I can't just check bias voltage and such by comparing to schematic. The receiver was long ago converted from 7591 to EL34. Bias was adjusted by lowering screen voltage. The tubes don't have any sign of strain even after running all day, no orange glow at all on the plates.

There is about 440 VDC at the plates, about 320 VDC (was 375) at the screens, and -20 VDC at the grids.

Dave

I changed the resistor on the bias bridge rectifier-to-ground from 15 ohms to 2 ohms and got the voltage to -24 VDC. This also, unfortunately, raised the fillament voltage on four 12AX7's higher than what is good for them, so I added a 16 ohm resistor in series whith them and have them right where they should be.

I'm a bit worried about reducing the bias resistor anymore. Not sure if the transformer tap and rectifiers are up to a lower resistance than this or not (not sure if it's up to the valule I have on it now.) Is -24 V close enough, or should I try an even lower resistor?

Dave

Is there any other source of heat? you mentioned the rectifier a bit ago.

Have you actually measured temp? 8 seconds is a long time when holding your paws to heat. There is a diff between hot and inducing damage, and hot and able to burn fingers. 85-C is not going to allow 8 seconds of good finger contact, I can assure you...

Still, none of my OPTx's run hotter than I can stand, so I would urge you to discover the heat source.

There is *NO* magic involved, so examine it thoroughly.
cheers,
Douglas

Dave

Also, some time ago I replaced the 1.2 ohm resistor between first and second stage in the high voltage supply with a 7.2 ohm. This is what lowered the screen voltage.

It gets complicated, but it does seem to play well and sounds wonderful. The things I'll do to save money. EL34's are still cheaper than 7591's. Besides, I love the sound of EL34's.

Dave

Yes, your output tubes are running too hot, plate current must be decreased. -26v is not negative enough.

Also, the heaters of the EL34s draw a LOT more current than 7591 heaters. This is dangerous. The original power transformer is barely adequate to run the original load. If I had to put EL34s in a 500C, I'd either use an external power transformer or remove/bypass some preamp tubes (or maybe even the tuner) to decrease the load.

>) you should change the "bias voltage" Ug1 to about -26V.
>If you don`t have adjustable "bias" pot, you can change
>the values of voltage divider resistors in "bias" supply
>to "enlarge" Ug1.

That's good advice. If this were my receiver, I would assume even worse, and modify the output stage to insert a cathode standoff resistor on the cathode of all 4 tubes, to further decrease plate current. I suggest a big 100 ohm wirewound resistor bypassed with a 1000 uF, 25v capacitor.

Another advantage to this scheme: the cathode resistor helps limit inrush current at turn-on.

Another thing I always do to old tube receivers is to put a large resistor in series with the AC mains input. 5-10 ohms at 10 watts also limits inrush, to help prevent power transformer death. Remember, AC line voltages have crept up over the last 40 years.

If modern EL34s conformed to the old data sheets, -26 would be adequate, but modern EL34s NEVER conform. They always seem to need more negative grid bias.

Will it hurt the power transformer to change the bridge rectifier-to-ground resistor to an even lower value than the 2 ohms I'm presently using (origional value was 15 ohms)? I've looked at schematics of amps from other companies and see that many times the bridge rectifier is hooked directly to ground without any resistors.

I was wondering earlier about using cathode resistors but didn't know how to adjust the values. I think I'll try this soon.

Dave