Sovtek EL84M

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Hi, I was curious if anyone has been getting any of these tubes failing early. I have used them a lot within the past 5 years with great results. Now this is the second time one has failed on me this year. I wonder if I should just switch suppliers, I won't name names on here. Or I wonder if this is a plant production thing and everyone is getting more duds and switching suppliers won't matter?

I also understand modern tubes suck and your bound to get a dud here and there.


Anyway, the amp is a Fender Blues Jr, cream board. Originally in for a bad power tube, original tubes from factory way over biased of course. I replaced them with Sovtek El84M because the guy wanted nice clean tone with a very rugged tube that will last a long time. I set bias to 22mA plate current. Tested fine on the bench, output looked great with no visible oscillation. I played it and it sounded great, I even go through a few on off cycles, on for an hour, off for an hour, on for an hour off for an hour etc... I do this after all repairs anyway but especially with power tube replacements, the duds usually fail early in life. This was back in September 2015, so almost 9 months ago. He says he brought it home, played it and it was fine. Hasn't touched it until recently for a gig and while he was playing he noticed a smell, well that was a resistor in the power supply burning a hole in the PCB:flame: The tubes have an internal short from g2 to g1, so pin 9 to 2, there was 300v on g1 pulling tons of current and burning up power supply resistor.

I threw another set of power tubes in and everything seems fine. Gonna eat something quick and maybe try and get it to misbehave a little, something is chewing up these power tubes. I have heard about oscillation issues with these amps in the phase inverter section. Here is some interweb deetz; Cream Board Phase Inverter Oscillation – Billm Audio

Tubes or amp? That is the question.
 
Hi, yes I have a 500k trimpot || with R51 (33k) to adjust bias without too much surgery. With the 6P14P-E I had the bias voltage at -12.4, now I have a pair of GE 7189's, they need a lot more bias but seem to be happy with -14v.

I quickly looked at the output and I see something at around 50kHz, 50mV in amplitude. It's present at R34 but not R33 so maybe this does look like PI instability? I shut the amp off and plan on pulling preamp tubes to see if it goes away.

To get everyone on the same page: http://support.fender.com/schematics/guitar_amplifiers/Blues_Junior_schematic.pdf

Edited: I had wrong resistor numbers
 
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Definitely something there. I put a 250pF cap across R30 and it helped but it was still visible.

I removed the cap and took a sheet of Aluminum and touched it to the chassis and placed it in close proximity to the ribbon connectors, sure enough the oscillation completely disappears, move the aluminum out of the way and it comes right back. The piece of aluminum sheet on the back panel of the amp forms a small amount of capacitance to ground effectively reducing the oscillation. Personally I feel that clearly with this amp it's not enough.

Not sure what course of action I will take here. I kind of want to find out what it is in the PI network causing it, possible feedback loop?
 
I would have anticipated PI instability showing up on the other side?

R33-36 could all be 10x greater, particularly if the tubes are fragile.

The worst part about this design is that the grid stop resistors aren't even on the PCB that the sockets are attached to, they are on the main board connected via ribbon connectors. I don't understands who designs these things? :confused:
 
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