1973 Fender Princeton non reverb bias question

[Warrenghall]

Good day. I have a 1973 Fender Princeton no verb. Until recently it had a set of tung sol 6v6’s but was sounding ratty. When I checked the bias one tube was at 20ma the other 10. Aha - bad tube - this explains it. I put in a set of NOS tubes I had and the amp sounds great, with good strong tremolo. Yet… when I checked the bias the tubes at 15 mA. Should I bring them to at least 20?

thoughts? By the way the amp was modded to add a bias pot by myself so changing the bias current is not a huge deal.

and for all those that helped with my 1973 Pro Reverb questions I retuned it and it sounds great.

thanks

warren

[phantomman]

What's your plate voltage look like at the 15 mA idle-bias figure?

[Warrenghall]

What's your plate voltage look like at the 15 mA idle-bias figure?

i actually only use an octal bias probe to check current.

[Bill Moore]

You need to know the voltage to figure the bias starting point.
Check this out:

https://robrobinette.com/Tube_Bias_Calculator.htm

[Warrenghall]

Checked the plate voltage. Around 500 v. Biased my new JJ’s at 17ma per tube.

thoughts?

[phantomman]

Too high. You should be seeing circa 420 VDC.

[Warrenghall]

Just to point out this is the non reverb version with a 5U4GB rectifier. What would I do to lower plate voltage?