock without getting feedback.

No problem with tube, tried several 12ax7. No problem with guitar or pick up (shilded cavity, EMG) it's just that CH3 simply have too much gain..

I have to wonder what you mean by "feedback". If anything is 'feeding back' with higher gain settings the two most likely causes would be a microphonic tube or pickup. Even if you've tried several 12ax7's in a given position it doesn't mean another positions tube isn't microphonic. Usually the first tube in the signal chain. It may be different circuit voicing that causes that channel to feedback when others don't. Or, if the amp is modified there may be stability problems and parasitic oscillation due to the circuit itself.

[Chuck H]

Since I do not have much experience with design if you could be so kind of suggesting what values to try and which exact capacitors - I can definitely try it out.

Can you tell me if the amp is currently modified in any way please?

[Emetal]

Can you tell me if the amp is currently modified in any way please?

Nope, no mods of any kind, brand new out of the box

[Chuck H]

Since I do not have much experience with design if you could be so kind of suggesting what values to try and which exact capacitors - I can definitely try it out.

Well... Looking at the design I can see that low value coupling caps for the first two gain stages are already in place and low value cathode bypass caps are used throughout the preamp. But you can still adjust the gain of the amp much better than swapping tube types by reducing the value of R55. That resistor pretty much sets the gain for the entire channel as the load resistor of the voltage divider just prior to clipping stages. It's 820k now. Rather than subbing a bunch of resistors on the board to find the preferred value I would try to temporarily put a 1M pot in that resistors place (wired as a variable resistor). You'll need to adjust that pot so that using the amps front controls feels intuitive for you regarding gain level. Then, without turning it, remove the pot and measure the resistance across the lugs you used as a variable resistor. Install a fixed resistor of the nearest standard resistor value for R55.

I really don't see this as much different from turning down the channel gain. But it will reduce overall gain for that channel without changing it much otherwise..

[Helmholtz]

Can you avoid the feedback by increasing the distance between guitar and speaker?

[Emetal]

Can you avoid the feedback by increasing the distance between guitar and speaker?

Yes of course, the further the better, I don't think that anything is broken it's just how this amp is, it has way too much dist on CH3

[Chuck H]

Having looked at the schematic now I may as well attend the original subject as well... You could put a 12ay7 or a 5751 in the V3 socket rather than V4 and get a gain reduction on channel 3. I wouldn't try it in V2 though because that sets up and executes the "cold clipping" stage and may change the overall wave form symmetry of the clipping for the worse.

EDIT: Also, using a lower gain tube in the V1 position will affect the gain of all three channels. So don't do that.

[Chuck H]

Can you avoid the feedback by increasing the distance between guitar and speaker?

Yes of course, the further the better,..

Then this is your guitar feeding back. No doubt. Gain is a factor but so is pickup microphony. I'll agree that channel 3 on that amp has a really stupid and redundant amount of gain, but here's the thing...

If you're turning the amp up to a gain and volume level that is causing this feedback then I have to assume it's because you WANT that level of gain and volume. Because there are knobs on the amp that control both of these things. So the solution to achieving that level of gain and volume without the feedback can only be to remedy the microphonic pickup problem. Logic.

[Helmholtz]

Yes of course, the further the better, I don't think that anything is broken it's just how this amp is, it has way too much dist on CH3

So you probably just have uncontrolled string feedback.
Such acoustic feedback mainly depends on both total gain as well as sound pressure received by the guitar and strings.

Guess you're playing very loud.That might work in a large venue/auditorium but not in a smaller club.

Nothing special to this amp.

Let's see if you like a lower gain tube help.

[Chuck H]

Yes of course, the further the better,..

So you probably just have uncontrolled string feedback.

I had considered string feedback, but Emetal's comment above makes it sound undesirable.?. When I started playing in bands I remember having to get use to controlling string feedback with high gain and volume but it was never undesirable. It's a big part of how things sound and operate contributing to sustain for fundamentals and harmonics. In fact it's this missing component that keeps all types of volume reduction for amps from being better. No way to get this goodness without gain and volume. So, if it is uncontrolled string feedback, just turn the gain down a little and the volume up a little. UNLESS that level of gain is what Emetal wants, but without the uncontrolled string feedback. In which case the answer, again, is not in modification of the amplifier, but rather in learning and practicing playing techniques to control string feedback.