Another Princeton Reverb Build

theprofessor

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I've built another Princeton Reverb -- this time for my college roommate. I built him a thinline two-P-90 T-style last year, and so this year, it's an amp. I've built several of these Princeton Reverb clones using the Doug Hoffman layout, and I decided to do that again (with a few modifications). Some new things for me on this build: I used ZERO cloth pushback wire, which I love. I also used a lot of stranded-core wire, versus solid core. Finally, I used all metal film resistors, with the exception of the 470k carbon comp I added before the grid of the phase inverter. We'll see how it sounds.

I also decided that I wanted to experiment with a slightly different aesthetic than the common blackface PR type. I got some Dumble/Vox style grill cloth and used that. Hoping it all comes together well. I think it will!

AS previously, I used a Hammond 290AX PT. I found a used Classictone 18002 Deluxe Reverb type OT, and I used that (I have that in my PR clone as well). A Mojotone reverb transformer. Cabinet is from Guitar Cabinets Direct, though I modified it slightly. And a Celestion G10 Vintage speaker, which I think suits this circuit very well.

Now I just have to check everything against the layouts and schematics, and then I plan to go through the startup procedure tomorrow. I'd appreciate it if you see any mistakes!

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King Fan

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Love me a beautifully built Hoffman layout. That’s impressive.

To go from the sublime to the ridiculous, you mention using stranded wire. The tinned-twist 'topcoat' kind, or the copper Medusa untinned kind? :)
 

theprofessor

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Love me a beautifully built Hoffman layout. That’s impressive.

To go from the sublime to the ridiculous, you mention using stranded wire. The tinned-twist 'topcoat' kind, or the copper Medusa untinned kind? :)
Medusa. That is one of the downsides and also one of the upsides of this sort of stranded wire: If there is a break, there are lots of other threads to make the connection. If there is a break, little bits of stranded wire can go all over the place, potentially wreaking havoc on your build! I've had to go through this thing with a flashlight and tweezers and remove lots of little single strands of wire -- or, snakes.
 

King Fan

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Thanks. You’re a brave man. To go from the ridiculous to the mythic, I’m a non-violent Perseus when it comes to Medusa... he cut off her head; I’ve banished her from my kingdoms.

In any case, what a great build. Time for a Twin… or a Trainwreck?
 

theprofessor

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Well it appears this Classictone OT is no good. I'd say that's the most reasonable guess. All the voltages were on track until I installed the power tubes and took measurements. Then the plates of those tubes were in the high 200's. I've got a similar Hammond that I'll put it now...
 
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theprofessor

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OK, everyone. I feel like a dunce. The problem is not with the Classictone OT. I also replaced the power tubes I was using with a different set. No change.

Here's what's happening: I'm going through the startup procedure, and everything looks pretty much on track until I put in the 6V6's and take measurements at the plates. Then I'm ending up at around 280 DCV with a 5U4GB rectifier. What should I check?

I'm using a Hammond 290AX PT and now a Hammond 1750H OT. Here's my voltage chart up 'til I install the 6V6's, and then the voltages plummet.

Princeton Reverb startup voltage chart

Before starting, do a continuity check from the sleeve of an instrument cable that is plugged into each input to the ground of the power cord. In addition, do a continuity check between the sleeve of an instrument cable that is plugged in to the speaker output jack and the ground of the power cord.

1. Plug in the amp

2.-3. Turn the amp on

Measured in AC volts


AC wall voltage: 122 ACV

Fuse 122.1 ACV

Power switch 122.1 ACV

6.3V pilot light works

Power transformer output (AC)

Heater voltage (AC) [goal: ca. 5.7 – 6.9 ACV] 6.92 ACV

Rectifier heater voltage (AC) [goal: ca. 4.5 – 5.5 ACV]

V7: 5AR4

Pin 2 (heater) to Pin 8 (heater) = 5.49 ACV

Pin to ground measured at pins 3 and 4, because I have SS diodes between 3->4 and 5->6

3->diode->4: 355 ACV

4->diode->5: 356 ACV

Heater pin voltages (AC) on power and preamp tubes (goal: ca. 6.3V):

6V6GTs


Pin 2 (heater filament) to Pin 7 (heater filament) = 6.9 ACV

12AX7s

Pin 4 heater to Pin 9 heater center tap = 6.91 ACV

Pin 5 heater to Pin 9 heater center tap = 6.91 ACV



4.-5. Install Rectifier tube and power on, using lightbulb limiter

Measured in DC volts


Rectifier output (pin 8): 486 DCV

B+1 measured at first filter node: 486 DCV

B+2 measured at second filter node: 486 DCV

B+3 measured at third filter node: 481 DCV

B+4 measured at fourth filter node: 478 DCV

Preamp tube plates (pins 1&6): V1, pin 1: 471 DCV

V1, pin 6: 471 DCV

V2, pin 1: 487 DCV

V2, pin 6: 488 DCV

V3, pin 1: 474 DCV

V3, pin 6: 475 DCV

V4, pin 1: 472 DCV

V4, pin 6: 478 DCV

OT connection to power tube (6V6GT, pin 3): V5, pin 3: 490 DCV

V6, pin 3: 490 DCV

All tube pin voltages (heaters are AC): heaters: 6.89 ACV


6. Install all the Preamp tubes and power on, using light bulb limiter

Measured in DC volts


12AX7 cathodes (goal: 1-2 DCV):

Pin 3: V1: 1.29 DCV;

V2 (12AU7): 18.6 DCV

V3: 1.37 DCV

V4: ----?

Pin 8: V1: 1.29 DCV;

V2 (12AU7): 21.4 DCV

V3: 1.44 DCV

V4: 63 DCV

12AX7 grids (goal: 0 DCV):

Pin 2: V1: 0 DCV;

V3: 0 DCV

V4: - voltage

Pin 7: V1: 0 DCV;

V3: 0 DCV

V4: - voltage

6V6 V5, pin 5 (grid)

_____ , then adjusted the pot all the way to the left (from the rear of the chassis) to get ____ DCV

6V6 V6, pin 5 (grid)

_______, then adjusted pot all the way to the left (from the rear of the chassis) to get _____ DCV

Fixed adjustable bias amp: Adjust bias pot for largest negative DC voltage possible. The pot moves between about ______ to ____ DCV with the 10kl pot and 22k resistor.



7. Install 6V6s, hook up to speaker, and take measurements

Measured in DC volts


The power tubes did not redplate

V5: 6V6GT
 
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theprofessor

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Hey everybody - Turns out that amps these days require high voltage to the bias supply for the power tubes to draw properly. Weird! So I had to add the red-and-yellow wire, since the amp was being so fussy... :)

IMG_4154.jpg
 

chas.wahl

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OK, I hope you're getting somewhere now.
Why all the heatshrink socks on the ends of many wires? Problems with PVC melt? Being able to identify both ends of a given wire?
 

theprofessor

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Hey @King Fan - Regarding my non-tinned-stranded-wire bravery: I'm not likely to do that again. And with respect to the Hoffman layout: I used it because I have had such good sonic results with it in several builds in the past. Very low noise, etc. I must say that I have come to dislike building it, though. The Fender shapes and orientations just make sense to me, and Hoffman's layout, while optimized for ground-points and several other things, feels like thinking backwards and inside-out. To further exacerbate the issue, his layout is incomplete and often looks nearly nothing like what an actual amp chassis built on his layout looks. I will not build a Hoffman layout again without making my own version of it in DIYLC (which I do not yet know how to use).
 

King Fan

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Yeah, I like the Fender layout a lot. I really appreciate your thoughts on Doug's. But I admire your build here. Elegant, modern beauty for sure. And I await your next PR: “Return to Fullerton.”

And yes, I find the right wire key for fun, easy construction.
 

BenTobith

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I've built another Princeton Reverb -- this time for my college roommate. I built him a thinline two-P-90 T-style last year, and so this year, it's an amp. I've built several of these Princeton Reverb clones using the Doug Hoffman layout, and I decided to do that again (with a few modifications). Some new things for me on this build: I used ZERO cloth pushback wire, which I love. I also used a lot of stranded-core wire, versus solid core. Finally, I used all metal film resistors, with the exception of the 470k carbon comp I added before the grid of the phase inverter. We'll see how it sounds.

I also decided that I wanted to experiment with a slightly different aesthetic than the common blackface PR type. I got some Dumble/Vox style grill cloth and used that. Hoping it all comes together well. I think it will!

AS previously, I used a Hammond 290AX PT. I found a used Classictone 18002 Deluxe Reverb type OT, and I used that (I have that in my PR clone as well). A Mojotone reverb transformer. Cabinet is from Guitar Cabinets Direct, though I modified it slightly. And a Celestion G10 Vintage speaker, which I think suits this circuit very well.

Now I just have to check everything against the layouts and schematics, and then I plan to go through the startup procedure tomorrow. I'd appreciate it if you see any mistakes!

View attachment 987696
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View attachment 987699
Who made your face and backplates?
 

theprofessor

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Who made your face and backplates?
The backplate is Mojotone. The faceplate is from Sandy Henry at Precision Design. She does great work. I learned about her from Jim Nickelson of Lil Dawg. She makes faceplates for his amps.

 

theprofessor

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Looks as if the Hoffman layout restricts cap choice? Not much space between the turrets.
You're right about there being a premium on space for capacitors. It does press one toward certain capacitors, but I've seen people get creative and shoehorn Jupiter caps on that layout.
 




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