Intermittent red plating on standby

[subway_savage]

hi there, i have a blackstar S1-100, it belongs to my friend who bought it used, this amp intermittently red plates once the power switch is flicked (on standby), it has EL34s, there's a distinct blue glow between the plates whenever it starts to red plate like crazy, within 6 seconds the tubes will be like charcoal.

i inspected all the components inside, everything seems to be fine, i checked the solder connections on the tube sockets and they look good. the only thing i could think of is that the Power transformer is shorting the heater filament supply with some high voltage making it supply high voltage or current to the heaters.

sometimes after a few shakes or relocation of the amp it works fine (6.5v across pins 2 and 7). but the odds are for red plating.

would appreciate any help thanks !

[Firestorm]

I didn't want to be the one to say it, but you might be out of your depth here. Blue glow is nothing; red plating something else entirely. How adept are you at working on things that can kill you?

[ToneMerc]

hi there, i have a blackstar S1-100, it belongs to my friend who bought it used, this amp intermittently red plates once the power switch is flicked (on standby), it has EL34s, there's a distinct blue glow between the plates whenever it starts to red plate like crazy, within 6 seconds the tubes will be like charcoal.

i inspected all the components inside, everything seems to be fine, i checked the solder connections on the tube sockets and they look good. the only thing i could think of is that the Power transformer is shorting the heater filament supply with some high voltage making it supply high voltage or current to the heaters.

sometimes after a few shakes or relocation of the amp it works fine (6.5v across pins 2 and 7). but the odds are for red plating.

would appreciate any help thanks !

If you had a plate to filament short you most likely have blown fuses. I would be checking the bias supply first.

TM

[subway_savage]

i'm not talking about the normal blue glow, this glow is only in the core of the tube and can be seen between the plates, and it only happens when the tubes red plate. when the amp runs fine that glow doesn't occur. all of this is on standby.

i have worked on and restored an old fender before. i know my precautions.

[subway_savage]

[/quote]

If you had a plate to filament short you most likely have blown fuses. I would be checking the bias supply first.

TM[/quote]

maybe the fuses installed are wrong values ?

i poked around the circuit with a chopstick hoping to find something loose, but everything seems good.

[Firestorm]

i'm not talking about the normal blue glow, this glow is only in the core of the tube and can be seen between the plates, and it only happens when the tubes red plate. when the amp runs fine that glow doesn't occur. all of this is on standby.

i have worked on and restored an old fender before. i know my precautions.

Sounds like screens. Happens on standby? Odd.

[katopan]

In standby usually there's no HT to the plates or screens, only heaters. If it's redplating when on, in play mode, then it's a bias problem or the valves are kaput.

[subway_savage]

[IMG:640:360]http://imageshack.us/a/img593/9139/3fkq.jpg[/img]

the blue glow that accompanied the red plating.

[IMG:640:360]http://imageshack.us/a/img850/3585/8eos.jpg[/img]


here's my data (all taken on standby):
- wrong value fuses installed (3.5A instead of 2A MAINS and 1A H.T) will replace them when i can.
- when red plating on standby plate voltage reads 425v
- amp will still red plate when switched off standby.
- on the test point (100mv recommended) the reading starts to increase from 0mv to more than 0.8v)

[xtian]

There should be no HT voltage and (no current flowing) on the power tubes when in standby, right? So, bad standby switch?

Also, definitely check your bias voltage on your screens. Should be around -40vdc. If you have no negative bias voltage, your power tubes will run away and red plate.

Great photo, btw!

[Firestorm]

Must be some kind of switch/wiring failure that permits HT but not bias. Awesome photo. The one after that is where the glass melts and sucks in. Saw one once. Sorry to question your skills. Being careful.

[JazzGuitarGimp]

The second photo is truely spectacular! Poor tubes though... Sheesh...

[Reeltarded]

Wow. What was the bias again? Like 9A per tube?

They are obviously getting some ultrasonic downline signal. Has to be. If you want to make another amp do that just to see what I mean set your (bad ass) meter to R and tap the junction of the output loads. Do not hold that. Just a tap.

Blue storm

[subway_savage]

with the tubes pulled out and amp fully running i get:
pins 3 = 496v, pins 4 = 490v, pin 5 = -1.5v on all tubes

so we got a bias problem here that somehow jumps in and out. any ideas what to check ?

EDIT: i knocked on the amp and the reading on pin 3 changed to -54v

[old tube]

Great Photos! I hope you don't mind but I posted your photo on a UK forum as there a lot more Blackstar's over here and one of their engineers regularly posts.

This was posted by him in response to your photo

Most likely the cable from the middle board to the OP valve PCB but if so it will be the first time I know of it happening on an S1. I have had it work loose on an HT-100 and routinely cable tie the lead to prevent it happening again.

If the problem is in the bias circuitry I strongly advise getting it back to Northampton because the bias is tied up with the DPR circuitry and that is not a job for the amp beginner. But again, there is no history of trouble with the amps in that regard as far as I am aware.

Hope this helps you resolve the issue.

[subway_savage]

returning it is not an option right now, as there are no blackstar authorised dealers where i live and the shop that it was bought from offered to exchange it with a 50w version (musicians warehouse Dubai)