Very Good question! W9NUP restored a butchered Dentron a couple of years ago and he found no center tap for the filament transformer (a Dentron save a penny decision). He found that the filament voltage was too high. He used the same trick that Roy Norgard, KL7FM outlines in his Heathkit SB-200 Rebuild Notes (which also uses 572B tubes).http://home.gci.net/~rnorgard/ampnotes.html
Here's what KL7FM did: The 572-B tubes recommended filament voltage is 6.3 VAC +/- .3 VAC. This means that the tubes should put out full rated power with 6.0 volts on the filaments.
When Bob measured his, they had 6.76 volts on the filaments. This was more than the maximum recommended voltage.
EIMAC did a study and found that for every 3% above the minimum filament voltage required for full output reduced tube life by HALF.
That could, in fact, be why I'm not getting as much power out of mine as yours. The heaters are getting weak. You probably don't have much time on your tubes. That's why you've got the extra power.
It's hard to find resistors to do the job. Each tube draws 4 amps filament current. That's 8 amps total. Each of the two coils of wire is about 30 inches long. They're made of 22 ga. Teflon wire. The resistive coils drop the extra voltage for me. I started with two 36 inch lengths and kept shortening them until I measured about 6.05 to 6.1 volts at the tube sockets. When I got the length right, I just coiled them up and hold them down in the corner with a stick-on and a ty-rap.
Now, how much wire to add?
The wire I supplied will drop about 10 milli-volts per inch.
So, take the voltage reading you just got and subtract 6.00 from it.
Here is an example:
You read: 6.76 Subtract: 6.00 You get: 0.76
Divide this by 0.01: .76 / .01 = 76
You need 76" of wire. But, half goes in each leg, so that's two pieces 38" long.
So... for the above example, you need two coils of wire 38" long. The wire will heat slightly when in use, and in so doing will increase in resistance, dropping more voltage. To compensate for this, use about 8% (4") less wire. Use two pieces about 34" long. Of course, you will have to figure this out with the voltage reading YOU take.
Give your Dentron a longer life!