Princeton Reverb Re-work

[tubeswell]

By the way, whats your favorite amp?

For the last 18 months I've been hooked on the 5G9 with a 6G15 slapped in front of it, even it it is a handful to lug around. (But even without a reverb unit the 5G9 is pretty damn tremtastic - a kind of late tweed rawness but with a hypnotic trem that slams you into the middle of next week)

I built a 5F6A/JTM45 hybrid with 6G9B trem and and sort of BF verb earlier this year which is pretty damn seductive, and an excellent surfin' machine, but is a heavy mutha (2 x 12 combo). Having said that a 5F6A is pretty nice too. But so is a 5E3.

I am building a BFPR clone as we speak - might have it finished in a week or two.

I am also gathering stuff for a 6V6 tweed super (5E4A).

Then I was thinking 6G3, and probably a 5E9A, and a 5F4, then I'd like to have a go at the harmonic vibrato in a few of the browns. I was thinking also about the Supro Thunderbolt, Matchless Spitfire, TW Express, and a Route 666 with a variable triode pentode control. So I'll be here for a while yet.

[frankeg]

Tangentially, a 100-ohm pot with each extreme connected to a power tube cathode, and the wiper to ground, is a good way to adjust for unbalanced tubes. And it has a nicer failure mode (no current flows) than a balance adjust in the bias supply.

- Scott

Hi Scott, Welcome to my dilemma! Have you read my 1st post which includes my original question? Balance is a good thing! How does it affect the sound from one end of the pot to the other and can the user tweek to suit?

[frankeg]

For the last 18 months I've been hooked on the 5G9 with a 6G15 slapped in front of it, even it it is a handful to lug around. (But even without a reverb unit the 5G9 is pretty damn tremtastic - a kind of late tweed rawness but with a hypnotic trem that slams you into the middle of next week)

I built a 5F6A/JTM45 hybrid with 6G9B trem and and sort of BF verb earlier this year which is pretty damn seductive, and an excellent surfin' machine, but is a heavy mutha (2 x 12 combo). Having said that a 5F6A is pretty nice too. But so is a 5E3.

I am building a BFPR clone as we speak - might have it finished in a week or two.

I am also gathering stuff for a 6V6 tweed super (5E4A).

Then I was thinking 6G3, and probably a 5E9A, and a 5F4, then I'd like to have a go at the harmonic vibrato in a few of the browns. I was thinking also about the Supro Thunderbolt, Matchless Spitfire, TW Express, and a Route 666 with a variable triode pentode control. So I'll be here for a while yet.

WOW! You're much more addicted and richer than I! If only you lived next door....

[tubeswell]

Nah I'm not very wealthy. I scrounge parts one at a time and mainly have been doing scratch builds. I have to sell stuff to make room for more stuff (and to fund the next build, but its not a money-making venture - I'm lucky to break even on materials - so its a hobby), so in reality I only keep 2 or 3 amps at a time - damn shame. But I must admit being badly addicted - so its an OCD hobby (that interferes with my other band hobbies).

[frankeg]

I like scrounging parts myself but the competition is growing all the time. E.B. is becomming a waste of time. I can't believe how some folks bid things up so much.

[frankeg]

So, I have the amp back and I measured that pwr tube cathode pot and it is 5k. I also noticed that when varied it changes the plate voltage. The plate voltage is HIGH - from 460 to 500 vdc through the pot. Sound-wise, the amp seems to like the higher voltage about 480. Thats with a bias voltage of -34 vdc. I just borrowed a fluke dmm from work to check the current and I'll have that info in a day or so. Assuming the current is fine; it's been like this for 25 years, should I ignore the hi plate volts? I'm thinking the higher voltage comes from (1) today's higher ac and (2) changing to SS rectifier in 80's mod.

[tubeswell]

Hi Frank

If you bypass the 5k pot, what is the plate voltage like?

Also what is the change in cathode voltage like with the pot?

[JoeM]

So, I have the amp back and I measured that pwr tube cathode pot and it is 5k. I also noticed that when varied it changes the plate voltage. The plate voltage is HIGH - from 460 to 500 vdc through the pot...

Did the mods include a new PT? Even with a SS rectifier that seems high for Princeton.

[MWJB]

Also bear in mind that actual plate volts is the differnece between the plate voltage and cathode voltage, if you have resistance at the cathode, NOT plate volts to ground.

425-455 depending on plate current is typical for a non-modded Princeton, plus whatever voltage you have at the cathodes.

If the owner says that he likes the 5K pot then I suppose that you should keep it. Never seen anything quite like that before though.

[frankeg]

I'm feeling a little dumb... Never thought about the cathode voltage because it's supposed to be a "fixed biased" amp. I'm sure thats it! I'll check that plus the currents this week-end... Thanks a lot guys... Based upon the tranny # it is the original PT from 1967. I got all the parts in to restore the reverb and do the cap job. I'm also replacing the 220k grid resistors on the 6V6's and adding a pot with new cap in the bias supply (ala. Hoffman). Do you think I should change all the power supply resistors while I'm at it? They're original 1 watt CC's. I've got 3 watt metal oxides. I am going to play around with the reverb mixer using suggestions I got here... I let ya know... Thanks again

[frankeg]

Among the parts I bought, I have a multi-cap can from CE Manufacturing with the code: 0907-01...How do I read this? Thanks

[frankeg]

Do I need to hook up both sides of triode?

The vibrato isn't being used on the Princeton. Currently, that side of the 12ax7 has B+ on pin 1, the cathode circuit intact and a 100 ohm resistor on pin 2 going to ground. The B+ on pin 1 isn't connected to anything else. The heater is connected as well. Question; can I disconnect everything on the un-used side leaving just the heater connection? Will this affect the used side which is the phase inverter? I've always used both sides of the 12ax7...

[MWJB]

plate, grid & cathode of the unused triode can be left without any connections.

[frankeg]

plate, grid & cathode of the unused triode can be left without any connections.

Thank you MWJB. I have disconnected it...frank

[frankeg]

Work's done, owner's happy

Well, the owner got his Princeton back today. He is very satisfied. Thanks again for the help and advice. I checked that variable cathode set-up and the highest plate voltage was 438 with 18 volts across the cathode or 420 volts real plate. The current measured 30.9 ma... I left it in there as the owner wanted. I also gave him a master vol which he had before I restored his reverb. I didn't think it was going to work out but it did. I kept the reverb mixer circuit stock (3.3meg/10 pf) and added a 1meg audio pot in front of the mixer to gnd. Works well. I was really surprised how tone full this amp is. I definitely want to build one someday. Got to sell some stuff first though....