Princeton Reverb 68 Custom hand-wired to blackface WITH AUDIO

Keefy · January 2021

Chassis marked for drilling for the eyelet board mounts. One of the positions marked turned out to be right under the OT - nope!

Eyelet setting in progress. Stand-offs are temporarily in place to stop me putting eyelets in the holes.

Keefy · January 2021

Eyelets are all set and chassis mounting holes drilled, time to test-fit the board in the chassis. So far, so good.

You can see 220pF ceramic on the reverb send and return sockets. I presume these are intended to suppress RF noise or something. I'll probably leave them for now.

zimmi_b · January 2021

Hi Keefy,
Really looking forward to seeing how this goes. Are you keeping an eye on costs involved in this? I'm building a 59 Tweed (from Stewmac) just now and might try this next.....

I guess you've seen the Rhett Shull video comparing hardwired to PCB Princetons?

Where did you get your eyelets/ fixing tool?

Zimmy

Keefy · January 2021

zimmi_b said:

Hi Keefy,
Really looking forward to seeing how this goes. Are you keeping an eye on costs involved in this? I'm building a 59 Tweed (from Stewmac) just now and might try this next.....

I guess you've seen the Rhett Shull video comparing hardwired to PCB Princetons?

Where did you get your eyelets/ fixing tool?

Zimmy

I’ve spent about £85 but that includes quite a few spares e.g. if I needed three 68k resistors I’d buy five. I also have the PCBs up for sale which I hope will bring down the net cost.

Yes I’m subscribed to Rhett Shull’s channel and I‘ve seen that vid.

The eyelets (and most of the other components) came from Modulus Amplification. The tool is from a kit for putting much bigger eyelets into banners and the like. It won’t reach to the middle of the board so I use a hammer and punch there.

Are you going to put your build up on here?

Keefy · January 2021

I realised I hadn't done the bias board. That phenolic board is a bastard to cut with hand tools! Anyway, now cut, drilled, set, and test fitted to the chassis end panel:

zimmi_b · January 2021

Yes, plan to upload project once I work out how to do the photos on here....

Thanks for the eyelet info

munckee · January 2021

I look forward to the keefy mod 68 princetons available in the classifieds : )

Keefy · January 2021

munckee said:

I look forward to the keefy mod 68 princetons available in the classifieds : )

Ha - I don't think my production rate would be that high!

Anyway, work in progress on the main eyelet board - ground busses, filter caps/resistors, B+ links, and now working my way across the board. I've soldered some eyelets but the rest will wait till I add the flying leads.

The diagonal red wire by the filter caps is the Stokes mod, which links the otherwise unused 3rd B+ node to the plate resistor of the phase inverter.

The red stickers are to stop me inadvertently using the mounting holes as thru-holes for flying leads.

Keefy · January 2021

No pics because iPhone appears to be dead against me linking to Imgur photos.

Keefy · February 2021

Progress from yesterday and today. Things seem to be going very slowly but that's fine, it is a pastime after all. Bias board in place and connected to the tremolo intensity pot. Most of the front panel wires now attached to the board. Board laid loose in chassis in last photo.

Keefy · February 2021

A productive and satisfying day. All flying leads completed, board installed, all connections made (I think) except filaments and mains.

BigMonka · February 2021

That’s really neat, it’ll be interesting to hear it when it’s done.
I’m fancying building an amp kit as my next project, and I also fancy a Princeton so this has totally piqued my interest!

Keefy · February 2021

Mains wiring completed. I had to extend the black and white/black (0V and 240V) PT primary wires. Each conductor got a j-hook solder join, a snug piece of heat-shrink, and then a piece of glass braided sleeving. I then slipped heat-shrink over both conductors together - you can just see it tucked under the mains switch to the right of the photo.

Just the 6V filaments to wire up now.

PeteC · February 2021

lovely job @Keefy - lots of care and attention there - it will be a great amp.
Are you going to come from above the sockets with your heater wiring ?

Keefy · February 2021

BigMonka said:

That’s really neat, it’ll be interesting to hear it when it’s done.
I’m fancying building an amp kit as my next project, and I also fancy a Princeton so this has totally piqued my interest!

It'll be interesting to see whether it works @BigMonka !

PeteC said:

lovely job @Keefy - lots of care and attention there - it will be a great amp.
Are you going to come from above the sockets with your heater wiring ?

@PeteC yes I am. I find this easiest to do after all the other socket wiring.

Keefy · February 2021

6V filament wiring completed - a pig of a job! I mainly re-used the original black and white leads but some were just too fat to fit back through the valve socket tags so I used different wire. Once I had finished, I noted that I had accidentally melted through the insulation on a section of the neutral mains cable, so I had to replace that. Off to bed now so that I can check everything in the morning with a clear head.

Keefy · February 2021

This morning I buzzed out all the connections for continuity and all seems good.

This afternoon I plugged into my Variac via a dim bulb tester (DBT) and wound up the voltage, but with no valves or HT fuse. HT and filament voltages were present, though somewhat elevated as they were off-load. Then I fitted the rectifier and confirmed I was getting rectified AC, and a bias voltage that gets less negative (i.e. hotter) as you turn the pot clockwise. Adding the HT fuse, I brought the voltage up again to form the filter caps, but had to stop short of full mains as I was already getting 500V at node A.

With the bias at its coldest I fitted the remaining valves and noted the voltage at each node. If you look at the photos you will see there is an extra voltage-dropping resistor between the HT fuse and node A. This is to get the voltages closer to the Fender specs. I am using the 240V primary on the PT but our mains voltage is typically 249V! I started with this at 100R, tried 470R, and settled on 330R. Each time I tweaked the bias for no more than 10W (70% of 14W) dissipation on each 6V6. I fitted a 1R resistor to each valve socket so that I can convert cathode voltage directly into cathode current. (At the moment they are not particularly well matched as I had to replace a noisy one with the only other one I had.)

I also discovered that I had had the reverb tank the wrong way round (putting the sensitive recovery pickup closer to the PT) so I fixed that.

This evening I plugged a guitar in and confirmed that all the controls work as expected, which they do, so I put the whole thing back together and had a play.

HOLY FREAKING COW! I never expected the rebuild to make this much difference! This is now a proper blackface amp in silverface clothing. Some of the standout differences from the already-partly-modded 68 Custom:

The volume pot works smoothly, whereas before there was a sudden jump at about 4 on the control.
There is more treble available.
You can turn the bass control up all the way without it being overpowering. I might end up tweaking this for a little bit more bass, which would be useful at home practice volume.
The reverb - omigod the reverb, massive and lush, and hum-free. Reverb at 10 was previously unusable but is now OTT ambient heaven.
Lovely crunch when the volume is turned up high (and that is LOUD).

I do need to tweak the tremolo intensity as it currently doesn't do much until about 6 on the control. I'm going to wait on the new valves before I do this, as the bias can affect tremolo intensity.

Can you tell I'm pleased?

vanlooy1 · February 2021

Nice one!!!

Keefy · February 2021

vanlooy1 said:

Nice one!!!

Cheers!

Another thing I forgot to mention - lower noise floor.

Look out for an audio demo...

vanlooy1 · February 2021

I certainly will!

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Princeton Reverb 68 Custom hand-wired to blackface WITH AUDIO

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