Princeton build with mod

[pontiacpete]

I built this princeton some time ago and was looking to modify it to get more power and to also deal with the reverb. But first my question is about the output power.
So the amp blew the power transformer, which I believe was caused by a bad rectifier tube since that went too. Whatever the reason was for the breakdown I thought this would be a good opportunity to try a deluxe PT and OT to see if I can get 20 watts RMS out of this. Did all the work only to find that it only produces 16 watts.
Does anyone know why this amp wouldn’t produce 20 or so watts? Is it the princeton circuit that I’m not understanding?
Thanks
pete

[loudthud]

I assume you mean Princeton Reverb. What schematic did you use ? What rectifier tube are you using ? Did you add a choke?

[Chuck H]

Does anyone know why this amp wouldn’t produce 20 or so watts?

I'm going to guess it's because the Deluxe is actually a 16 watt amp.?.

You didn't say which "Princeton" the original amp is based on or which "Deluxe" the new iron you purchased is for. In some cases changing Princeton iron to Deluxe iron will actually reduce output.

[pontiacpete]

I used the schematic for a blackface Princeton reverb. Ah but I didn’t use a choke! Would you suggest putting it in place of the 1K resistor on the first node?

[Helmholtz]

BTW, the difference between 20W and 16W is less than 1dB.

[loudthud]

20 Watts is somewhat optimistic and probably not obtainable with anything except NOS USA tubes which can stand the abuse. Even with good tubes, screen Voltage might be dropping too low under full power conditions in the Princeton thus limiting power. Everything in the power supply is critical to getting that 20 Watts. Rectifier tube and filter caps especially.

I noticed that the Deluxe Reverb uses 470 Ohm 1 Watt screen resistors. That may be what keeps the tubes from burning up.

[pontiacpete]

I just clipped in a choke in place of the 1k resistor and what a difference. I’m measuring 14.25vac on the output.

[g1]

I’m measuring 14.25vac on the output.

That must be with some clipping?

[Chuck H]

I just clipped in a choke in place of the 1k resistor and what a difference. I’m measuring 14.25vac on the output.

The Priceton Reverb doesn't use a choke and it has no screen resistors. So you're probably pushing the screens too hard with high voltage and fixed bias.

[Tom Phillips]

I just clipped in a choke in place of the 1k resistor and what a difference. I’m measuring 14.25vac on the output.

Are you monitoring the waveform with a scope? Note that Fender specifies the output power at something like 5% distortion. There are lots of variable that contribute to most Deluxe Reverb amps only able to put out around 16 to 18 watts clean.

[pontiacpete]

Thanks for all the suggestions. This is what I’ve done so far. Installed the choke, added 470R screen resistors and 1.5K grid resistors. Added a bias pot and lowered the bias. The previous measurements were taken when the tubes were running really hot at about 40mA. Now with the tube biased at about 20mA there is 12.7vac on the output which I calculate to be about 20 watts. It clips at 9 or 10vac.

[Helmholtz]

It clips at 9 or 10vac.

As amp output is always specified before (or at the onset of) clipping, this means your amp delivers around 12W into 8R.

[Chuck H]

It clips at 9 or 10vac.

Is that being read with a RMS meter or on your scope? I assume you're seeing the onset of clipping on a scope and not just just listening for it? If that 10V at the onset of clipping is being read on the scope screen, that is not RMS. And your power would actually be a paltry 7 watts converted to RMS.

My own amp with a pair of el84's has 365Vp, cathode biased and only makes about 13W RMS using a Hammond 270ex (144mA) and a 1608 (8k). I too was disappointed that I wasn't getting the standard "17 watts" but the amp operates perfectly and I have to believe that many wattage claims from the past were exaggerated.

[Helmholtz]

BTW, acc. to Dave Funk's book the blackface Princeton Rev. was a 12W amp and IIRC that's exactly the number I once measured.

[Tom Phillips]

A recent 1965 Princeton that I worked on measured final power output of 13W clean and 19.6W full hard clipped.
(Waveform observed on a scope and voltage measured with a true RMS reading meter.)
The amp had a new matched set of Ruby power tubes and the line voltage was set to 120V.