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Increase idle current on my KT77's?

tomman888

Member
2006-11-03 4:46 am
Hello all-- I've got a question about a modern KT-77/EL-34 based amp. It's a Rogue Audio Cronus & the power tubes are a relatively new matched quad of Genalex Gold Lion re-issue KT77s(Russian).

The manufacturer advised me to bias them the same as the EL34's it shipped with were biased; that is, 35mA per tube. Luckily this model has a builtin bias meter and pots for each tube so it's easy to play around with different settings.

Normally, I would leave it alone but quite frankly I feel that this amp is way too polite for a tube amp (it sounds more like SS). I have already rolled NOS ECC82's and ECC83's into the front-end FWIW.

I briefly tried setting up the KT77s at 40mA per tube and did not experience any redplates.

What would you do? Is it alright to increase? The amp doesn't run too hot and is well ventilated...
 
Not really enough info here to give you an absolute answer.
I will assume (educated guess) that the high voltage rail to the output tranny is 450 Volts.

At 40mA and fixed bias that means at idle each KT77 is dissipating 450 x 0.04 = 18 Watts (Anode + Screen dissipation). That is well within limits. I would set that in preference to 35mA every time.

45mA =>20.25 watts - that would be OK too.
50mA =>22.5 Watts - that is about the limit I would ever set for a KT77 but I've certainly run KT77 at this level without any problems.

DO NOT set anything higher than 50mA

Hope this is of some help,
Cheers,
Ian
 
This amp claims 55W per channel. From a pair of EL34 that probably means Class B, which in turn means low quiescent current. It is not simply a case of keeping within the allowable heat dissipation, but ensuring that small signal gain is not too much different from large signal gain. Increasing the current is likely to introduce distortion in the mid-power region. Of course, this may be what you want as you seem to be complaining that it sounds too good at present!
 

tomman888

Member
2006-11-03 4:46 am
Hi I rebiased em all to 40mA each and it certainly sounds more 'effortless' if that makes sense.

I believe the plate voltage is 500V. but I cannot seem to get a solid answer; and to measure it would entail taking the amp apart which is a very tedious process for this amp.
 

tomman888

Member
2006-11-03 4:46 am
Hi sled108--I definitely don't think it's 'polite' anymore. After cranking up the bias values it's as if the highs & lows suddenly have magic in them. Like a different amp, really.

But of course the downside is that I'll be replacing the finals more often.

Not too worried about that--I'd rather have it sound better.
 

tomman888

Member
2006-11-03 4:46 am
My local hifi dealer expressly told me not to buy any JJ tubes.

I spent a small fortune retubing the amp's stock SED 'winged-C' EL34s for these Gold Lion KT77s but I think it was worth it.

FWIW, I took his advice re: the JJ's since I ordered two new JJ 12AX7's in 2007,, for use in a restored Stromberg-Carlson integrated amp's phono section, and one went microphonic within 3 weeks.