New project

[gui_tarzan]

I'm going to sleep on this, I'll get back at it tomorrow.

[Tom Phillips]

...the resistor read just shy of 820 ohms...

The big clue to the problem area remains that the V2 pin 3 cathode is at 0V and should be ~1V. Did you measure the resistance from the V2 pin 3 socket pin to chassis ground. (Not just across the resistor? Also double check the voltage reading. Is it really 0V?

[gui_tarzan]

810 ohms from ground to pin 3. I must have had my meter set too high last night to read the voltage, this morning I have 1.11v on pin 3 with the meter on the 20v scale. So if I'm getting adequate voltage on the tubes it should have normal volume unless there's something wrong with the OT. I wish I had a scope... especially since the doggoned thing played great that one night.

Yeah, I know how to use a meter, I just forgot to set it to the right range last night.

[Tom Phillips]

810 ohms from ground to pin 3. I must have had my meter set too high last night to read the voltage, this morning I have 1.11v on pin 3 with the meter on the 20v scale. So if I'm getting adequate voltage on the tubes...

OK. So that suspected issue was a false alarm.

...it should have normal volume unless there's something wrong with the OT...

There could be many many other things causing the problem. The OT could be one of them but I'd say that the probability that it's the OT is very low. There is a high tenancy to blame the OT in discussion forums. It's the electronic form of hypochondria. "This signal is low. It must be a bad OT" = "I don't feel good. I must have cancer."

An organized troubleshooting approach is the way to go. There are other unanswered questions and maybe untried suggestions in the recent posts that may lead to a solution.

[Jazz P Bass]

At this point, have you actually measured the Vac at the speaker terminals.
Set the amp for as clean as it can get before breaking up.
Bang a few chords & note the voltage at the speaker.
V X V divided by the speaker resistance = output wattage.

Also, you can measure the Vac at the output tube grids.
This will tell if the preamp is doing it's job of trying to drive the output tubes.
I would expect a nice healthy signal (ie: 90 Vac) at the grids.
If that is so & the output tubes are known to be good, if you still perceive low volume, that leaves the output transformer as suspect.

[gui_tarzan]

OK. So that suspected issue was a false alarm.


There could be many many other things causing the problem. The OT could be one of them but I'd say that the probability that it's the OT is very low. There is a high tenancy to blame the OT in discussion forums. It's the electronic form of hypochondria. "This signal is low. It must be a bad OT" = "I don't feel good. I must have cancer."

An organized troubleshooting approach is the way to go. There are other unanswered questions and maybe untried suggestions in the recent posts that may lead to a solution.

I agree with you, it just seems odd that it worked fine when the NFB was hooked up wrong and now it doesn't.

Maybe a little more information... I put my signal generator on the 2 & 7 of V1 and have a quiet tone. Same with pin 2 on V2. Same on pins 2 & 7 of V3. When I put the probe on pin 5 of V4 & V5 I have much more volume.

[g1]

Have you tried it with the NFB disconnected?

[Justin Thomas]

Thinking put loud... Is it possible that you're getting so much negative feedback that it's practically nulling the signal? Does the resistor actually measure its stated value? The reason I ask - I was using a pot to figure out what value of feedback resistor to use in a small amp, and as it approached one extreme, the sound would start to lose volume, then distort, amd eventually disappeared. I couldn&t tell much difference at the other end so I skipped the loop entorely. Is it possile to kill a signal by doing that - total cancellation, or did I just make aml kistake in my own wiring?
Resistors have been off in value by magnitudes of lots - don't trust the color codes automatically; I've also read them backwards if the tolerance band is something other than none/silver/gold. I had one just the other day - had a red band in BOTH ends, and I was reading it wrong. A quick meter check fixed that...

Just a thought...

Justin

Edit - sorry if I missed any info, forgot to read pqge two of replies frok today by Tom and g-one...

[gui_tarzan]

Have you tried it with the NFB disconnected?

Yes, no difference. Something weird is going on in the pre section but I'm having trouble finding it. I measured all the resistors in the amp today and they all read accurately (within tolerance). I have changed pre-amp tubes several times. I can inject a 1khz signal as mentioned earlier at the input stages of V1, V2 pin 2 and the PI and I get the same quiet results. As soon as I inject it in the power section the volume increases quite a bit.

[g1]

Your PI plate voltages don't jive with the cathode voltage. What is the resistance if you measure from PI pin 3&8 to chassis?
What is the value of the presence pot?

Edit: Never mind about the cathode voltage, bad math.

[gui_tarzan]

18K from pins 3 & 8 of PI to ground. The presence pot reads 6.6K in the circuit.

[g1]

If you are getting ok level when injecting at power tube grids, work backwards from there.
Inject at pin 7 of PI and measure AC volts at Pin 6.

[g1]

Another thing, maybe a typo, on your drawing the .0001 coming off vol.2 wiper, not drawn as per 5F6A.

[gui_tarzan]

You're right, it's on volume 1 on the 5F6 schematic. It's actually on volume two and there isn't one on volume one of my build.

[g1]

Well partly, but what I meant was that yours is shown in series with the wiper, it's actually supposed to go across the pot, from hot to wiper, not in series at the output of the pot.