fisher 800c phase inverter adjustment 2

[slhoward]

Is there a way to cheat on the phase inverter adjustment, that is to get it fairly close without all the expensive test equipment. I have an ociliscope and a volt/ohm meter or vtvm.

Also would it be possible to run el 34s in this amp. I realize the bias would have to be adjusted, perhaps a pot for the bias etc. Seems like the pin arangement is the same?


Steve

[gingertube]

As a first approximation of where to set that pot - use the multimeter.

Some theory.
If you check the schematic you see a 68K in the cathode of the concertina splitter with a 180K going up to B+.
Remember that B+ is an AC (signal) ground so the AC cathode load is 68K in parallel with 180K = 49.35K (You might want to actually measure those resistors and redo the calc).
Now you want the anode load of the concertina (which is 33K + whatever the 25K pot is set at) to equal that same value - so adjust the pot until the combined resistance reads 49.35KOhms.

Once you know where that pot setting is, you can actually screw it a little each way while listening to see if you can detect any difference in sound. My guess is you won't and that setting will be the best.

Cheers
Ian

[Eli Duttman]

Also would it be possible to run el 34s in this amp.

Not really, for 2 good reasons. EL34 heater current draw is greater than 7591 heater current draw. Do you want to burn the power trafo out? The 2nd reason is drive voltage. The 7591 is as easy to drive as an EL84, voltage wise. Fisher took full advantage of that fact. The 12AX7 voltage gain block into 12AX7 concertina phase splitter is a carry over from EL84 designs. Even if the power trafo could support EL34s, sufficient drive voltage is not available.

Is there a way to cheat on the phase inverter adjustment, that is to get it fairly close without all the expensive test equipment. I have an ociliscope and a volt/ohm meter or vtvm.

Unlike some folks, you seem to be willing to make changes and don't lionize Avery Fisher. Remember, 7591s are as easy to drive as "12" W. tubes. If you build an external "12" VDC supply to power the preamp section 'X7 heaters, the 150+ mA./"50" V. negative supply becomes available for other purposes. You then could "import" the differential splitter/driver employed in "El Cheapo". The only instrument needed to adjust that circuit is a multi-meter. BTW, NOS Mullard made 12AT7s are still affordable at about $25 per tube.

[slhoward]

Thanks to both of you for the great advice.