Bias setting for 5F6A

[Johnnydoright]

I completed a new 5F6A build and did initial startup, checked voltages, ect. All seemed ok, no unusual noise, but when trying to set the bias, I am getting a reading of -58.1 volts DC with the bias pot set fully counterclockwise. Turning it clockwise the reading decreases, slightly to -59.4 volts DC. This is the full range of adjustment I get at the "BIAS RANGE " pin on the board. I am puzzled, as I thought there would be a more significant change in voltage as the adjustment was made. Additionally, as this is the first amp with adjustable bias I have built, I am not sure what the value should be. This is a 5F6A Hoffman board which I obtained most of  the parts at the Hoffman site. Weber transformers,  JJ tubes.
6.8 volts heater voltage, 5.5 volts rectifier heater. 710 volts transformer voltage.
Any thoughts/suggestions/help are certainly appreciated. Thanks.

[PRR]

Something important is not connected. I can't see it from here. But a clean mind, a bright light, and a yellow marker on the schematic, you will probably find it.

[sluckey]

I completed a new 5F6A build and did initial startup, checked voltages, ect. All seemed ok, no unusual noise, but when trying to set the bias, I am getting a reading of -58.1 volts DC with the bias pot set fully counterclockwise. Turning it clockwise the reading decreases, slightly to -59.4 volts DC. This is the full range of adjustment I get at the "BIAS RANGE " pin on the board. I am puzzled, as I thought there would be a more significant change in voltage as the adjustment was made.

The turret marked "Bias Range" is AC volts and is not supposed to change. Connect your meter to pin 5 of V4 or V5 to see a change of negative voltage.

[Johnnydoright]

Pin 5. Ok, that's the one! I think there is some math involved as well. I am now seeing the reading change significantly as the trim pot is turned. I set the amp at -48 volts after I did more reading last night, but think there are calculations that should be performed. I am still reading through this info, but making progress. Also tube type is important. Currently there are 6L6GCs in the amp, and I believe they are rated at 30 watts. Thanks so much. I will read on. :-)

[sluckey]

Read this to learn how to bias your amp...

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

The 1Ω resistors mentioned by Lord Valve are already installed on the Hoffman 5F6A board near the bias pot.

[Johnnydoright]

Thanks Sluckey. Yes the 1 ohm resistors are there, and I read that post a couple times. Going to read it again, and make some notes, do some adjustments and maybe plus in a guitar later today. I will keep you posted on my progress. I want to be careful not to screw anything up.

[EL34]

My board install instructions are here with bias info
https://el34world.com/Hoffman/instructions.htm

[Johnnydoright]

Thanks guys. After reading the links provided, I adjusted the bias a few times, reading voltage at power tubes, and across the 2 1 ohm resistors. I ended up at -48.0 volts, which is in all likelihood a bit low for this amp, but I figure to go easy on the tubes at first. Voltage at pin 4 is 457 volts. A cross the 2 1 ohm resistors I got 29.8, and30.9 milivolts, yielding power output of 27.87 watts. I will let the amp run a few hours cans see how it goes. Much thanks for all your help and advice.

[sluckey]

Voltage at pin 4 is 457 volts.

No, that's the screen voltage. You need to measure pin 3 for plate voltage.

[shooter]

for lots of fun, after you're happy, put 10hrs on the PA tubes, re-check bias.
then break out some gator-clips, meters and monitor the 1 ohms while playing hard for a few minutes, let your eyes and brain solve "the average" value as you play.  (some meters will capture min max and avg)  Then decide where you want to set bias, for tube life or musical life.

[PRR]

29.8, and30.9 milivolts, yielding power output of 27.87 watts.

No, that is power INput, at idle. Idle dissipation. Which has no direct relation to power output.

My sick Honda idles 600rpm some days, 1300rpm other days. When I FLOOR it, I get the same max power no matter what it idles at.

It is possible to idle a high-power amp real cold. The 5F6a traditionally is set up that way. When you play LOUD you will see those 30mVs rise over 100mV as the power OUT exceeds 40 Watts.