DIY Standel tube amp with el84 suffered breakdown

[kingplank]

I've been running this home built Standel-ish amp for 2 months now, lately it has a faint motor noise, and the last 2 days the volume gone down to almost completely faded.
A circuit trace check point out to cathode bias cap and resister both gone open circuit.


My plate voltages can be changed from 240V to 200V. I am wondering if it is biased too hot (150 ohm 5 watt resistor)
Any suggestions?
Thanks in advance

[tubenit]

Have you tried new tubes?   What are the current voltages you have on the tubes?   Do you have any hi-rez pictures? 

The more information you provide us, the more likely we can offer some useful help to you.

I would suspect tubes first.  Oscillation problems with power tubes could be an issue?

Can you confirm the grid resistors to the EL84's are grounded?

With respect, Tubenit

[kingplank]

Thanks for the reply.
At first I replaced the el84 with new ones, and the amp had very little volume.
I had since pulled out the bias cap and resister, both had gone bad.
The resister has a bit of burn mark on it
I thought I check with this forum before I replace the parts.

[sluckey]

150Ω is OK for a pair of EL84s but I would use a 10 watt resistor.

[shooter]

If the R burnt open, I would replace your plate diodes, I would verify your rectifier is still putting out DC NOT AC.   And I'd verify the OT isn't open

[PRR]

5W should be OK for two EL84. The tubes together are only good for 24W. The bias voltage is less than 1/20th of the P-K voltage, so <2.4W for plate current. EL84 add 20% for screen current, 2.64W in bias resistor. Yes I'd be happier with 2X oversize, 10W. But a 5W should last half a lifetime.

I wonder if the signal grids got loose. Bad connection on G1 resistor(s) or leaky coupling caps.

[kingplank]

Thanks for all your replies.
The amp is now fixed, with the following changes
- cathode resistor replace with new R150
- bypass cap is now 100u 630V
- replaced coupling caps whit .27u, made no difference, .1u put back in
- R50 22k plate load replaced, was burnt
- replaced R41 & r42 grid leak from 150k to 220k, now R50 is not hot.
Not the most fun Covid stayhoe project though.; but I am happy that it works.


I still have no idea what failed first and caused the chain of events of failure;maybe some of you can give some insight into this.

[DummyLoad]

...


wiring error in the schematic with 150K EL84 grid leaks - now 220K i believe - the are supposed to installed AFTER the .047uF coupling caps - E.g., connected to the 1.5K grid-leaks.


--pete

[sluckey]

+1

[kingplank]

Thanks Dummy load for pointing out my wiring error.
It is now fixed, the amp is now a little quieter and a little louder.
Gave it a little blast test, no smoke and fire.


Gerald

[DummyLoad]

test attachment .


--pete

[kingplank]

Schematic and voltages updated.
Calculated plate dissipation per tube + 67.5%.
I hope no breakdowns for at least a year


Gerald