Amp Help?

[squirrels]

I’m not sure of this is the proper forum for amp debugging but I know some of you work on amps and I’m not on any other forums, so...

I picked up  a Valve Jr. combo a few weeks back to modify. Long story short, I had one years ago that I did a few mods and component tweaks on that I had to sell due to financial reasons. I’ve been casually looking for one recently and picked one up for $80. Sounded pretty good right off the bat. So far, I’ve only added a master volume to get a cleaner tone at lower volume to use as a pedal platform. A few days ago the volume cut out completely. If I plug in a patch cable I get some sound but it’s pretty low. The other day I used a can of compressed air in the tube sockets and the amp worked normally for a little while.

I measured all of the resistors and a noticed that R2 (68k) had a bump on it and R7 (1m) would only give me a measurement of 501k only when the volume is turned down. I clipped both out and replaced them... no change. R11 was giving me no reading or it would fluctuate, I haven’t replaced this resistor yet. I also changed R5 to a 250k pot for a master volume. I’ve also reflowed most of the solder joints. Some of the wires on the output transformer look like they were a little melted as well, possibly an OT problem?

Any help would be appreciated or if someone knew where I could find info that would be just as helpful. I am also aware that tube amps can be dangerous to work on so I assume all responsibility on any advice given.

Here is the schematic I found online:

[Travis]

Have you powered it up with the chassis open and tubes installed? How do your voltages compare to the schematic when the tubes are in?

You might be able to observe something going wrong if you watch it as it’s powered up

[squirrels]

The chassis is out sitting on top of the cabinet. The tubes all glow and lower gain tubes create less volume. I haven’t checked voltages and haven’t looked at the board during power up. I’ll read up on how to check the voltages and do it later. Thanks Travis.

[Travis]

Since the heaters are DC you can measure each pin of the tube socket with your meter set to DC, just make sure your meter can handle the high voltage on the plates

The reason I say it’s better to measure with the tubes in, is that the voltages will go up significantly without the load of the tubes

[squirrels]

I’ll have to check if my meter can handle it. I’m assuming that R11 would be the bleeder resistor or am I wrong? I was going to snip on leg and measure it since it’s acting funny soldered in.

[Travis]

Yes you’re right about R11 and that sounds like a good idea

[squirrels]

I measured R11 and it’s OK. I also lifter a leg from R7 and tested and it was fine as well. My meter is good for 600v max but I don’t have a reliable set of probes. They broke and all I have are alligator clips with one of the ends of a probe that I clip in. Fine for pedals, not going to even try it for an amp.

So then I tried blowing air into the sockets again and it fired right up. Maybe all I need is some contact cleaner or new sockets?

[squirrels]

...and I put the chassis back into the cab and back to low volume.

[Frank_NH]

I’m not an amp guy by any means, but I would check all of the bypass electro caps C3, C4, C5.  It’s possible one or more of them is bad, which would definitely reduce the output.

[squirrels]

I’m positive that it’s actually the sockets. I’m going to get some cleaner tomorrow and see if that helps. If I use some compressed air in the sockets, it fires up normally for a few minutes then goes quiet.

[squirrels]

Definitely the sockets. I sprayed a bit of contact cleaner on them and after I saw a little bit of lightning-in-a-bottle when my EL84 started to spark (due to one of the speaker wires coming loose...oops) it came back on and has been running well since yesterday. I wound up bidding and winning on a used Valve Jr. transformer because I thought the low output was because the OT went bad. I’m kind of glad I have it now just in case.  I know I’m very to the party with the VJ but it’s a decent little amp for pretty cheap. It’s nice to have a cleaner amp for a pedal platform to go with my OR15. Thanks for your help everyone.