M11 Rickenbacher Big Box Cathode Voltage

[plexi50]

I saw an old post on the forum here with a couple links to amps that are very similar to the M11 BB. It shows +16.00vdc cathode voltage.I am getting +20.5 vdc on the cathode using a 250 ohm resistor. Grid voltages per 6V6GT tubes are +34.5 & +25.5 vdc.Plates are 340vdc / Screens 356vdc. Why the big difference in the grid voltages?

[plexi50]

Schematic from link. This is the Big Box?

[sluckey]

Grid voltages per 6V6GT tubes are +34.5 & +25.5 vdc.

That's very bad! Should be zero. Disconnect both of those .05 coupling caps. Any better?

[shooter]

as I was typing You are fast on the keys!

Grid voltages per 6V6GT tubes are +34.5 & +25.5 vdc.

i'm in second guess mode from heat/humidity but don't believe there should be VDC at G1.  are you measuring cathode to grid vdc?


does swapping the tubes make the voltages "follow the tube"

[plexi50]

Doing the caps now It doent matter what value cathode resistor i try 150,200,25o the voltage stays roughly the same at +20 +21. Removing caps. Measuring Cathode to ground / Grid to ground.

[plexi50]

Lifted 05 caps and now get +23.8mv & +15.5 mv. Cathode to grid -19.05vdc

[sluckey]

Lifted 05 caps and now get +23.8mv & +15.5 mv.

That's a big clue.

[plexi50]

Ok hold on. The schematic i posted is nothing part value wise the same. Look at the new board pics.

[plexi50]

Ok here are the better pics. Had to change phone cameras.

[labb]

Take a look at the schematic for the Fender Delux 5B3. Think it and the M11 are pretty much the same. Just a note, I had a lot of trouble with the available octal preamp tubes being microphonic. That was a while ago. Maybe better now.

[plexi50]

Swapped out a pair of Magnavox NOS 6V6GT tubes for JJ tubes. Put 05 caps back on. Grids now measures +14mv and +17mv. Cathode is now a solid +20.2 vdc. But isn't that cathode voltage a little high?

[plexi50]

Take a look at the schematic for the Fender Delux 5B3. Think it and the M11 are pretty much the same. Just a note, I had a lot of trouble with the available octal preamp tubes being microphonic. That was a while ago. Maybe better now.

Yes that's where i found that schematic. But it's not the same as this M11. This is suppose to be the M11 Big Box Model.

[plexi50]

This amp uses (1) 12AX7 & (12AU7) or 12AT7.

[AlNewman]

Are all the caps in that amp ceramic?

[shooter]

But isn't that cathode voltage a little high

do the math to know that answer, start at ~~~60% 80%(just realized self bias)idle, check again at roaring, even better check dynamically over time (AVG on some meters)
EDIT: play n listen, change and repeat

[plexi50]

Are all the caps in that amp ceramic?

Yes they are.

[plexi50]

There is no bias cathode capacitor on this amp where the 250 ohm cathode is.

[shooter]

336 - 20.2 = 315.8vdc
20.2 / 250 = .0808A
315.8 * .0808 = 25.51664W
since 2 tubes on common cathode resistor;
25.5W / 2 = 12.8W per tube

[plexi50]

336 - 20.2 = 315.8vdc
20.2 / 250 = .0808A
315.8 * .0808 = 25.51664W
since 2 tubes on common cathode resistor;
25.5W / 2 = 12.8W per tube

That looks right to me. It's a great sounding amp. It has that different tone that is unique! That explains a lot!

[PRR]

336 - 20.2 = 315.8vdc
20.2 / 250 = .0808A
315.8 * .0808 = 25.51664W
25.5W / 2 = 12.8W per tube

This is Plate plus Screen power, no?

Rule-thumb for 6V6/6L6: screen is 5% of total cathode current. So plate is 95% of cathode current, or 0.95 of total heat, or 12.16W per plate.

The 12W number is for mass production without measuring. If you measure all you build, 14W is the factory rating. By experience, 14W is not even a strain.

When Miller and Offenhauser built race-car engines, they took-back blown-up engines. They studied the cracks and added more metal in that area. Over the years the castings doubled in weight (so I heard once). It was just cheaper to add metal than to work all winter and then lose the big race on the last lap. Or for Harry and Fred to listen to unhappy customers moan and gripe about overpriced cheap engines. I suspect everybody in the 6V6 racket played a similar game. Beefier metal and bigger plates were cheaper than arguing about burnt tubes. Someone pointed a recent Fender Champ idles near 19W plate dissipation.

[plexi50]

336 - 20.2 = 315.8vdc
20.2 / 250 = .0808A
315.8 * .0808 = 25.51664W
25.5W / 2 = 12.8W per tube

This is Plate plus Screen power, no?

Rule-thumb for 6V6/6L6: screen is 5% of total cathode current. So plate is 95% of cathode current, or 0.95 of total heat, or 12.16W per plate.

The 12W number is for mass production without measuring. If you measure all you build, 14W is the factory rating. By experience, 14W is not even a strain.

When Miller and Offenhauser built race-car engines, they took-back blown-up engines. They studied the cracks and added more metal in that area. Over the years the castings doubled in weight (so I heard once). It was just cheaper to add metal than to work all winter and then lose the big race on the last lap. Or for Harry and Fred to listen to unhappy customers moan and gripe about overpriced cheap engines. I suspect everybody in the 6V6 racket played a similar game. Beefier metal and bigger plates were cheaper than arguing about burnt tubes. Someone pointed a recent Fender Champ idles near 19W plate dissipation.

Plate &Screen power.

[shooter]

This is Plate plus Screen power, no?

Si
4 lines of equations is more than enough already 
12.6 lets me know the house won't burn down, hooking up the 4X12 with guitar tells me if I can rock the house down, 12.16 is for over achievers