KT-77 Einstein question ref biasing

badgergman

badgergman

Member
Re-tubed Einstein combo with JJ KT-77's.

2nd batch of these as first ones I could not get to bias in correctly. They were much hotter than EL-34's I had in place and would not balance at all. The tech I got them from suggested that my Einstein may be happier with a lower rated pair of KT-77's, wth similar readings to EL-34.

Seems he may be right, I got them biased in but after a short time I found that 1 tube was displaying 40ma and one 30ma in order to achieve 70ma on the pair.

Is this an issue? I bought 2 pairs of KT-77 as a matched quad and both are reading the same. Concerned that amp may need something doing to it.

Low volume tests suggested that they sound great, got a couple of shows this weekend in order to fully evaluate.

Approaching 2 years with Einstein now and still loving the amp!!!!
 
Bad news. Soundcheck at gig last night.

Amp lasted about 4 mins before dying completely. Tube fuse LED lit up. One tube looked really bad, blown to bits (see pic). Replaced with new pair of KT-77's, sounded dreadful. Seriously thought amp was toast. Quick 1 hour round trip back home to get EL-34's and bias meter.

Installed and said a few prayers.....

Thankfully amp not destroyed. Gig went great. Testament to amps design that even a tube blowing so horribly didn't cause internal damage......

Think I like EL-34 BHT's better than KT-77's...... Sigh of relief to say the least.....
 

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No problem Peter. Tubes are mysterious.... Will speak to my supplier....

Just eternally grateful that the amp survived!! Sounded awesome last night!!

Ref the bias question? Is my meter method okay?
 
Most units and myself are measuring the current
on the cathode. Here we get the anode current
and the screen grid current. The grid current
on the EL34 and KT77 types can be high and
different between the "matched" tubes.

Some manufactures do the matching only on the
anode current. In this case the cathode current
between the tubes can be differ on the @matched
set concerning the cathode current.

Important is the current through the output
transformer and this is just the anode current.

More tolerances are given by the components
around the tube but this should be a minimum.

Of course we are talking about class AB and pentode
schematics.

Simply use your ears. Best is what sounds best .... as usual.

Best,
Peter
 
Hi Peter. Thanks for the response.

However, I am a little confused now.

I was under the impression that my meter measured cathode current?

Regards
 
Peter would you recommend the Gold Lions over the JJ's? Personally I ABSOLUTY hate JJ's for this reason. Tried EL34 JJ's twice in 2 different amps and the tubes last a month in one and 6 months in the other. Personally I'll never use JJ's again.

So Peter or anyone else that has experience with KT77, are the Gold Lion superior to the JJ's?
 
badgergman":7t17g354 said:
Hi Peter. Thanks for the response.

However, I am a little confused now.

I was under the impression that my meter measured cathode current?

Regards
Click to expand...

Yes, your method is right but don´t know the method
of the tube dealer. Maybe because of this can be a
certain mismatching. I tried to explain but my English sucks.
Sorry.
 
No worries Peter, your English is way better than my German!!

Thanks for the clarification.

I am reverting back to Ruby EL-34 BHT for now. they have given me no trouble in 2 years (replaced annually).

I think you are right. Let my ears be the judge for now.

Regards.
 
tripstan":3pqshk0g said:
Peter would you recommend the Gold Lions over the JJ's? Personally I ABSOLUTY hate JJ's for this reason. Tried EL34 JJ's twice in 2 different amps and the tubes last a month in one and 6 months in the other. Personally I'll never use JJ's again.

So Peter or anyone else that has experience with KT77, are the Gold Lion superior to the JJ's?
Click to expand...

I know the issues with JJ but the KT77 has big maximum ratings.
So they are relaible if you drive them at 50%.
To be honest, I never took care about the Gold Lions because
they are overpriced for OEM manufacturing. Let me order another
set to check out the performance.

Cheers,
Peter
 
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