Bogner ecsstacy help please, tubes red plating

[Stratjeep2]

Hi guys, I've been here before but it was with a different email account, and I can't find the password so I had to reregister :-(

I have a Bogner Ecstacy 101B in the shop, i'm reasonably experienced with tube amps, but can't find any info on this online. I am repairing it for a local company, it's under warranty, and Bogner has been less then helpful. They won't provide schematics even to their dealers.

Anyway, amp was taken to another tech, for new tubes and bias. When returned to client it immediately red plated but was shut down shortly after (I'm told 1-2 min).

I've taken voltages with no tubes in, on pins 3, 4, and 6 I'm getting almost the exact same thing, 540-541V. I brought the bias way down to about 20ma, and a quad off JJ's EL34's will last a few minutes voltages constant and then they will red plate.

Shouldn't there be a bigger difference between the screen voltage and the plate voltage? Isn't the screen voltage high for EL34's?

Client is screaming, Bogner will talk to me and I have talked once, but they suggest the tubes are not hardy enough and I should put in Winged C's. Though they use JJ's themselves.

Should I lower the screen voltage, and if so what would be the best way?

As far as I know the amp worked fine before the owner decided to retube it, and I can't see anything suspicious from the last tech. I've gone through 3 sets of JJ's and 1 quad of Shuguang's, some would last a few min, others would redplate almost immediately. I'm considering asking if the client has the original tubes.

Unfortunately I'm not an old expert on this stuff, most amps are not an issue, but this one is a problem child.

Any insight would be greatly appreciated, amp is on the bench so any additional info I can provide as needed.

Thanks!!

[Jazz P Bass]

You really should be monitoring the bias voltage for stability & lack of Vac ripple.
In my mind it is not the voltage that kills the tubes, it's the current.
If for whatever reason you lose the negative bias voltage, the tubes have to red plate.

[Stratjeep2]

I can do that, it's just I can't monitor it long once they start to red plate :-) The bias voltage initially would drop down to -9 volts or so even without tubes, but after I resoldered the board, and redid the tube sockets it seems to remain constant. I'll chuck in some tubes and monitor the voltage. Thanks!

[Jazz P Bass]

Yeah, -9 volts will turn the tubes on real hard.

[Stratjeep2]

Yeah, -9 volts will turn the tubes on real hard.

Yeah, I know :-). Amp has been running now for an hour and a half and no red plating yet :-(
I'm pretty sure it's not fixed though. Haven't done anything except been monitoring.

Thanks, appreciatte your time.

[g1]

Suggest you monitor the bias voltage at the tube sockets with the output tubes removed. This way they're not there to get hurt them when the bias disappears.
Are the output tube sockets board mount type? Do all 4 redplate or just one or a pair?
I'm suspicious of a bad solder connection somewhere in the bias circuit, or at the sockets, which may have been triggered by board flexing when the customer changed the tubes.

[Mick Bailey]

Did you measure the 540v after dropping the current? This will push the voltages up some. You also need to measure the screen current to see if the screens are operating within their limit.

[Stratjeep2]

Thanks guys, I have monitored it with and without the tubes, initially I'd get -9 with no tubes on one half. After resoldering everything it seemed stable with no tubes, but one half red plates, can be either half :-(

Today it's run for three hours without issue, the 540 was after dropping the current, but it was still in the 530's before hand. I didn't measure screen current, I figured with 540 volts on them they wouldn't be happy anyway, but I will do so.
Has 1k 5 watt resistors, I was thinking about upping them to 2K 10 watt to maybe make the tubes slightly happier.

Thanks guys, really appreciatte the thoughts and guidance, sincerely.

David

[Jazz P Bass]

It can be difficult to 'see' the actual screen voltage drop.
Personally, I don't worry about it.

[g1]

Don't think there is ever a need for 10 watt screen resistors, unless you want to guarantee that it won't possibly act as a fuse.
I'm not sure if originally the problem was a particular pair of sockets but is now random? Could it be that your resoldering repaired the socket fault but you are now dealing with a dodgy tube?

[Mick Bailey]

Increasing the screen resistors won't make a huge difference, unless you go much higher. Then there's a risk of starving the screens under load and reducing output. Assume that the designers knew what they were doing when they chose the particular values in the amp (not always the case, I know). A screen resistor fusing can be a good thing, so I stick with relatively low wattage FP resistors. I don't think that the screen resistors are the problem.

Check the heater voltage to make sure your amp isn't over-voltage to begin with.

[Stratjeep2]

Thanks again guys. I took out the board today again and went over all the bias and tube socket joints, reflowed and cleaned the board. Amp has been on for a couple of hours. Hasn't acted up in two days which makes it difficult to fix.

Mick, I'm embarrassed to say but I haven't checked the filament voltage, will do that tomorrow morning. How much more than 6.3 v would be too much?

Thanks! Truly appreciatte all the consideration.

[Mick Bailey]

If the heater voltage is over, this can point to the B+ being over as well, roughly by the same % increase. If it's a lot more than 6.3v it shortens tube life, though some 70s Fenders here in the UK have been running heaters at around 7v for decades. If it's 6.something I usually leave it. If it's more than this I look to see that the voltage selector or transformer strapping is correct.

[Jazz P Bass]

I think that the OP will find that the Bogner's run the heaters high.
Close to 7 vac.
I would not sweat it.
If the amp has run for a few hours, consider it fixed.

[Stratjeep2]

I think that the OP will find that the Bogner's run the heaters high.
Close to 7 vac.
I would not sweat it.
If the amp has run for a few hours, consider it fixed.

Amp has been up and running six hours and one gig, thanks guys, truly appreciatte all the help.
Far as I can tell it must have been a bad solder joint in the end. The factory soldering certainly looked sketchy.

Cheers!

David