Epiphone Valve Senior problem

[g1]

Can you repost the schematic? The one in your first post is not working.

[bigdrums925]

Here's the schematic againvalveseniorjventura4bjpg voltages.pdf

[bigdrums925]

So I haven't seen any responses on this post for a couple days now, am I doing something, or not doing something correctly?

[g1]

How about a resistance check from the junction of R12/R15 to ground?

[bigdrums925]

4.3K, 4.6K with j13 removed. Do you see that as what should be? seems right to me, thanks

[MarkusBass]

You need to help us to help you. To do it I would perform some basic tests:
- remove preamp tubes and provide the signal directly to the power amp (C13 capacitor). Does the problem still occurs?
- plug in the V2 tube (without V1). Does the problem still occurs?
- when the problem occurs, is the REVERB pot fully counterclockwise, or not?

These are just initail tests, which I would perform in order to learn about the root cause of the problem.

Mark

[bigdrums925]

Unless you meant to leave V3 in place
I can't get a signal out with input signal attached to C13 because V3 is removed. I presume you meant C24?
Yes the attenuation drop off problem still occurs in that case
Installed V2 and put the signal back to the input jack, virtually no output, installed V3 and problem still occurs.
All tests done with reverb pot fully counterclockwise

It appears the problem is in the power amp section, yet I can't seem to isolate further than that.
What do you suggest to try next?

[MarkusBass]

Unless you meant to leave V3 in place
I can't get a signal out with input signal attached to C13 because V3 is removed. I presume you meant C24?

Let's make it clear: preamp tubes are V1 and V2. When I said "remove preamp tubes" I meant: remove V1 and V2 (and nothing else). Power amp tubes are V3 (phase inverter) and V4 and V5. The input capacitor to the power amp is C13 (marked on the schematic as C13/C14). C24 capacitor is not an input capacitor. I refer to the schematic posted by you in post #17.

Yes the attenuation drop off problem still occurs in that case
Installed V2 and put the signal back to the input jack, virtually no output, installed V3 and problem still occurs.

As I said, tests without V3 do no make much sense. Do not remove V3.

It appears the problem is in the power amp section, yet I can't seem to isolate further than that.
What do you suggest to try next?

- I would verify that the phase inverter is working correctly. To do this you need to remove preamp tubes (V1 and V2) and output tubes (V4 and V5). Check with the oscilloscope that on V3 you have inverted signals (you may need 2-channels oscilloscope to do it).
- I would verify that that the MASTER pot is wired correctly. To test it you may disconnect the pot and replace it with two 220k resistors. This test should be performed with V3, V4 and V5 tubes in the amp. The input signal should be provided to C13.

Mark

[bigdrums925]

Thanks for staying with me Mark.
Yes, the output of V3a/V3b are inverted 180 degrees taken at R17/R18, signal input at C13
The problem still occurs with only the power tubes in, and with V2
Reverb pot fully counterclockwise
I'm at a total loss where to go next, what do you think?

[Alex R]

I'll just add, because maybe you don't seem convinced, that internal oscillation is the first suspect and we haven't eliminated that possibility yet. You see if Enzo says something the rest of us just nod in sage silence, but really it's what we're all thinking

Alas we still have some confusion about tube numbering. Isn't this the correct function of the preamp tubes:

V2 = preamp input and stage 2
V1 = reverb driver and recovery
V3 = phase inverter

So are you saying that with the phase inverter V3 unplugged the amp still responds to the volume control by showing the fault condition? Don't seem possible. If V2 and V3 are still in you've only eliminated the reverb. We're trying to work out which section of the amp has the problem.

So try injecting a signal with V1 and V2 unplugged, if that's not what you've already done. Well, do it anyway and tell us what happens, in full detail (your posts are a bit shorthand tbh).

Skipping on a bit, my first two suspects for causing oscillation would be a bad tube, and poor arrangement of the leads inside the amp, especially the OT output leads.

[MarkusBass]

Yes, the output of V3a/V3b are inverted 180 degrees taken at R17/R18, signal input at C13
The problem still occurs with only the power tubes in, and with V2
Reverb pot fully counterclockwise

I told you to remove V1 and V2 and work with V3, V4 and V5 (signal to C13 capacitor). Now I see that you are doing tests with V2. Or, is it just a typing mistake and you meant V3?
In such difficult cases I alway bring the circuit to the simplest possible in order to see why it doesn't work. That's why I told you to remove V1 and V2, replace the MASTER pot with two fixed resistors.
I would stick to the problem reported by you that touching the cathodes of V3 with multimeter causes the output voltage drop. A multimeter has input resistance in a range of 1 to 10 Mega Ohms. I would solder 1 MOhm resistor there and see what happens. The voltage drop shouldn't happen in this case.
This can be something very simple like e.g. ground wire attached not correctly to the ground of the amp. Could you post a photo how the amp looks inside?
Another tests that I would conduct:
- remove negative feedback resistor. What happens in this case?
- increase C24 to 1n (this is a capacitor that prevents oscillation). Does this solve the problem?
- increase capacitors in the power supply (especially C32) - just solder another capacitor in parallel.
- when the problem occurs move OT wires. Does this change anything?

Mark

[bigdrums925]

Supporting Member

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Join Date:Mar 2007Location:UKPosts:1,247

I'll just add, because maybe you don't seem convinced, that internal oscillation is the first suspect and we haven't eliminated that possibility yet. You see if Enzo says something the rest of us just nod in sage silence, but really it's what we're all thinking

I have complete respect for Enzo and his ability to get at the problem area right off, as well as everyone here, and thank you all! I only have not been able to properly setup and see some evidence of oscillation as yet. I have searched with the scope for it however. I am happy to try any suggestion and report the results. If I have not done that it's because I ran out of time/parts and had nothing conclusive to report just yet. Either that or I was unsure about how measure the suggestion correctly

Alas we still have some confusion about tube numbering. Isn't this the correct function of the preamp tubes:

V2 = preamp input and stage 2
V1 = reverb driver and recovery
V3 = phase inverter

Yes I believe that is the correct function of the preamp tubes.

So are you saying that with the phase inverter V3 unplugged the amp still responds to the volume control by showing the fault condition? Don't seem possible. If V2 and V3 are still in you've only eliminated the reverb. We're trying to work out which section of the amp has the problem.

I told you to remove V1 and V2 and work with V3, V4 and V5 (signal to C13 capacitor). Now I see that you are doing tests with V2. Or, is it just a typing mistake and you meant V3?
In such difficult cases I alway bring the circuit to the simplest possible in order to see why it doesn't work. That's why I told you to remove V1 and V2, replace the MASTER pot with two fixed resistors.

I apologize for not being clear. After rereading Mark's suggestions I followed them with V1 & V2 and V4 & V5 removed, signal input to C13, verifying correct 180 degree inversion of the signal by V3, measured at R17 & R18. (I was a bit confused as to where I should check for the inversion at first.)
I will follow up with the results of removing the master pot and replacing it with fixed resistors.
And thank you all for helping me, I value the suggestions and education more than you know!
Lets see it this posts correctly................

[MarkusBass]

I will follow up with the results of removing the master pot and replacing it with fixed resistors.

If the MASTER pot is soldered to to PC board, I would leave it as it is. Try other tests I mentioned.
The board does not look that bad but it seems to me that the reverb transformer wires are very close to the tube and to wires that go to the reverb spring. I assume that we excluded problems with reverb but have you tried moving the wires a little bit aside?
On the PC board there is a name of the circuit designer. So if you are located in the States, you could try calling Epiphone and talking to the designer directly.

Mark

[bigdrums925]

I called Epiphone and all they could offer was a copy of the other schematic I mentioned near the top.
V1 has been removed for some time, 2 weeks?
Carefully unplugged each wire 1 at a time, labeled and routed as far from the board and each other as possible
Except the pairs which I twisted together, then tightened each spade lug.
Changed the tubes 1 at a time more than once.
Added capacitors to C24
Changed C23 to 47uf
Removed the feedback loop wire
Checked all the grounds
None of these has changed the attenuation drop off at a certain level, either gain or master
I realize I haven't followed the suggestions to the letter, but did what I could with what I have
What next?

Here's another picture:

and a close up:

[Alex R]

please do follow the simple suggestion of removing the preamp valves leaving only the phase inverter and power valves, injecting a signal, increasing it, and seeing if the problem persists. Really simple thing to do surely? Then we'll know if it's the preamp or power amp. Attacking possible causes is often a waste of time unless you have found out where in the amp the problem occurs. Separating the preamp from them power amp is step one. We haven't yet taken it.