Rocket EL84 pair red plating.

[sethyrish]

Hi
I’m trying to work out a fault on my Rocket causing a pair of EL84’s to red plate. On stand-by, one isn’t glowing so I guess that’s causing its partner to red plate also. I had a 100uf reservoir capacitor after the GZ34/protection diodes which I think might be the root of the problem. Between B1 and B2 there was a London power SB84 VVR.
The amp started distorting until I noticed the red plating.
So I’m not sure if the problem is the rectifier, one EL84 or VVR which I’ve removed for now. The capacitors are all measuring the correct uF.
What’s my next step without damaging any other components?
I’ve replaced the 100uF with a 20 for now and removed the VVR.
If a tube has red plated does that mean it permanently stuffed?
The voltages don’t look right at:
B+1 423
B+2 423
B+3 418
B+4 418
B+5 417

[statorvane]

Your B+ appears far too high for EL84s. Does your PT handle multiple supply voltages? Possible it is wired incorrect for your mains supply?

If they are not both glowing on standby, that means one tube has no heater filament or voltage. Could be a problem with the tube socket or heater wiring.

Also I do not see how B+3 and B+4 are the same voltage. There should be a ~9K dropping resistor between these nodes reducing the B+4 value.

I'd try eliminating the power scaling first and square away the power rail and heater wiring. Then add it in once your B+ looks correct..

[Stevem]

Are the preamp tubes even lighting up, as they are not pulling any current if the power supply voltage is not dropping down as was questioned in the first reply to you.

[sethyrish]

AC is correct at 260 and 300v to the rectifier. 5v to rectifier and 6.5 to heater filament. The dropping resistors are there and were working before. I'll check for short and re-do power supply. Only one EL84 not glowing on standby. All other valves glowing. Changed position of of faulty valve and it's the valve, not the socket.
I did change v1 from both sides to 1 side. I'll have to re check all wiring I guess.
I heard GZ34's don't like a large capacitor directly after. So I thought That must have been the problem.

Thanks for your suggestions.

[Bob S]

Looks like a diode bridge is working & the GZ34 went south.
300ac in will give those B+ voltages with a diode bridge.
100uF is too hard on the recto tube.
Wiring problem ?

[Stevem]

The 84 doing that in standby is shot!
Yeah, the spec for the recto is 60 mfd, but I have found that due to its slow turn on that if you make it feed a filter and then have the standby switch after that then they will pump up a 100 mfd cap without failing!

[sethyrish]

Cheers

Thanks for replies