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In Reply to: RE: help with Dynaco Pas line stage circuit check posted by xaudiomanx on May 03, 2013 at 19:13:11
hi, I've gone back to Circio instructions and installed .33uf 400v caps as output caps , the first .0075 for each channel is still replaced with wire,some other forums I have read mention that you need another cap instead of the jumper wire so that there are two caps after the final stage otherwise you get DC at the output, I'm currently getting 0.4mv DC on the output RCA's, is it supposed to be zero?
Thanks for your help
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So, this is what I have running at the moment, I just installed the .33uf cap (which I might increase to .47uf) and it's been suggested on another forum that it's a good idea to install a second output cap between eyelets 4 & 6 possibly a .47uf or 1uf instead of the jumper wire, and to increase the second .02uf cap to to .22uf and the possibly the first one to .1uf
Thanks again
Whenever I did a mod to the Dynaco Pas anything I always got rid of the stock board and went with one of the mod boards(not that there are that many). That got rid of all the tone controls and everything in the circuit that isn't needed.
Your preamp should consist only of the RCA input to the grid with a resistor to ground(your volume control is just that), a load resistor on the plate(470K), coming off the plate of the first half of the tube a coupling cap going into the grid of the second half of the tube(.02UF) with a resistor to ground off the grid(1.2M), plate load resistor on the second half of the tube(100K), with a coupling cap coming from the plate of the second half of the tube to the RCA outs(.02UF). Off the RCA out there is a 510K resistor to ground. and along with all that there are resistors to ground coming off the first half(4.7K) and second half cathodes(1K) to ground.
All the other stuff is totally eliminated but in that board they may have to be bypassed with a wire.
That is the simplest design which many have resorted to. As for what to do with the existing stock board I can't say but I would go back and follow the Curcio instructions right to the letter. And yes, there should be no DC voltage on the RCA outputs. That is what any cap cap does. Lets AC pass and blocks DC.
Hope this helps!
I initially used the pre with the stock line board and was thought it sounding amazing, with layered imaging and very musical.I did try an upgrade line board with a different circuit, and after living with it for about six months and thinking there was something missing I reinstalled the stock board and found that I still preferred the stock board even in its current condition, with original components that were still in spec. the original board is getting old and some of the traces are starting to come off, so I'm going to just replace the board and copy the circuit as is, I just want to make sure nothing important is missing. Dynaco changed the coupling cap coming from the plate of the second half of the tube to the RCA outs from .02uf to .22uf in the Pas 3, ( I have a Pas 2)so I was going to do the same, is that the output cap? its the last cap before the output and the only thing connected to it other than the output is the feedback resistor in parallel with the 33pf cap, in which case do I need the .33uf cap?. most other modders of this pre have a anything up to 1uf there. and yes your post did help, I was able to read the circuit a bit better after your short explanation of what should be where.
Thanks
Hi again, the confusion on my part is this, the original Curcio tone removal instruction say to jumper BOTH of the original .0075 caps with wire, and some other people say to install a cap up to 1uf so that there are two output caps in series after the second half of the tube to stop DC as apparently one was not enough, maybe thats why Dynaco changed the cap from .02 to .22
The reason for increasing the size of the output coupling cap to .22UF might be just for that reason of DC blocking but it also increased the bass rolloff to a lower frequency. I'm sure the preamp can handle it.
My suggestion would be tofollow the Curcio instructions right to the letter but in my opinion I have a better circuit that was used many years back by Audio Dimensions in the Tu Be or Not Tu Be manual. I did like the results much better than stock but I think you had to increase the B+ to 400VDC. They tell you how in their manual. And if I am not mistaken there was no global feedback which is a huge plus IMHO.
I would rebuild the preamp you have using the values listed on your schematic without the feedback loop and see how it sounds, also increasing the output coupling cap to .22UF.
I would take out all the stuff after the second half of the tube and just run a .22 right to the RCA outs. See how it sounds. All that other stuff I think is killing the sound but it might be with that circuit you might just need the feedback. I have no idea as I never used the stock board.
I was basically working the same with just the final .02uf cap and the feedback loop, I definitely plan to increase the final .02uf cap to at least .22uf. I'll remove the .33uf that I added and test for DC at the outputs, if there is none or the same as now (0.4mv) I'll leave it out. The other circuit I tried was no feedback as well, and while all the detail and bass was there, I thought it just sounded a little flat and un-involving to me, the stock board is more fun to listen to. So maybe I just like the sound of negative feedback, crazy as it sounds. Before I put the stock board back in, I bought a Pas3 Aikido circuit kit from Tubes4hifi, which I will try before doing anything to expensive, its going to have to be pretty good to top the stock board.
I've always agreed on the Dynaco amps that it sounded great stock so why mess with success. I know in the ST-70 and whatever other products used the 7199 tube getting good ones is a real horror but if you change to a different front end board it is not a Dynaco any more even if it is called a "modded" Dynaco which means what to me. The front end board in any amp or preamp is the most important. The rest is academic in any amp.
The Tubes4hifif circuits are quite good but in my opinion I do like stock the best myself, even with out of tolerance parts the distortions created can be quite rewarding. I don't get into all the hype about boutique parts or this had to be this or that. The only part of the circuit in my opinion that should be as close to spec is the resistors in the bias supply(270K or something like that) in the Mk III or ST-70. With everything being as close to equal in the bias supply, the tubes would be easier to operate at their best possible spot.
I like a bit of distortion. Gives some color or character to the sound. That is why the most favorite preamp to me is the 26 tube preamp. Lots of color but the sound can't be matched by anything out there. Very large sound stage.
Hi, I am happy to keep it as an original Dynaco circuit, but I am wondering, what would happen if I moved the single coupling cap from directly after the second tube stage to after the feedback point, that would stop DC getting to out RCAs but would it allow DC to get into the feedback loop?, and is there anything to stop it from getting back to the first tube stage, and what would happen if it did? just trying to minimize the part count if possible.
Thanks
Don't know much about what you are asking. Any time I ever modded something was following the directions of the mod kit. If I ever built something it was to the exact schematic. I very rarely ever went off on my own and did anything. I am not a circuit designer. I try things as what others thought and went from there.
I would go on the DIY Tubes site to the Asylum and ask these questions. Those guys are very smart and have great ideas to try with anything ever built. They might have better mods than what is already tried. Just tell them you are new to DIYing and they are more than willing to get into it with you no matter how novice the question.
As far as moving the cap to after the feedback loop I couldn't tell you but I know in DC the voltage is single direction so it can't come back to it's point of origin so if in your thinking voltage is getting by coming backwards then I would have to say that it is incorrect. If the directions don't say to do it then I wouldn't. I don't think you will damage anything. I just don't think it will sound right.
Whenever I build anything I try to build without feedback.
Hope this helps!
Paul
Thanks for all your help, I'll probably just stick recreating the circuit as is for now, I might think about other mods at a later date, but I'm pretty happy with the stock sound.
Thanks again
Graham
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