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Fender 'Super -Amp' 5F4 build

Thanks for this input. I was not aware of the difference between cathode and fixed bias operation here. I looked at www.ampage.org/td/vtd6l6.html, and saw 6k6 for Class AB1, but this doesn't mention cathode bias operation. The original (but unknown) organ amp was definitely cathode biased.
 
Having used Rob Robinettes' tube bias calculator, I'm confused about the problem with using my transformer with cathode bias. It may be because I don't know how to draw and interpret a load line....

But, with the present bias arrangements, and using 5881 tubes, I entered the following values with the amp idling:
Plate to cathode voltage: 298V
Number of tubes sharing cathode resistor: 2
Voltage drop across cathode resistor: 25V
Cathode resistor value: 248R

The results are:
Cathode current 100.8 DCmA
Total plate current 95.3 DCmA
Plate current per tube 47.7 DCmA
Leading to a plate dissipation per tube of 14.2W (61.7%)

I then checked this by measuring the actual plate current: 52mA
Resulting in a calculated plate dissipation per tube of 15.5W (67.4%)

Now, to go and read more about how to draw load lines.
 
Dandy said:
Having used Rob Robinettes' tube bias calculator, I'm confused about the problem with using my transformer with cathode bias. It may be because I don't know how to draw and interpret a load line....

But, with the present bias arrangements, and using 5881 tubes, I entered the following values with the amp idling:
Plate to cathode voltage: 298V
Number of tubes sharing cathode resistor: 2
Voltage drop across cathode resistor: 25V
Cathode resistor value: 248R

The results are:
Cathode current 100.8 DCmA
Total plate current 95.3 DCmA
Plate current per tube 47.7 DCmA
Leading to a plate dissipation per tube of 14.2W (61.7%)

I then checked this by measuring the actual plate current: 52mA
Resulting in a calculated plate dissipation per tube of 15.5W (67.4%)

Now, to go and read more about how to draw load lines.
Click to expand...
If someone ever offers a load line class, sign me up
 
Dandy said:
Still pondering which way to go with the bias issue, I came across this mod for High Voltage Tap Adjustable Bias for Center Tap PT: https://robrobinette.com/5F6A_Modifications.htm#High_Voltage_Tap_Adjustable_Bias

He also has a nice tube bias calculator, but it seems like moving over to fixed bias would make best use of the OPT.
Click to expand...

There are other ways too. Some use a small 120V>12V transformer connected in reverse to 6V filament power. Gives around 50V AC to the bias supply circuit.

I just bought a small (2.4VA) 115V>48V transformer for bias supply. Running off 120V and considering the low regulation of the small transfomer, it's putting ~60V AC to the bias supply. That transformer cost me about $8 US, new, from DigiKey. The bias circuit draws about 9mA, this transformer is rated 50mA, so it's way more than enough, even small as it is.
 
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