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First tube conflagration

Jeffery

Jeffery

High Powered Mutant
Subscriber
I'm pretty much a tube noob, so I'm not sure what to make of this.

I got a nice Dynaco ST 70 with a VTA low gain driver board, SDS PS cap board, no selenium.

It's been running superbly until tonight. I had it on, then turned it off for about 40 minutes while I ran an errand. Came home, turned it back on and got a little zap sound from one side. Then about the time I started towards the amp, the right front output tube went up like a Roman candle. I got it shut off almost immediately.

OK, the output tubes were JJ EL34, which I gather ain't the cream of the crop. Would this be unheard of for one of these?

Was 40 minutes too little time to restart?

The amp appears undamaged underneath, should I pull an output tube from the other side, bias it, and see if it goes up? Or just get a new quad of tubes, I kind of want to give some KT77s a whirl, anyway, but not if there's an inherent problem that'll smoke the new tubes.

Thanks for any guidance.
 
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Uh no, I've restarted mine after 2 minutes. Couple of thoughts- 1- what rectifier is in this thing??

2- One thing i've learned about the SDS cap board, check the solder joints going to the board, if it wasn't solder properly- and they are a tight fitting board with all the wires going into it, connections can come loose, don't ask how I know :D

While you have it apart, check all the wiring, going to and from the tubes, into the SDS and the vta board. Don't know what caps you have, the russian PIO's have no shielding, they are silver and if they touch ground spark city.
 
Also- the SDS board has provision for bias wiring for the stock board, this is not used when the VTA board is installed. Don't know who built or wired your amp, make sure no wires are going to "bias" on the cap board.

PS- i've been running JJ's in my amp since I built it, I've adjusted the bias once , it was off by 10mv, never replated, sparked- nothing. You should have each tube set to .40v DC.
 
Jeffery said:
I...Then about the time I started towards the amp, the right front output tube went up like a Roman candle. I got it shut off almost immediately....
Click to expand...

I assume that this means the surface of the tube that you can see through the glass, the plate, turned bright red or orange.

That happened to mine after a power supply rebuild and replacement of the coupling caps. One of the caps was VERY leaky, quite bad. Did not properly block the DC voltage from the phase splitter. Instead of the negative 32 ~ 35 volts on the grid of the output tube, I think its pin 5, I had something like negative 25 ~ 27 volts. That permitted too much current to flow through the tube turning it bright red.

I would remove all the tubes, open it up and SAFELY check the voltage on pin 5 (again I think that is right) of all 4 output tubes. If one of them is more positive then -32 V or so then I would suspect the coupling cap is bad. If so, replace all 4 of them. You want a minimum voltage rating here of 400 V and even going all the way to 630 V would not be amiss.

Good Luck and post back.

Shelly_D

Also, I use solid state diodes in mine on the SDS cap board. I was having issues with the tube rectifier and wanted more reliability.
 
So if a coupling cap on a vta board goes bad this can happen? What about a resistor going bad? Interesting.
 
It'll be a day or so before I get around to it, but at least I have some things to look at.

It has a Ruby rectifier. The failure wasn't a red plate, it was more of a flame, like burning steel wool in the tube.

There is a broken wire from FIL on the VTA board to the preamp socket on the back on the side that failed. Cause or effect, I don't know. I'll fix that and check the voltages and report back.
 
bktheking said:
So if a coupling cap on a vta board goes bad this can happen? What about a resistor going bad? Interesting.
Click to expand...

The output circuit of the amp is the same no matter which driver board you are using. The most common cause of red plating is too much current through the tube. In the Stereo 70, the cathode resistor in the output is there only to measure the voltage across it and therefore, indirectly, the current through the tube. It does almost nothing to limit the current flowing through the output tube. The most likely cause of the problem is the grid of the tube is too positive, that is the negative voltage on the pin is not big enough (absolute value). To be clear, I mean that -32 to -35 volts is good, -25 volts is bad, anything positive is VERY bad. No matter what the circuit on the driver board, the coupling caps need to be part of the circuit and in good condition to provide DC isolation between the 2 sections of the amp. If they go bad, it could allow that voltage to rise.

I am not sure what bias circuit of the VTA board looks like. If there is a resistor that could go bad, either short or open in the bias section that could allow the bias to rise toward the positive then that too could be the culprit. Not knowing the bias circuit I could not rule that out. Does that help?

In either case, the voltage on the grid of the red plating tube is the first place to check.

Shelly_D
 
Jeffery said:
It'll be a day or so before I get around to it, but at least I have some things to look at.

It has a Ruby rectifier. The failure wasn't a red plate, it was more of a flame, like burning steel wool in the tube.

There is a broken wire from FIL on the VTA board to the preamp socket on the back on the side that failed. Cause or effect, I don't know. I'll fix that and check the voltages and report back.
Click to expand...

That sounds like that tube may be toast. Sorry for your loss man.:tears:

Shelly_D
 
I'm not sweating the tube, as long as nothing in the hardware is farked I'm fine. I just don't want to make a habit of it.
 
Jeffery said:
It'll be a day or so before I get around to it, but at least I have some things to look at.

It has a Ruby rectifier. The failure wasn't a red plate, it was more of a flame, like burning steel wool in the tube.

There is a broken wire from FIL on the VTA board to the preamp socket on the back on the side that failed. Cause or effect, I don't know. I'll fix that and check the voltages and report back.
Click to expand...

That wire is used to measure the bias, not sure if a wire touching ground would cause that and I don't want to try it:D
 
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