Fisher X100 Bias Setting

[tube-a-lou]

Hi all,
Doing a little work on my x100 and wanted to know what is the Bias
setting of the amp, I know it's 40 on the X100B but what is it on the
X100??

Thanks
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[audiodon]

Do you mean the bias of the output tubes? 40 is on the bleeding edge on the high side. I like 28-32, especially in summer.

 

[tube-a-lou]

Hi Don,
There's a control by the 12AX7 tubes, which I believe is for the output tubes. Should I lower the bias from
40 to 30 on the X100B. I made a cooling fan for my amp which keeps the amp really cool in this hot summer
were having in the North East so just wondering.


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[mibag6]

The 40V bias is also the supply for the heaters on 4 of your 12AX7s which are connected in series so you might run into distortion issues if you drop that voltage too much. Right now you get 10V per tube to run the heaters, if you drop that to 7.5V I think you'll have issues.

 

[audiodon]

I'm not familiar with that amp. Is that a 7868 output amp?
Make sure you're not messing with the controls for the phase inverters.
When you say 40, I interpreted that to mean 40ma bias across a 10 ohm resistor on the cathode of each tube. Was I correct? If yes, that's the dissipation level. I don't push my amps that hard, but it can be done.
If it's a balance control, we're talking about an entirely different thing.

 

[tube-a-lou]

It's not a Balance control on the X100B, I measure Pin 3 on each 7868 tube
and get -40 across the board, when adjusting this control, I was asking about
the X100 because it has the same type of control and where to measure it on
the 7189 (6P14P-EV) tubes.

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[TheRed1]

I think it's the same. Sams doesn't give instructions for setting the bias on X-100 but on other similar 7189/EL84 amps you measure the voltage on pin 3 of the output tubes. I believe the figure given on the schematic is a conservative value. The 500-S schematic (7189s), which has instructions for setting the bias, also shows 40V on pin 3 but the instructions for bias adjustment give a range of 40-45V.

 

[Dadbar]

The 40V bias is also the supply for the heaters on 4 of your 12AX7s which are connected in series so you might run into distortion issues if you drop that voltage too much. Right now you get 10V per tube to run the heaters, if you drop that to 7.5V I think you'll have issues.

+1

Dropping the voltage on the cathode of your 7868's will starve your 12AX7's and burn up your 7868's. Too much cathode voltage will increase longevity of your 7868's while burning up your 12AX7 heaters. Keep the cathode voltage within 40-44V for the best compromise (the exact value may depend upon your 7868 matched set)

 

[audiodon]

Ah yes, I see what we're talking about now.
The X-101-C has a similar pot in a cathode biased application using 7591s.
I agree with Dadbar on how to deal with it.

 

[tube-a-lou]

Thanks all, this is good I'm just getting ready for the Power Transformer to arrive
and get this guy going. Still checking resistors on it, some of the Carbon Comps are
off a little so I'm checking the rest.

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[MickeySmile]

From my service manual for X-100, the bias setting at 44V