Improving Fisher's Heater String Cathode Bias System

[gadget73]

Its less that you're clipping the output tubes, they'd do that anyway, but you won't have crossover distortion issues, and generally the idle current will also be lower with the removal of the shunt resistor. Overall it tends to be an easier life on the output tubes with that shunt resistor not in the picture. I'd expect more output power as well, or at least no loss of power from the stock configuration.

Cap-coupled cannot drive into grid current, the cap itself will not pass enough current to allow it.

These were my results with a Fisher TA-600, which used two 12AX7 heaters and no shunt resistor. Single 22v diode vs two 11v, but electrically equivalent. Since I didn't pull a shunt the idle current won't have changed, it just limits the shift when actually passing signal.

so I finally bought a couple of round tuits and a zener diode. Went with a 22v 5w part. 1N5358 for those playing along at home. I mounted it right along side the cathode cap, hanging off the two phono tube sockets. Space and access are not your friends in a TA-600 and that was the least difficult spot to add it.

Stock voltage at idle: 20.0
stock voltage at full pwr: 26.6
full power, both driven 9.5 wpc

Zener voltage at idle: 20.0
zener voltage at full pwr: 22.05
full power, both driven 11 wpc

Thats full power before the tops start to flatten out. Crossover distortion in stock form sets in about 8.5 watts. With the zener in place I get no visible crossover distortion at all. I didn't measure the THD but visibly it looks a lot better. Its not enough extra power to make a difference but hey, free power and less distortion along with easier life on the phono tubes for under 50 cents. I'll call it a win.

 

[dcgillespie]

Pistachio — With the cathodes clamped at 22 volts, power output can only go up over the original design. With just under 300 volts dropped across the tubes under quiescent conditions, and even less as the B+ drops as power output increases, I wouldn’t expect much more than 9 or 10 watts RMS per channel out of the design anyway. There is no earthly way the design is capable of 18 watts of power output (unless that’s a peak music power rating), even if only a single channel is driven at a time.

Dave

 

[gadget73]

could be 18 watts total, though "20+20" in the model number definitely implies otherwise. I think Fisher claimed the TA-600 at 40 watts, even though it won't do 20 single-channel and absolutely not 40 with both driven.

Could be PMP, but PBS (pure BS) is just as likely.