6L6 weird issue. Please chime in.

  • Thread starter Thread starter 1meanplexi
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1meanplexi

1meanplexi

Well-known member
Good Morning.

I picked up a 6505+ for a steal. I knew something was not exactly right with the head so decided to purchase it regardless of the issue. It fires up sounds "good". I swapped out the power tubes with new Mesa 6L6's. The pre's are new as well.

Taking a look at the power tubes I noticed a blue hue in tubes 1,2 & 4 (normal) Tube 3 did not have this. I put my hand near the tubes and realized that tube 3 was warm ( I could touch it and not lose skin) but no where near where the others were at. I swapped tubes 3 & 4 around thinking it might be tube 3. Nope.....tube is fine. So, what do you guys think? Why would tubes positions 1,2 & 4 be operating at normal operation but the 3rd position is not firing like the afore mentioned.

Thanks crew :rock: on!
 
Have you confirmed the bias isn't running away on the amplifier and is stable?

Do the heater filament voltages look stable?

The current limiting screen grid resistors on these amps are on the underside of the power tube board. Could be that someone did a botched repair.

If the bias is stable, the tube isn't arcing on startup or shutdown, and the heater filament voltage reads normal, I wouldn't be too concerned. You could have a tech pull the power board and inspect it if you'd like.
 
Yep... what was said above... and have you checked to make sure there are no burn marks/carbon traces on the sockets? Any oxidation/carbon build up in the tube socket contacts? Screen resistors... check with a dmm... and look for any burn marks. I'd also confirm all tubes are idling at the correct bias with a dmm or bias meter as well.
 
glpg80":1kugfmka said:
Have you confirmed the bias isn't running away on the amplifier and is stable?

Do the heater filament voltages look stable?

The current limiting screen grid resistors on these amps are on the underside of the power tube board. Could be that someone did a botched repair.

If the bias is stable, the tube isn't arcing on startup or shutdown, and the heater filament voltage reads normal, I wouldn't be too concerned. You could have a tech pull the power board and inspect it if you'd like.
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I did not do any measuring except for the bias. It reads fine (single bias point) and is at 36ma with a degree of 2 or so on the drift. As far as the other items listed....I will not attempt that on my own. Kind of like the guy who buys a Harley, because you own the bike it does not make you a mechanic.
 
If you're comfortable doing it you could measure the voltage drop across the screen grid resistors for each tube at idle (assuming it has separate resistors for each tube). The voltage drop should be about the same for each tube. You could also measure the resistance from cathode to ground on tube 3 (with amp off) just to make sure it's grounded (compare with the other tubes, I'm not familiar with the 6505). And check the socket itself.
 
SpiderWars":yrjqumnd said:
If you're comfortable doing it you could measure the voltage drop across the screen grid resistors for each tube at idle (assuming it has separate resistors for each tube). The voltage drop should be about the same for each tube. You could also measure the resistance from cathode to ground on tube 3 (with amp off) just to make sure it's grounded (compare with the other tubes, I'm not familiar with the 6505). And check the socket itself.
Click to expand...

Bit more complicated on 5150's being as though everything is upside down on the power tube board and nothing is marked. If you're not used to measuring components on an amplifier and cannot see the component you're measuring, it makes things a bit more challenging.

+1 to checking the tube socket isn't totally fucked. That's also a great recommendation. Since the amp is somewhat questionable though, take it to a tech for anything socket related. The filter bank on these amps self-drains but only in normal operation and the time it takes differs to each amplifier depending on the health of the caps. You don't want to be poking around if you don't know what you're doing - especially in an amplifier with a questionable history.
 
Do you guys think some doxit in the sockets with a shot or two? I read some say it is fine and others that you should not do it.
 
1meanplexi":ni9xxeqs said:
Do you guys think some doxit in the sockets with a shot or two? I read some say it is fine and others that you should not do it.
Click to expand...

Do not spray anything down the sockets. These are PCB mounted ant the traces on top are exposed - you could short something fairly easily. I'd recommend taking an old tube, spraying the pins, shaking it off until it's not dripping cleaner, and pulling/pushing into the socket to clean them. Don't move the tube side to side - that's what bends the pins.
 
I'm betting the sockets need the spring retainers tightened up. They spread out pretty easily in the Peaveys, and should be firmed up.

Tighten them up, and see if that's not your problem. If it is, and you're not making good contact with the pins, it'll go from little glow, to red-plating, and then you'll have an SGR problem.
 
napalmdeath":2z5otylj said:
I'm betting the sockets need the spring retainers tightened up. They spread out pretty easily in the Peaveys, and should be firmed up.
Click to expand...

Try this. I had a similar problem and it was the sockets being stretched.
 
Worked fine until you changed tubes?? I'm confused... The seller told you it had issues?
Did you bias it?
 
BrokenFusion":3ahrg316 said:
napalmdeath":3ahrg316 said:
I'm betting the sockets need the spring retainers tightened up. They spread out pretty easily in the Peaveys, and should be firmed up.
Click to expand...

Try this. I had a similar problem and it was the sockets being stretched.
Click to expand...

Another +1 on this. Tighten them up and then use contact cleaner as was indicated earlier in the thread.
 
The cold tube very likely has an open/burned screen resistor.
 
Thanks for all the responses. I took it to my tech yesterday. I will let you know what he found. Again...thank crew.
 
And the winner is.....Blown Screen resistor. All good now. She is ripping.
 
1meanplexi":31lgsx0a said:
And the winner is.....Blown Screen resistor. All good now. She is ripping.
Click to expand...


Nice. I’ve had several blow through the years. I end up putting in 1k/5w resistors. It will change the tone slightly.
 
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