5A5 TV front biasing question

[ChubeRules]

I've got a Fender 5A5 TV front cathode bias.  When I checked the power tube dissapation they were running at almost 115%.  The tubes seemed to be running unusually hot- is why I decided to take some readings.  The PT looks original from the braided wires coming out of it, which tells me it was probably designed with a 110VAC primary.  It's putting out 480VDC B+ under load, running a 5U4 rect.  The amp had a 250 ohm 10w cathode resistor.  I've already bumped the cathode resistor up to 300 ohms with resulting plate diss. down to about 107%. My question is is this the right approach to getting the tubes running at spec -- should I bump the cath resistor up to 360 ohms, or should I be looking at reducing the B+ overall.  (the heater VAC is about 6.6, which gives me only a little wiggle room.) Or, is there some other approach I'm missing.

[Papa Jim]

describe the connections of what you are referring to as the cathode resistor.

[tubenit]

I am thinking this is a 1947-1953 era Fender TV front Tweed Bassman 5A5?   Is that correct?

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_tv_front.pdf

http://ampwares.com/category/era/tv-front/

I am understanding that you are referring to the 5881 power tubes shared cathode resistor.  Correct?

Some of what you may be facing is higher wall voltage which you've alluded to already. 

Do you know what the current draw is for the power tubes?  AND are both tubes drawing the same current? 

With respect, Tubenit

[sluckey]

Take the resistor on up to 470Ω and see where the bias point lands. Increasing that resistor causes the plate current to decrease, but that decrease in plate current causes the plate voltage to rise. Sometimes the decrease and increase seem to offset each other, making it seem like you're not getting anywhere.

[ChubeRules]

describe the connections of what you are referring to as the cathode resistor.

The cathode resistor (

I am thinking this is a 1947-1953 era Fender TV front Tweed Bassman 5A5?   Is that correct?

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_tv_front.pdf

http://ampwares.com/category/era/tv-front/

I am understanding that you are referring to the 5881 power tubes shared cathode resistor.  Correct?

Some of what you may be facing is higher wall voltage which you've alluded to already. 

Do you know what the current draw is for the power tubes?  AND are both tubes drawing the same current? 

With respect, Tubenit


This is the Fender Tweed TV front 5A5 Pro circa 1947.  not the Bassman.  Yes this is the share cathode resistor. 
The current draw with the 300 ohm resistor is 49mA and 60mA respectively.  The 5881s are supposed to be a matched set.

[ChubeRules]

Take the resistor on up to 470Ω and see where the bias point lands. Increasing that resistor causes the plate current to decrease, but that decrease in plate current causes the plate voltage to rise. Sometimes the decrease and increase seem to offset each other, making it seem like you're not getting anywhere.

OK, I'll go incrementally up in resistance until I get the dissipation to something below 100%.  I'm thinking 360 ohm should do it. 

[PRR]

I would not fret about running modern 5881 somewhat past Pdiss spec. They were very-improved 19W 6L6 types, and (from bad experience with 6L6 designers) were rated very conservatively. Anyhow most you buy today will be re-bottled 6L6GC guts, 35W. (Or you can put 6L6GC or KT66 in 5881 sockets.)