Other Amps

Princeton Reverb Build

1

Having built a few deluxe reverbs, a stereo 7189 based deluxe reverb and a couple of vibroluxes. I thought I might build a Princeton reverb for around the apartment and the quiet almost acoustic rehearsals the Colorifics seem to have these days.

I have all the bits:(though the reverb transformer is on the way)

-An old bogen VP20 chassis and transformers. Gutted though still some work to do on the chassis.

-Battered but cool light plywood cab that housed a less than nice regal amp.

  • Picked the the rest of the components today and had a lot left over from previous builds.

  • either the JBL D123 pictured or an old (and lighter) rola 12" I have. i also have some 10's I could try.

Including the reverb transformer I guess I have $125 into the amp already..whew!

I have a couple of questions for those in the amp know.

The Cathodyne PI, is it a big deal when mostly playing dead clean?? Would a long tailed pair achieve a nicer sound? I like clean and bright but with some bottom

And if so can I just use the extra triode after the reverb and the triode used of the cathodyne and make a LTP pair with them rather than adding another tube?

I'll be adding a mid pot to the EQ as that's what I'm used to.

2

PR’s are what they are. You either love them or loathe them. Ive built a bunch, with all the various mods to clean them up. The Stokes mod helps by feeding the PI more voltage. The VP20 runs EL84’s and probably isn’t making 300v on the B+, so you’re fighting that battle to start with.

For low volume playing around the apartment and acoustic jams, I would think a straightforward PR with the D123 would be fine. I’ve read that the Howard Dumble mods for the Tweed Deluxe help balance out the PI and clean up the PR some. If you change to a LTPPI and add a gain stage, you have a DR.

3

To change the PI to a long-tailed pair, use a 12DW7 as the reverb driver/recovery. Use Section 2, pins 1, 2, 3 to drive the tank, and section 1 pins 6,7,8 to recover that will give you the extra triode you need for the LTP

4

Yes I was going to recommend the Stokes mod as well. The higher voltage is going to give you more headroom.

5

To change the PI to a long-tailed pair, use a 12DW7 as the reverb driver/recovery. Use Section 2, pins 1, 2, 3 to drive the tank, and section 1 pins 6,7,8 to recover that will give you the extra triode you need for the LTP

– WinnieThomas

I'm curious (though I know of the 12DW7 and have had one but sadly not now)

You're suggesting a two triode (one tube) reverb circuit rather a 3 triode (12At7 and 1/2 12ax7) right?

Doesn't that leave me with two extra gain stages with a PR?

6

You'll need one of those two stages for the vibrato circuit, and yeas, drive the reverb with the 12AU7 triode and recover with the 12AX7 triode. you really don't need to double the triode as the driver.

7

Weeelll I mostly follow existing schematics and sometimes mix them up.

Are you suggesting do the exact same circuit but with just one triode or will this require redesign. I'm not an engineer nor do I have much electrical theory.

I build amps much like I bake bread. I follow a recipe and after I get comfortable with the recipe then sometimes add elements but nothing involving serious redesign. I like the idea though. Have you tried that in the existing fender reverb circuit?

How dies it work? How does the reverb sound?

8

Yes, use the existing schematic, but only one triode to drive the reverb. The 12AU7 triode has less gain than the 12AT7, but that will actually give you more variation on the reverb control. The reverb comes up a bit more slowly but there's always plenty of it.

9

Mr Thomas you might like this impetuous experiment. As the reverb driver tube socket relevant pins are just jumpered together I realized that if I could separate them I could put in a 12AU7 and test the idea of 12DW7 single tube fender reverb circuit sonically to see if I liked the results.

Looking at the reverb driver tube socket on my BF vibrolux build I decided that desoldering would be a PITA.

Then I remembered I have LOT'S of 12AU7. So I made a half one by clipping off the appropriate pins (plate grid cathode) and plugging it in carefully. Lo and behold, as you said, it seems to work just fine. A little less reverb but still lots.

Any possible long term ill effects?

11

No long term ill effects. I've built several with a 12DW7 and used the extra triode in an Elizabeth for the tremolo circuit.

12

Toxophilite, let me know if you need the tube. I have a stash of 7247 Mullards.

13

Thanks Powdog, PM sent

Chassis reconfigured with new faceplate and old tube socket holes covered. I got started on the tube socket holes and drilling out on the back edge. Also pictured is my backup PT

I plan to make basically a small deluxe with one triode (half tube) tremelo, one tube reverb and a long tailed phase inverter but with little transformers. Actually the OT is about right but the PT is definitely Princeton reverb sized and seems to produce 320-0-320 VAC. which as I will be using an SS rectifier might do the trick. (no voltage drop from tube rectifier) Small and light seems to be a consideration these days.

If I decide I want more voltage, and well, more everything, I have the backup PT from an old stereo hifi that produces 390-0-390 VAC and has two separate 6VAC heater taps, one for the power amp and one for the preamp, Fancy!

14

390 0 390 will likely produce over 450V on the filter caps. They make amazing explosions when they let go at that voltage

15

Well my variac isn't the most exact, It could be 375-0-375. I have had one explode in another build a while back. It was most exciting and smoke came out of all the chassis holes! Cool!

I usually have one rated at 500VDC in the first position after the rectifier just to forestall such excitement.

16

Some progress on the build. Sockets installed , face plate made and drilled. AC section installed( jack, switch , fuse, pilot light) though not yet wired up. Cone boards and standoffs installed Trying a new thing with the bit of cone board above the PT. For the rectifier circuit and either the first filter cap or the bias supply.

Lot's still to do 9turret strips , ground points (for power amp and preamp) etc etc.

i like to recycle and save some $$ if I can so using some old Hammond organ cone boards and turret strips. All actual components will be new, except for some older tubes.

17

OKay more done. Board mostly stuffed, heater done, terminal strips in for B+ and grounds, a few other odds and ends.

I have a question for the esteemed Powdog;

Can I use 20 gauge solid core wire for the B+ 'rail' It's insulation is rated to only 300V ?

I do have some automotive primary stranded 18 gauge wire that's rated at 600V but it's kind of clumsy to work with and takes up a lot of space on my terminals.

18

I use solid core hookup wire rated at 600v. 20 GA is fine for B+ since it is usually less than .2 amps.

19

Thanks! and oops i forgot the picture of my crazy build.

20

Well my reverb transformer hasn't arrived and because the rest of my amp is done (at least for testing) and as I'm mad and impetuous I wired in a little 3-5 watt OT that has the wrong input impedance (250 ohms on the driver side and 1.4 ohms on the tank side)

After the usual mishaps( parts backwards, electrocution, 1meg instead of 100K resistor, ooops no coax cable connecting the reverb mixer with the PI) I got the amp going and it sounds good. Whoopee!

However I guess as expected , even with an SS rectifier my voltages are lowish in the power amp, 350VDC biased to 25 on the 6V6s 170 on the preamps and 180 ish on the PI Had to do some messing around with the dropping resistors. probably achieving around 14-16 watts

it might be a thing if I want more om pah pah to put in my backup PT that produces 375-0-375 Right now I'm maybe getting around 16watts which isn't bad.

Humourously I have reverb with the wrong impedance transformer and the 12DW7. Fine sounding reverb, not quite enough to be Dick Dale but plenty for everyday use . Still will get the proper transformer.

By the way thanks a lot Powdog, Tubes arrived today! You are a amp-saver and a gentleman!

21

That is a lovely build! I recycled a few Hammond AO-35 chassis into 18 watt Dr. Z Stangray type clones, which are actually note-for-note AC30 clones with a weirdo tone control. But I digress. It's fun to recycle, even though that cone board is weird to work with.

Nice job!!

22

I totally agree, very fun to use existing things especially if you got them on the cheap. I got this mostly defunct Bogen VP20 mono turntable probably 9 years about for $20. Almost all my other amps are built on hammond organ amp platforms (AO-43s and AO-39s and an AO44)

Heart of this amp mostly finished, just have to track down a little buzz in the tremelo. Biased to 25ma it and 150 on the plates of the 6V6s gives me about 16 watts. Sounds good! Reverb works very well, i found this tiny hammond (organ) reverb driver lying around and it seems to do the trick in my one tube version of the fender reverb circuit. As usual my builds are unconventional and look it. Odd assortment of knobs etc.

it will be mounted in the funky Regal amp cabinet pictured above but with a new baffle cut for a 12" speaker but using thesame grill cloth and logo so it'll be a sneaky sleeper amp.

24

Here it is (mounted in the ratty little regal cabinet I had), with a friend. I have a JBL D123 in it which sounds great of course. originally I had a lighter 12" Rola alnico speaker from an organ and it sounded really nice except it didn't have much oomph and would crap out on the bottom very easily. The JBL is HEAVY though.


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