August 16, 2020 0 By admin

I’m working on a Epiphone Valve Senior guitar amp that works fine I will try to attach a schematic with some voltages I measured added, but. Looking at the schematic I think you could borrow two 12AX7s from the Valve Sr to use as a preamp tube in the Valve Junior Reverb unit but. Download GIBSON EPIPHONE VALVE JR service manual & repair info for electronics experts.

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Page 1 of 2 1 2 Last Jump to page: Results 1 to 35 of Epiphone Valve Senior problem. Thread Scjematic Show Printable Version. When it is slowly turned up by either the gain or master volumes will cause the output to drop to near nothing with noticeable distortion.

Then put it back on standby for a few seconds, take it off standby and it comes back to life until the gain reaches a certain level. Then it drops off again.

I’ve changed the tubes, checked the heater and plate voltages and verified most all the values of the resistors, and checked most of the caps. I don’t understand why it resets itself when I turn on and off the standby senlor the plate voltages don’t change with the drop off.

I will try to attach a schematic with some voltages I measured added, but had valge doing that last time. As always any suggestions are much appreciated! I have a second “updated” schematic from Gibson but it has no call outs, r3, sfhematic, etc.

Its a better drawing and largely the same but no labels. My first guess is that at elevated gain settings, the amp is going into oscillation. It is oscillating at frequencies above audio. Senuor is what you’re left with after you have forgotten what you have learned. I don’t see anything I can detect as oscillation on the scope.

Any recommendation on a proper test for that? I would also vote for oscillation. How exactly are you checking this? With a dummy load, signal generator and oscilloscope on the output?

Epiphone Valve SENIOR | Harmony Central

Since you cannot see it, I would suggest detecting at which stage the signal dissapears. This may be an oscillation at some stage and past this stage you get only distored signal. I would check the amp stage by stage. In this way you will learn at least at which stage the problem appears.

Yes I am checking with a speaker load, signal generator and oscilloscope. I was reluctant to probe with the scope around the plate voltages so used a meter set to AC volts. Are my concerns about using a scope on those high voltages warranted? First checked that all plate voltages did not change with the signal output dropping.

Then checked the heater voltages on each tube, also stayed constant when the output dropped. I put a few notes on the schematic one of which says when I touched the VOM probe to junction between the cathode pins on V3 it caused the same effect as turning up the gains.

Can you tell me anything about the 4 resistor array at that location? What is that supposed to do? That was the only place where I could get a changed response.

As far as oscillation, I looked for any trace of such on the speaker output with the scope starting at the lowest output and raising gain until output dropped. Originally Posted by bigdrums Another way to check for correct NFB, disconnect R If gain actually decreases with R15 disconnected, then the OT is phased wrong. Originally Posted by The Dude.


But, I did learn something. There are protons, neutrons, electrons, Experience is something you get, just after you really needed it. So I checked the feedback and it appears to be correct as the output goes up when the feedback is removed Then I chased the signal backwards. AC voltage at the plate of V4 climbs steadily with the increase in volume or master volume pots to 50 VAC, then drops to 1 VAC when the volume at the load drops as would be expected of course but pin 5 of V4 rises from 0 to 8 VAC, then jumps to 9 VAC during the same test.

TROUBLESHOOTING: Epiphone Valve Sr Combo – Ultimate Guitar

DC voltage remains steady on both plate and screen grids at and respectively with light bulb current limiter in line of seniorr supply AC Heater voltage pin 2 or 7 referenced to ground starts at 3 VAC until standby switch is turned off, then stays steady at 2. Seems to me the problem is in the power tubes, but they are brand new and matched.

My only suggestion is that the OP removes the ‘lamp limiter’. It is serving no purpose. Originally Posted by Jazz P Bass. I removed the lamp limiter, before attenuation pin 1, V3 is VDC, VDC after attenuation Pin 6, V3 before is VDC, and VDC after attenuation I am starting to think epiphoen may have something to do with high freq oscillation as Enzo said because the amp will attenuate sooner with a higher freq input.

Anybody suggest which components I should start with to curb such a problem? Can you repost the schematic? The one in your first post is not working. Here’s the schematic again valveseniorjventura4bjpg voltages. So I haven’t seen any responses on this post for a couple days now, am I doing something, or not doing something correctly?

Do you see that as what should be? You need to help us to help you. To do it I would perform some basic tests: Does the problem still occurs? These are just initail tests, which I would perform in order to learn about the root cause of the problem. Unless you meant to leave V3 in place I can’t get a signal out with input signal attached to C13 because Vaalve 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 scheematic 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?

It appears the problem is in the power amp section, yet I can’t seem to isolate further than that. Thanks for staying with me Mark. I’ll just add, because maybe you don’t seem convinced, that internal oscillation is the first suspect and we haven’t eliminated that sdnior 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: If V2 and V3 are still in you’ve only scematic 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.

Last edited by Alex R; at Supporting Member Alex R’s Avatar Join Date: I will follow up with the results of removing the master pot and replacing it with fixed resistors. I called Epiphone and all they could offer was a copy of the other schematic I mentioned near the top.

Epiphone valve Senior reverb

V1 has been removed for some eipphone, 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 eliphone 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?

Really simple thing schrmatic 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 swnior 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. Yes, it looks like you are conducting every possible test but not the basic one we suggested: What happens in this case?

Heater voltage pin 2 or 7 referenced to ground starts at 3 VAC until standby switch is turned off, then stays steady at 2. If it’s oscillation then output transformer leads are the suspect – I see you twisted and rerouted them. If you are comfortable working with the amp on, and can avoid the various high voltage sources, my next step would be leaving the preamp valves out vavle producing the fault condition, then moving leads around to see if it changes and trying to stop it that way.

Swapping in known good PI and power valves also worth a try, especially if they are microphonic. That’s not the ultimate cure, or not a good one, it just tells you where the problem feedback is. If that doesn’t find the problem you need to eliminate the PI stage, again a cap to ground on the input to see if that stops it.

If it’s none of those things then we might have to ask what is the next suspect problem – but it does sound very much like oscillation. I think my next suspect would be a bad OT but that’s a long way second to oscillation. There are wiser heads than mine on here and they may adapt these suggestions to good effect.