Low volume, possible cathodyne issue, help needed.

[eschertron]

if you can get some kind of test signal (PC sound card? smartphone app?) then you can measure accurately what's happening.

To get an idea of each stage's gain, measure the AC volts at the grid, and then at the plate. That (plate/grid) will give you the actual amplification factor of the stage. What is likely happening in your amp, is the signal is being lost as it goes through the tone stack with no recovery stage after. A standard FMV stack can lose 20-30dB, a reduction that results in a loss of 90% or more of the signal! If you haven't played with the Duncan tone stack calculator TSC then try it.

[Chuck H]

This stage is only increasing the 2v to 3v at the PI grid.

Well... That's 33%ore signal idn' it.?. If you had significant voltage going into the PI a 33% increase would matter more. You can use the Wizards design with the grid stopper at the PI for it's benefits, but you need a more typical/correct voltage in the first place.

[wozt]

Full pre-amp schematic.

[Chuck H]

Where did that tone stack come from. It would help for evaluation if we had component values for C5, C6, R14, R15, R16 and if R14, R15 and R16 are user adjustable from the control panel? If not, what are the tapers and fixed positions used. All the same that tone stack looks VERY lossy. Not to mention that it's complicated with opportunities for error.

[Dave H]

Full pre-amp schematic.

I don't think there is a fault. There just isn't enough gain in that pre-amp.

[Chuck H]

I don't think there is a fault. There just isn't enough gain in that pre-amp.

I agree. That's sort of what I was getting to. Replace the TS with a more typical "tweed" type or a simple variable LPF and TA DA!

Still interested in where that tone shaping circuit comes from.?.

[wozt]

I agree. That's sort of what I was getting to. Replace the TS with a more typical "tweed" type or a simple variable LPF and TA DA!

Still interested in where that tone shaping circuit comes from.?.

It's a Hiwatt tone circuit.

[Chuck H]

Ah... So it is. It's a lossy stack

The thing is, you have more attenuation behind the tone stack and less amplification ahead of the tone stack compared to a Hiwatt. Also, Hiwatt amps were intended to be LOUD and CLEAN. I know that didn't stop Pete from cranking them into some distortion, but they weren't the grind boxes that Marshall was building. In other words, they had less gain than other amps and yours has less than a Hiwatt. Cobbling circuits from different type amps together doesn't often work out. It can with similar type amps, but you have a tweed Fender style mixer and cathodyne PI and then a Hiwatt tone stack. Notice that most amps using a cathodyne PI have the preceding gain stage going straight in with virtually no voltage division. You're feeding your cathodyne with a tone stack with probably 20dB or more attenuation! It's time to rethink the whole design or make a drastic compromise in the tone stack options like moving to a variable LP filter. A plain ol "tone" control.

[Dave H]

I don't know which 6V6 power amp you are using but my cathode biassed 6V6 amp has 23V bias and a fixed bias 6V6 amp could be 30V or more so you are going to need about 30V peak to drive the 6V6 grids and that tone stack could have 20dB (x10) attenuation so the tone stack driver needs to produce 300V peak which it can't. The amp needs a gain stage between the tone stack and the cathodyne.

[wozt]

OK i get the picture. I'm going to try running v1 a b as serial stages, then tone stack then using v2a as a recovery and see how that goes. I think my next build is going to be a a non-cobbled together one.

[Chuck H]

Cobbling is OK. It get's better when you become more familiar with the circuits. Something look out for during your rewire is to keep the preamp moving from input toward the output. If you, for example, just rape one of the input triodes for the recovery stage the preamp will be doubling back on itself and the output will be coming from near the early preamp stages. And that's asking for stability problems.

[wozt]

Given I have 3 halves of 12ax7's available, would you go for

12ax7-tone stack-12ax7-12ax7. or

12ax7-12ax7-tonestack-12ax7.?

[Chuck H]

One recovery stage after the tone stack should be ample. A 12AX7 has considerable gain. Placing the tone stack as late in the circuit as possible is most desirable for amps that will spend some time clipping the later stages. I'll vote for the latter. Stage/stage/tonestack/stage.

[wozt]

kept the 12dw7 in the socket while resoldering and I have burnt out the tube grid I think, maaaaaaaaaaaaaan.

[g1]

I think you meant to say Canola up there Chuck. (#26)