Bias a Silvertone 1474

[jamesedmunda]

$ new matched JJ 6l6gc are getting 330 vdc plate to cathode and my bias probe says 50ma, according to the RobRobinette bias calculator I should be around 84ma. The 1474 has a 39 ohm and 220 ohm 5 watt
bias resisterst hat power a couple 12ax7 heaters. My question is if I change the 220 ohm resister to 160 will the heaters be ok.
 Thanks in advance James

[Latole]

This is the circuit you are talking about ;

[sluckey]

I'd leave it be. The 12AX7s heater voltage will be affected.

[Soulfetish]

I agree with sluckey, I would leave it. Forget what Robinette's calculator says in this case. This amp is not a cathode bias/class A design. It was not designed to have an idle bias of 95%. Much like in the 1473, they use the series heater arrangement to generate the bias voltage, not the other way around. Only, in the 1474, they added the resistor in parallel with the heaters to provide the additional bias current, and the additional 39R series resistor to for a larger bias voltage (because the 1474 has a higher plate and screen voltage).
Perhaps more importantly than the how it will affect the heaters, you're attempting to increase the steady DC working load (and increased AC RMS current) on the power transformer by about 68% in the HT winding.
In addition to the increased demand on the PT, that additional 68% DC current has to pass through each of the 2 output transformer primaries. There is almost no chance that those transformers were designed to handle that additional DC current. They are pretty small as it is, and have been know to fail under the operating conditions the amp was designed for.

[Latole]

6L6's Kathodes voltage show 27 volts on schematic ( 2 X 12 volts 12AX7 heater ? )
What are you reading ?

 

[Soulfetish]

6L6's Kathodes voltage show 27 volts on schematic ( 2 X 12 volts 12AX7 heater ? )
What are you reading

There is an additional 39R resistor in series with the 12AX7 heaters. But, in my experience, the heater arrangement doesn't quite make 24V. I usually read from 18-20V (or so) across the heater arrangement.
I tried messing around with the value of the other 2 resistors on my friends amp to try and bring the heater voltage up to 24V, but the bias voltage on the 1473 schematic lists 20.5V, so I leave it.
The amp is quiet, the 12AX7s seem to work fine, the 6L6s last forever and the amp sounds kind of kick ass. It's a strange set up but it works :)

[Latole]

Thank's for reply.

[jamesedmunda]

Thanks for the replies, I'll leave it
 The amp is so loud and breaks nicely, I couldn't see how it could be better,
 its already a trainwreck.