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Revisiting an old friend : the Dynaco PAS-3

dividebytube

dividebytube

Well-Known Member
I recently picked up another Dynaco PAS preamplifier from E-Scam. I have owned three of these the past nineteen years, but always sold them for 'something better'. I planned to use this one with my McIntosh 250 solid-state amplifier that is used in my 'living room' system. The PAS still had three Dynaco labeled tubes that turned out to be smooth plate Telefunkens. The other 12AX7 turned out to be a Realistic 'Lifetime' tube that appears to be a U.S. made GE. The seller shipped it quickly and I got it within 2 days.

It's a fairly late unit and the faceplate is in very good condition too. I added an SDS power supply board which replaces the original filament suppy and the B+ can capacitor with modern components. I also bypassed the tone controls and loudness switch. Old Black Cat coupling capacitors are always suspicious, so I have an order of replacement Obbligatos coming my way. However I did replace two .02uf cap with some new Russian K40s .022s I had stashed away in my junkbox. I may be replacing the volume control, but right now it seems to track quite well. Shuguang 12AX7C9s are currently in place but I'll be rolling in the Tele's once I have everything else done.

Now it sounds very good driving the McIntosh 250 amplifier. Seems to have added some detail and fatten up the bottom a bit compared to running the cd player directly in. I'm always surprised how adding a preamp actually seems to help - perhaps the 'added distortion' helps the op-amp output of the CD player? Who knows - but at least now I can add a turntable. The 12AX7 with feedback is hardly an 'optimal' tube to be using, but it sure is musical. There is something very pleasing about these old designs.

I need to get a matching Dynaco tuner for it and I can have the total 'retro-audio' look. Throw in some better speakers than the vintage Wharfedale W60s and I'll have an excellent second system.
 
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Ha! I just won a PAS-2 from the same site. I have some rather big plans for it, like a complete re-vamp of the circuit with something like a CCS loaded 6H30/6BL7/6BX7/12B4 linestage output and a passive-type RIAA preamp. Design some new boards maybe :scratch2: we'll see. :D

I was sitting a long time on making a preamp, this 'box' will quickly speed up my efforts.
 
Always like reading threads about the PAS. Could you tell a difference with the few cap changes you made Dividebytwo? I'm sure you'll let us know the outcome of the future cap replacement, right?
 
The RIAA curve can be improved by quite a degree. I can't find the link, but basically;

1. Remove the 750pf caps from the selector switch.

2. Remove the 27k resistors from the selector switch.

3. Replace the RIAA 68pf cap on the PC 6 board with a 820pf (silver mica works well)

4. Parallel the 100k RIAA resistor with a 2M (metal film)

5. Replace the 2750pf RIAA cap with a parallel combination of 2200pf and 470pf.

6. Replace the 4.7M RIAA resistor with a 2M (metal film)

7. Do not remove the 47K feedback resistors between the cathodes of each tube.

To take care of the LF roll-off issue; jumper the .1uF output coupling caps connected from the plates of the second stage(s) with a solid wire. Then change the value of the .1 output coupling caps, connected between terminals 1 and 3 (Left) and 7and 9 (Right), to .47 X 400vdc.


After all this, you can use any of the three low level inputs for turntables (they will all be compensated). RIAA error improves from +/- 2db. to +/-.2db.



Hey Micah, are you going to try to retain the original boards?:scratch2:
 
Hey Jay,

I'm not sure about the boards yet. I'm gonna give the original circuit a listen and see how I like it, but I'll probably end up changing the circuit anyway, and make some new boards. Or maybe I'll just keep it stock...I donno yet. I'll have to wait and see till I get it :dunno:
 
Yeh, I know what you mean. The boards on these tend to be awfully leaky at this stage of the game, but they can be cleaned with acetone and a brush. Denatured alcohol makes a good rinse. They come out really nice looking after you get the waxy stuff off.
 
thanks for the RIAA info jaymanaa - the original PAS phono circuit always had a bit of a bass hump. Still pleasant though in the stock form.

DaWoofer - I couldn't tell much about the cap change - I did the SDS power supply board, tone control bypassing and .022uf cap change all in one fell swoop. I was just happy that it worked! Seems that all that work really 'tightened' up the sound making it less vintagey sounding. I'm trying to make this thing as reliable as possible since it will be used by the wife and son. I'll update this thread once I plop in the Obbligato caps - I've used them before and they're a very nice cap for the $$$.
 
I'm using Auricaps in the last one I did, with Tele smooth plates, and it seems like a nice combination. Funny, there aren't hardly any circuits that I like Telefunken 12AX7s in, but they really work to my liking in the Pas 2/3s.
 
Update : I installed the Obbligato caps and they sound very good - nice detail and a little less murky than I remember. I made sure to keep the capacitor lifted above the board to stop the metal body from making any bad contact. Next stop - speaker upgrade!

img1480sc2.jpg
 
Looks like it turned out great.

I'm beginning to do the same work to my PAS 2. I received the SDS power supply board, but I'm a bit confused with which wires go to the fuse. I notice you don't have a fuse in the above picture - did you skip that step?
 
Kuma said:
Looks like it turned out great.

I'm beginning to do the same work to my PAS 2. I received the SDS power supply board, but I'm a bit confused with which wires go to the fuse. I notice you don't have a fuse in the above picture - did you skip that step?
Click to expand...

I admit I left the fuse out - mostly because I didn't have one on hand and didn't feel like making a trip to Radio Shack. I will install it when I have the time. I would have also turned the board 180 degrees to make the AC line closer to the fuse. I thought the directions for the board were a little spotty to say the least.

DaWoofer - those Pyramid caps are actually not in circuit (since I bypassed the tone controls). I left them their for the old-timey look. ;) I think on the phono stage there is one .1uf (per channel) still in circuit though.
 
dividebytube said:
I admit I left the fuse out - mostly because I didn't have one on hand and didn't feel like making a trip to Radio Shack. I will install it when I have the time. I would have also turned the board 180 degrees to make the AC line closer to the fuse. I thought the directions for the board were a little spotty to say the least.
Click to expand...

Ha! I did the same thing. And yes, those instructions are terrible.

If I were to do it again I would have first soldered in new wires to every connection on the underside of the new board, run them through the holes in the chassis, and THEN mount and connect the other ends of the wires. I ended up with some pretty sloppy solder points due to the lack of space.

And in order to prop the board up a little bit, I had to make a hardware store run and buy a 41 cent bolt to use their spacer that came with the kit.

All in all, mine sounds better in the bottom end - but now I have a slight hum when I crank the phono section up past half volume (not that I would ever listen it at those levels).

Next up for me is cap upgrades and bypassing the tone controls.
 
I hear ya Kuma, it was a very tight area with the stock wires. I wish I had wired the board first. Mine was so tight I wired it in Upside down. Looks very unprofessional to say the least. If I were to sell it I would take it out and rewire, but with the cover on no one but me can tell and I have no other issues with it. My next move is to do what Dividebytube as just finished, but money is so tight right now and other more important projects are in the fire like "where is next months mortgage payment gonna come from?"
 
I agree the instructions for the SDS cap board were bad! I never did get mine installed correctly, however the customer service rep was great.. still I just gave after destroying the traces with multiple soldering accidents..installing it upside down ext...The unit had other issues so I shelved it.. But i got a new one that I'm just now recapping and have a couple questions; I'm going to be using Vitamin Q's .01uf 300 volts in the phono section and i"ll have a couple left over, could I run them in parallel on the tone control. at present there is a .02 400 volt cap there. I know .01+.01 =.02 but I'm concerned about 300 volts? thoughts. Second Question: where did you get your Obblgato's?
Kirk
 
kirkleclaire said:
I agree the instructions for the SDS cap board were bad! I never did get mine installed correctly, however the customer service rep was great.. still I just gave after destroying the traces with multiple soldering accidents..installing it upside down ext...The unit had other issues so I shelved it.. But i got a new one that I'm just now recapping and have a couple questions; I'm going to be using Vitamin Q's .01uf 300 volts in the phono section and i"ll have a couple left over, could I run them in parallel on the tone control. at present there is a .02 400 volt cap there. I know .01+.01 =.02 but I'm concerned about 300 volts? thoughts. Second Question: where did you get your Obblgato's?
Kirk
Click to expand...

You can parallel capacitors to get the value you need - but it might be a little tight in there. Make sure the metal Vit-Q cases do not touch any of the metal connections on the board.

Obbligatos are available from Ebay and/or DIY HiFi Supply (same vendor).
 
Dividebytube- it looks like you used two different .02 caps from the first stage and at the second stage. Why did you choose to do this? (In the linestage.)
 
Glad to see this thread resurrected.

I bypassed the tone controls and couldn't be happier. Like someone else said, it's like taking pillows off the speakers. I was very against such a move initially, but I'm now a convert - don't bother bypassing with caps, IMO, just bypass them entirely. It will all sound better in the end.
 
Kumas,
So the tone bypass information was from the Dynaco doctor site? Seems I read a post sometime ago that claimed some information was incorrect? has it been corrected? Is that where you got your info? I'm planning on by passing mine and want to be sure...
Many thanks,
Kirk
 
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