~5e3 Bias

[Jeru]

5e3 build is pretty much done. Everything works, I need to clean up the grounding to get
rid of some hum, but that will wait. It's running Pair of old RCA 6V6s, possibly from the
donor console chassis but I can't remember. With shared 250 ohm bias resistor the bias
is hot -- 33ma on each 6V6 at 314 Plate volts = PD of 10.36 = roughly 86% of max of 12.

Question / contest: What 6V6 cathode resistance value do you think gets me
down to ~70% of max PD..?

Closest to the actual value wins the prize, which is just my gratitude. (I know -- drag)

My plan is to wire up a couple of those 5W white ceramic brick resistors in series, hoping
I can get this done with parts on hand. I have recently had four projects on go at the same
time, and the number of rather small AES orders I've placed lately for ~a couple little things
is moderately embarrassing.

THANKS.

 

[King Fan]

Not gonna win the contest, but isn't the 5E3 cathode biased so safe (and probably desirable) to run up to 100% or higher?

Edit: If your plate voltage is 314, is your B+ around 320? That's nice and low, and may explain the cool bias -- many folks aim their 5E3 at something like 370. What PT are you running?

Most of all, does it sound good?

 

[D'tar]

Cathode bias 100% = good

Fixed bias 70%= good

You are well within safe zone. How's it sound?!

 

[Ten Over]

With shared 250 ohm bias resistor the bias
is hot -- 33ma on each 6V6 at 314 Plate volts = PD of 10.36 = roughly 86% of max of 12.

Is the 314V from plate to cathode or from plate to ground?
Are there 66mA across the 250 Ohm resistor for 16.5V from the cathode to ground?

 

[Jeru]

Thanks all. I'll go ahead and reveal my ignorance -- I thought that single ended cathode biased amps
could safely be at ~100% but (wrongly) that cathode-biased class AB / P-P amps needed to be at ~70%.

I will re-measure taking all of your input and suggestions to heart and will post findings (likely next wknd).

Many thanks folks.

 

[red57strat]

I'm not sure if you'd even like the sound of your 5E3 at 70%. The bias on the old GE 6V6s in mine is 87% and they've held up well for over 10 years.

My plate voltage is 357 and it's "just right".

 

[JPKmusicman]

I was at about the same power dissipation and just left it as others suggested.

 

[Intubator]

Don't worry about the question, I also had the same confusion on mine and inquired about it a couple of years ago. King Fan was kind enough to clear it up for me as well. Yes, even though its Push/Pull it's still cathode biased and can run at 100% safely. I actually ended up reducing the value of my cathode resistor to 200 or 220 ohms to get my tubes up to 100% dissipation from about the mid 80's originally. If you think your overall B+ voltages are too low then consider adjusting the dropping resistor values so the last leg is driving your V1/ V2 plates well enough.