How far can you go?

[Richard]

When modding an amp, how much can you change before it's no longer a Princeton (or 5E3 or whatever)?

[Enzo]

As soon as you change anything, it is no longer stock or vintage. That is for the real collectors. If you add a three wire cord, or upgrade the filter caps, it is still a Princeton.

ANy further, you now have a modified Princeton.

[rathmann]

I thought a modified Princeton was called a Boogie! ;-)

[Richard]

I thought I'd kick this out to get opinions. I wouldn't go crazy with a blackface but if you're changing stuff, why not make the best of it? (it worked for Paul Rivera, Randall Smith and others) On the other hand, there's something to be said for respecting the work of Leo Fender (or Jim Marshall). There's also mad props for the people who built something that will last 40 years. (Yeah your Apple computer and Playstation 3 will be around in 2047)

[Matt T.]

A Princeton is a good example. IMO, you can replace every e-lytic in the amp and even add screen grid resistors (so now you're adding something that was never there) and even do the power supply mods that everyone has been doing for...ever. I think that's still a Princeton. These mods don't change the basic tone of the amp...they just allow it to run a little safer, add a tiny bit of headroom (or take a tiny bit away). It's when you change the circuit such that it doesn't sound or 'feel' like a Princeton that I think it's no longer a Princeton.

[Richard]

To me, changing parts values isn't a big deal, but changing the circuitry can be. Working on my Princeton I have gained mad respect for a "student" type amp that's still holds it's own after 38 years. Fuck, I work in the building industry and most buildings I draw won't be around in 40 years. I can't help but tinker but not many things last that long, even non electronics.

[Zoe_N_Iain]

(Iain) I think the main exception to the 'keep it totally original' philosophy, at least for me, is adding a post PI master volume. It doesn't change the character of the amp at all with the MV wound up full, so I'd consider an amp with that done to be an 'improved classic' design, with an easily removeable modification should someone buying it not want the function.

If you swap the mains switch for a three position on/standby/off toggle, you can easily use the 'spare' hole for the MV, so you aren't even drilling any holes and the amp can be put back to stock. Well, as long as you don't lose the original switches in the meantime.

[Bruce / Mission Amps]

The absolute best sounding black face Princeton Reverb amp I ever heard was one I did for a recording studio where, as the request was, I change it to cathode bias and use their NOS tubes.
But, I also replaced the filter cap and cathode bypass caps, used a 5AR4 rectifier, installed a fixed bias phase inverter, replaced all the coupling caps with stock Mallory 150s, tweaked the tone stack, some coupling cap values and the vibrato circuit... still worked in cathode bias, just not as well.
Then the final kick inthe arse, I removed the old baffle board and used a 12" baffle board with a reissue Celestion G12 AlNiCo Blue.
This amp still is still regarded as the Princeton Reverb that "Out Princeton Reverbs "any other ones compared to it.
When driven hard, the Celestion gives it a little voxy AC15 tone, which is terrific sounding.
Sounds like a major project but I only had about two and half hours into it, (not including the baffle board) and it still sounded like a Princeton Reverb but with more of "everything" that makes one sound great, plus, the surprise "Voxylady" tone.

[Richard]

Bruce, what is the fixed bias phase inverter? I assume this is still the split load type. What did this do for the sound?

[Arthur B.]

Bruce, what is the fixed bias phase inverter? I assume this is still the split load type. What did this do for the sound?

I'm thinking the same thing. Where was the bias supply derived from?

[Bruce / Mission Amps]

The bias of the cathodyne driver's triode is derived from the 1000 to 1500 ohm resistor under the cathode.
A fixed bias PI, sometimes called the paul c mod, uses a voltage divider from the triode's plate B+ supply to form a positive DC voltage of about 1/3 the plate voltage and applies it to the grid of the triode while holding the plate and cathode from B+ and ground respectively with the same high value resistance.
In the Princeton, and many others, it would be the two 56K, plate and cathode resistors.
A good example of this is drawn up and by a fellow AMPAGER, Sean Kilback and shown on:
Mike's Tube Amp pages.

http://people.cornell.edu/pages/mt24...aul_c_mod.html

http://people.cornell.edu/pages/mt24...matic_comp.jpg

This mod does not work as well on cathode biased power tubes as it does work well on fixed bias power tubes, such as used by the Princeton.

[Richard]

Do those mods work on the ab984 circuit, the values seem pretty similar? Also, why doesn't the grid need to be biased with negative voltage?

[Arthur B.]

Also, why doesn't the grid need to be biased with negative voltage?

It is biased with a negative voltage with respect to the cathode. It seems it would be easier to directly couple to the previous stage, though. Going back to the original topic, I think it depends on the particular amp. If it's a valuable museum quality piece, I think historical accuracy will be the highest priority. If it's a junker, modding it might be doing the amp a service.