Valve amp bias

[Cyrus]

I'm struggling to understand why a valve amp that is not self biasing has been set up so it is (seemingly) not easy for an owner to bias it.

It will be easy once you know how - calculate the bias current you want at whichever pin it is, tweak pots and measure the current represented as a voltage at each valve base. That's roughly how mine goes. I'm sure biasing instructions will be available for a John Wood KT88 online somewhere.

[Audio Al]

Hi Alan,it’s a kt88 as in my sig, John wood told me the bias is set at 30 me but I can’t for the life of me remember where he showed me to put the meter probes, I know roughly, I’ve emailed him to ask but thought I’d see here as well. I will post a close up pic later if that helps.

I’d like to check the bias because I seem to have a slight drop in gain and after he adjusted the bias last time it sounded great, the power valves according to him were about 63% but I wonder if some new ones might help?

Jamie should that read 30 mv ? and not 30 me ?

[karma67]

Lol yes 30 mv

[Audio Al]

I thought the JW KT88 amp was 70 WPC

[karma67]

would that not explain the low MV bias?

[struth]

if maker says 30ma, then leave it at that. he ought to know the best setting

[anthonyTD]

30 ma per valve is just the idle current ie; with no signal, the output stage is not class A, as you drive the amp with signal, the output valves will draw more current!
Hope this helps.
A...

[karma67]

well ive just done it,i set it with the amp on hooked up to a reciever and speakers connected,should i have done it without anything connected then?
1 thing i did notice is the 1 valve (no 3 from left to right) was out compared to the others,this was the same valve that was out when john adjusted it,does this point to a worn valve?

[karma67]

For future reference,
I just heard from JW, it’s 35ma for each tube.

[Cyrus]

Good on you; doing this kind of thing for yourself pays dividends, literally.