Are "C" receivers harder than "B" receivers on the 7591 output tubes?

[monkboughtlunch]

The "C" series tube receivers deliver about 5 watts per channel more than the "B" series (30 wpc vs 25 wpc, respectively).

To what extent do the "C" receivers drive the 7591 harder than the "B" receivers? Is there a significant difference in output tube life between the two series?

 

[bberkom]

A recommended modification in the output section of the 500c is to swap out the coupling caps and grid resistors, as the original values used drove the 7591's to their limit. It's been said that they didn't care as much about driving the tubes hard since they were so cheap to replace. These mods reduce strain on the tubes as long as they are biased correctly. I don't have experience with the b version to make a comparison, so not sure if there's something similar with the b version. May be a difference in OPTs as well, but one of the more knowledgeable fisher guys will chime in soon I'm sure.

 

[larryderouin]

The output sections on the "B" and "C" models are identical with regard to surrounding grid return resistors, coupling caps, etc. The main difference is the output transformers. Neither is really harder in my experience than the other unless you stuff Russian output tubes in one or the other and run them stock WITHOUT the mods. With the mods they run approx the same with regard to tube life.

 

[Dave451]

C also used higher plate and screen voltages compared to the B, right?

 

[larryderouin]

Yes. But you adjust them identically, and do the math to get the optimum output. Watts is Watts. Higher voltage will equate to a higher current on the cathode , hence a lower(more negative) pin 6 voltage on the "C".
800B output


800c Output.



The numbers for pins 6 on the "B" and "C" are based on maximum dissipation of 19W for the 7591. Optimally for longer life of the tube (less stress, etc) we won't run them flat out. especially the russian tubes. So an operational envelope of 70% to 80% of maximum dissipation is recommended for all tubes. You can run them higher than 80% if you have $$$ burning holes in your pockets, which in addition to killing the tubes also puts undo stress on the caps and resistors from the P.I. to the outputs.

You want say for example 75% of max dissipation on the 7591 which is 19W.
The following are Examples based on schematic values. Your voltages will undoubtedly be different for the plate and screen voltages. Ideally you want to run the 7591 between 70% and 80% of maximum dissipation. 70% is 13.3w and 80% is 15.2w 75% is a good medium value of 14.25W
After getting the initial ma, subtract 3 ma for screen dissipation. (So bump the ma by 3 ma to compensate).
75% of 19 is 14.25W. On the "C" model divide 430v by 14.25w and you get 30.175ma. This is your cathode draw in ma that you would get for 75% on the "C". Now subtract 3ma for screen dissipation. the ma is now 27.175 and wattage is 11.61w so bump the ma up to 33 to compensate for screen dissipation.
Your actual wattage with screen dissipation is actually 14.25W.

On the "B" divide 393v by 14.25w and you get 27.57 ma. The same applies for the "B" as the "C". Add 3 ma to 27.5 to compensate for screen dissipation.

As the plate goes up you need more cathode current to make 14.25W. Conversely as the plate voltage goes down, you need less to make 75%

FOR EXAMPLE on my Sansui 1000A (which is essentially a Jap 800c) The plate Voltage is 480V. I run them at 75 % so we'll use 75% to be consistant. 480 divided by 14.25 = 33.68ma add 3ma to compensate for screen dissipation and you have a cathode draw reading of 36.7(rounded up.).

You get the same wattage at different plate voltages with different bias voltages and ma's. The output wattage is the same! Ohms LAW at Work.

 

[longle]

If tube life is a concern on any vintage hardware a sure-fire way to extend their life is to use a bucking transformer to reduce line voltage. When I restored my Fisher C-810 there was a very noticeable reduction in heat off the chassis when I used a bucking transformer. A Radio Shack 13v, 3a transformer dropped the line voltage from 123v down to 110v with no loss of performance. A secondary advantage is with the bucking tranny installed in a box with a switch you can leave the power switch on the receiver turned on and turn the rec on and off via a switch on the box. Saves wear and tear on sometimes impossible to find on/off switches.

Here's one I put in an old wooden cigar box. Fused, switched, power indicator light, and one outlet.