Fender Princeton ----> Twin

[Chuck H]

That "Output Power at class A" is a useful feature when analyzing a cathode biased amp for operating class.

Yes it is. I ran some calculations for a little amp I did a few years ago where I literally just used some "best guesses" for some things affecting operating conditions. I did do some research regarding typical design parameters but I was working rote and had never built a single ended amp before. And (embarrassed) tube charts still confuse me for the most part. I just seem to have a hard time wrapping my head around them. Anyway... Using the calculator you linked it turns out I was spot on. Which was good to see since that amp is in the hands of a local guy

[Chuck H]

Thanks Pete and Helmholtz for the clarification on the g1 figure. I had incorrectly assumed the calculator was indicative of AB1.

[J M Fahey]

Again, not sure what are discussing here or what that customer wants.
A Princeton is a *twelve* watt amp.

With best efforts, biasing it colder, replacing OT, what can we expect? 15W?
Still way too far from a Twin or even a Super Reverb.

That Customer is not realistic.

Besides, nobody buys Princeton for it's cleans but because it can be turned high and crunch/compress at lower levels.
Which it is doing very well.

[Chuck H]

Again, not sure what are discussing here or what that customer wants.
A Princeton is a *twelve* watt amp.

With best efforts, biasing it colder, replacing OT, what can we expect? 15W?
Still way too far from a Twin or even a Super Reverb.

That Customer is not realistic.

Besides, nobody buys Princeton for it's cleans but because it can be turned high and crunch/compress at lower levels.
Which it is doing very well.

Well sometimes one might find themselves owning an amp for one reason or another only to find LATER that they want or need more power. At which point they probably want to preserve the tone they're familiar with but still want more of it. It seems obvious enough to most of us here that if you turn the amp up and it's not loud enough then you need a bigger amp. (<period) But ideas about tube amps being "tweakable" abound on the web. Like tales of how Randall Smith modified a Fender Princeton for the same power as a Bassman to create the first Boogie (paraphrasing). Players reading this stuff don't realize this isn't something you can do with a few tweaks and these unrealistic desires come up sometimes. You don't get anything if you don't ask. You can try to educate your customer but because of web lore and rumors you stand the real possibility of putting them off to seek what they want elsewhere. And I know your position about that would amount to "Well good riddance then." but most service folk, if they're being honest, would rather not dissapoint a customer I think.

JM2C

[Helmholtz]

.... and had never built a single ended amp before. )

Here's what I meant:
Every so often the question arises if some cathode biased push-pull amp runs in class A or AB (e.g. AC30, tweed Deluxe etc.).
When it's class AB, the calculator will give a loadline having 2 sections with different slopes.
The flatter part corresponds to class A operation and the steeper part to class B operation.
"Output Power at class A" is the power produced within the flatter (class A) part of the loadline.

[pdf64]

That "Output Power at max g1" makes no sense with guitar amps, where no significant positive grid drive is possible.

Even more confusing is that the uppermost plate curve often belongs to a positive grid bias and one needs to identify the relevant Vgk = 0 curve by using the pointer.
I coudn't figure out what makes the calculator choose the max. positive grid bias.

For "Load" one needs to enter the Raa with PP amps. The slope of the loadline then correctly corresponds to Raa/4 in class B and Raa/2 in class A.

Unfortunately all former settings are lost when changing the tube type.

That "Output Power at class A" is a useful feature when analyzing a cathode biased amp for operating class.

I found that hovering and clipping could be used to show the grid voltage of the various plots, eg here's a nice example showing how 4k is a good, 'through the knee' load for KT66

[Helmholtz]

Yes, placing the mouse pointer on the plate curve will display the grid bias.

As with all power stage calculators it's essential to use full clean output B+ and screen voltages.
Otherwise power results can be way too high.

[pdf64]

nickb 's loadline calculator is really good, and very usefully provides additional data regarding valve dissipation, which is handy http://bmamps.com/ivds.html

[Helmholtz]

nickb 's loadline calculator is really good, and very usefully provides additional data regarding valve dissipation, which is very useful http://bmamps.com/ivds.html

Yes, it is (if you don't forget to set grid drive to twice the bias voltage).
(I think the valve dissipation number is misleading as it seems to correspond to full power, which isn't the actual maximum.)
But I like the vtadiy because it takes into account the class A part of the loadline.
Also helps to determine the cathode bias resistor.
Sometimes I use both.

[g1]

FWIW I spoke to a tech at Hammond when I was reproducing my proto amp for Dean Markley. He said (paraphrasing) that you could use 16xx series OT's at half their rating for any guitar amp and they would be bullet proof. And this has indeed been my own experience.

Did you mean double their rating?

[Chuck H]

Did you mean double their rating?

Well, yes. I think I meant to type "half the rating" but finger memory completed the wrong word.?.

[Axtman]

Great information here! Thanks everyone for jumping on this thread.

I have been very busy this week and have not had time to do anything to the amp. The owner told me that he is buying a Celestion G10M Greenback and wants me to install it. I hope that he likes it.

[Chuck H]

Great information here! Thanks everyone for jumping on this thread.

I have been very busy this week and have not had time to do anything to the amp. The owner told me that he is buying a Celestion G10M Greenback and wants me to install it. I hope that he likes it.

Seems like an arbitrary move sideways. That speaker has the same sensitivity as a stock Jensen for that amp. I couldn't find specs for the stock Oxford used in those amps and I can't know what is actually in his amp now. But at 95dB the G10M is only on par with other ten inch speakers. Maybe he'll like the tone better than the stock speaker though I wouldn't really think so. I wouldn't expect the amp to seem any louder or cleaner for it.

[Axtman]

Chuck,
I agree. I was looking for a more sensitive speaker for him. My guess is the amplifier is working so well (patting my back) that it is overpowering the existing puny little Oxford speaker. If he still does not like it with the new speaker I will explain to him that he needs to sell his Princeton Reverb and buy a Deluxe Reverb or Jazz Chorus or something else. I am not going to butcher up the Princeton!

[Helmholtz]

Chuck,
My guess is the amplifier is working so well (patting my back) that it is overpowering the existing puny little Oxford speaker.

What output power does the amp measure before clipping?
Does the amp use a 5U4 or a GZ34 rectifier?