Adjusting bias on vintage Princeton amp

[repoman]

I have a '66 Princeton (Non reverb) that is showing 4.5-5.1 plate dissipation, 13.4 ma plate current, the amp sounds a bit low on volume and does not really break up at max volume.
Anyone where to begin in figuring out why these values are so low? I'm guessing that if the plate dissipation is more around 8-12 the amp will have more volume and break up?

There is no bias adjustment pot on this amp.
Would I need to change the 220k grid resistors by increasing them to bring up the plate current and dissipation?

[andy_tchp]

I have a '66 Princeton (Non reverb) that is showing 4.5-5.1 plate dissipation, 13.4 ma plate current, the amp sounds a bit low on volume and does not really break up at max volume.

I can't really assist with the technical aspects regarding what you've measured, but that's what non-reverb Princetons do. Feels like a mini Twin, but nicer. You should have quite a lot of volume on tap though, depending on the speaker that's in there.

One of the greatest sounding amps ever IME.

[JSett]

Andy is 100% correct. They are mini clean monsters. Best thing to do is goose the front end with a ton of boost and try and overdrive the preamp a little, it'll still be hard though.

There are ways to cook the valves more but it's more than a simple bias adjustment. But have you ever heard a Twin reverb up loud enough to overdrive? I've done it and it's not a pleasant sound...you'll just get a mini version of that if you manage it.

[repoman]

Ok

I did get some info from some guys on TGP and I wound up switching the bias circuit resistor from a 27k to a 22k and this brought the plate current up to about 8.8, there is a tiny bit more overdrive when dimed.

Strange amp...it doesn't really sound very loud compared to my PRRI but when I used a decibel meter they were pretty comparable.

[Embenny]

Strange amp...it doesn't really sound very loud compared to my PRRI but when I used a decibel meter they were pretty comparable.

Lots more goes into our perception of loudness than what you'd get from a single measurement. SPL meters have different ways of weighting different frequencies, with A-weighting (dBA) being one scheme that aims to imitate the human ears response curve.

But then there's the time dimension and average amplitude. Two amps could be producing similar peak amplitudes, but if one is compressing more, it'll have a higher average amplitude and sound much louder. My guess would be that your PRRI is overdriving more, which is providing the extra compression that boosts its perceived loudness.

[andy_tchp]

I'd be interested to know which speaker each amp was running and their respective efficiency (if known/published) too.

[repoman]

Stock speakers in my PRRI and this OG Princeton. I did try a new Jensen MOD speaker with it and there was quite a difference, the stock one was definitely much more mellow.