Help with Hoffman 5f1 build

[xochipelli]

Built the Hoffman 5F1 Champ and am having a problem with high B+ voltages.
Using a 5Y3GT EHX Rectifier and a Mercury Magnetics PT and OT.
The PT has a center tap that I connected to the first filter caps (-) terminal. I'm assuming that is correct.
Wall Voltage 120V
HT 640 Vac
Heaters are 6.70V and 5.37V
B+1 418VDC
B+2 386VDC
B+3 335 VDC
EHX 6v6GT plate 416VDC, Screen 130VDC, Cathode 7.78VDC
EHX 12AX7 plates 223VDC and 220VDC, cathode 1.74VDC

The voltages seem much higher than those given off the schematic. When playing the sound starts to break up pretty early.
Would lowering the B+ off the rectifier help? I'm wondering if i can use a voltage dropping resistor between the rectifier and the first filter cap.
Any help would greatly be appreciated, i am learning as i go along.
Thankyou.

[dwinstonwood]

...Screen 130VDC...

How did you measure the 6V6 screen voltage? That seems low. It should be close to the plate voltage.

[xochipelli]

I measured the screen voltage from pin 4 to ground. the screen voltage goes thru a 470/3W resistor before it hits the tube.

[sluckey]

I measured the screen voltage from pin 4 to ground. the screen voltage goes thru a 470/3W resistor before it hits the tube.

If the screen voltage is really only 130V, I suspect you have a 470K/3W rather than a 470Ω screen resistor.

That low screen voltage would account for the high plate voltage and the low cathode voltage. Show us a hi-rez pic of your board.

[xochipelli]

Ahh, I think you may be right Sluckey, thanks!!
I can't post a Pic right now, says the file size is too large, I need to learn how to reduce it but...
I ran the resistor color code and it is a 470K. I'll have to change that when I get home from work.
Aside from that do you think the other voltages are acceptable?

[sluckey]

I ran the resistor color code and it is a 470K. I'll have to change that when I get home from work.
Aside from that do you think the other voltages are acceptable?

All the other voltages will come down when you get the correct 470Ω screen resistor installed. Then check voltages and report back.

Another way to lower voltages is to replace that EHX 5Y3 rectifier tube with a real NOS 5Y3.

[xochipelli]

Replaced the screen resistor with a 470 ohm and the voltages did go down. my plate dissipation seems a bit high though.
B+1  384V
B+2  327V
B+3  282V
6V6GT  PLATE 370V  SCREEN 324V  CATHODE 20.19
 My bias resistor is 469 ohms with 20.40 V across it.
So 20.40/469= .0435 = 435 milliamps.
So 370Vplate x .0435 Aplate = 16.095 Wplate dissipation.
I think that might be a little high. But the amp does sound good.
My heaters are running at 6.62V and 5.3V
Does this sound right? It's my first time biasing an amp.

[xochipelli]

I forgot to mention i am also running the NFB on a 100K pot in series with a 10K resistor. I've played around with it a bit and it works better than i expected.
I'm wondering if the NFB pot setting will contribute to changes in my plate dissipation. i will have to investigate that further tonight, gotta get off to work now.
I'm learning as i go along.

[sluckey]

So your plate voltage is 370v and your cathode voltage is 20.19v. You must subtract the cathode voltage from the plate voltage to get the voltage that is across the tube. Use this voltage to calculate plate dissipation.

370 - 20.19 = 349.81

349.81 x .0435 = 15.2W plate dissipation

That's 109% max dissipation (based on 14W max). It's very common for these Champs to run that hot. You can replace that 470Ω with a 750Ω to bring the dissipation down and make the tube last longer. I would just leave the 470Ω in the circuit.

[xochipelli]

Thanks Sluckey. I think I am going to leave it as is.

[dude]

Champs sound best with high dissipation as long as you use a robust 6V6, preferably a American made 6V6GT. Dissipation of 15 watts at least is the standard for many champ fans. If you bias at 12 to 14 watts or less, the amp sounds like a solid state amp,