R24\25 burning in KEL84

[mussolo]

Can a mod please edit the heading to KEL84 and not KEL34...ooops

I have been noticing a slight burning smell in my 2006 KEL84 and decided to open it up.

R24 and R25 are blackened and appear to be on their last legs...any ideas on the cause and should they be replaced with uprated resistors?

The amp only has about 200 hours of playing time on it...

[mussolo]

I see that the amp's builder did not take heed and leave the required 15mm gap between some of the resistors and the PCB as specified in the building instructions.

Since this is an older model I may as well replace R10\11\12\13\14\15 as well with uprated resistors as recommended [as well with R24\25\26\27 with uprated resistors].

It looks like I need to remove the PCB to do this....not a simple task!

[honeymonster]

Might also be a good idea to put some reflective foil tape underneath the power resistors, to help minimise any burning of the PCB.

[John Caswell]

Contrary to what Honeymonster says DON'T place reflective tape on the PCB as it will only drop off as the adhesive softens and cause other problems.
Stand R24/5 away from the PCB by about 10-15mm as in the original instructions and uprate them to 3Watt Wirewound types.

John

[mussolo]

Thanks guys...all resistors replaced...amp back to former glory.

I see the front LED now glows a nice red once the amp is warmed up...before it only managed a faint orange so obviously some voltages were not correct with the failing resistors.

Now to get a new set of EL84s....one of the valves is glowing very red and is hotter than the other 3. Any recommendations?

[Dave Senior]

Get EL84M's. Significant improvement.
Dave

[mussolo]

which of these should I get?

http://www.ebay.it/itm/EL84M-Sovtek-...item5ae5a2751f

http://www.ebay.it/itm/EL84M-6BQ5-So...item5ad4f14e5b

http://www.ebay.it/itm/EL84M-RUSSIA-...item5adeb30884

[mussolo]

Should I try something different in ECF80 as well?

I have the Ei ECF80 mounted now...

[bikerhifinut]

Now to get a new set of EL84s....one of the valves is glowing very red and is hotter than the other 3. Any recommendations?[/quote]

That may not necesarrily be a failing EL84.
If a coupling cap is leaking and passing dc, that will cause the relevant valve to pass more current and eventually it will fail, possibly taking other components with it, especially cathode resistors as the valve passes more current. Paper in oil capacitors can suffer from this, but its not impossible for plastic dielectric caps to go bad.
You could try swapping the valve with one of the other EL84's. If the fault follows the valve, then its likely a bad valve. If the fault stays with the socket then get the coupling and bypass caps sorted pronto! And before you put the cover back on check the voltages across the cathode resistors and on the anodes.
This could also be a reason why the cathode resistors failed as well as being close to the PCB.

[mussolo]

The glow follows the valve so it is definitely the valve.