Dynaco Dyna PAS3X tube preamp basic inspection and updates

Here I look through an original and factory wired Dyna PAS3X and do the very basics to inspect its voltages and update the heater voltage wiring.

[@ekbanjosworld4926]

I too have a PAS 3. I especially like your keeping it original approach. No BS ! If you'd wanted a Frankenamp, you'd have built it ! Kudos

[@jothanankrogh]

I have bad news :( It's suddenly picked up a bad channel imbalance, and I have just swapped in new balance and volume pots as I found no other imbalances in resistors and caps and tubes, it's lost a few original parts in fixing this.

[@janetyer7147]

There was a significant upgrade to the Dynaco PAS 3X that used to be available from Audio By Van Alstine (AVA) called the Super-PAS Three preamplifier. AVA used to offer that upgrade either installed by AVA or as a kit which you could install for yourself. I looked at AVA's website prior to posting this comment, and they no longer mention it there, but perhaps they might have something in inventory if you contact them. It would be worthwhile if available.

[@julesl6910]

Have two of these PAS preamps, they are glorious

[@jothanankrogh]

Hello Jules, what do you use them with? In a surprising twist, I recently ran this preamp into a Mcintosh mc2105 and the result was not nice at all. I need to keep my PAS + Mk3s dynacos setup intact.

[@julesl6910]

Dyanco ST-70 series II and an old SCA-35. Obviously it was designed for this use and sounds great. Might not be a good pairing for a McIntosh amp... I will add that in the regular preamp stage (right pcb) I'm running 60's nos Tung Sol 12ax7 's and in the other unit the original dynaco west germany tubes. Might want to experiment with putting in gold lions or something to pair better with the McIntosh@@jothanankrogh

[@julesl6910]

Look up your amp's input impedance, it may be a bad match for the PAS@@jothanankrogh

[@jothanankrogh]

@julesl6910 the mcintosh mc2105 has a stated input impedence of 200k. I am not sure how much of what I hear is an impedance mismatch or sensitivity mismatch or excess gain? But together they do sound nasty.

[@julesl6910]

Yep that's about twice the standard input impedance of most amps, it will require a mcintosh preamp or something like that lol. I envy your "problem" and hope to have something like it some day.@@jothanankrogh

[@jordanch68]

The selenium rectifier has internal resistance greater than modern diodes have. Usually replacement involves diodes and a resistor to make up the difference. You could re-design your board to include that and incorporate the fuse as well.

[@tomsherwood4650]

I had a PAS 3 I got cheap in the 80s. I did upgrades as I could. First I got a repro face plate somewhere because of the labels worn off. Then I had a source of Dale metal film resistors and PolyP capacitors. So I restuffed the circuit boards with those new components. Then I tried rolling tubes. The Telefunkens sounded OK in it but original Mullards had a better sounding midrange to me. Did not try domestic or Russian tubes or such.

[@jothanankrogh]

I have kept the telefunkens in place. I have a small stash of classic old 12ax7s, but sadly the Mullards among them are a bit noisy and microphonic.

[@taralewis9916]

Hi Jonathan, I have Pas 3 and with a HK Citation I Ithat I have had since the mid 80's. Both was completed rebuilt by a Micheal Fraser in Los Angeles. His design is based off Herbert Keroes from the 50's. In the Pas 3 he has added two 12au7's, not sure why. In the phono section there are two small vertical boards added, maybe for the RIAA. The tone controls are removed. He needed the area for two 450V, 100 MF's each, two zener are added on top of the selenium rectifier. A choke has been added. Numerous capacitors are stacked on the phono and output borads. A couple friends brought their PAS3 over for comparison and the Fraser kill them. The Citation II is completed changed. No more 6 volt TV tubes in it

[@jothanankrogh]

Hello Tara, sorry for the late reply. I would GUESS that the added 12au7s are for cathode follower stages for the line output, maybe even for the phono too. They probably did something with the power supply to address the extra current needed also. Very interesting to see how complex or how simple mods of this platform emerged.

[@taralewis9916]

@@jothanankroghAre there out here in the Los Angeles CA area that still work on tube gear.

[@robmoffatt4997]

I feel your pain! I have a couple Heathkit SP2 dual channel preamps in cue....

[@holgers5216]

that fuse holder, as far as I can see, is rated for automotive use, ie 12VDC, and so I would not use it for mains 120VAC..irrespective of the fuse used...just my 2cents worth!

[@NikitaNicholas]

Excellent! I was just thinking about all these! whether to get a couple of Mark III, ST70 etc .

[@ClearOutSamskaras]

Did you do a video on the E.A.R. clone? Edit: Never mind, I see it now. :)

[@johnbravo7542]

All tube amplifiers I've built have no tone controls.

[@jothanankrogh]

I generally don't use them but some LP pressings seem off and I would enjoy with a little correction

[@francoisroberge5882]

Want your Mark IIIs to sound real good? Get a better preamp (e.g. ARC SP16, Rogue 99 etc. for around $1500 in Canada). You will be amazed. The Dyna is fine but has it's limits no matter what you mod it with. I hooked up a Rogue RP-5 ($3500 US) to a DYNA ST-70 and WOW. Goes to show you how good these simple Dyna power amps can sound when properly fed with a higher quality preamp.

[@jothanankrogh]

Advice appreciated! But now I am eyeing those chinese clone preamps of those models on ebay

[@wa4aos]

I ASSUME you know that a leaky electrolytic does NOT just mean electrolyte physically leaking from its pins or end. You should measure every section for value and ESR, equivalent series resistance. Often is the case, aging electrolytic caps start exhibiting internal resistance and start acting more like a resistor than a cap. I generally toss electrolytics older than 30 years unless it's a Nichicon, Rubicon or other high quality caps. Those I still check and if the value and ESR are good, maybe, allow more like 40 years. Bottom Line, electrolytic caps don't last indefinitely. While on the bench and under the hood is a good time to do this work. AND keep in mind, there are a lot of fake caps with brand names like mentioned. DigiKey and Mouser are a good source for quality parts.

[@jothanankrogh]

Yes all electrolytics were replaced, I just wasn't sure at first if they were the original parts.

[@garagepunkfan]

I would convert those Special and Tape Head inputs to line level

[@jothanankrogh]

Yes, I have thought of doing two inputs as phono and the rest as lines, but the truth is there is so much crosstalk between selections I have not bothered and do source switching before the PAS

[@ekbanjosworld4926]

Yeah, good choice of music ! These dumbass mods on KZbin aren't intelligent enough to recognize the final movement of Beethoven's 9th Symphony 😂