Proper bias for 7581A ???

NOVA70

Silver Supporting Member
Messages
229
Hi,
Any bias recommendations for bias in ma. for a pair of 7581A in a Bassman clone? Never had a pair of these, just need some input.
Thanks,
Pat
 

NOVA70

Silver Supporting Member
Messages
229
Thanks for the reply,
I set them around 30ma, is that ok?
Pat
 

philster

Member
Messages
861
I think that you have to keep a few things in mind. Setting bias is not as simple as saying that 6L6 should be 35 to 40 ma's, for example. Plate voltages vary on different amp circuits. As does the negative current draw.

Power tubes should be functioning within their range of "static watt dissipation", so it is hard to say that a 6L6 or an EL 34 should be set for a specific bias accross the board.

FWIW, I run the 7581A's in my Blockhead Splitfront at about 43ma's.


Phil
 

NOVA70

Silver Supporting Member
Messages
229
So how does one go about setting the bias and considering all those factors. would I need special equipment? or can it be done with an digital tester?
Thanks,
Pat
 

Reeek

Member
Messages
1,196
Max dissipation / plate voltage * 65% to 75% (idle power) = safe BIAS range


30 / 450 *.65 = 43.3 mA
30 / 450 *.70 = 46.6 mA
30 / 450 *.75 = 49.9 mA
 

Blue Strat

Member
Messages
30,194
Tubes with glowing red plates are WAY past the maximum dissipation...likely at 200% or so, so this is NOT how to bias an amp.

As Reeek stated, it's a matter of plate voltage (450 is usually safe to assume for 6L6GC type amps, you'll never be more than 10% off which is fine) and the maximum power dissipation for the particular tubes you're using. 6L6GCs and 7581As have a max power dissipation rating of 30 watts.

The easisest and safest way to bias an amp is with a "bias probe". These are available from several sources including www.webervst.com ("Bias Rite").
 




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