Dynaco PAS

trem · « **on:** 26 Jul 2009, 03:13 am »

I have a Dynaco ST-70 that is being "refurbished". Seems like it will be a decent power amp. But what about the PAS preamp that were made? I have read a lot of unfavorable reviews concerning their overall quality. But I already have one sitting around. Do you think it is worth putting a lot of effort into? Would I be better of buying a better/newer preamp?
Thank You

ltr317 · « **Reply #1 on:** 26 Jul 2009, 03:18 am »

It's a cheap platform to mod the unit for better sonics. I have a heavily modded unit that was done by someone who knew Dynacos well, and the sonics now compared favorably with much more expensive preamps.

trem · « **Reply #2 on:** 26 Jul 2009, 03:44 am »

Quote from: ltr317 on 26 Jul 2009, 03:18 am

It's a cheap platform to mod the unit for better sonics. I have a heavily modded unit that was done by someone who knew Dynacos well, and the sonics now compared favorably with much more expensive preamps.

Thanks for the reply. If you have any gut shots......
I really do not know how good the PCB's are in my preamp. The selector switch looks like a nightmare. To tell the truth, I only need to have an input for a CD and a turntable. Do not need Tape, FM, or anything else. If any body has any schematics, gut shots, or suggestions, I am all ears/eyes.
Thanks Again

ltr317 · « **Reply #3 on:** 26 Jul 2009, 04:08 am »

I don't have any gut shots, but I could take some in the next few days, though it's been so heavily modified that it bears little resemblance to your stock model. One of the mods was to install a bypass from the other inputs for a cd direct. Another mod was to install dual volume pots and eliminate the tone and balance controls. I believe the phono stage has not been changed.

xecluded · « **Reply #4 on:** 26 Jul 2009, 07:16 am »

I have two Dynaco PAS preamps that has Van Alstine mod done to it (i didnt do the mod.) They really sound nice.

trem · « **Reply #5 on:** 26 Jul 2009, 08:00 am »

Quote from: xecluded on 26 Jul 2009, 07:16 am

I have two Dynaco PAS preamps that has Van Alstine mod done to it (i didnt do the mod.) They really sound nice.

Thank You -
I will do a search here and on the web, but is there a schematic by any chance? I do some work with guitar amps, but Hi-Fi is all kind of new to me.

mightym · « **Reply #6 on:** 26 Jul 2009, 12:01 pm »

I too have an un-molested Dynaco pre-amp. Since I can't bear to get rid of anything....

See if some of these don't help( there's a schematic in there somewhere ):
home.indy.net/~gregdunn/dynaco/schneider.html,
home.netcarrier.com/~rstevens/pas.html,
www.dynakitparts.com/store/.

The Dynakitparts link has original Manual downloads available on the lower left sidebar, I believe there's a schematic there too. I'm sure a search for Dynaco will reveal a lot of other stuff( like a link for Van Alstine's site which I no longer seem to have ). Since I'm not going to be using mine for a main system, I'm just going to repair what ails it, and put it in the bedroom.

I'll say this, " you can spend more re-building an old Dynaco, than you would on a new tube pre-amp". So choose carefully.

Hope this helps.

John

trem · « **Reply #7 on:** 26 Jul 2009, 03:37 pm »

Quote from: mightym on 26 Jul 2009, 12:01 pm

I too have an un-molested Dynaco pre-amp. Since I can't bear to get rid of anything....

See if some of these don't help( there's a schematic in there somewhere ):
home.indy.net/~gregdunn/dynaco/schneider.html,
home.netcarrier.com/~rstevens/pas.html,
www.dynakitparts.com/store/.

The Dynakitparts link has original Manual downloads available on the lower left sidebar, I believe there's a schematic there too. I'm sure a search for Dynaco will reveal a lot of other stuff( like a link for Van Alstine's site which I no longer seem to have ). Since I'm not going to be using mine for a main system, I'm just going to repair what ails it, and put it in the bedroom.

I'll say this, " you can spend more re-building an old Dynaco, than you would on a new tube pre-amp". So choose carefully.

Hope this helps.

John

Sage advice indeed. Guess I need to back up a little bit, I have probably been a little misleading. I have schematics for the ST-70 and PAS. What I am hoping to find are schematics of preamps that will work for a phono and one for a CD that I can build inside the PAS chassis.
I have a "basic" knowledge of electronics, and I can build, drill holes, solder, lay out wire, etc. But I am not any kind of hobbyist engineer. I could never think up my own Phono/CD preamp. I am capable of building from a schematic however.
And like "mightym" mentioned, you can easily and quickly, spend more money on a PAS than it is worth. So I guess that is my real question, is there a print/schematic for building a decent preamp inside of a PAS chassis?
Thank You

boycephoto · « **Reply #8 on:** 26 Jul 2009, 04:18 pm »

Hey Trem,

The Audio by VanAlsitne Super PAS 3 is a great upgrade for your stock PAS. I built mine a few years ago and it's a real winner especially for the price, even if you have Van Alstine do the upgrade for you. I use it with a Van Alstine Ultimate 70 (Dynaco Stereo 70 upgrade) and a pair of vintage Rogers LS3/5's. A Super PAS 3 will easily preform in the $2000+ catagory of preamps.

I would be glad to answer any questions I can.

Dave

Wayne1 · « **Reply #9 on:** 26 Jul 2009, 04:39 pm »

Curcio Audio (Dynaco Doctor) has most of anything you will need to repair or mod the PAS

Curcio Audio PAS

Another vendor for boards for mods is tubes 4 HiFi

The Dynaco Tube Audio Forum might also be helpful.

You also might want to search through the back issues of Audio Amateur and Glass Audio. There have been many articles published about PAS mods. AudioXpress back issues

trem · « **Reply #10 on:** 26 Jul 2009, 06:11 pm »

OK, Great!
You guys have given me a lot of info/ideas. This should keep me busy for awhile. Don't mean to keep bumping this up. But I just wanted to say Thank You one more time.
I have a Scott LK-48-B that I am working on, and need some advice with that also. Will start a new thread for the Scott.
Thanks Again
I Really Appreciate It

JoshK · « **Reply #11 on:** 26 Jul 2009, 06:35 pm »

I've seen a lot of posts on diyaudio's tube forum about this preamp. I haven't been interested in the topics so I haven't read them, but its worth checking out and doing some searches over there, lots of knowledgeable folks there.

I know one of the biggest problems with these units were the tubes they shipped with. Apparently that accounts for a lot of the unreliability. I've also seen many folks happy with just a few mods.

avahifi · « **Reply #12 on:** 26 Jul 2009, 11:14 pm »

The first thing you should look at in an old original Dyna PAS is the heater supply. This consists of the big square selenium rectifier stacked on top of the two electrolytic capacitors held in place by a bracket next to the power transformer.

This antique type rectifier becomes resistive as it ages and the heater voltage (that should be in the 12.6V DC range) drops off to 10V or less. This turns off the tubes, drastically reducing their open loop gain, and provides wall to wall musical mud.

The quick and inexpensive fix is to replace the rectifier assembly with two modern 1N4006 or similar diodes, and replace the electrolytics with a couple of 10,000 uF 16V axial lead electrolytics or similar.

The most common reason for having a channel dropout is a dirty Tape/Input slide switch. Cycle this back and forth many times and hit all of the switches and controls and jacks with Deoxit D-5. If necessary, you can buy a replacement switch at Radio Shack for a couple of bucks.

Replacement power transformers are available from Triode Electronics for about $40 if necessary.

That old preamp can be turned into something nice with a little effort and expense, and even better if you are up to trying one of the several do it yourself rebuild kits available out there.

Regards,

Frank Van Alstine

Nels Ferre · « **Reply #13 on:** 27 Jul 2009, 12:12 am »

Frank was a bit modest. His Super PAS 3 is the rebuild kit to get. If I had a nice looking Dyna like yours, that's what I would do.

The kit is easy and fun to build. The instructions are concise and make success pretty much a certainty. Frank will even check it for you when you are finished. And the sound? Lovely- far better than the original Dyna circuit.

trem · « **Reply #14 on:** 27 Jul 2009, 02:41 am »

Once again, thanks for all of the replies/help. My wife and I made good money for 25 years. The last 3 years have been a disaster. I got hurt real bad, have had lots of operations, and she has not been able to buy a job since January 2006. A lot of people are in our same situation, or worse. As they say, you don't know what you've got 'till its gone. I was accustom to having a lot of play money. Now I do not have much "extra" cash and it is real frustrating. I am probably like a lot of guys; I would rather eat peanut butter sandwiches and have plenty of money for my expensive hobbies. Oh Well!!!!!
I have a lot of thinking to do regarding this PAS. Thanks for all of the great help!!!

slbender · « **Reply #15 on:** 15 Aug 2009, 10:25 pm »

While it is perhaps a good idea to get rid of the selenium rectifier, since these can emit poisonous particles into the air if they fail, I'm literally quite shocked and surprised that Mr. Frank Van Alstine is so completely unknowledgeable of the Dynaco PAS Parts, Voltages, and Circuitry... Those Selenium Rectifiers in the filament circuits of all versions of the PAS, were always known to be highly resistive - by design, even when they were brand new, as that is part of the circuitry to protect the power transformer from high-surge over currents, especially at turn-on. Use of power selenium saved the cost of adding in a large wattage power resistor or two.

The Heater Voltage in the PAS should be at exactly 11.0 DC Volts not anywhere near or above 12.6 Volts by design - which allowed the circuitry a distortion null point using the original Telefunken smooth plate tubes - as supplied by Dynaco; as well as insuring in excess of (projected) 50 years of tube life. I have highly used original PAS Telefunken smooth plate tubes that still measure at 100% (+/-2%) 40+ years later, and probably will continue to be highly usable and measure close to "new" well beyond my lifetime.

Throwing in some silicon with a pair of 10,000 uF capacitors instead of the original 2,000 uF caps is a sure way to cause pre-mature failure of the (now) 40 year old power transformer sooner, rather than later. This is foolish, and a bad second bit of advice. Now if one adds some thermistors and power resistors into that circuit, to emulate the selenium parts used in the original circuit, then silicon can be used, along with higher values of capacitance without peak current surges overheating and overloading the power transformer.

Another cap across the output leg of the high voltage cap, since there are already multiple resistors present would be a reasonable thing to do. Say a 22 or 33 uF cap rated at 350 Volts of low ESR if one worries about ripple due to aging of the triple cap in the high voltage power supply.

I've never heard a PAS producing wall to wall mud but spending a minimal few tens of dollars replacing the pairs of coupling caps and the output caps with Polypropylene's bypassed with either Polystyrene or Teflon caps is more likely to do the sound some really good improvement, as some older caps may be leaky after a few decades, however, the old Black Cat caps often stand the test of time fairly well, and replacement of coupling caps can be minimal to still maintain something of the old "Tube Sound" without starting to be shrill or modern sounding like a transistor laden preamp.

For sure, the Tape Monitor switch is all important, and should be replaced with a new one, that is a very common failure point..

Lastly, maybe an IEC if all other equipment in the system is IEC grounded might be useful, but then grounding issues come to the fore, well perhaps that is best left alone unless some potential chassis to chassis issues can be measured (yes a powerful pun!).

-Steven L. Bender, Designer of Vintage Audio Equipment

Quote from: avahifi on 26 Jul 2009, 11:14 pm

The first thing you should look at in an old original Dyna PAS is the heater supply. This consists of the big square selenium rectifier stacked on top of the two electrolytic capacitors held in place by a bracket next to the power transformer.

This antique type rectifier becomes resistive as it ages and the heater voltage (that should be in the 12.6V DC range) drops off to 10V or less. This turns off the tubes, drastically reducing their open loop gain, and provides wall to wall musical mud.

The quick and inexpensive fix is to replace the rectifier assembly with two modern 1N4006 or similar diodes, and replace the electrolytics with a couple of 10,000 uF 16V axial lead electrolytics or similar.

The most common reason for having a channel dropout is a dirty Tape/Input slide switch. Cycle this back and forth many times and hit all of the switches and controls and jacks with Deoxit D-5. If necessary, you can buy a replacement switch at Radio Shack for a couple of bucks.

Replacement power transformers are available from Triode Electronics for about $40 if necessary.

That old preamp can be turned into something nice with a little effort and expense, and even better if you are up to trying one of the several do it yourself rebuild kits available out there.

Regards,

Frank Van Alstine

hdspeakerman · « **Reply #16 on:** 15 Aug 2009, 11:05 pm »

Bill Thomas who often posts on this forum customized my PAS and I am very happy with it. He is very knowledgeable and great to work with. He also has a supply of original Dynaco parts. I give him my highest recommendation. If you want to do the work yourself he may still be able to offer advice.
Howard

boycephoto · « **Reply #17 on:** 16 Aug 2009, 02:39 am »

I know exactly what Frank Van Alstine is talking about, I have heard a PAS that produced musical mud and lots of it. It got soooo deep it got on my boots. Maybe it was the same PAS that Frank had.

dB Cooper · « **Reply #18 on:** 16 Aug 2009, 12:49 pm »

What a classless flame job on the part of Mr. Bender. I wonder how many of AVA's PAS power transformers have failed over the 20+ years he has been producing them. They should be dropping like flies if Mr. Bender is correct. Regardless, there is a way to disagree respectfully, and a way to disagree disrespectfully. Mr. Bender chose the latter.

And why does he look so much like Borat???

boycephoto · « **Reply #19 on:** 16 Aug 2009, 02:24 pm »

I looked up the definition of the word "vintage" on dictionary.com and here is some of the results:

vin⋅tage /ˈvɪntɪdʒ/ Show Spelled Pronunciation [vin-tij] Show IPA noun, adjective, verb, -taged, -tag⋅ing.
Use Vintage in a Sentence
1. the class of a dated object with reference to era of production or use.
2. representing the high quality of a past time.
3. old-fashioned or obsolete.
4. Old or outmoded.

Isn't it an oxymoron that mr. benders uses the word "vintage" as part of his signature?

Enough said.