12AX7/12AU7 Preamp Supply Voltage Lowered

[tubeswell]

I lowered the plate R to about 17k.

12au7
plate resistors: 17k/pin 6 and 22k/pin 1.

I still don't believe I heard much difference this time as well.
The thing I did noticed, however, I did not really get an increase in headroom... which I thought might happen by lowering the 12au7 plate R since the preamp is running about 50 or more volts less than it was designed to. But this was my initial inquiry.

Also, when a chord is struck the sound sometimes cuts out and fades back in ("sag" or compression?) with what I believe to sound like some "hard clipping" as well.

Can you post a schematic of the whole amp?

[Intergalactic Tourister]

Can you post a schematic of the whole amp?

I only have the preamp. I currently test it by plugging into the "Power Amp In" on my combo Fender Champ 25 SE... which has a 12ax7 phase inverter. This is temporary, though.

But yes, I can draw it out (preamp only though) and post a picture.

[Intergalactic Tourister]

Here is a picture of the schem and power supply. Hope I attached it correctly.

***Oops. 12ax7 pin 6 goes to pin 2 (NOT pin 1).

[Chuck H]

A couple of things stand out for me...

1) Why the 22k load at the input? It's a VERY small value for that part of the circuit.
2) What is the 2nd triode doing? It looks like the plate is connected to the prior plates output and then on to the third triodes grid with only a large resistance at the second triodes grid and the cathode is grounded. What is this doing?
3) Also on the second triodes plate is a resistor in series with a capacitor going to the first triodes cathode. If the second triode were doing anything this would form a positive feedback loop. Due to the capacitor value the positive feedback would only be in the very high frequencies. I can't understand how that could be a good thing for a guitar amplifier since it only really promotes oscillation and hiss and does almost nothing to the useful guitar frequencies. But, since the second triode doesn't seem to be doing anything It equates to a negative feedback loop between the first triodes plate and cathode which may be useful for negating oscillation and lowering hiss. Still, why is it connected there instead of directly between the first triodes plate and cathode?
4) At the output of the third triode, following the coupling cap you have a string of resistors for the load totaling 1115k. But I don't know what the purpose of the string is. Is the tone stack connected to one of the junctions to form a voltage divider? And why three resistors?
5) The cathode bypass cap on the fourth triode is such a small value for that circuit that only frequencies over 10kHz are being affected. This is also above the useful frequency range for guitar and really only serves to increase hiss and promote oscillation.

On the power supply I would say that if low voltages have been a concearn you should greatly lower the value of the series resistors that decouple the stages. You could get good separation of the nodes with only 1k and decrease the voltage drop across those big resistors a little.

If there is anything useful hapening in the circuits I noted I apologize and plead ignorance and am happy to learn something about a circuit I was not aware of.

[Intergalactic Tourister]

Thank you for your input, Chuck.

1) Why the 22k load at the input? It's a VERY small value for that part of the circuit.

I've had different grid leak resistors in. Its sounds mellow at 22k. But depends what I'm using as an input.

2) What is the 2nd triode doing? It looks like the plate is connected to the prior plates output and then on to the third triodes grid with only a large resistance at the second triodes grid and the cathode is grounded. What is this doing?

I made an error in the schematic [see my last post]. The 1st triode plate goes to grid (NOT pin 1). But yes, the cathode is grounded.

3) Also on the second triodes plate is a resistor in series with a capacitor going to the first triodes cathode. If the second triode were doing anything this would form a positive feedback loop. Due to the capacitor value the positive feedback would only be in the very high frequencies. I can't understand how that could be a good thing for a guitar amplifier since it only really promotes oscillation and hiss and does almost nothing to the useful guitar frequencies. But, since the second triode doesn't seem to be doing anything It equates to a negative feedback loop between the first triodes plate and cathode which may be useful for negating oscillation and lowering hiss. Still, why is it connected there instead of directly between the first triodes plate and cathode?

The resistor in series with the capacitor does do something [noting schematic error]. The preamp was previously used for different things like mics or different instruments- hence letting the highs in. However, it does not introduce oscillation or hiss at all but, yes, nothing for guitar frequencies.

4) At the output of the third triode, following the coupling cap you have a string of resistors for the load totaling 1115k. But I don't know what the purpose of the string is. Is the tone stack connected to one of the junctions to form a voltage divider? And why three resistors?

Yes. The coupling cap is connected to a 100k resistor then to the 1M Bass Pot. The 1M Bass Pot is then grounded with a 15k resistor (not 1115k string of resistors). That whole area I wrote "Tonestack." Basically the .022 coupling cap goes into the tonestack (1M Bass & 1M Treble) and right back out into the grid. It was simply an omission because I was spending too much time figuring how to draw it. I also thought the tonestack would be irrelevant here.

5) The cathode bypass cap on the fourth triode is such a small value for that circuit that only frequencies over 10kHz are being affected. This is also above the useful frequency range for guitar and really only serves to increase hiss and promote oscillation.

Yes. I've used 1uf and it sounds good. That bypass cap had been changed before I received the preamp. Again, it was used for different things.

On the power supply I would say that if low voltages have been a concern you should greatly lower the value of the series resistors that decouple the stages. You could get good separation of the nodes with only 1k and decrease the voltage drop across those big resistors a little.

Ok, thanks. I believe those resistors were part of the design since it was originally getting 220v B+. I thought about it already but the low voltage kinda sounded neat to me. So, I'm not entirely sure yet if I want to lower the resistors.

If there is anything useful hapening in the circuits I noted I apologize and plead ignorance and am happy to learn something about a circuit I was not aware of.

Yeah I feel really bad making that error in the schematic. I feel like I just caused a lot of confusion. I'm very sorry about that.

[Intergalactic Tourister]

On the power supply I would say that if low voltages have been a concearn you should greatly lower the value of the series resistors that decouple the stages. You could get good separation of the nodes with only 1k and decrease the voltage drop across those big resistors a little.

I lowered the 10k to 5k. No audible difference.

I then lowered the 22k to 10k- also no audible difference... maybe it wasn't enough to make any difference.
I might try as you suggest and use 1k.

Other than that I like how it sounds. Thank you tubeswell and Chuck H.