Princeton Reverb 68 Custom hand-wired to blackface WITH AUDIO

Keefy

My medium-term plan is to gut the innards and install a blackface AA1164 circuit on turret board, with a different speaker and some useful mods (switchable NFB, bias pot). Before I start down that road, I want to enjoy the amp for its own sake for a bit, get to know it, and work out what I do and don't like. (NAD thread is here.)

First job is to see what valves it's loaded with:

V1 12AX7 pre-amp - Groove Tubes ECC83S - original fitment
V2 12AT7 reverb send - Mullard CV4024 - nice!
V3 12AX7 reverb recovery/mix - JJ ECC83S - I like JJs, might try a 12AU7 in here to make the reverb more controllable, but then I do like a bit of 'cavernous' too
V4 12AX7 vibrato/PI - unidentified 6N4 [12AX7 equivalent], also marked CV400_, can anyone identify it from the pic below?
V5 and V6 6V6 - Groove Tubes GT-6V6-S - original fitment

On my Orange valve tester, 3 of the 4 preamp tubes test 'bad', but only because the test algorithm rejects any double triodes where the test scores are more than 5 points apart on Orange's arbitrary scale of valve worthiness! The power tubes score exactly the same, indicating that they are well matched, so they can stay.

I was also intending to fit a 12in speaker of some sort (cue much poring over forum discussions), but having looked inside the cab I am uncertain whether this can be done without some woodwork. I have been advised to stay away from woodwork, especially if I want to keep the same number of fingers and thumbs as I have now... There is a satisfying amount of bottom-end on top with the stock OEM Celestion, so I might look to fit a different 10in instead (cue further poring over forum discussions).

Mystery 6N4:

fandango

Keefy said:

I was also intending to fit a 12in speaker of some sort (cue much poring over forum discussions), but having looked inside the cab I am uncertain whether this can be done without some woodwork. I have been advised to stay away from woodwork, especially if I want to keep the same number of fingers and thumbs as I have now... There is a satisfying amount of bottom-end on top with the stock OEM Celestion, so I might look to fit a different 10in instead (cue further poring over forum discussions).

@Keefy ;

Fitting a 12 " speaker is easily done.

A new baffle easily fitted. To do that you will need to remove approx 2" of batten along the lower edge from the combo cabinet to allow for the larger speaker. Again very easily done. And a very small area of matt black. and you're good to go.

Fitted a Neo Creamback and very very pleased.

The baffle made by one of tFB's very own - Richard at Alder & Ash Pedalboards - @richalderandash ;

Keefy

fandango said:

Keefy said:

I was also intending to fit a 12in speaker of some sort (cue much poring over forum discussions), but having looked inside the cab I am uncertain whether this can be done without some woodwork. I have been advised to stay away from woodwork, especially if I want to keep the same number of fingers and thumbs as I have now... There is a satisfying amount of bottom-end on top with the stock OEM Celestion, so I might look to fit a different 10in instead (cue further poring over forum discussions).

@Keefy ;

Fitting a 12 " speaker is easily done.

A new baffle easily fitted. To do that you will need to remove approx 2" of batten along the lower edge from the combo cabinet to allow for the larger speaker. Again very easily done. And a very small area of matt black. and you're good to go.

Fitted a Neo Creamback and very very pleased.

The baffle made by one of tFB's very own - Richard at Alder & Ash Pedalboards - @richalderandash ;

Cheers @fandango, I was actually going to use the same baffle, as I have read (on the Internet, therefore it must be true) that it makes little difference to the sound from the new 12in speaker. As you say, some batten would need to be removed. I haven't looked closely yet, I've just been enjoying plugging in various guitars to see how they sound; in a word, glorious!

fftc

I might just be missing something here but if you are planning to go that far would you not be better starting afresh with a custom build?
Or is it your dislike of woodwork that stops you doing that?

Zoolooter

Alessandro SC64 10" or 12" was made for a Princeton. They have the right balance of everything. Plenty of info on the forums.

https://www.lean-business.co.uk/eshop/eminence-george-alessandro-10sc64-10-guitar-speaker-20w-8-ohm-p-2951.html?gclid=EAIaIQobChMIybOG1qLF3QIVQ-h3Ch0powieEAQYASABEgKqZvD_BwE

Keefy

fftc said:

I might just be missing something here but if you are planning to go that far would you not be better starting afresh with a custom build?
Or is it your dislike of woodwork that stops you doing that?

I'm happy to do electronics work and can make a good job of it. I don't have the space, the tools, or the skills to be building wooden cabinets and fitting tolex and grille cloth, or making up face-plates. A PR kit from Modulus would cost me more. (Not knocking Modulus btw, I built one of their excellent 5E3 Deluxe kits into an existing cab.)

The finish and appearance of the PRRI as it stands are excellent, and there's no point in me trying to improve on that. But for not too much outlay, I can take a good amp and make it something a bit special, enjoying the process as I go, plus I have the option to make circuit changes if I wish - something that is rather easier on turret board than on a PCB.

I might go to a 12in speaker, I might not. The world is my bivalve mollusc!

fandango

Keefy said:

fandango said:

Keefy said:

I was also intending to fit a 12in speaker of some sort (cue much poring over forum discussions), but having looked inside the cab I am uncertain whether this can be done without some woodwork. I have been advised to stay away from woodwork, especially if I want to keep the same number of fingers and thumbs as I have now... There is a satisfying amount of bottom-end on top with the stock OEM Celestion, so I might look to fit a different 10in instead (cue further poring over forum discussions).

@Keefy ;;

Fitting a 12 " speaker is easily done.

A new baffle easily fitted. To do that you will need to remove approx 2" of batten along the lower edge from the combo cabinet to allow for the larger speaker. Again very easily done. And a very small area of matt black. and you're good to go.

Fitted a Neo Creamback and very very pleased.

The baffle made by one of tFB's very own - Richard at Alder & Ash Pedalboards - @richalderandash ;;

Cheers @fandango, I was actually going to use the same baffle, as I have read (on the Internet, therefore it must be true) that it makes little difference to the sound from the new 12in speaker. As you say, some batten would need to be removed. I haven't looked closely yet, I've just been enjoying plugging in various guitars to see how they sound; in a word, glorious!

And for grill cloth, you can get this from a couple of places:

1. https://www.tubeampdoctor.com/en/shop_Coverings_Grill_Cloth/
2. https://www.tube-town.net/ttstore/Tolex-Grillcloth/Grillcloth:::111_120.html

I decided not to bother with piping, so mine looks like this:

Keefy

fandango said:

Keefy said:

fandango said:

Keefy said:

I was also intending to fit a 12in speaker of some sort (cue much poring over forum discussions), but having looked inside the cab I am uncertain whether this can be done without some woodwork. I have been advised to stay away from woodwork, especially if I want to keep the same number of fingers and thumbs as I have now... There is a satisfying amount of bottom-end on top with the stock OEM Celestion, so I might look to fit a different 10in instead (cue further poring over forum discussions).

@Keefy ;;

Fitting a 12 " speaker is easily done.

A new baffle easily fitted. To do that you will need to remove approx 2" of batten along the lower edge from the combo cabinet to allow for the larger speaker. Again very easily done. And a very small area of matt black. and you're good to go.

Fitted a Neo Creamback and very very pleased.

The baffle made by one of tFB's very own - Richard at Alder & Ash Pedalboards - @richalderandash ;;

Cheers @fandango, I was actually going to use the same baffle, as I have read (on the Internet, therefore it must be true) that it makes little difference to the sound from the new 12in speaker. As you say, some batten would need to be removed. I haven't looked closely yet, I've just been enjoying plugging in various guitars to see how they sound; in a word, glorious!

And for grill cloth, you can get this from a couple of places:

1. https://www.tubeampdoctor.com/en/shop_Coverings_Grill_Cloth/
2. https://www.tube-town.net/ttstore/Tolex-Grillcloth/Grillcloth:::111_120.html

I decided not to bother with piping, so mine looks like this:

Nice work! But for me the fun is in the electronics. Fender Mexico have done a fine job with the cab and the chassis, so they are staying as they are

Keefy

Zoolooter said:

Alessandro SC64 10" or 12" was made for a Princeton. They have the right balance of everything. Plenty of info on the forums.

https://www.lean-business.co.uk/eshop/eminence-george-alessandro-10sc64-10-guitar-speaker-20w-8-ohm-p-2951.html?gclid=EAIaIQobChMIybOG1qLF3QIVQ-h3Ch0powieEAQYASABEgKqZvD_BwE

Thanks @Zoolooter I will check that out.

Keefy

A correction - V2 (12AT7) is the reverb driver that people recommend swapping to a 12AU7 for better control. I tried the swap, but because the reverb return is a bit hummy, it just meant I got more hum from having to turn the control up higher! Original valve replaced, but it turns out my CV4024 may not be a Mullard, even though some people on eBay are claiming they are! 

I put a 1976 Sylvania 12AX7 in V1 - I like that, I think it has a bit more detail to it than the JJ.

A Facebook post in a valve amp builders' group tells me that V4 is a generic Chinese 12AX7 so I've put another Sylvania in there.

Any ideas to reduce the hum from the reverb?

Keefy

Sorted - it was the reverb tank, a Ruby Tubes 2-spring long tank. I had an old Accutronics 3-spring long tank put by so I swapped that in and the reverb now runs much quieter.

I've also restored the original pre-amp tube line-up while I work out which of the replacement tubes is a bit microphonic!

EDIT - the 3-spring didn't sound as surfy, so I dug out an Accutronics 2-spring - that's better!

Keefy

Well you can all stop posting in this thread because I like this amp so much, I've decided not to mod it. I might try a different speaker at some point if I can find a bargain, but no rush there. Nothing to see here...

Keefy

Zombie thread resurrection!

During the last year I have replaced the stock speaker with an Eminence Legend, restored the NFB to 60s spec, and reduced the excessive bass. The original Reverb tank has gone back in as I needed the other one for another amp (or something). I discovered there is already a bias pot fitted, which I found was set a bit cold so I tweaked that up closer to spec. All the B+ voltages are on the high side, despite the fact that I did the mains voltage correction.

I am now looking again at doing a full conversion to blackface specs, but incorporating the Stokes mod, on eyelet board (rather than turrets - I’ve gone off turrets!). Rather than install a 4-way ‘cap can’ I have bought four separate F&T 20uF 500V caps. I will use an extra-long board so that I can accommodate these. Bias components will go on a separate board as per the original design.

I can always sell the PCBs once removed as they are all in good working order.

Just waiting for Modulus to raise the shop shutters and then I’ll get an order in.

Keefy

Parts ordered form Modulus.

I pulled the chassis to take some measurements to transfer to a full-size layout diagram. Then I spent quite a while staring at it and thinking, 'Do I really want to do this?' I remembered about the high B+ voltages and decided that yes, I do. It will be much easier to swap around voltage-dropping resistors on an eyelet board.

The PCB is fixed with 10 bolts that pass through stand-offs and into threaded holes in the chassis. My eyelet board will be a touch narrower so I plan to use some of the fixing points on the pot side and install new ones on the valve side. That will give a bit more space between the board and the valve sockets than there is at present. Btw if you have one of these amps and need to pull the PCB with the pots on, be aware that it won't come out without first removing the main PCB (yes, 10 bolts to undo). Guess how I know that...

The new board will be about the same length as the existing PCB and will accommodate all four filter caps. I am undecided as to where to put the bias board. There may be room for the components on the and of the new main board. Otherwise I'm thinking I'll mount it on the end panel next to the PT - I'll see how that looks on the layout diagram

The cardboard guard around the speaker and footswitch sockets was very annoying so I've removed it.

Keefy

The eyelet board arrived today, along with most of the parts that I don't already have. I need to make more progress on my layout drawing!

Keefy

Layout drawing mainly done, except for:

• Mains connections, heater filaments - would just clutter up the page;
• Bias - need to finalise where the bias board is going, look at some alternatives to how the pot is wired (to prevent loss of negative bias if the pot loses contact); and
• Stokes Mod - I'll add the changes when I'm all done.

Has anyone here done the Stokes Mod, and was it worth it?

I'll probably pull the PCBs tomorrow and get them up on eBay to take advantage of a sell-for-£1-max deal.

Never go anywhere without a map!

Keefy

I have stripped the PCBs out of the chassis and put them on eBay in the hope of defraying some of the (admittedly modest) cost of this project. The empty chassis looks rather sad...

munckee

I’m not sure modding is the correct term for what you are doing here!

assume we can have a video when done?

Keefy

munckee said:

I’m not sure modding is the correct term for what you are doing here!

assume we can have a video when done?

Extreme modding maybe? Yeah it’s pretty much a complete rebuild.

I hadn’t thought of doing video but I might manage some audio. For the moment here’s what it sounded like before I gutted it. The reverb is from one of my DIY spring units, not the amp itself.

https://m.soundcloud.com/guitargraeme/hello-guitar-solo

Keefy

More stuff removed. I was going to keep the filament wires but I prefer to twist mine tighter, and besides they get in the way when wiring up the sockets so I prefer to do them last.

I cleaned up all the socket tags and snipped off all the push-on connectors. The mains safety earth push-on has been replaced by a sturdy solder joint.

Keefy

The holes for the pots needed widening to accommodate the new CTS pots. I was going to do this by attacking the steel chassis with a step drill and carefully hand-reaming the face plate but there was one thing stopping me removing it - the pilot light. I assume they use some special low-profile spanner to fit the backing nut to this, because no tool that I possess could get a purchase on it. I used the step drill on both parts together, then swung the face plate up to go round the holes with a de-burring tool.

I connected the 5V filament supply and wired up the input sockets. The latter took two goes because I had reckoned without the angled front panel, which left the grid stoppers shorting to ground on the bottom of the chassis. I first wired everything up with the sockets mounted on the outside of the chassis, then moved the assembly inside - that corner is a very tight space!

I checked my layout diagram and made a few changes and corrections. I cut the phenolic board down to 35cm, covered it in masking tape, and marked all the holes for eyelets, mounting screws, and through-holes, as well as the position for the HT fuse. It looks like it will be a good fit.