Single-Channel AB763 Deluxe Reverb in a Princeton Reverb Chassis

ElliotKnapp

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At the risk of blasphemy: consider making the vib oscillator and driver (follower) with transistors or an IC or 2 and eliminate the need for the extra tube hole.
I've said this before. If you use a 12DW7 to drive and recover the reverb it will free up half of a 12AX7 so you can convert it to a long-tail pair phase inverter. That will make a big difference.

You guys are clearly several levels above me here...wouldn't even know where to start or know if I was doing it right!
 

NTC

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The solid state part is a bad idea - it would take a bit of electronic magic to make the power supply for the driver portion. You could make 5v from the heater winding, but that might not be high enough for the driver. It would work for the oscillator. Easier to use a tube.

Ignore the man without a tele.
 

sds1

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The solid state part is a bad idea - it would take a bit of electronic magic to make the power supply for the driver portion. You could make 5v from the heater winding, but that might not be high enough for the driver. It would work for the oscillator. Easier to use a tube.

Ignore the man without a tele.
LND150 is popular in tube amps because you can drop it in for 12AX7 etc. with zero reconfiguration to power supply or otherwise. It works great in any transparent application including tremolo oscillator circuit like you suggested.

Hardest part is mounting it. You can use a terminal strip.
 

NTC

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I had some of those and they seemed to blow up when you soldered them down. I never got a circuit to work. Be great if the do - it really does do the same thing...
 

sds1

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I had some of those and they seemed to blow up when you soldered them down. I never got a circuit to work. Be great if the do - it really does do the same thing...
Yeah those butane torches aren't the best choice for tube amps, I recommend something flameless especially when working on silicone.

I'm kidding! Dunno why you had problems, I've had no problems soldering and even resoldering those little guys. I've not ever received a bad one either. Got them from a reputable supplier?
 

zook

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Try this:

princeton_reverb LTP poweramp aa1164_schem clear.jpg
 

trancedental

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Similar to Zooks PR circuit above, 3 preamp tube Princeton Reverb, with no transformer 1 tube 12DW7 reverb, no vibrato with cathode bias (or fixed bias option).

Using no reverb transformer / choke saves almost the same cost as an output speaker transformer.

I saw this a while back on EL34world forum. Relevant credits are written on the circuit diagram.

PR no transformer reverb no vibrato cathode bias .png
 

ElliotKnapp

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I've said this before. If you use a 12DW7 to drive and recover the reverb it will free up half of a 12AX7 so you can convert it to a long-tail pair phase inverter. That will make a big difference.

This idea piqued my interest and although I own a 500 series DI that uses a 12DW7, I'd never really investigated this tube before. What I still don't understand is, if the original Fender circuits call for 2 triodes of a 12AT7 in parallel to drive the reverb, how can a single 12AU7 in the 12DW7 be a sufficient replacement? @zook are you willing to school me on this?
 

elpico

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When you're low on available driver power like this you can drive the tank from the OT. I know, Fender never did it that way, but other companies did and it works.
 

zook

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This idea piqued my interest and although I own a 500 series DI that uses a 12DW7, I'd never really investigated this tube before. What I still don't understand is, if the original Fender circuits call for 2 triodes of a 12AT7 in parallel to drive the reverb, how can a single 12AU7 in the 12DW7 be a sufficient replacement? @zook are you willing to school me on this?

You really don't need the two triodes to drive the reverb tank. One will suffice, and the 12AU7 half of the 12DW7 will be just fine. If you want to hear the difference, sub in a 12AU7 in place of the 12AT7 in a fender reverb driver. You will find that you can turn the reverb up more before it becomes overwhelming.
 

ElliotKnapp

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Hi- I was wondering what value resistor you settled for? Sounds pretty good!

Not my FB video but I did just finish a Princeton Reverb build and included a switched mid boost--I auditioned 10k and 15k mid resistors and preferred the 15k. The 10k, although it's the top of the range for what's found in AB763 amps that actually have a mid potentiometer, was a little too subtle for my tastes. 15k wasn't too over the top to my ears, but it definitely provides a serious mid bump. I was expecting it to diminish the effect of the Treble and Bass controls more than it seems like it does. Also important to note that the higher you go, the more you're leaning toward defeating the tone stack entirely and simply boosting all of the output--with 15k there is a very slight overall boost happening but nothing too far in the direction of a "Raw" switch.
 

ElliotKnapp

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Also, since it's the original subject of this thread, I did complete my Princeton Reverb-sized single-channel Deluxe Reverb back in December--here's a gut shot. It is great! I don't miss the normal channel and using the can cap is so much build-friendlier than the doghouse, and not having all of those power wires makes noise management much easier. The board layout is tight, with cathode resistors/capacitors stacked vertically to make everything fit. The build includes push/pull pots with Bright switch, negative feedback on/off, and a boost accomplished by eliminating the ground of the 220k resistor that replaces the Normal channel--I prefer this to a Raw switch. Also, individual bias adjustment circuit to accommodate an unmatched pair NoS pair of 6v6s, and a Master Volume, which I've since removed. It's small enough to fit on all of my Mojotone Lite cabs. Great little amp, and definitely scratches a slightly different itch than the Princeton!
 

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Phrygian77

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Hi- I was wondering what value resistor you settled for? Sounds pretty good!

Not my FB video but I did just finish a Princeton Reverb build and included a switched mid boost--I auditioned 10k and 15k mid resistors and preferred the 15k. The 10k, although it's the top of the range for what's found in AB763 amps that actually have a mid potentiometer, was a little too subtle for my tastes. 15k wasn't too over the top to my ears, but it definitely provides a serious mid bump. I was expecting it to diminish the effect of the Treble and Bass controls more than it seems like it does. Also important to note that the higher you go, the more you're leaning toward defeating the tone stack entirely and simply boosting all of the output--with 15k there is a very slight overall boost happening but nothing too far in the direction of a "Raw" switch.

It's a 33k mid resistor. I've done up 75k also with an NFB switch and rattled stuff off the walls. 33k seems to be a good compromise spot. It's more noticeable than 10-15k, and still more than a 25k pot.
 

joulupukki

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Also, since it's the original subject of this thread, I did complete my Princeton Reverb-sized single-channel Deluxe Reverb back in December--here's a gut shot. It is great! I don't miss the normal channel and using the can cap is so much build-friendlier than the doghouse, and not having all of those power wires makes noise management much easier. The board layout is tight, with cathode resistors/capacitors stacked vertically to make everything fit. The build includes push/pull pots with Bright switch, negative feedback on/off, and a boost accomplished by eliminating the ground of the 220k resistor that replaces the Normal channel--I prefer this to a Raw switch. Also, individual bias adjustment circuit to accommodate an unmatched pair NoS pair of 6v6s, and a Master Volume, which I've since removed. It's small enough to fit on all of my Mojotone Lite cabs. Great little amp, and definitely scratches a slightly different itch than the Princeton!
@ElliotKnapp this looks really interesting! Do you have a layout diagram posted anywhere I could reference? I’m interested in building one of these. Also I’m curious about how the MV knob performed when you had it in. Were you able to get into distortion with the MV turned lower and the volume turned up?
 

ElliotKnapp

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@ElliotKnapp this looks really interesting! Do you have a layout diagram posted anywhere I could reference? I’m interested in building one of these. Also I’m curious about how the MV knob performed when you had it in. Were you able to get into distortion with the MV turned lower and the volume turned up?
Hi hi! I didn't do a layout in DIYLC or anything since I followed the AB763 circuit so closely, with the exception of a 220k bleeder to ground (compensating for the missing Normal channel) and the switchable mods I included for NFB and bright. I probably still have the paper layout I used to build and drill the turret board--could probably make a copy and mail it to you if it's helpful.

The master volume I used was the simple "Lar-Mar" type shared on Rob Robinette's site. It worked great at opening the way for higher preamp distortion, I just messed up the hole spacing on the front panel and the asymmetry was killing me so I removed it...that, and I commonly use attenuators so the MV wasn't really a must for my setup.

The biggest build challenge I'd call out is accommodating the larger power transformer and extra 9-pin tube hole. I don't have a lot of metal-working tools so it was drilling and filing to get me there for the PT and a step bit for the tube hole. After doing some more builds since then I'd probably recommend going with a Weber chassis and power transformer--you'll still need to pop out a 12AX7 hole (which will be tougher since the chassis is stainless) but their Princeton Reverb chassis is already specced to fit the same PT as the Deluxe Reverb. Fun build--I personally wouldn't ever build a regular DR after having this more compact version!
 

joulupukki

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Awesome. Thank you @ElliotKnapp for explaining. I think I can see enough of what you did from your previous gut shot to figure out a layout that would work for me. I have some old scrap metal that I may attempt to make the chassis myself so I won't have to worry about a PT fitting into any hole. In fact the PT I've got is the stand-up type so I really will only have to drill a couple of grommets. *IF* making my own chassis works out then I can probably increase the size of the board and not have to have everything so cramped.

I'm thinking too at this point to build it as a combo amp but build the cab 2" taller than a normal PR so that the cab is a tiny bit larger for a 12" speaker. If I can figure it out, I may also use two LND150 MOSFETs in place of a 12AX7 for the tremolo oscillator tied into the bias circuit so that it's a smoother tremolo (I'm not a huge fan of the opto trem). The only thing I've gotta figure out with that is how I would do individual bias adjustment – and if that's not gonna work, I'll just do the typical bias adjustment with a 10KL pot.

Good to know about the MV. I also use an external attenuator, though it's handy when moving amps around to not also have to move that thing around too, which is why the MV sometimes sounds attractive, but I really would only want to have that to have distortion at lower volume levels ... and it wouldn't really be driving the power tubes into distortion. I'm on the fence on whether to add that.

In any case, it's really cool to see that you had a successful build and thank you for sharing!
 




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