New JJ 6v6S very odd issue

[pjd3]

Hello,
Well, just cant theorize what just happened here.

I built a Slucky Plexi 6V6 a couple of years ago and this has been my main gigging amp which I use frequently. Its been problem free, and sounds really good. I dont beat it, I run it clean, like a pedal platform (which it sorta is)

I just received a pair of Aphex matched JJ 6V6S's from "Thetubestore". Before I installed them, I did a very thorough check of the critical voltages, B+, plate, 6.3v, 5v, bias voltage and bias current as measured with a one ohm cathode series resistor to each of the 2 cathodes on the 6V6's. Every thing looked good and stable except a 4ma difference between the NOS 6V6's that I got from KCANOS tubes a couple of years ago. They still sound good but, I was very curious about all the good reports I heard about JJ and wanted to give them a whirl - Plus I needed a spare for gigging.

I first set the bias voltage to about -50 volts just to start with low emissions and work up. I figured about 430 plate volts @ 20mA would give me about 70% emissions on the JJ's. As I was decreasing the negative bias voltage and measuring both B+ and bias current, there was a smoke puff from the pilot light area. Its a Fender style pilot light with a couple of 100 ohm resistors from each leg to ground to create a faux ground classically found with Fender 6.3 volt pilot lamps. I thought I may have had a slipped probe or my sleeve crossed something or other, but, didn't find anything so moved on. So far, I had hit my 20mA mark at 433 volts so, felt pretty confident that all was working fine. Then, I began to increase the amps volume while watching input and output voltage on my scope. I was using an 8 ohm dummy load with a sine wave at 1khz. As I was increasing the volume the output waveform went flat, then I heard crackling and saw smoke once again at the pilot light. I turned the amp off quickly to inspect, and it looked as though both the 100 ohm resistors had been burnt, practically black.
So, this is where I"m at now.

Does this sound familiar to anyone in any way? Having never used new production power tubes before, I'm left with a complex about new production tubes! Damn. I've always bought and used NOS, always.
Has anyone heard about issues with the new JJ 6V6S's? Filament to cathode shorts or anything? I'm nervous to try anything now but, I will remove the new JJ's and see if all else is normal, if I even have a filament coil that is working. I visually inspected the amp and everything looks to be fine, no loose probes, no detached wires. This was only after installing the JJ's.
Ahhh,, OK. Back to the bench. Check all my voltages, make sure they are still there.

Thanks and any insight or input would be very valuable! I wanted to like these tubes !
Best,
Phil Donovan

[pjd3]

Checked a few things.

Primary PT fuse was black and blown.

With new JJ's removed, put in a new fuse and checked voltages.

I was happy to see heater V of 6.5vac and a B+ of nearly 460vdc - with the power tube loads removed I would expect that.

So, I suppose it would be prudent to ohm out the new JJ 6V6's pins to see if there were any filament-to-cathode shorts, or anything funny at all, and making sure the 6.3 heaters are still intact. Then, guess I have no choice but to contact the distributor and ask what they can do. Besides having built and used a few tube amps I'm also a senior electronic tech by day so, I feel very confident I followed good safety rules and didn't inadvertently introduce any adversities to the amp.

Thank you and surely, any feedback is welcome.
Best,
Phil

[pjd3]

JJ 6V6S's don't measure anything odd at least from an ohm meter check perspective.

Heaters are intact at 1.9 ohms and no cathode to filament conduction. Of course I suppose when the tubes have all their power applied to them other things probably can occur that can't be seen with a simple ohm meter.

I'm going to put my original NOS Sylvanias back and see what happens. My instincts tell me that all will be fine but, I'll report back after I get there. Thank you,

Best,
Phil D,

[sluckey]

The most common thing to burn those resistors is a tube short between pin 2 and 3. Look closely at both tube sockets for signs of an arc trace between pins 2 and 3. Look at bottom and top of both sockets.

[pjd3]

Put NOS Sylvania's back in, amp works as been for the last 2 years. Hit 18 watts until sine wave beginning to clip at output.

Time for a Bruins game.

Thanks for comments and stopping by, tubes frighten me. ha.

Best
Phil Dono

[sluckey]

You should replace those two 100Ω resistors.

[pjd3]

Thanks Sluckey,

I just did install two more 100 ohm resistors where the others burnt, but did run the amp for a short while without them. The amp was noisy. I'll try it again tomorrow see if it quieted it down, and also check the sockets for arcing/shorts.

Best,

Phil

[bepone]

there is no 100% warranty that the new tube will be ok. new tubes are just random quality. this is coming from hand made work, and quality is proportional to worker's concentration, stress, physical and mental health.. etc..

like mentioned if 6.3V resistors are burned there is short circuit between pin 3 (anode) and pin 2 (heater), can be permanent or intermittent, this tube is not good.

[statorvane]

I've had the same thing happen with a pair of EH 6CA7s last year. Anode to heater was shorted on one tube. Those were great sounding tubes and can't get them anymore.

I would check both tubes to make sure there is no continuity between pins 2 and 7 and the other pins.

[pjd3]

Thanks everyone for coming by.

I'll likely send the tubes back and ask the vendor to please check them, send me a new pair, tell me to go hit the road Jack, something.

I was really curious to try the JJ's though. I've heard alot of interesting feedback on them, mostly good, and wanted to hear for myself. Maybe I just got some bad luck on a bad tube. I got them Apex matched, perhaps I should pay the extra few buck for the "burn in service", maybe give them more opportunity to expose any inherent problems.
And for what its worth, my "100 ohm resistors" were actually each three 300 ohm metal film resistors in parallel. So, they had a much larger accumulative rating than just a single 1/4 watt resistor.

THank you all,
Best,
Phil D

[sluckey]

I've bought quite a few JJ tubes, EF86, EL84s, 6V6s, GZ34, etc. Never had any problems. I haven't bought any tubes since covid.

There's another current thread with a new bad 6V6. Didn't mention JJ.

https://ampgarage.com/forum/viewtopic.php?f=6&t=35953

[Colossal]

I have had good results with the JJ 6V6S over the years, but no longer use their EL84s. The glass is very thin and fragile. Plus they don't sound as good to me as other EL84s, but I suppose that is application dependent.

[Reeltarded]

The Mullard copy EL34 was pretty good but failed what I would describe as 'often' and always biased weirdly, then I started servicing amps with EH ca7s and those sound like my expectations and didn't drift 20% 6 hours later.

I have no idea what to buy now.

A Katana.

[pjd3]

Just to respond back to a couple of the question that came up here:

All looks well at the tube sockets - no apparent arcing signs between any of the pins, specifically looked between pins 2 and 3.

Really ey? You can get a short between the plate and the filament? I have heard of cathode-to-filament shorts since they are quite in proximity of each other but, hadn't thought about a plate-to-filament short - figured they were too far apart for that but, I guess not if others have experiences that. The plate and/or B+ voltage ranged from about 458 down to 420 depending where my bias was and what set of tubes I was using. I always start by setting the -bias voltage much higher than set point for low starting emission and work up from there. So, for instance, on my Sylvania 6V6's I"m seeing 437 @ about 20mA (with 16mA for the less powerful tube - they have drifted over time but still sound pretty decent).

I'm going to warm up the amp tonight and do one final bias/voltage check to make sure all looks as it should (and as it has) before buttoning it up.

Thank you all for your valuable input once again.
Best,
Phil

[LOUDthud]

All looks well at the tube sockets - no apparent arcing signs between any of the pins, specifically looked between pins 2 and 3.

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

Voltage spikes on the plate of a power tube can reach several thousand Volts in an overdriven guitar amp. I've seen tube sockets breakdown, but never a short within a tube. Warning: those spikes will kill most scope probes if you try to look at the spikes. You can look at the spikes across the speaker (not a dummy load resistor) where they will only be hundreds of Volts. Still could harm a scope probe though.

If you don't have a scope, try looking at the bottom of the tube sockets while playing loud with the lights out.