Serious electronics expert help needed!

Hi all,

I wonder if I could get your opinion on something! I'm a little stuck and you may recognise the problem.

I have some valve monoblock amps with a set of valves in them. The valves are KT90's. There are 8 in each amp. I have an engineer looking at them at the moment (for a checkup). One of the amps will not sustain a stable bias (after 1 hours warm up). The other one will. With no music signal input the measurements are jumping all over the place at maximum 30mv + or - on the bias of 75mv. I.e. not stable. He's checked all the resistors etc. but doesn't have a valve checker. This guy is very clued up with electronics (it's his business). Does this sound like the valves are on their way out? The previous owner (6 months ago) told me that the valves were new.

He's a really top pro so be as technical as you like and I'll pass on the info.

Any ideas?

Many Thanks, Tim

 

Go and see Andy round at Harrords sound and vision, he's very much a Valve virtuoso and then some. 01242 519581

 

It would be better to ask on a more DIY orientated site like www.worldaudiodesign.co.uk/forum or www.diyaudio.com

For my pennies worth.

• Swap the valves over between the amps to see if it is a valve or not.
• What type of PSU is it. Maybe there is a fault there (smoothing cap or faulty choke or dodgy rectifier tube if valve regulated).
• If not then with the valves out measure the voltage on the last smoothing cap and then the first anode point for the KT90.
• There may be a resistor in line and he should be able to see whether this is breaking down (my first thought, but you said he had checked)
• What biasing voltage and is this stable?
• Maybe there is a problem in the coupling of the parallel pairs, loose track or wire. This would alter the voltage draw due to a valve effectively switching on and off.
• Bad grounding so that the obvious ground is intermitant and therefore the voltage could be floating.

I'm no expert by any means, but good luck.

Oh and a final thought. I assume they are EI KT90's? KT90's made from 6 to about 24months ago where very dodgy. Changes at the factory didn't go well. I had 8, 2 of which failed and blew an amp. If it turns out to be faulty KT90's, consider changing to KT88EH which are more reliable. (do a google of Jim McShane and you'll find out) or go for KT90 mkIV which are now available and appently better.

 

Cathode decoupling cap leaky? eg electrolytic cap bypassing resistor from cathode to ground to allow maximum gain? Can he measure a wobbly current in series with the [quiescent] valves - or is it, as LiloLee said, some resistor/component is varying its value where the bias is measured [failing]..????????