Potential amp problem

[Dazza1004]

I've just picked up a cornford MK50, when I checked the bias there is a big difference between the 2 output tubes, one is reading 28, the other 42mA. The difference is pretty consistent across different sets of tubes (old and also new matched pairs)

The difference between the 2 seems outside of normal specs to me (I have emailled cornford but no response yet), am I ok to keep using the amp and any ideas what could the problem be ?

[Dmoney]

If you put known matched valves in it, it should be roughly same.
28 and 42 does seem like a big difference.

Is it always the same socket that has the low reading?

[Dazza1004]

If you put known matched valves in it, it should be roughly same.
28 and 42 does seem like a big difference.

Is it always the same socket that has the low reading?

I think so but I need to double check

The amp sounds ace but still worried something's amiss

[richard]

E-mail Cornford. They've had some tough times but lately they've replied to me very quickly.

[jpfamps]

How are you measuring the bias?

I would firstly swap the valves to different sockets and check that the current draw follows the valve.

If it doesn't there is either a problem with the bias supply of the amp, or your measurement method.

If it does, then the valves are not that well matched.

As long as the hotter valve is not biased too hot this won't cause any damage, but you may experience some hum and possible some unpleasant distortion artefacts.

I expect (hope!) that the valves were matched for current draw before installing into the amp, however as valves age current draw can vary and mechanical disturbance can also affect valve parameters (I know this is true!!).

[Dazza1004]

Well, it's f**king blown up, smoke from one of the power tube sockets and then death

Great, had it 2 weeks  :(

[Dmoney]

sorry to hear that, man!
did you get it direct from cornford? can you send it back? did they ever reply to your email?

[HTH AMPS]

sorry to hear about your amp man, but try calling them - that old thing called the telephone is still the most direct way of 'speaking' to someone.

if there was smoke out of the amp, its likely just a screen resistor (easy fix).

mind you, if this was a brand new amp, it doesn't fill you full of confidence in the company.  still, valves can vary in matching if they're not burned in properly - I've had it happen myself.

[Dazza1004]

Martin Kidd did get back to me, I am going to try and call him tomorrow and arrange for them to fix it

I bought it 2nd hand, let that be a lesson to me !!

Which couriers are good ?

[HTH AMPS]

Martin Kidd did get back to me, I am going to try and call him tomorrow and arrange for them to fix it

I bought it 2nd hand, let that be a lesson to me !!

Which couriers are good ?

I normally just use Parcel2go - they subcontract out to DHL, Business Post etc... but you get a better price than going direct to any of those couriers.  I'd expect to pay around £15.00 for the courier with the amp insured for the full value.