Vintage Epiphone Comet amp. Low Volume. - diyAudio
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Old 5th February 2011, 10:24 PM   #1
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Default Vintage Epiphone Comet amp. Low Volume.

I just picked up a mid 60's epiphone comet tube amp. All the functions work but the volume is quite low. when you turn up the volume knob, you really don't get any volume til 7 and there is little difference between 7 and 10.

The amp runs on (2) EL84/6BQ5, (1) 12AU7 and (2) 6EU7. I noticed that both EL84 power tubes are just barely hanging in their sockets. They are so loose. Could this be the issue with the volume. Should I try to retension the sockets?

Here is the schematic.

http://www.webphix.com/schematic%20h...2rvt-comet.pdf

Any help would be much appreciated.
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Old 6th February 2011, 04:02 AM   #2
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Location: Victoria, BC
Welcome to DIY Audio!

How much experience do you have working on amps?
What do you have in the way of tools & test equipment?
The answers will determine what sort of troubleshooting you will be able to do.
I've never had much luck re-tensioning sockets, but it certainly can't do much damage. (Unless the socket area is awfully crowded, I've just replaced the socket with new/NOS.)
Make SURE that the power supply capacitors are discharged before you start messing with the sockets.

Do you have another amp that you can use to test these tubes, or have you tested the tubes?

Cool amp, BTW, and a change from the usual (Fender) circuit.
John

Last edited by VictoriaGuy; 6th February 2011 at 04:09 AM.
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Old 6th February 2011, 04:14 AM   #3
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Default epiphone amp

I do have some experience working on tube amps. I know about discharging caps. I have recapped several of my amps. I build guitar pedals also. So I do have some electronics knowledge. I have a volt meter, oscilloscope and tube tester (although i'm not sure how accurate my tester reads tubes).

All the tubes tested good on my tester. So I'm sort of stumped.
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Old 6th February 2011, 06:57 AM   #4
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Tom-
You shouldn't have any trouble tracking down the problem- you have the tools and enough experience.
Sounds like the tubes are OK- though most testers don't work tubes very hard.
If I were looking at your amp, I'd....
Definitely get those sockets tightened up if you can. Also, cleaning the sockets and tube pins can sometimes help- DeOxit or similar. I just read a suggestion about using an old guitar string (A?) to clean sockets - I haven't tried this.
Next thing would be to record some voltages, see if they all look OK. If you do that and post a schematic with the voltages written on it, you might get somebody more expert than me to suggest something.
If the voltages look OK, I'd start signal tracing with the 'scope.
Afterthought..
If one of the PS caps is leaking, it could pull down the B+ without blowing the fuse. (Is the fuse the right value?)

John
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Old 7th February 2011, 10:56 AM   #5
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By far the most common cause in valve amps is the anode loads of the triodes, these go high or O/C.

So check R6 100K, it's the only one in the audio chain (the rest are in the reverb section).
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Old 9th February 2011, 09:40 PM   #6
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Thats the first time Ive seen an interstage coupling transformer used as a phase splitter in a guitar amp. That is not likely to be your problem, but very unusual. Scope should be your friend here. Do you have some way to make a constant tone like 100K hz? Dont try to troubleshoot with your guitar as a signal source. Look on the center of the "loudness" control with the scope and see if it has that "all at once" effect that you hear with your ear. Also swap tubes of the same kind in position. Just some things to try.
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Old 2nd March 2011, 03:01 AM   #7
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I checked this amp out with a scope. Here are the results...any help would be appreciated.

-B+ 340V has 25V of AC on it.

-On the schematic...V1A appears to be amplifying normally yet the signal on the grid of V2A (2nd stage) is much smaller then the signal on the anode of V1A (preamp)

-Injected guitar signal into the grid of V2A and it was just as loud as if I plugged into the input jack

-I suspect I have an issue between V1A and V2A.

Any thoughts?
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Old 2nd March 2011, 08:11 AM   #8
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Have you checked the voltage on the anode of V1A?, R6 is a resistor that commonly goes duff in such amps, and causes low volume.

There will be loss between V1A and V2A, as the volume and tone controls are there, what is the signal like on the top of the 'loudness' control?.
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