New JJ 6v6S very odd issue

[sluckey]

Really ey? You can get a short between the plate and the filament?

Absolutely. And the number one cause for plate to filament short (pins 2 and 3) is banging away on the guitar when the speaker is disconnected. When the OT secondary circuit is open, ie, no speaker, and you hit a power cord, a huge signal will be coupled back through the OT secondary circuit and stepped up to the primary circuit (flyback). This high voltage signal will be present on pin 3 and will arc to pin 2 because pin 2 is only 100Ω away from ground. Pop goes the resistor if you are lucky. Many times the arc is short lived and the only clue is the burnt 100Ω resistor. But sometimes the arc will burn a path through the plastic, or phenolic tube socket to pin 2. This burned carbon track will actually measure very low resistance. When this happens, you are in danger of burning the OT and/or PT. Just hope the fuse does it's job!

[cys]

I recently acquired a vintage amp that would short pins 2 and 3 of one of its 6L6 tubes after 10 minutes of playing with the volume dimed. In this case the tube shorted internally. It was repeatable with another brand of tubes. Once it did it a second time, I hooked it up to a scope and saw a significant flyback transient. I used high voltage differential probes for the measurements. I then fitted MOV's across each half of the OT primary to the center tap. No problems since.

[LOUDthud]

Two common remedies for the pin 2 to 3 short are:

Resistor across the speaker, 180 Ohm 10W for a 50W amp.

Diodes from the power tube plates to ground. Usually three 1N4007 in series.

Ok, three: Ceramic Tube sockets.

[cys]

Two common remedies for the pin 2 to 3 short are:

Resistor across the speaker, 180 Ohm 10W for a 50W amp.

Diodes from the power tube plates to ground. Usually three 1N4007 in series.

Ok, three: Ceramic Tube sockets.

MOV across each primary half to the CT as fourth remedy (?).

[LOUDthud]

MOV across each primary half to the CT as fourth remedy (?).

That can work. That was used on the Traynor YBA-1A in the 70's. When you find a used one, those parts are either missing or burned up. I built a solid state amp (no OT) that allows the spikes up to the breakdown Voltage of the output transistors. I used an MOV across the speaker.

A simple R-C snubber network across the OT primary is seen on some Fender designs starting around 1990. I tried an R-C across the OT secondary, .1uF and 10 Ohms 2W IIRC. That didn't look like it worked good enough. The object is to knock the spikes down to a manageable level, not kill them completely.

[pjd3]

I took cys as meaning the secondary ct as I not familiar with any primary coil with a CT but, maybe there is.

Phil D

[pjd3]

A Plate to filament short would make sense due to the immediacy of the 100 ohm resistor burnout. It wasn't a slow cook, it was like an instant crackle/smoke/pop/black all at once. And I lost an OT secondary output wave form as I was increasing the volume with a 200mA p-p sine. I was using an 8ohm power resistor at 1Khz to try to emulate some acceptable impedance.

I left the amp on all day yesterday with the NOS Sylvanias. Stayed pretty steady. Except, the plate voltage slowly decreased over a couple of hours from around 447 down to 436 over a few hours. The bias current seemed to stay in the same range, maybe fall a milliamp or so. I had to wonder why the plate voltage would would drop but, it wasn't enough for any red flag concern, just a curiosity

Thanks all,
Best,
Phil

[martin manning]

I left the amp on all day yesterday with the NOS Sylvanias. Stayed pretty steady. Except, the plate voltage slowly decreased over a couple of hours from around 447 down to 436 over a few hours. ... I had to wonder why the plate voltage would would drop but, it wasn't enough for any red flag concern, just a curiosity

Monitor the mains voltage at the same time. That could explain most or all of it.

[pjd3]

Ah yes, thats right, a little fluctuation on the mains can look like allot on the HT.

Thanks,

Phil

[pjd3]

Guys,

Can mistakenly plugging a 16 ohm speaker (V30) into an 8 ohm tap exacerbate the shorting issue? I haven't heard a thing about that but, just looking under all stones. I may have done that.

Phil

[sluckey]

not likely.

[cys]

I took cys as meaning the secondary ct as I not familiar with any primary coil with a CT but, maybe there is.

Phil D

I was referencing the primary on the output transformer.

[pjd3]

Thanks for the clarifying cys

and Sluckey for the comment.

Phil

[MeanMrMustard]

I don’t buy the JJ 6V6s anymore, even Apex matched which is standard from CE. Sucks about the price difference against all others but too many have shown up very microphonic and CE won’t take returns based on that since JJ doesn’t consider that a defect. Just had a heater to grid short in one that’s about 3 yrs old from a pair in an amp I built for a local guy. Ordered a pair of the TAD red bases to try. Hopefully worth the added cost. We will see.

[pjd3]

For my amps I'm going back to NOS.
I've simply had very good luck with them. The only issue I ever had was a Mullard 4004 for a PI that started whistling after a year or two. Every other one of those 4004's just kept chugging right along. I've usually got my tubes from KCANOS and I really couldn't ask for better service and luck, or maybe not luck at all, just good tubes from a source that carries good stuff.

If it ain't broken.....

Best
Phil D