princeton reverb spin.

[DummyLoad]

piddling around in between building the dual EL34 SE amp i posted a while ago.

doug's recent post on his princeton reverb piqued my interest in another princeton reverb, i built a rack mount version for a friend's studio, however, i was left with the impression that i'd like more power: 35-40W would be nice.

please allow me to digress for a moment: i have this ancient combo that richard and i designed that has been basically used as a 1x12 cabinet for nearly seven years now. it was conceived by richard around 4/2008 and we built two, complete with about a half dozen or so mechanical and machining flaws, since we were noobs at the time with CNC and amp building.

so, to cut to the chase, i have this chassis that's about 20" x 8" x 2" that was built for a nice combo cabinet. it's got some cosmetic flaws, but does have a 660VCT PT and 6.6K:8R 25W OT that were part of the original concept (it used 6BG6GA output tubes) that i was not satisfied with. richard still has his and i gutted mine a few years ago and shelved the iron.

in this build, both PT & OT are edcor transformers, however, there are comparable components available over the counter.

what i did was basically look at the mullard EL34 cut sheet and found a cathode bias condition that is perfect for the parts i have. the PT should make around 460V B+ at idle current specified by the mullard condition. it also states 450V B+ with 6.5K Ra-a with 465R under each bottle: i have a 6.6K, with 460V...i don't think EL34s will care. ;-)

i made some minor mods to the PS and circuit mods as well:
1) deleted tremelo ckt. (1/2 12AX7 is unused).
2) deleted bias PS.
3) tweaked the SLPI for a bit more drive and moved the power tap for it.
4) added a choke and changed to FW SSR.
5) added a dwell pot to tame the surf if desired, and i used a smaller summing resistor so it'll break a bit sooner.
6) changed the PI - EL34 coupling caps to shave off some bass.
7) trimmed the bypass caps on the gain stages to shave some bass.

the 3.3K NFP resistor is a guesstimate - i'll tweak it once built. schematic attached.

happy rosin smokin' y'all!

--pete

PS, this spin on a PR has probably been done. no offense to anyone who has done so is intended.

[tubenit]

That is a fantastic build idea!  I like it!   

I am going to put this in the SCH Library of schematics.   Have you already got a SCH layout for it?  If not, I may try to modify an old PR layout to go with this.

THANKS for sharing it.  With respect, Tubenit

[printer2]

Just had a thought inspired by the jack on the reverb return. Could use a switched jack with the cathode bypass capacitor switched out if a jack is plugged in. Then take a signal off the 12AT7 cathode to an output jack.  Or just a switch to kill the cap and also to switch from the reverb return to the external cap.

[tubeswell]

Or for insane rocking' gain, you could make the driver/cathodyne tube into a LTP, and use the spare triode from the absent LFO as an extra driver stage? (more like an Allen Old Flame typology)

[DummyLoad]

Or for insane rocking' gain, you could make the driver/cathodyne tube into a LTP,

thanks for the input TW, but for now, i'd like to keep the mods minimal. all i want is a princeton reverb with more power. for now...

it would be interesting to breadboard your concept in the future. perhaps that would be a good location for MV as well, between the last gain stage and LTPI. your proposed topology would be similar to the trainwreck express, only with reverb and without the presence control.

the tremelo could be added with modification to modulate the the last gain stage, but then it's not a PR power tube bias vary tremelo that has a characteristic of it's own.

tuebenit, thank you! regretfully, i don't have a .sch layout but i'm working on a visio layout to mod the franken-chassis i have    and you're welcome to have that when completed.

should anyone have interest, i would encourage use of doug's turret board and layout and just change what's need, of course the down side here is that the fender chassis isn't an option any longer... :-/

iron suggestions are doug's bassman PT, doug's SR choke, doug's reverb driver OT, and the classictone 6k6:8 35W OT.

click me: link to classictone 6k6 OT specs

--pete

[DummyLoad]

i have most of the franken-chassis layout is done. the PT/OT placement will swap. most of the control stuff is pretty much set though.


this is a tough chassis to work with. the pots cannot be removed when the control turret board id installed. that means all of them have to be pulled at once with the turret beard to get to the one: it is also the reason for the grounding scheme. might as well be a PCB amp... :-\


another pair of eyes would be appreciated to help check the layout. schematic is in the first post and is accurate.


TIA,


--pete

[sluckey]

I see two issues with your layout. The pots are wired wrong. As shown, the left lug should connect to ground. All of the little tube sockets are wired backwards. For ex., you have the input jack connected to V1 pin 8!

EDIT... Looks better if I hold a mirror up to the monitor. Well, except for the writing... 

[eleventeen]

I can tell you that the SF PR I modded 30+ years ago with Dynaco mono-35 watt PT & OT and 6L6's is just about my favorite amp. I will readily confess I did zero calculations when I did the mod, because I didn't know about them! All I knew was that I had to get more negative volts to bias a 6L6 vs 6V6 but since I was replacing the PT, that (reconfiguring the bias) was on the table anyway. I don't recall at this point whether the Dyna PT had a bias tap or not.


Originally, I hacked together an add'l gain stage using the tremolo tube; pre and post gain controls; again, without the benefit of much knowledge or planning of any kind, and I ended up with a barely-under control vicious monster of a little amp. Altec 12" speaker jammed in there. People marveled at that amp, it was great fun to play because it would break into feedback, it seemed, right where I would want it to. It could also play very clean. 


As the components aged, it kind of got less and less controllable. Ten+ years went by, by the time it clearly needed the standard re-capping, I had no idea nor documentation of what mods I had done, so I pretty much ripped them all out and started from scratch and that's how it sits today. With the extra power from the 6L6's and those Dyna trannies, no trem, 2 blank holes in the front panel, LOL. Still a great amp.


I will also say that the $175 used Peavey Valveking I bot (with the same Altec 417-8H speaker installed) is about 90% as good as the PR.

[DummyLoad]

I see two issues with your layout. The pots are wired wrong. As shown, the left lug should connect to ground. All of the little tube sockets are wired backwards. For ex., you have the input jack connected to V1 pin 8!

EDIT... Looks better if I hold a mirror up to the monitor. Well, except for the writing... 

thanks steve. i'll fix it shortly. 


--pete

[DummyLoad]

I can tell you that the SF PR I modded 30+ years ago with Dynaco mono-35 watt PT & OT and 6L6's is just about my favorite amp. I will readily confess I did zero calculations when I did the mod, because I didn't know about them! All I knew was that I had to get more negative volts to bias a 6L6 vs 6V6 but since I was replacing the PT, that (reconfiguring the bias) was on the table anyway. I don't recall at this point whether the Dyna PT had a bias tap or not.


Originally, I hacked together an add'l gain stage using the tremolo tube; pre and post gain controls; again, without the benefit of much knowledge or planning of any kind, and I ended up with a barely-under control vicious monster of a little amp. Altec 12" speaker jammed in there. People marveled at that amp, it was great fun to play because it would break into feedback, it seemed, right where I would want it to. It could also play very clean. 


As the components aged, it kind of got less and less controllable. Ten+ years went by, by the time it clearly needed the standard re-capping, I had no idea nor documentation of what mods I had done, so I pretty much ripped them all out and started from scratch and that's how it sits today. With the extra power from the 6L6's and those Dyna trannies, no trem, 2 blank holes in the front panel, LOL. Still a great amp.


I will also say that the $175 used Peavey Valveking I bot (with the same Altec 417-8H speaker installed) is about 90% as good as the PR.

this good to read. much thanks for the feedback. 


--pete

[DummyLoad]

pedernales rev. 2. i think i have it right (mostly?) this time... i'll re-route the NFB signal path under turret board and under the PS board and finish the rest of the mains.
 
again, much thanks for everyone's time.


--pete


EDIT: missed a signal connection from reverb summing summing node to final gain stage grid.