Epiphone Valve Standard Mystery Component

[cyrusparty]

Hi Guys, I've had a Valve standard for a few years that quit working the other day. It powered up but I got no sound coming out. I opened it up and one of the heater windings on the transformer had seen a lot of heat. Attached to it was a component that was fried and blackened. I originally thought it was a capacitor, but I found this thread:

http://music-electronics-forum.com/t8459/

and in post #12, one of the replies states:

"The filaments also had a MOV in series with the transformer. It was burned. We never did find any reason for the windings to open up or the MOV to burn up. Power tubes all good & not intermittently shorting."

I checked the windings on the PT and everything is still putting out voltages near the stated values. The only problem is that the MOV is not shown on the schematic (attached below). Perhaps it was acting as a protection for the transformer? Does anyone have a current schematic? Or at least a general idea of what value to replace with?

I also found another thread on the gibson forum discussing potential problems with the PT:

Epiphone Valve Standard Blows Up? - Gibson Brands Forums

From post #2 :
"The other cause, and just as likely because the PT is underpowered and does not supply enough current to properly heat the tubes -eventually burning out those particular windings in the PT. Expensive fix is replace the PT with something more robust - cheap fix (and what I did) was to buy a small 12V 3A transformer, mount in open space on chassis and supply heater current from this instead."

It looks like the transformers are quite cheap and I'm wondering if this might be a better solution in the long term. Does anyone have any thoughts on this?

thanks
Cyrus

StdSch.2005.10.21.pdf

[The Dude]

I didn't see a schematic in your post, but it probably matters not if the device isn't shown on the schematic. If it's in series with the filament windings, I'm wondering if it is not an NTC thermistor. It would be there for a sort of "slow on" filament to prevent a surge on the filament winding as the filaments warm up. That's just a guess. Can you read any numbers on the device, or is it burnt black?

Also, is this the problem? Do the filaments light? What is your measured filament voltage at the tubes?

[cyrusparty]

That's very possible! Here's some photos of the component. It might be a black thermistor. I'm seeing some damage on top of the component and the lead of the transformer was blackened as well, so I'm guessing something happened here! On one side of the component we have 0908 (possibly) and an R.....

"Also, is this the problem?" I think it might be part of the problem, I initially opened up the amp because I was getting no sound.

"Do the filaments light? What is your measured filament voltage at the tubes?" I actually removed the component and, in the process, while I was removing some of the Transformer connectors, some traces lifted from the PCB. I'll put it back together tonight and get some readings for you.

Oh, and I edited a schematic in the bottom of the original post. Apologies if its not clear, still getting used to the set up of this forum!

[cyrusparty]

Alright,

Measured between pin 4 & 5 on the EL84 (V4 and V5) I'm getting 5.3VAC. On the 12ax7's (V1, V2, and V3), I'm measuring between 4 and 6 and getting under 0.1VAC.... If (on the 12ax7's) I measure between pin 4 and ground, I get 2.7VAC and between 6 and ground it's under 0.1VAC...... Could something be amiss in the pre-amp section? I've re-attached the schematic, because in my original post it was showing up as a link, I probably checked the wrong box somewhere......

[Jazz P Bass]

Uhmm.
The 12Ax7 heaters are pins 4/5 & 9.

Pin 6 is the high voltage plate.

Please recheck the voltages.

[cyrusparty]

oops, I misread the schematic! I am getting 5.84VAC between 4 and 9 as well! sorry about that!

[J M Fahey]

And between 5 and 9?

[cyrusparty]

I'm getting 5.84VAC across pins 5 and 9 as well. I should of clarified that as well, thanks for your patience!

[cyrusparty]

So, thinking about this with a fresh brain, I'm getting heater voltage to all tubes and, despite the iffy looking component and the burn marks on the transformer lead, it appears to be doing its job. It seems possible that, in terms of the immediate problem (no sound), that this might be the wrong path to look into. I'll take another look a little later today, and see if anything pops up.