My 6V6 sparks when I play loud

efnikbug

Member
Messages
632
Over 20 years ago, before kits, I built a Tweed Champ from scratch. I've been playing it for its simplicity and for its early break-up. To me, it sounds pretty lousy at low volume, but when I turn it past 2:00, it wakes up a bit using my Fender Classic Players 50s Strat. It's when I put a Dunlop Jimi Hendrix FuzzFace in front of it is when the amp livens up and I have fun.

I like playing the amp turned passed 3:00 and the FuzzFace w/ fuzz slightly backed off from max and the volume slightly above unity gain. I go from smushy all-out fuzz to glassy clean using the volume on my guitar.

It's when I have everything cranked, I notice that there's something flashing inside and near the bottom of the tube where the leads are.

Stupid question: Is this bad? And what's going on?
 

HotBluePlates

Member
Messages
13,660
... It's when I have everything cranked, I notice that there's something flashing inside and near the bottom of the tube where the leads are. ...

Using your tube has "blue glow" and you're seeing that pulsing when you play, the flashing is probably bad. Some kind of white/yellow sparking/flashing is almost certainly arcing between elements in the tube.

What we don't know is whether it's a defect in the tube, or something being caused by the amp. Assume it's a tube defect first, and swap for another 6V6 to see if the flashing stops.

Nothing good can happen from repeatedly playing an amp that's arcing.
 

efnikbug

Member
Messages
632
Previous tube did the samething and it was doing fine for awhile. It finally gave out last month.

Either way, I'm going to take out the tube, spray the pins, and run it in-n-out of the socket to try to clean it out, and then see what happens.
 

YBA-James

Member
Messages
123
1) It's bad 2) Seems you've got an arc at or near the tube sockets; carbon buildup may make it worse over time.
Once a tube arcs and leave a carbon trace on the socket, does that socket absolutely need replaced? Or can the carbon be scraped/brushed off? I recently repaired an amp that arc'ed pretty bad. I fixed the problem, cleaned the socket and all seems well, but I've been afraid to push it too hard.
 

PushedGlass

Member
Messages
835
Once a tube arcs and leave a carbon trace on the socket, does that socket absolutely need replaced? Or can the carbon be scraped/brushed off? I recently repaired an amp that arc'ed pretty bad. I fixed the problem, cleaned the socket and all seems well, but I've been afraid to push it too hard.
In my view, if the arcing is going on underneath (i.e., the wiring side) of the socket, I feel like it would be really difficult to clean and so you're not really in the clear until that socket is replaced.
 

HotBluePlates

Member
Messages
13,660
... I recently repaired an amp that arc'ed pretty bad. I fixed the problem, cleaned the socket and all seems well, but I've been afraid to push it too hard.
In my view, if the arcing is going on underneath (i.e., the wiring side) of the socket, I feel like it would be really difficult to clean and so you're not really in the clear until that socket is replaced.

It would be very interesting to know what was causing the arcing. As in, could there be an arc on the wiring-side of the socket that results in flashing seen inside the tube? Or is that flashing (and the path available for the arc) only within the tube and not a fault of the wiring/socket?

I don't know; hoping others who have resolved arcing in amps chime in.
 

stratified

Member
Messages
213
I’ve resolved arcing problems in three amplifiers, one of my own and two that belonged to customers.
The arcing was on the tube side of the socket in one case and on the wired side on the other two.

As per post #5 of this thread, once the carbon from the burning Bakelite makes a good pathway, it will continue to arc until a more catastrophic failure occurs.
In addition to the carbon, on two of the amps I could see redeposited metal from the arc vaporized points on the tube pins and socket connections. It was copper colored in one case. This is probably what’s happening if your amp has ceramic sockets, which would not be prone to burning and yielding carbon.

In all cases, I replaced the tube socket and any heat compromised wiring.

With my amp, (a ‘63 Vibroverb) I wanted to source a period correct replacement socket so I took a Dremel tool to the problem area on the socket and removed the burned bit. This temporary solution allowed me to continue using the amp until the replacement socket was located.

One scenario that would make arc over more likely is an impedance mismatch with the speaker impedance too high.
At full drive level you’d be outside design spec and closer to the occurrence of “flyback”.
 

slider313

Gold Supporting Member
Messages
8,552
Are you using an RCA plug like the originals or a 1/4" jack for the speaker connection?
 

slider313

Gold Supporting Member
Messages
8,552
1/4"
I'm not seeing the significance of that.

If the speaker jack is not making good contact then you'll have the exact symptom you described but it will be at the socket. Those old RCA jacks were not the best choice for the speaker connection.
 
Last edited:

eolon

Gold Supporting Member
Messages
504
Hello efnikbug:
I just finished working on an amp with the same symptoms. It is an Evil Robot Phil X Custom 214, C 30/18. It is a cathode-biased Class A amp with 4 6V6s. It measured a static dissipation of 15 Watts (!) per tube at 450 VDC (also !).

One of the tube positions was also red-plating, understandably.

The arcing damaged the 6V6 in that position, and the others weren't so great either.
In talking to John Kasha, the designer, I found that this was an early amp. In later versions he modified the circuit with bigger screen resistors, and a B+ dropping Zener diode on the transformer secondary, and a higher value cathode resistor, and a different fan orientation.

I dropped the B+ voltage, made the other changes, and the problem was solved. It still runs hot, at about 13 Watts, but it is Class A, so I used JJ 6V6S, which are rated for higher plate voltage and 13 Watts. It's a great amp (now), but it is pushing the envelope.

Anyhow, in your case, if you are fortunate it is just a failing tube. If not, then I would check the static dissipation and the plate voltage. Also, bad filter caps can sometimes induce that arcing in a tube.

Best Regards,
Don
 




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