Fisher 500C with JJ 7591S - Optimum Bias Current?

[Scott Novak]

I've got a Fisher 500C and have just installed a set of JJ 7591S output tubes.

The bias voltage was low because of a weak selenium rectifier, so replaced it with a silicon bridge rectifier. The voltage increased substantially so I replaced the 15 ohm series dropping resistor, between the electrolytic filter caps, with a 30 ohm resistor and the filament voltage for the 12AX7As is now 22.8VDC. The Sams schematic shows 22 VDC for the filaments, but that was for a design center of 110 VAC believe. My line voltage hovers around 120 VAC most of the time. The 22.8VDC filament voltage should result in a very long life.

Any opinions on the optimum filament voltage for the 12AX7As

The filament voltage on the 7591S is 6.60 VAC

I'm thinking that I should add series resistors to reduce the voltage.

Any opinions on the optimum filament voltage for the 7591S output tubes?

I saw one suggestion for a bias current of 35 mA per 7591. With the bias adjusted 35 mA per output tube, my B+ measures 465 VDC vs 430 VDC on the Sams schematic, and the screen voltage measures 410 VDC, vs 375 VDC on the Sams schematic. The reduced bias current is probably why the B+ is higher.

Any opinions on the optimum bias current for the JJ 7591S?

Any opinions on the 465 VDC B+ and the 410 VDC screen voltage?

Scott Novak

 

[porkchop61]

Scott-
I have an 800b with the JJ 7591's and I'm running them at the 35mA bias. They sound good to me, and I've been running them at this bias for 6 months now without much change (new tubes).
I also had to put a dropping resistor of 100R on my heater line to get it closer to the schematic. This was after replacing the Selenium rectifier with a new bridge rectifier.

I'd search on the Audiokarma web site (I'm a member there) for some more information, as this topic is heavily discussed there.

http://www.audiokarma.org/forums/forumdisplay.php?f=14

Glenn

 

[Miniwatt]

I've got a Fisher 500C and have just installed a set of JJ 7591S output tubes.

The bias voltage was low because of a weak selenium rectifier, so replaced it with a silicon bridge rectifier. The voltage increased substantially so I replaced the 15 ohm series dropping resistor, between the electrolytic filter caps, with a 30 ohm resistor and the filament voltage for the 12AX7As is now 22.8VDC. The Sams schematic shows 22 VDC for the filaments, but that was for a design center of 110 VAC believe. My line voltage hovers around 120 VAC most of the time. The 22.8VDC filament voltage should result in a very long life.

Any opinions on the optimum filament voltage for the 12AX7As

The filament voltage on the 7591S is 6.60 VAC

I'm thinking that I should add series resistors to reduce the voltage.

Any opinions on the optimum filament voltage for the 7591S output tubes?

I saw one suggestion for a bias current of 35 mA per 7591. With the bias adjusted 35 mA per output tube, my B+ measures 465 VDC vs 430 VDC on the Sams schematic, and the screen voltage measures 410 VDC, vs 375 VDC on the Sams schematic. The reduced bias current is probably why the B+ is higher.

Any opinions on the optimum bias current for the JJ 7591S?

Any opinions on the 465 VDC B+ and the 410 VDC screen voltage?

Scott Novak

You must mean something else when you say 'filament voltage'
a 12AX7As with 22.8VDC heater voltage? 😕 Not a long life I'm afraid, or am I missing something here?

 

[Scott Novak]

In the Fisher 500C, pairs of 12AX7As are wired in series, hence the nominal 22 VDC. Running each filament at 11.0 VDC will result in longer tube life, but with lower transconductance.

My concern was what 12AX7A filament voltage sounds best with stock component values.

Scott Novak

 

[Miniwatt]

Ah yes, of course. I learned something today😀