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Dynaco PAS-2 or PAS-3 update

tcdriver

tcdriver

AK Subscriber
Subscriber
Dynaco is famous for making some of the best sounding tube equipment of the classic tube era. The PAS preamps were among the best if not the best sounding preamplifiers one could buy up until the time they went out of production.

There are two circuit differences between the earlier PAS units and the PAS-3X. The two differences are:

1) The "X" version had tone control potentiometers that were constructed to be electrically removed from the circuit when set to their flat position.

2) A 1uF electrolytic capacitor was placed in each channel’s tone control circuit to block the DC from reaching the audio output.

Whereas the owners of an earlier PAS preamp (PAS-2 and PAS-3) may not be able to find the specialized potentiometers that were used in the "X" version they can and should add the 1uF capacitor to eliminate the DC Voltage in the output of the preamp. This is especially important if the amplifier that receives the signal from the preamp has a direct coupled input such as the Dynaco ST-70. The DC Voltage from the preamp will upset the bias point of the input stage of the amplifier and nothing good will be the result.

The original PAS-3X used a 1uF, 25V electrolytic capacitor, but, I would suggest using a polypropylene or polyester capacitor with the same voltage rating or higher for the update. These capacitors may be found at any good electronics supply dealer.

To add the capacitors:

Replace the wires connected between eyelet terminals #5 and #12 on the line-amplifier circuit board and the lower terminals of the bass controls with the 1uF capacitors. That is, remove one wire and replace it with one capacitor. Next, remove the remaining wire and replace it with the other capacitor. That is all there is to it.

tcdriver
 
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Mine came like that, is there any special marking on the 3X? Been thinking about pulling the lytics and trying poly or oil.
 
You need to try a nicely working EICO HF85 some time you may forget your first sentence. This would be in the same entry level cost on the used market as a Pas preamp. There were many really great preamps back in the day. The Scott 130 and LC21 will walk all over a Pas 3 in every respect for instance IMHO. But these preamps are much more complicated to refurbish and command some serious $ for entry. The PAS were the best bang for the buck preamp of the day. The HF85's only downfall is it can be a noisy beast and takes some talent to tame in this respect. The biggest flaw it has is the 12AX7 heaters use AC even on the phono section.

Great advice on installing the output coupling caps try upping ther value to .22uf it will tighten the bass response ever so slightly.

Craig
 
Hi, here's my vintage preamp two cents.

I've got some PAS preamps, and have had fun with them. Also have an HF-85 that sounds good. Both sound flat and small compared with a pair Lafayette KT-300s (each is half a KT-600). I never compared directly with a Citation 1, but was surprised at the large effortless sound of the Cit 1 -- but look at the size of the power supply on it! I sold my Ampex 350s, but if you have the space, these are the ones for a big but clean sound.

I've got a Fisher tube tuner-preamp (is this rare? haven't seen others) that I remember as sounding good, though not particularly clean... I let go of an MX-110 without doing much comparison, but it was an impressive sounding unit.

For newer stuff, a Lazarus Cascode has a deserved cult following for good bang-for-buck, but a head-to-head with a deHavilland Ultraverve showed how much detail was missing!

Low-mu octal-based triodes: a natural wholesome goodness you can't get out of puny 9-pins... or so it seems to me. So I'll be selling most of the preamps mentioned above. Maybe AK Fest would be a good place to let folks look them over.
 
I personally have a preference for those little 9 pins. I think the octal preamps just don't give the balance and I have owned a few and listened to them all the way up to the $4500 range. I think it is strictly a perception thing. There octal they look cooler so they have to be better.

Craig
 
NOSValves said:
I personally have a preference for those little 9 pins. I think the octal preamps just don't give the balance and I have owned a few and listened to them all the way up to the $4500 range. I think it is strictly a perception thing.
Click to expand...
Matt Tenma used his access to SOTA Hewlett-Packard test equipment to show that the lowly 6SN7 has lower distortion than any commonly-used 9-pin. I think his article was published in Sound Practices way back when.

On older gear especially, I like the reassuring contact of those big pins on octal tubes too. Who hasn't had enough of cruddy contacts between those little pins and their sockets?

Size matters.
 
I would have to do the distortion test myself before I would just swallow that as the gospel. Besides its the end product that counts not what the tube can do under controlled test procedures.

I have very little problems with tube contacts on small signal tubes its a power tubes that give you trouble whether there 9 pin or octal.

Craig
 
Great Vintage Tube Preamps

NOSValves said:
You need to try a nicely working EICO HF85 some time you may forget your first sentence. This would be in the same entry level cost on the used market as a Pas preamp. There were many really great preamps back in the day. The Scott 130 and LC21 will walk all over a Pas 3 in every respect for instance IMHO. But these preamps are much more complicated to refurbish and command some serious $ for entry. The PAS were the best bang for the buck preamp of the day. The HF85's only downfall is it can be a noisy beast and takes some talent to tame in this respect. The biggest flaw it has is the 12AX7 heaters use AC even on the phono section.

Great advice on installing the output coupling caps try upping ther value to .22uf it will tighten the bass response ever so slightly.

Craig
Click to expand...

I have had an EICO HF85, and still have a Scott 130. I respectfully disagree that they are better than the Dynaco PAS preamps. The AC on the EICO HF85's heaters makes it more difficult to get a quiet, hum free signal and its ugly. The Scott, though more attractive, does not have the clarity and resolution of the PAS-3X IMHO.

tcdriver
 
PAS-3X markings

Thatch_Ear said:
Mine came like that, is there any special marking on the 3X? Been thinking about pulling the lytics and trying poly or oil.
Click to expand...

There are no special makings on the PAS-3X to indicate its "X" status. There is an easy way to tell if you have an "X" version without lifting the cover. The special tone controls have a more limited rotation than the earlier, more standard tone controls. The non - "X" controls rotate about 270 degrees from stop to stop. The "X" controls rotate about 210 degrees from stop to stop.

tcdriver
 
NOSValves said:
I have very little problems with tube contacts on small signal tubes its a power tubes that give you trouble whether there 9 pin or octal.
Click to expand...
Really? That surprises me. I can't tell you how many times I've had some problems with noise or cut-out or whatever on some old piece of gear, when the culprit turns out to be a small signal tube that needs to be wiggled about to make good contact. In my experience it's rarer with output tubes, and then only with 9 pin minis like 6BQ5.

On the flip side, I have had more problems with microphonics with octal tubes than with 9-pin minis.
 
tcdriver said:
I have had an EICO HF85, and still have a Scott 130. I respectfully disagree that they are better than the Dynaco PAS preamps. The AC on the EICO HF85's heaters makes it more difficult to get a quiet, hum free signal and its ugly. The Scott, though more attractive, does not have the clarity and resolution of the PAS-3X IMHO.

tcdriver
Click to expand...


When dealing with the Scott 130 you have to factor in that it is not some basic design like the Pas 3 or even the HF85 and it requires a serious rebuild to get it operating as intended. The pas 3 can be rebuilt by almost anyone with minimal skills as it should be since it was simple and very cheap kit to begin with just like the HF85. I said the HF85 has a noise issues but only on the phono section AC on the heaters on the other tubes are of no real issue if you can't get the line stage quiet you have some other problem.

Craig
 
tubino said:
Really? That surprises me. I can't tell you how many times I've had some problems with noise or cut-out or whatever on some old piece of gear, when the culprit turns out to be a small signal tube that needs to be wiggled about to make good contact. In my experience it's rarer with output tubes, and then only with 9 pin minis like 6BQ5.

On the flip side, I have had more problems with microphonics with octal tubes than with 9-pin minis.
Click to expand...


well let me explain something first. Before I ever fired up any piece of gear the very first thing I do is clean all the controls, retension/clean all sockets whether 9 pin or octal and test the tubes/clean the tube pins. I mean after 40+ years its hardly fair to expect this stuff to just work.

Craig
 
Tcdriver,

I don't think I need the special pot as I always prefer "flat". It looks like I can handle this mod. Thanks very much for a clear explanation the differences between pas2 and pas3x. I thought about buying a pas3x but ended up with the pas2 because of the $125 BIN.
 
Rebuild

NOSValves said:
When dealing with the Scott 130 you have to factor in that it is not some basic design like the Pas 3 or even the HF85 and it requires a serious rebuild to get it operating as intended.
Click to expand...

The Scott is a different design than the PAS preamps. I have rebuilt my Scott 130. I respectfully disagree with your assessment that it is a better sounding unit. I can imagine that depending on the other components used in a music system that one may prefer the Scott over the Dynaco. Our preferences are just that.

NOSValves said:
I said the HF85 has a noise issues but only on the phono section AC on the heaters on the other tubes are of no real issue if you can't get the line stage quiet you have some other problem.
Click to expand...

I agree.

tcdriver
 
Well I traded some art glass I got in a garage sale for mine (just checked the controls and I don't think I am getting 270 out of them, and the NP lytics look old as dirt, original Tele 12AX7s, wooden cabinet) and am using it in the biamp system to master the sub woofers' amp. I plan on running the phono thru there and out the Tape to my SET since the voltage should be high enough for the 50K pot.
So far I like it fine. I will see how much I like it after I get the phono hooked up. I do have a Heath AA-11 that I will put up against it after it has been gone thru. The filament transformer gets hot as a firecracker. Is that normal?

6SN7 is supposedly linear compared to a 12AU7. Course you can't hear linearity any better than all the distortion that shows up on test gear when measuring a good SET. Numbers are only guidelines anyway. You can't hear them.
 
1uF or 0.22uF

NOSValves said:
Great advice on installing the output coupling caps try upping ther value to .22uf it will tighten the bass response ever so slightly.

Craig
Click to expand...
Dear Craig (NOSValves),

I know this is an older thread, however, your comment has caused some confusion in a later thread and I would like you to clarify your recommendation.

The original advice was to use a 1uF capacitor. Here you are suggesting "upping ther value to .22uf". I am confused, the 1uF capacitor is larger than the 0.22uF capacitor. How is it that one uses a smaller capacitor to up the value?

Sincerely yours,

tc
 
Hey!!!,
Since you're only blocking a low voltage, there is no reason to use very small caps. 2 or more will be fine. I'd put a reasonable limit of ~10 uF on that component. I've gotten good results from 7.5 uF motor-run oilers in my own designs...:)

A mike or so of 50V polystyrene would not be possible with most linestage topologies, due to voltage that's usually being blocked. The polystyrene is a very nice dielectric...
cheers,
Douglas
 
tcdriver said:
Dear Craig (NOSValves),

I know this is an older thread, however, your comment has caused some confusion in a later thread and I would like you to clarify your recommendation.

The original advice was to use a 1uF capacitor. Here you are suggesting "upping ther value to .22uf". I am confused, the 1uF capacitor is larger than the 0.22uF capacitor. How is it that one uses a smaller capacitor to up the value?

Sincerely yours,

tc
Click to expand...

The answer is fairly simple..........I had a brain fart. For some reason my brain was thinking the factory cap size was .1uF not 1uF sorry.

Craig
 
NOSValves said:
I personally have a preference for those little 9 pins. I think the octal preamps just don't give the balance and I have owned a few and listened to them all the way up to the $4500 range. I think it is strictly a perception thing. There octal they look cooler so they have to be better.

Craig
Click to expand...
Really, I have always wanted to try an octal preamp figuring it would be better, but have only used 9 pin types. Thats what counterpoint and conrad johnson always used, prehaps that is the reason! I have some cool looking 56 tubes ( sort of like tungsol 6550) I have often wanted to try in a line stage, basically just because of their appearance. I would have to dig them out to be sure, but I think they are 5 pin or something funky like that.

Russellc
 
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