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FDP Forum / Fender Amps: Vintage (before 1985) / SF Princeton Bias



Jun 26th, 2018 10:32 AM   Edit   Profile  

I have a 1970 silverface Princeton. Years ago I did the Hoffamn bias pot mod, introducing a minipot to permit me to adjust bias. I have recently changed power tubes (from Mesa Boogies to JJ) and changed my rectifier to a electroharmonix. for some reason now the maximum bias current possible seems to be around 18 mA, whereas I swear, years ago, I was able to adjust up into the 30's. Any idea of what could have changed. Even when I put the boogies back in the amp, with the original rectifier, the max bias current I could achieve was around 19 mA. Is there another component which could have failed / drifted to have caused this change - or am I just going batty?

BTW amp sounds good as biased


Too Near Atlanta GA

Amp Tech Emeritus
Jun 26th, 2018 11:18 AM   Edit   Profile  

Generally (!), that model output circuit is happy in the 17 to 24 mA range. Components and conditions that can cause variations in what you are seeing as measured cathode current draw can be line voltage, 6V6 filament voltage, bias voltage input dropping resistor (usually a 100K 1 W prior to the diode), the bias shunt resistor (stock is usually 22K), plus the condition of the bias cap, the plate voltage applied to the 6V6s, etc...lot's of choices.


(This message was last edited by willie at 01:19 PM, Jun 26th, 2018)



Jun 26th, 2018 11:37 AM   Edit   Profile  

Thanks so much for the quick response. So - can I safely leave it "don't worry about it and play" zone? I was worried some other component had failed / drifted and was causing the change. Went visually through the whole circuit, as well as component tapping with a chopstick - and all seemed good.

thanks again


Too Near Atlanta GA

Amp Tech Emeritus
Jun 26th, 2018 01:04 PM   Edit   Profile  

Most likely it would not present a problem doing so. It is curious as to why your readings have changed and it is quite possible that a component has changed value...but as long as your tubes are operating within the parameters I mentioned (I like 18-20 mA) you should be fine. Keep an eye on things however and watch for any further changes.




Jun 27th, 2018 05:38 AM   Edit   Profile  

Just as a follow up its important to remind oneself when things don't make sense there's usually a reason. As I had mentioned I was using a Hoffman bias checker I had made. AS a precaution I had ordered a new one through Amazon. Using this yesterday showed my plate current in the 60 mA range with bias pot wide open. The problem seemed to be my previous bias checker. Fortunately this was identified prior to blowing the tubes - but at least it makes sense. I will adjust bias tonight to 24 mA.

thanks for all the advice - and it goes to show - if your gut tells you something doesn't make sense - its probably right.


Too Near Atlanta GA

Amp Tech Emeritus
Jun 27th, 2018 08:14 AM   Edit   Profile  

Good catch...that explains the variation in readings...play on. ;)


FDP Forum / Fender Amps: Vintage (before 1985) / SF Princeton Bias

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