Peavey 6505+ power tube problem

[maxxi]

Hi everyone

i recently purchased a used 2007 Peavey 6505+ head. It still had the original Ruby tubes in it (A new set of Sovtek 6l6WXT+ are on their way).
When taking the chassis out of the headshell for cleaning i decided to fire it up to check what the power tubes were doing.

There i found out 3 power tubes have a blue glow when in operating mode, but the far right did not.
So i decided to put the far left tube in the far right socket and vise versa, to check if it was a tube or a socket problem.

The problem did not follow the far right tube as it glows blue in the far left socket. The tube in the far right socket did not glow blue now, as opposed to before.
I checked if the far right tube was getting hot like the others, but it remained quite cold leaving me to believe that only the heater is getting power, but the rest of the tube is not when turning the amp into operating mode.

Could this be a screen grid resistor that burned out, or would the amps fuse blow when this was the case?
An other option I am considering is a cracked solder joint, as 5150's/6505's are apparently sensitive to this.

 

[Trouble Free]

Blue glow can be normal in most cases, It could just be the bias is set on the hot side. I would check how many watts each tube is using, and adjust as needed.

 

[paul-e-mann]

Hi everyone

i recently purchased a used 2007 Peavey 6505+ head. It still had the original Ruby tubes in it (A new set of Sovtek 6l6WXT+ are on their way).
When taking the chassis out of the headshell for cleaning i decided to fire it up to check what the power tubes were doing.

There i found out 3 power tubes have a blue glow when in operating mode, but the far right did not.
So i decided to put the far left tube in the far right socket and vise versa, to check if it was a tube or a socket problem.

The problem did not follow the far right tube as it glows blue in the far left socket. The tube in the far right socket did not glow blue now, as opposed to before.
I checked if the far right tube was getting hot like the others, but it remained quite cold leaving me to believe that only the heater is getting power, but the rest of the tube is not when turning the amp into operating mode.

Could this be a screen grid resistor that burned out, or would the amps fuse blow when this was the case?
An other option I am considering is a cracked solder joint, as 5150's/6505's are apparently sensitive to this.

You should move this thread over to the workbench section so the techs see this. Im no tech but have been learning tube amp troubleshooting this year, i suggest you follow the startup procedures from robrobinette.com to verify everything is working, and then look at his troubleshooting section.

 

[Rudy v]

It is a screen grid resistor that is gone bad. I have had the same issue with my 5150's i put a new one in problem was solved.

 

[maxxi]

It is a screen grid resistor that is gone bad. I have had the same issue with my 5150's i put a new one in problem was solved.

Thanks for the answer. Did you replace it with the stock value, or did you change it to a higher one?

 

[Rudy v]

Thanks for the answer. Did you replace it with the stock value, or did you change it to a higher one?

At first just stock, but later replaced with 4 470ohm 5 watts, and I bias around 31ma to 35ma

 

[maxxi]

At first just stock, but later replaced with 4 470ohm 5 watts, and I bias around 31ma to 35ma

OK. Do you know if there are other things I might have a look at while the amp is being worked on? Because I don't really get a high quality feeling from this amp

 

[Rudy v]

I did have a loose solder joint on the board, at the connection that comes from the power transformer, also look at the big connector that goes to the power board for some burn marks j14 and j15 is the plate and screen, but so far i have no burn marks jet.

 

[maxxi]

I did have a loose solder joint on the board, at the connection that comes from the power transformer, also look at the big connector that goes to the power board for some burn marks j14 and j15 is the plate and screen, but so far i have no burn marks jet.

I just got around to measure the screen grid resistors, and one is indeed not measuring anything. I also took a look at the plastic connector between the power tube board and the main board, and no burn marks are to be seen. Don't know if it will ever burn seeing the amp is going since 2007. When the new set of tubes arrives I'll take the amp to my tech and let him replace the screen grid resistor(s) and let him do a general inspection of the amp. Thank you for your help!

 

[Rudy v]

Graag gedaan

 

[maxxi]

I've been somewhat naughty and took the power tube board out myself. Went quite well to desolder the screen grid resistors. I ordered some 1K 10W resistors, but those are way too big to fit on that board. Ordered some 470 Ohm 5W ones, hopefully they'll arrive quickly

 

[maxxi]

Update:
Soldered in the 470Ohm resistors, reassembled the amp and fitted a matched quad of Sovtek 6l6wxt+'s.

Amp fired up as normal, all tubes are getting current now. The amp actually sounds more punchy now

 

[Rudy v]

That's good to hear, don't over bias the power tube's, a stock amp has a bit of a low bias probable around 29ma ,if it sounds good for you no problem.

 

[maxxi]

That's good to hear, don't over bias the power tube's, a stock amp has a bit of a low bias probable around 29ma ,if it sounds good for you no problem.

I checked the bias current at the amps test points for now. Peavey recommend -55v, but the lowest I can get the tubes is -58v, this is by turning the pot all the way anti-clockwise. I ordered a bias probe and a 6.8K 1/2W resistor in case the bias voltage is too low when measured with the bias probe. Also, I thought turning the bias pot anti-clockwise lowered the current but it seems it's the other way round?

 

[Rudy v]

I ordered the biasmaster +4 octal probes, from the tube amp doctor, in germany so i can check every tube one by one, and bias on the highest current drawing tube, it's a nice tool to have.

 

[maxxi]

I ordered the biasmaster +4 octal probes, from the tube amp doctor, in germany so i can check every tube one by one, and bias on the highest current drawing tube, it's a nice tool to have.

Got the resistor and bias probe in today. I settled on a bias of 28mV, which corresponds to +- 54V measured on the amps bias test points (before the change in resistor this would only go up to 58, so you can imagine how cold those tubes were biassed).
The first thing i noticed is that the amp sounded a lot warmer (especially on the clean/crunch channel), but did lose some of that 6505 filth. It's a bit more polished sounding now, but that's not a bad thing I think.

 

[Rudy v]

That filth is crossover distortion at your output.