Modded Epi Valve Jr [v.3] with very slow power tube oscillation problem...?

[norstroms]

I read somewhere that the Epi Valve Jr is prone to oscillation problems when mods are applied, and it now appears I've fallen down that hole.

I bought the head-only version back around 2008 or so, have loved it as an un-modified go-to amp for those rare moments I have these days for plugging in and reliving what it's like to play an electrified geetar. [Background: I happily became a father in 2011 and so my priorities have necessarily shifted... I'm slowly gaining back some time these days after everyone's off to bed to get back into our shared area of interest  ]

I always knew I was going to mod this thing, I just never got around to it until recently. It started with visiting thetubestore to get some recommended replacement tubes, and I saw that they offer this BitMo Trio kit. $50 for a collection of parts that couldn't have cost more than $10, but I recognize they put work into creating this kit so I gave it a shot. No big whoop. I suspect that this is a topic that was discussed at length about 5 years ago and I'm just now showing up to the party. Bear with me, thank you for your patience.

The gist of the kit is a three-way voicing switch and a tone control with pull-boost. Great. Installed it according to instructions, and have enjoyed it for the last couple weeks. Noticed sometime last week that one time I was playing with it the EL84 unexpectedly went into a shut-off state. The pilot is on, the fuse is not blowing, but the power tube seems to have lost it's heater supply or something. After maybe 30 seconds the tube powered up again. Didn't think much of it at the time, just filed it away for a future look-see.

Last night I added my own negative feedback circuit (22k, as described in the 5E1 schem) and was very impressed with the results. I hope to make that a three-way switch as well, something like 22k/1meg/disconnect, to adjust how much neg feedback or none (stock). I hope to end up with a switch that takes it from Champ to something-in-between to stock crunchy Valve Jr.

After doinking around for a few minutes with the new neg fb mod in place, the EL84 goes into a power-down state again. Comes back up after about 30 seconds. Doink around for a moment, then EL84 power-down. Repeat. I switched out the tube to another one just to make sure that tube wasn't faulty, same result. And that's where I stand at the moment.

I did a cursory search for this and haven't found much besides the post that indicates that the stock circuit is prone to oscillation when mods are performed. So I looked at the BitMo mod and noticed that the design is kind of effed-up. I can't think of any reason to run all those lines from the new switches to the ground side of the pre-amp cathode resistors, seems like there's a lot of unnecessary wiring that could better be grounded directly to the pot they come off of. I could run a ground buss from the volume pot across the input ground to the new tone pot. So my next step is to clean up my install of the mod and remove/correct some unnecessary details. I also saw where an engineering student added a grounding shield over the back side of the 12AX7, inside the amp on the circuit board. Might try this...

Anyway, my only guess is that there is a very low frequency oscillation hitting the power tube. I'm not hearing any tone issues when the circuit is actually working. The tone with the neg fb is excellent, very clean compared to the grit of the stock circuit. I can post pics and/or schems if anyone's interested. Any advice on solving this would be welcome. Thanks!

[sluckey]

Power on, power off, power on, power off, etc... Is this what you are calling "a very low frequency oscillation hitting the power tube"?

The filament is easy to see in the EL84. Do you lose the glow when the amp powers off? Same for the other tube? Can you measure some voltages? Do any voltages drop out when the amp powers off?

[norstroms]

Yeah, this is not the sort of oscillation that you can hear like a sine wave or farting or something, it's a power on/power off cycle. The tone, when the circuit is passing audio, is great and I'm not hearing any whistles, sine waves, farting, or other audible evidence of oscillation. If there is some runaway oscillation happening it is in an inaudible frequency.  And yes so far the only clue I have is the filament appears to be cycling on and off. I'll make some notes about measurements taken and post over the weekend, I hope. Thanks for any input.   

[shooter]

I had a similar problem with el-84's, but never 30seconds, more like 5-10.  The wise folk here had me check drive to the EL's, walla, I was hitting them with about 5times max drive shutting down the tubes, but that's a left field idea, keep following Sluckey and  you'll stay outta the weeds! 

[PRR]

Heater cycling is a bad connection.

I have seen just that on a tube which was tossed into a dumpster. (And it was a metal tube so I couldn't see the glow and didn't tumble for some time.)

Since you have changed the tube, back-track all the heater wiring to the power transformer.

The "Head" once had a crude DC supply for the heaters. I don't see how this could "cycle"-- I would expect a diode to spew its guts and quit forever (or smoke the PT).

Are you safe/competent to measure voltages? Are maybe ALL voltages going down together? Like the 300V? There is probably a Thermal Overload cut-out inside the PT. On cheap gear, these are usually one-time cut-outs, but there are self-resetting cut-outs also. And a 30 second cool-off would not be unexpected.

And then the question is: why is it cutting out? Is the EL84 red-plating? Is the PT getting too hot to touch? If the amp runs reasonable but the PT cuts-out, maybe they got a bad (too sensitive) cut-out. While brave men can open the PT and investigate, IIRC eBay has piles of ex-Junior transformer sets from people who replaced the iron with better stuff. Which of course is another option for you.

[drew]

It's probably the issue discussed in the second post in this thread:  http://forum.gibson.com/index.php?/topic/49880-my-valve-jr-stopped-working/

I had a similar problem with a Valve Junior that sat unused for several years before I modded it into a Beast Champ.  It would work, then it wouldn't work, then it would, then it wouldn't.  Since cleaning the contacts in the heater fuse holder, it's been fine.

[norstroms]

Whelp... I did indeed find the heater fuse holder was the likely culprit. I found that the plastic casing was in a rotated state which made the fuse connection itself intermittent. Secured the fuse holder [the fuse was intact] and haven't had the heaters flip on and off since. After establishing that much, I cleaned up the mods a bit, grounded them properly, and added the negative feedback switch. As mentioned elsewhere the negative feedback ended up not making much of a difference. You can hear a little of the effect when it's cranked up, but it's pretty useless at lower volumes. Oh well! Maybe one day I will install better components (quieter resistors in places, bigger caps where necessary, transformers) but... meh, it's pretty decent left alone at this point.

Thanks again for the thoughts people chipped in. Love this board.