EL 34 Red Plating - What to do?

[Tim D]

+1 on what others have said.

A variac and/or DBT would be highly recommended for the next power-up. At the cost of match quads that would be a cost effective way of not blowing out another tube. (Making a DBT is simple for anyone that can use basic tools.)

 

[PE9ZZ]

Have you measured anything? No use swapping out tubes if you don't know what is going on. I would be interested in the cathode voltages of both tubes. They should be around 30 volts. But equal. Glowing plate inducates gross overload. Also check that 250R/25W resistor.

 

[Perkri]

+1 on what others have said.

A variac and/or DBT would be highly recommended for the next power-up. At the cost of match quads that would be a cost effective way of not blowing out another tube. (Making a DBT is simple for anyone that can use basic tools.)

Thanks!. Had not thought of using a DBT as a "permanent" part of my amp's power source! I have one - not pretty - but can easily build a second more "permanent" one. It's going to need one heck of a bulb as the draw is pretty big!

Have you measured anything? No use swapping out tubes if you don't know what is going on. I would be interested in the cathode voltages of both tubes. They should be around 30 volts. But equal. Glowing plate inducates gross overload. Also check that 250R/25W resistor.

Thanks, will do! No, have not measured anything... As it is an auto bias, I have never needed to measure a thing, so I don't know how to do that?

 

[PE9ZZ]

Maybe now it's time to get help from a pro? This amp can kill you.

 

[Perkri]

Maybe now it's time to get help from a pro? This amp can kill you.

Oh, I know!!! EXTREME caution is being applied!!! Hence, no live measurements having been done up to this point. And I always let the amp "run down" and drain caps with signal before opening... The 250R/25W measures correctly

 

[Perkri]

 

[gadget73]

I'd honestly be looking at bad/leaky coupling caps. If the grid resistors are good and the tube sockets are fine, thats really all that remains here. The problem stays with the socket, so its not the tube. Its cathode biased but only happening on a single tube, so no failure of the bias supply is to blame.

Coupling cap will connect to the 1K resistor coming off pin 5 of the EL34 socket. Quick and dirty check, simply disconnect the cap and see if the tube stops red plating. The amp won't work correctly but if the tube quits going Chernobyl you have your answer.

More advanced check, measure the voltage from that point to ground with the cap installed and amp powered up and see what it does. It should not creep positive. If it does, the cap is leaking. Use extreme caution if you do this, lots of high voltage floating about.

 

[Perkri]

I'd honestly be looking at bad/leaky coupling caps. If the grid resistors are good and the tube sockets are fine, thats really all that remains here. The problem stays with the socket, so its not the tube. Its cathode biased but only happening on a single tube, so no failure of the bias supply is to blame.

Coupling cap will connect to the 1K resistor coming off pin 5 of the EL34 socket. Quick and dirty check, simply disconnect the cap and see if the tube stops red plating. The amp won't work correctly but if the tube quits going Chernobyl you have your answer.

More advanced check, measure the voltage from that point to ground with the cap installed and amp powered up and see what it does. It should not creep positive. If it does, the cap is leaking. Use extreme caution if you do this, lots of high voltage floating about.

OK!, I think, I may have discovered a "problem"... Will update shortly. Thanks!!!

 

[PE9ZZ]

Agree with @gadget73. Disconnect the amp, hook the red probe to pin #5, black probe to chassis. Then switch on. Voltage should stay near zero. Culprit will be the big brown C connected to it if it does creep up. Cathode voltage would have shown this too.

 

[Perkri]

Agree with @gadget73. Disconnect the amp, hook the red probe to pin #5, black probe to chassis. Then switch on. Voltage should stay near zero. Culprit will be the big brown C connected to it if it does creep up. Cathode voltage would have shown this too.

After I make the kids some crepes, I will list what I think I my have done wrong during the re-cap. Looks like I connected cap to tube 1 differently than how I did to tube 4... I will draw out a schematic and post in a bit. Amp is disconnected...

Thanks!!!

 

[audio3000]

Thanks!

OK, here is what I found:

1. They are both 9.99ohm
2. The 1K and 220K are both measuring right
3. Where do I connect meter to check junction voltage? Will check tomorrow
4. Where do I connect meter to check cathode voltages? Will check tomorrow
.

Please see diagram below. Red arrows point to grid voltage measurement point (item 3 above). Blue arrows point to cathode voltage measurement point (item 4 above). Connect the ground lead of your DMM to the chassis.



Another way is just to disconnect the .22/600V cap from the circuit to see if the tube still red-plated.

As others have already pointed out, tube amp has lethal voltages.

 

[whoaru99]

Thanks!. Had not thought of using a DBT as a "permanent" part of my amp's power source! I have one - not pretty - but can easily build a second more "permanent" one. It's going to need one heck of a bulb as the draw is pretty big!

Having a DBT permanently connected is not what was suggested.

If your mains voltage is materially higher than the nominal input voltage rating of the amp then a bucking transformer may be beneficial...but not a DBT. DBT is for testing.

 

[peterh]

Thanks!

OK, here is what I found:

1. They are both 9.99ohm
2. The 1K and 220K are both measuring right
3. Where do I connect meter to check junction voltage? Will check tomorrow
4. Where do I connect meter to check cathode voltages? Will check tomorrow





Ooooops. My bad. They are reading 227K...

Thats ok.

 

[bbqjoe]

Just curious, because I don't know, what happens to a tube amp if hooked to a speaker going bad?
Would that cause tubes to go south?

 

[gadget73]

Probably not, but what do you mean by "going bad" exactly?

 

[bbqjoe]

Probably not, but what do you mean by "going bad" exactly?

I don't know, any number of things.
Voice coil shorting, crossover problems, speaker blown all together?

 

[Tim D]

Thanks!. Had not thought of using a DBT as a "permanent" part of my amp's power source! I have one - not pretty - but can easily build a second more "permanent" one. It's going to need one heck of a bulb as the draw is pretty big!




Thanks, will do! No, have not measured anything... As it is an auto bias, I have never needed to measure a thing, so I don't know how to do that?

I wasn't recommending a DBT for permanent installation, but for your next power up while you figure out the problem.

 

[PE9ZZ]

No, unless your OPT goes phut. Which it won't. And if it does, not in a way your speakers get fried. In contrast to semiconductors that WILL blow up your speakers if they dump the supply voltage on the output without a protection relay.

 

[Pio1980]

Thanks!

OK, here is what I found:

1. They are both 9.99ohm
2. The 1K and 220K are both measuring right
3. Where do I connect meter to check junction voltage? Will check tomorrow
4. Where do I connect meter to check cathode voltages? Will check tomorrow





Ooooops. My bad. They are reading 227K...

Close enough. Check socket tension pin 5 on the suspect.

 

[airtime]

In contrast to semiconductors that WILL blow up your speakers if they dump the supply voltage on the output without a protection relay.

Even with a relay I have blown speakers when SS outputs short out. It just takes one milcro blast of DC on those tweeter coils and that's it.

Oh yeah, I did actually FRY a pair of woofers, but I don't remember if a relay was involved???