5F6A Bassman Bias / Current / Watts

[plexi50]

I want to make sure i have my thinking right. The reason i ask is i am using a pair of 1958 5881 power tubes and want to be sure i have them biased a bit on the cold side. Or should i say not hot,not cold.

I have:
450 Plate VDC
-44.0 grid VDC
50.0 ma cathodes each 5881

I come up with 450 x .05 = 23 watts. The tubes are rated @ 26 watts each. Is this correct?  Max dissipation is 26 watts.
What percentage of dissipation do i have now @ 23 watts? I missed class again.

[sluckey]

88.5%

[plexi50]

Ok. I have a 2.275 voltage drop between the plate and CT. I need to read the ohms between the standby switch and plate and divide?
Divide the voltage drop by the ohm reading to get the plate current in amps.

88.5 is to hot for those tubes. I am using a 47K bias r and a 380-0-380 MM PT.

So a -46 / -47vdc grid would lower the plate dissipation correct?

[sluckey]

read this...

http://www.duncanamps.com/technical/lvbias.html

Better yet, bookmark it.

[plexi50]

Thanks. Reading now. I am going to learn all the things that i never fully mathematically should have long ago.
The basics is not good enough and i want to get better with the numbers more precisely.

[sluckey]

23/26=.885

.885x100%=88.5%

[plexi50]

Plate 455 / Grid -46volts / 40.6 ma = 70% dissipation? Edit: 70% dissipation of 26 watts. 18.2 watts?
Still reading. I'm getting it slowly.

[plexi50]

Plate 463VDC X .046 ma = 21 Watts.
18 / 26 = .692
.692 x 100 = 69.2 %  plate dissipation?  The wattage does not add up though if i am correct.

[PRR]

Try 40mA.

But note that Tung-Sol 1962 gives maximum plate voltage as 400V. This is Design Center, for pre-planning production with inevitable deviances. If you are actually measuring, you can probably go 20% up. 1.20 * 400 = 480V. You are right up there. A 3% jump in wall-voltage puts you right-at the limit.

Also note the Design Center limit on Pdiss is 23 Watts. This at 70% leads to 34mA at 465V.

[HotBluePlates]

Plate 455 / Grid -46volts / 40.6 ma = 70% dissipation? ...

The 5F6-A schematic shows 432v plate, 430v screen, and has a bias of -48v. Why not just stick with the -48v and double-check that your tubes aren't overheating (it sounds like they won't be).

[plexi50]

Plate 455 / Grid -46volts / 40.6 ma = 70% dissipation? ...

The 5F6-A schematic shows 432v plate, 430v screen, and has a bias of -48v. Why not just stick with the -48v and double-check that your tubes aren't overheating (it sounds like they won't be).

HBP i originally did that and used a 340-0-340. I had 430VDC on the plate, -47 grid but had no 1 ohm resistors at the time to put on the cathodes to measure the cathode current. I was not sure what my plate dissipation was.

Over the years i have seen a number of vintage 59,60 Bassman Tweeds that had a Triad 8087 390-0-390. I even had an early 1960 white bassman head come in 2 weeks ago with a Triad 8087 PT in it. Pic below.

So i got a MM 380 PT and put it in a few days ago. The ones with the larger plate voltage that i have seen here really do sound great.
I was trying last night to set the bias,get the plate voltage and ma current all within close specs as they should be. But i couldnt do it swapping resistors all night. This morning i decided to put in a small trimmer pot for the bias and BINGO! I was able to bias and achieve Plate,Grid,ma voltages where i wanted them to be.

455 x 0.042 = 19.11 watt plate dissipation per 5881 with 0.042 = 42ma cathode current each 5881.
Grid -45.8

[plexi50]

Early 1960 Bassman head with Triad 8087.