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Valve - plate red hot !

G

Gerard124

pfm Member
Last night I was using amps and I looked over and one of the C6A7s was red hot at the plate. I quickly shut down and noticed there was that toasty/burning smell in the room. What has happened? And is there any possibility of any damage to the amp (Tsakiridis Devices - Apollos)
 
Your amplifier has developed a fault, the valve is taking too much current. There are a number of possible reasons, faulty valve, faulty bias, leaky cap from previous stage - not knowing the circuit it`s not possible to be more specific. If you are not familiar with valve circuits you would be advised to find someone qualified to look at it.
 
As above - it could be many things and remember, there are several hundred volts in there which can do you damage.

Let your dealer know, pronto.
 
Gerard said:
So this is an amp problem and not a failed valve.
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Could be either, as I said but the burning smell doesn`t bode well for the amp anyway - it needs to be checked over by someone with experience - did I read that these amps have a 5 year warranty? Not likely to include valves or damage caused by faulty valves though.
 
It could be the valve. If you feel confident, you could swop it and be prepared to switch off the mains quickly.
 
In which case it would be worth swapping the "hot" valve with it`s pair, but as Dave says, be prepared to switch off quickly. If the red hot anode stays with the original valve position it`s the amp, if it moves with the valve it`s the valve. I gather the amp was biased up with the valves you have, if so changing for new valves will probably involve going through the process again for optimum performance.
 
I decided to check the other mono block (left channel) first and then I will move on to the one with the hot valve.

One of the valves is showing zero bias? but 'looks' ok? - the amp was playing fine last night.
 
If you smelt magic smoke that amp needs to see a repair man IMO. Tubes don't go stinky - the tube itself may well be dead, but any smell will have come from it frying resistors or caps as it passed to the other side.
 
Gerard said:
I decided to check the other mono block (left channel) first and then I will move on to the one with the hot valve.

One of the valves is showing zero bias? but 'looks' ok? - the amp was playing fine last night.
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I think you were lucky you saw it on time or more serious damage could of been done. I vote you take it to an amp repair man - before trying it out again.
 
This appears to be valves - I've isolated 4 good valves (Electro Harmonix 6CA7s) and tested each mono block independently (running for over an hour) and everything sounds ok, albeit in Mono as I only have enough working valves to test one amp at a time.

So I'm now in the market for new valves - decisions, decisions El34, 6CA7, 6550, KT88 or KT120? all work with the Apollos.
 
FWIW when I had the Prima Luna Prologue 2 I preferred EL34s to '88s, not as crisp, punchy or tight, but something lovely about the mids. I ended up with Svetlana 'Winged C' EL34s and very nice they were too. It's what works best in your amp and with your speakers that matters though, there are no absolutes with tubes at all!
 
I'd be looking for a bulging cap and a charred resistor at that valve position. When el34s went on one of mine the big cathode resistor and associated cap were taken out each time, in spectacular fashion.
 
TheDecameron said:
I'd be looking for a bulging cap and a charred resistor at that valve position. When el34s went on one of mine the big cathode resistor and associated cap were taken out each time, in spectacular fashion.
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Not going inside a valve amp !

What is the likely hood of damage? everything sounded ok when I tried them earlier.
 
Always a sucker for KT88's here... I'd be a bit wary though with these these different valve options... EL34 and 6CA7 are equivalent and generally use a 1K screen grid resistor.... the other options are also equivalent to each other but would normally use a 470R resistor.... the bias is also quite different from one to the other... by about 18V... that could easily be encompassed with a generic bias pre-set but it would be a large range to see in practice.... just saying ;)
 
Gerard said:
I understand you can use any of those valves, some info here :

http://www.ikonaudioconsultants.com/apollospec.html

I've ordered a set of J J Tesla EL34L from Watford Valves, not impressed with the failure rate of the Electro Harmonix 6CA7s
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I'd have stuck with the Electro Harmonix if I were you! Or better still Svetlana...
I wouldn't touch JJ with a barge pole... Although do I hear they have tightened up quality control in the last year or so.
 
Arkless Repairs said:
I'd have stuck with the Electro Harmonix if I were you! Or better still Svetlana...
I wouldn't touch JJ with a barge pole... Although do I hear they have tightened up quality control in the last year or so.
Click to expand...

Out of 12 Electro Harmonix 6CA7 in the last 10 months only 6 still function! the JJs are going to have to be very poor to be worse than that?

The people at Watford Valves seem quite keen on the JJs
 
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