Help Needed: Plate Voltage and Bias Point for 6V6GT Tubes in Bogen DB212 Amp

[Avogadro33]

Need Help From Tube Amplifier Gurus

First, the bottom line: I would like to know the bias set point for 6V6GT tubes in a Bogen DB212 amplifier.

Some background: I acquired a fine old Bogen DB212, but it has the wrong output tubes. Will do a typical cap-job as needed and purchase a set of used 6V6GT tubes.

What seems clear: I found the formula for bias point: I = X/100 x P/V.

P ---- We will assume the plate dissipation is 12 watts (I am aware that a few sources say 14 watts, but most go with 12 watts).

X ---- We will assume the preferred bias percentage is no more than 70%.

The issue: What to use for power tube plate voltage (“V”) in the formula?

A look at the schematic:

The schematic for the DB212 shows the 6V6GT with a 380V output. But I am informed that this is a cathode bias amp (beyond my knowledge) and therefore such voltage must be subtracted from the 380V figure in order to arrive at the value for plate voltage, “V”. If wrong, please feel free to correct this.

The schematic has another connection to the 6V6GT with designation 45V. And there is yet another connection to the 6V6GT with designation –25V.

So, if either, would I use 380 - 45 = 335, or 380 - 25 = 355 for plate voltage (“V”) in the formula to derive the bias point (“I”)? Maybe neither!

Again, I would like to know the bias set point for 6V6GT tubes in a Bogen DB212 amplifier, under the parameters of 12 watts plate dissipation and maximum 70% bias percentage.

Any help?

Thank you.

 

[gadget73]

The -25v at the grid figure is shown with a square. I don't see the explanation on the schematic, but fair bet its the measurement from cathode to grid. Ballpark of -25v difference there doesn't seem insane just at first blush.

Plate voltage is 380 to ground, but with +45v at the cathode the voltage across the tube is 380-45 or 335 volts. Screen voltage is 370 to ground, or 325 for all practical purposes.

Plugging all that into a tube curve calculator, 28ma per-tube would bias at -25 from cathode to grid.

The thing with all this stuff is that on the schematic it never quite adds up. Just as an example, it shows 150ma on the power supply line feeding the output tubes. The curve calc says 112 total which isn't completely bonkers. If you do the math for the voltage drop across the output transformers though, thats about 100ma per-tube, which is impossible.

http://pacifictv.ca/schematics/bogendb212schematic.pdf

anyway I think if you end up with about 45 volts at the cathode and roughly 20 volts drop across R33 and R64 it's fine.

 

[Tom Bavis]

The preamp heaters are also in series with the 6V6 cathodes, but aren't part of the bias network - cathode bias resistor is 330 Ohms, for each pair of tubes. 10W resistor seems odd, since it will dissipate less than 2W. Depending on which number you believe (-25V g-k or 21V across Rk), cathode current is 38 mA or 32 mA per tube - probably on the lower side, since it stays within the EZ81's 150 mA rating. No bias adjustment available or needed, though a matched quad or two matched pairs will be best.

.035 x (380-45) = 11.75 W plate plus screen dissipation. This is class A, usually biased hotter than AB.

 

[Mr Boochie]

I enlarged the schematic. Click on the photo to enlarge it.

 

[Avogadro33]

Thank you gadget73, Tom Bavis and Mr Boochie.

I spoke with our local technician today. He looked at the DB212 schematic and your comments (which were in general beyond my knowledge). Interesting.

In essence, he agreed with the bottom-line comment of Tom Bavis: “No bias adjustment available or needed.”

What I learned: Whereas, for example, my Scott and Fisher receivers have DC balance and other adjustment pots, and some Pilot amps, the MAC 1500 and others additionally have individual bias controls built in, the Bogen DB212 amplifier has none of this and is essentially self biasing. All one need do is to install the 6V6GT tubes (preferably a matched quad or two matched pairs) and that’s it.

I am nonetheless glad I went through the exercise of trying to figure this out; it surely will help the next time around.

Thanks.