Tried a 12au7 last night. Helped some but still not quite there. The 12au7 sounded kind of dark. I like the brighter sound of the 12ax7 but the decay is just too much. For now, the control works pretty good with a 100k tacked across the pot - a little easier to dial in. Will wait for another tank before I mess with it any more.
Put a 25k pot in line with the feedback resistor. Works very nice. Also added a switch to change the V1a bypass cap from 25u to 5u. Ended up with 2 separate caps instead of running them in series and shorting one. Works great - no pops. Pretty subtle but does tighten things up quite a bit when cranked. May try a 1u once I get a chance to play it some more.
Pretty much done... Faceplate should be here by the end of the week. Going to order a new reverb tank if I can figure out which one...the one Norm suggested doesn't seem to be available any more. Been reading up on reverb tank codes trying to decide on the best substitute.
When it comes to input / output grounding on reverb tanks does that make a big difference in use?
I prefer the 9 series pan in a medium decay, much better Reverb in just about any fender circuit. No need to do any other mods with that pan. I don't think a Princeton needs a master volume either. I would try the Deluxe Reverb OT though. Better low end.
I recently replace a Reverb tank in a Pro reverb with the Belton 4 series long decay and could never get rid of the long tail decay and obnoxious reverb using all the classic resistor and cap tweaks, 12au7. Finally put in a 9 series medium decay pan and it was lovely, full, lush but great sounding reverb. It is a real shame the Accutronics are no longer made in the usa, the belton made ones are not as good sounding. Oh yeah, and used to have to glue the coils to the transducer lams on the Accutronics, the beltons are already tight, still don't sound as good .
Thanks Billy. I was planning on ordering that tank since the 8 series Norm suggested doesn't seem to be available. Not to mention it will line up with the holes I've already drilled in the cab.
The 8 series is a short pan for mesa Boogie and some Marshall, it does work well in newer fenders too.
The 9 is a 3 spring, more complex delay line. The 2 spring ( or 4 as they are called, since they are coupled together into two longer springs) can be good too. I just found I really like the sound ot the Medium decay, whichever model you get. It does not suffer from those long decay trails which I find annoying. Here is the model I use which ever brand you get.
9AB2C1B
Belton bought Accutronics and they are now made in China too. Probably in the same plant, I have not compared them side by side yet, they might even be the same. The beltons do have tighter coils than the old accuronics.
Got the Bellton replacement for a 9AB2C1B and installed it today. MUCH better. I liked it before but the decay was too boingy (boingee?) for me. Sounds very much the same just without the funky (boingy ) decay. Still have 2M2 for the mix resistor and a 100k resistor across the 100K reverb pot.
Also swapped the 4.7u cap on the cathode of V1a (via a toggle between 4.7u and the stock 25u) with a 1u. The 4.7 was barely noticeable. The 1u seems to be maybe just a bit small at low volumes but I think will be perfect for it's intended use - to tighten the low end when cranked.
Found a new problem.... When I crank the treble pot, all is fine until the very last bit where I hear a pop and then lots of noise (hiss not hum). Not sure what that's all about. Bad pot perhaps? Used CTS from Mojo - seem to be OK but the volume pot is kinda funky too....no volume till you hit "2" then it works like normal.... I have an Alpha I can swap it with.... May have to get one for the treble too...
Deric wrote:Tried a 12au7 last night. Helped some but still not quite there. The 12au7 sounded kind of dark. I like the brighter sound of the 12ax7 but the decay is just too much. For now, the control works pretty good with a 100k tacked across the pot - a little easier to dial in. Will wait for another tank before I mess with it any more.
Hi Deric - I thought the suggestion for the 12AU7 was as a replacement for the 12AT7 as the reverb driver (not replacing one of the 12AX7s) . I was wondering how that would go.
When I removed the negative feedback on one of my cathodyne builds I had to use a 220k grid stopper on the input of the inverter. It did change the sound. It tightened up the bass, which seemed to make it sound more like a long-tailed pair. I liked it well enough to keep it, but you may not want to chance losing the swirl Princeton reverbs are known for.
I know I am arriving late to the party with this since the sockets have already been wired, but the other suggestion is to use a 12dw7, with the au side (pins 1,2 and 3) as the inverter. I built mine that way and like it, but I have a Mullard 12dw7 in there. Maybe all I am really hearing is the NOS goodness of that tube. I was inspired by Sunn and Ampeg designs that used a single bottle pentode/triode like the 6an8 and 7199. The triode side used for the inverter in those circuits is similar to one half of a 12au7. I'd be interested in hearing from others here if they think there is any real advantage to the 12dw7 in this slot. Maybe a Mullard 12ax7 would sound just as good?
Deric wrote:Got the Bellton replacement for a 9AB2C1B and installed it today. MUCH better. I liked it before but the decay was too boingy (boingee?) for me. Sounds very much the same just without the funky (boingy ) decay. Still have 2M2 for the mix resistor and a 100k resistor across the 100K reverb pot.
Also swapped the 4.7u cap on the cathode of V1a (via a toggle between 4.7u and the stock 25u) with a 1u. The 4.7 was barely noticeable. The 1u seems to be maybe just a bit small at low volumes but I think will be perfect for it's intended use - to tighten the low end when cranked.
Found a new problem.... When I crank the treble pot, all is fine until the very last bit where I hear a pop and then lots of noise (hiss not hum). Not sure what that's all about. Bad pot perhaps? Used CTS from Mojo - seem to be OK but the volume pot is kinda funky too....no volume till you hit "2" then it works like normal.... I have an Alpha I can swap it with.... May have to get one for the treble too...
Thanks for all the help!
Sounds like a Crappy pot with a bad audio taper. Try a PEC if you can fit it in.
Hi Deric - I thought the suggestion for the 12AU7 was as a replacement for the 12AT7 as the reverb driver (not replacing one of the 12AX7s) . I was wondering how that would go.
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