Conrad Johnson PV12 AL

Hopefully hifiman5 will weigh in here as he has one that was modified and he is quite happy with it.  Good luck.  Regards......

I have had my PV12L extensively modified by Bob Backert of RHB Dezigns.  He focused on the power supply section of the line stage, bypassed the circuit board  traces with gold coated silver wire and added bypass boards to improve the performance. It is still a cj at heart but is more open sounding with a tighter more responsive bass response.

The PV12 used a 3 stage power supply. A pre-regulator fed a separate regulator for each side.

In addition to putting in better caps, or bypassing existing, adding some heat sinks to the regulators (really just transistors) could be useful in increasing reliability.

Also the resistor used in the power supply CRC circuits could be improved.

I’m really not a fan of mixed-metal conductors though.

Best bang for the buck on those is really the output caps though. They make much better caps than the PV10/12s had. If you are a noob, start there. :) Watch the cap voltage ratings carefully though.

Best of listening!

Erik

I want to thank Carmenc, Hifiman, and Erik for responding. I know capacitors have vastly improved since the first PV12, and that's why I'm looking into these modifications. I have a schematic of the PV12, and it looks like the diodes could stand an upgrade as well since there are some faster diodes; Shotsky or something like that is the name, that are used by Musical Design.

"Musical Design"; is close to me, and they do a lot of mods. If they are still in business I'll give them a ring. I plan to rebuild the PV12 if I ever get one. It's incredible how much improvement you can get if you're a technician, and can do good soldering work. The biggest problem I've ran into is finding room for those huge new caps, that I use to replace the electrolytic capacitors.

Once you find an old amp with a good design, it's simply a matter of finding enough room for new parts.


Enjoy the music.

Yes, you may very well need to stand them up.  Depends which direction you go. :)

Plan on standing them up and adding an extension.  So, a little soldering, and maybe a little heat shrink, with some solid copper core.  Also will need a dab of hot glue. Don't go nuts, in case you want to change things later.

Honestly, having done this before, I would encourage you to just do the output caps and stop there. :) After that it's a lot of diminishing returns, but those two caps can really do a lot.

Best,


Erik

Forgot to mention, Chris Johnson (previously of Sonic Frontiers) at Parts Connexion has a lot of experience with this, send him an e-mail after you acquire your victim, er, preamp.  He'll give you great suggestions, and may even give you a discount on caps.

Best,

Erik

Erik, you seem to be quite an expert on these modifications. Right now I'm looking at a schematic of the PV-12; I've already replaced the electrolytic capacitors with Black Gates, when they were still selling them. Which capacitors do you suggest replacing? I had good luck in a speaker crossover with top of the line Jensen poly caps. The PV-12 I'm looking at wont work unless I get a transformer.

I imagine you have a schematic as well; are there any specific caps that you recommend replacing, I always use the same value and voltage of course.

BTW I e-mailed Chris, and told him that you recommended him; I even got a reply; this is just the beginning.

Those of us who are handy with a soldering iron can go a long way on improving most components in our rigs.

Good luck on all your future mods.

@orpheus10 Sorry for the delay.

For the PV-12, there are 2 sets of caps which are really key. As I wrote before, the output caps are really most critical If they are 2uF, upgrade them to 4uF, maintaining or exceeding the original voltage. Alternatively use a pair of 2uF caps on each.  How crazy you want to go is up to you. The sky is the limit in your spending.

After this there is a large electrolytic which I believe is on the heater or something like that. It basically just maintains a DC voltage to the tubes, but it’s part of the signal instead of the DC supply, so it’s not fed directly by the regulators.

After these two, the PV-12 (if memory serves) has a 2 level voltage regulation stages. A pre-regulator, and then 1 more per channel. You can identify the stages by the chain of zener diodes in each.

Don’t go too crazy, but it may be worth replacing or bypassing the filter caps on the outputs of each of these 3 stages. Save your money for the output caps though!

Lastly, add heat sinks to each of the transistors used as regulators. They go out frequently and sinks are cheap. :)

One more advanced mod which I never did and now don't remember is to lower the total gain. That preamp has too much gain, and you don't need it.  Lower the gain and you'll get much less noise.

@orpheus10  I have experience with Musical Design and agree that you might visit them, especially if they are in close proximity.  But first...finding that PV-12.