Low volume in preamp stage

[Realist2]

Greetings, I have a simple Montgomery wards, tube amp using, two preamp tubes, a phase inverter, and two 6L6 output tubes.
My problem with the amp is low volume. While troubleshooting I fed the power amp with another preamp feed, at the channel mixer output, and it was loud and proud.
The voltages are close to spec. With the exception of off the last filter cap, which is low by 25v. This is the supply for the preamp where the problem seems to be.
The reduced amplification is can be seen with an Oscope right off the first stage plates (pin1) in either channels preamp tubes.
I am wondering if changing the value of the resistor between #3 and 4, filter cap in an attempt to raise the voltage to spec would be a bad idea?

http://music-electronics-forum.com/a...56a-edited.pdf

[J M Fahey]

Irrelevant.

You may rise gain at both input stages by bypaing cathode resistor with, say, 1 to 10uF x25V caps.

[eschertron]

@Realist2 -
A couple questions for you:
Test (signal) voltages are not printed on the schem you posted. How do you know there's low gain at the plates of the first valve?
If v1 is something other than a hi-mu 12AX7, it may not give you the amplification that you want. Have you checked/swapped it?
Have you checked/swapped v2?
Have you tried a guitar-level signal into the amp with all controls maxed? How not loud and proud is it?
Does it look like someone may have modified the amp in any way before you got it?

edit:
if the second to last filter cap is 360vdc and the last filter cap is not 350, but 325vdc, then there's (360-325)/22k = 1.6ma across the 22k resistor. Actually a lot less than what I'd expect, especially since the drops across the 4 preamp plate resistors (using the schem values) add up to 2.8ma. Juan is right. It's the numbers on the schem that are completely out of whack, not the amp.

[Realist2]

Irrelevant.

You may rise gain at both input stages by bypaing cathode resistor with, say, 1 to 10uF x25V caps.

The amp is only producing about 1/5 of its normal volume. My intent is to repair rather than modify, but I'm at my wits end with it.

[g1]

Those resistors on the grids of the input tubes are less than 1/5 of what I'd expect to see there, especially considering they are supposed to be for "microphone". Are they really 150K ?

[Pedro Vecino]

The value of screen grid resistors is 470K. I imagine it's a mistake in the schematic.

[Enzo]

If that 350v supply node is only 325, that is less than a 10% variance, and hardly matters. it certainly will not have a material effect on loudness.

All those inputs are marked MIC, and those are Hi-Z mics, and probably intended for cheap crystal mics, which actually put out a pretty hot signal.

[Enzo]

If that 350v supply node is only 325, that is less than a 10% variance, and hardly matters. it certainly will not have a material effect on loudness.

All those inputs are marked MIC, and those are Hi-Z mics, and probably intended for cheap crystal mics, which actually put out a pretty hot signal.

[Realist2]

eschertron-
(Test (signal) voltages are not printed on the schem you posted. How do you know there's low gain at the plates of the first valve?)
I was able to compare the sine wave with an Oscope to another amp that is working.

(If v1 is something other than a hi-mu 12AX7, it may not give you the amplification that you want.)
All three are 12ax7 and I have swapped them.

(Have you tried a guitar-level signal into the amp with *all *controls maxed?) Yes all controls fully maxed its equal to a deluxe reverb just shy of 3.

(Does it look like someone may have modified the amp in any way before you got it?) No, other that the last filter cap was addled bypassing the D cap in the can.

(If the second to last filter cap is 360vdc and the last filter cap is not 350, but 325vdc, then there's (360-325)/22k = 1.6ma across the 22k resistor. Actually a lot less than what I'd expect, especially since the drops across the 4 preamp plate resistors (using the schem values) add up to 2.8ma. Juan is right. It's the numbers on the schem that are completely out of whack, not the amp.)
I don't doubt that. These plans were edited. The diodes were originally drawn backwards. The actual voltages across the filters are, A 395, B 383, C 365, D 325.
Channel 2 plate voltages are 2V over the schematic and channel 1, are 8V under schem, but that difference follows the tubes, and not the circuitry.

G1-
(Those resistors on the grids of the input tubes are less than 1/5 of what I'd expect to see there, especially considering they are supposed to be for "microphone". Are they really 150K ?). As far as I know, I have checked all of the values against the plans and with a meter.

[Realist2]

What would you think may be preventing the tubes full amplification in that first stage Enzo?

[Jazz P Bass]

Have you checked the Volts ac ripple on the power supply nodes?

[Realist2]

Good morning Jazz P Bass, no I haven't, I don't know how or what exactly that is or what symptoms it would produce. I can only guess that you mean AC on those DC voltages?

[Jazz P Bass]

Yes.
An AC component on the CD will actually lower the DC voltage.

In the worst case, if it is the decoupling capacitor (C22) for the preamp, the AC will let the signal 'ride' on the DC voltage, thus lowering the output.

[Realist2]

One thing I did do was temporarily replace the filter cap feeding the preamp, and there was no change, so I replaced the original cap.

[g1]

You are speaking of lack of gain of the first stage, I believe you are probably looking at input signal at the input jack rather than at the grid when you are calculating gain. I believe those grid resistors may be responsible for your signal loss.
Try this: inject your sine wave at the input jack. Measure the peak to peak volts with your scope at the following locations:
input jack
V1 pin2
V1 pin1
Alternatively, just replace the 150K resistor with a 1meg and see what effect it has.