Grey smoke from a KEL84 - Help! - World-Designs-Forum

misterc6 · #1 8th April 2023, 11:47 AM

This morning I decided to try a different set of 4 EL84s in my KEL84. Lovely music for about 1 minute then a crack and the dreaded grey smoke.

I have looked inside and it's clear that R29 and R31 (100R 0.5W) are badly burnt, C15 (100u/35v) is bulging at the top, and I suspect that C13 has also suffered some damage. I now think that R27, and possibly R25 (270R 1W) are also damaged.

Questions:
Is that likely to be the extent of the damage?
Will the Output transformer be damaged and how can I test this?
When I've replaced the components what should I check before refitting a known good set of EL84s?

I have replacements for R29 and R31, but not R27 and R25, nor the capacitors. Is anyone able to help me with some replacements? I will of course pay for these and the postage.

TIA,
Malcolm

John Caswell · #2 8th April 2023, 01:29 PM

Hi Malcolm
From your description the EL84s you put in were faulty, as that have taken out the screen grid g2 oscillation stoppers and also the cathode k bias Rs and Cs, the valves were probably Screen grid g2 to control grid g1 short circuit
I doubt very much the OPTx is damaged as the 100Rs would probably have protected it and also you switched off pretty quick.
PM with address and I will see what I have in stock.

John

misterc6 · #3 8th April 2023, 02:23 PM

Quote:

Originally Posted by John Caswell

Hi Malcolm
From your description the EL84s you put in were faulty, as that have taken out the screen grid g2 oscillation stoppers and also the cathode k bias Rs and Cs, the valves were probably Screen grid g2 to control grid g1 short circuit
I doubt very much the OPTx is damaged as the 100Rs would probably have protected it and also you switched off pretty quick.
PM with address and I will see what I have in stock.

John

Many thanks, John.

PM sent

Richard · #4 9th April 2023, 05:26 PM

Hi both, I've got a set of 4 x 100uF 35V cathode caps if you want them just drop me a pm

John Caswell · #5 9th April 2023, 05:39 PM

Hi Malcolm
Various resistors in the post Tuesday - you can be as pedantic as you like over them.

John

misterc6 · #6 10th April 2023, 10:01 AM

Thanks, Richard, that's very kind. I should be OK with the one spare Rubycon I have here which is the same as the remaining three. I'm trying to minimise the number of components I have to change. Please reserve the four you have for me and I'll let you know if I need them.

Thanks John, I look forward to receiving the package.

Regards,
Malcolm