Fisher 12AX7 Heater Voltage

notdigital

AK Subscriber
Subscriber
I'm still poking around my recently acquired 500-c trying to work through a few remaining issues.

Although I never really thought to pay too much attention to the 12AX7 heater voltage when I had changed out the rectifier in my other units, (some previous owner had already changed it out in this one), I felt it became economically necessary once I popped in a couple of 10M's in V-14 & 15.

I measured the voltage coming off the rectifier @ -25.9. I know there is some wiggle room here isn't that a little on the high side? The schematic calls for -22. If so, a) do I simply splice a resistor into the wire leading to the tubes? and, b) how do you calculate the value needed (mind you, I sat in the BACK of math class :D)



Thanks all in advance :thmbsp:
 
In my modified 400, I added a 10 ohm 2 watt resistor in the lead going from the heater/bias supply over to the heaters. After the resistor (i.e. the heater side of it), I also added an additional cap to ground (+ = ground, 2200 uF, I think), which improved the filtering to the heaters.

Do not change the existing dropping resistor, as increasing it will reduce the bias voltage to the output tubes. By adding the additional resistor and cap, the voltage to the heaters is dropped, filtered better, and the output tube bias voltage is basically unchanged.

In some of the final pics of my unit in that thread, you can see these components added in over near the heater/bias supply.

Calculating the value of such a dropping resistor is more of a trial and error process, as the resistive characteristics of the heaters varies according to the temperature they operate at.

Dave
 
Mr. D: Thanks! I happen to have the needed components on hand and I'll plug them in while I'm inserting screen resistors (I wonder whose idea that was???? :thmbsp:) I should have added, I have already installed Drew's IBAM so there's no concern for bias ouput voltage.

I'll hunt down your 400 thread and peek at your installation altho, in my case, there's an empty lug to ground right where the dual bias caps live; I'm thinking of using that.

As always, thanks for your help!
 
Good info guys :thmbsp:

I've lined up a 400 repair job over the Xmas/New Year holidays so hope to give Dave's modded 400 thread a work-out.
 
Back
Top Bottom