6BQ5 push pull

[Bob Meyer]

I have found a bias mod that works very well with push-pull 6BQ5 tubes..

Remove cathode bias resistor(s) to ground and cap(s)
 Add 50UF 25WVDC from each EL84(6BQ5) to ground.
USE 2 7805 Vreg IC TO220 3 term. 5V neg Vreg IC.one for each EL84(6BQ5)

connect Vin of reg ic. to cathode.
connect ground to ground.
connect V out to 150ohm 1/2 watt resistor to ground.
Do this for each EL84(6BQ5)
This will give APPX 12V from cathode to ground.

This mod will make more stable cathode bias and will auto adj cathode voltage to meet 
tube bias current.

This is good for tube life as they go un-bal threw the life.

I did this mod on a heathkit EA-2 and workd very well.
 you can also use SV83 tubes from SVETIANA..
go to www.svetlana.com and go to techinical bulletin No 29.
I did not try the bias mod on SV 83 tubes.
SV 83 tubes have slightly different bias req.
drop resistor (cathode to ground) buy appx 20%
see tube spec for bias current.

This bias mod should work on most output tubes.
set resistor for bias current.
I did not try it yet on 6L6GC tubes yet.
bias mod is ok for 6V6 tubes.fine tune resistor.
you could use a 317T reg with a smaller resistor to ground and a small pot on ADJ lead.
to have ADJ bias.I did not try this yet but will work.

Bob 
Rascal at Utec.net

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