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while still running in all its original glory , i will either sell them as original or maybe upgrade them and use them for another 10 yrs as i still think the st35 is the best el84 power amp and i have had them all
its sounds superb with the eico hf85 preamp which i plan to upgrade as well with new parts
this is my kitchen system so i only use it at weekends when i cook
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Not for the preservationists, but VTA also has ramped up their PAS mod kits, and now offers package deal for phono, line, and PS upgrades, or as separates. I like Roy's designs generally, and have them in the ST70. The linestage mod uses 4 12AU7s in an Aikido design. I implemented this recently and like the results. Also quite affordable.
I know many folks say 'scrap the PAS', but I got mine and an ST70 as a package deal, and wanted to keep the ensemble. Plus, I just like keeping the vintage look while still wanting to see how far I could push toward improved sonics.
Cheers,
K.
after 35 yrs of horns and triodes ,i am still learning
thanks
Years ago I did the Koren mods for the PAS. Have been more than happy. You can read about it here.....http://www.normankoren.com/Audio/Spice_preamp.html
Tones of dynamics. Can handle long interconnect runs. Only gripe is a little too much gain in the pre-amp stage. However, since most of my listening is with vinyl it works out great for me. When I do switch over to DAC or CD I have to remember to turn it down significantly. I know that some don't like cathode followers but I can't fault it.
Here you can see someone elses build with boards that can be bought from classicvalve.ca
http://www.diyaudioblog.com/2011/04/starting-norman-koren-dynaco-pas-preamp.html
Edits: 07/26/15
Check out the EFB modification for your ST-35. It only costs a few dollars in parts, and the measured as well as the subjective performance of the amp will improve. Not to mention that your output tubes will last a lot longer. Jim McShane didn't want to sell me a quad of Gold Lion EL84s for use in my SCA-35 until he found out I was doing the EFB mod. Then he was happy to, because the tubes see 27 mA of current after EFB, as opposed to 35 mA prior.
As for the PAS...I did the David Vorhis "Last PAS" mod on one of mine many years ago, and am still using it. At the very least, I'd follow the suggestions given by a few other posters, and upgrade the filament supply with schottky diodes and cram in as many µF as you can fit.
My first mod years ago was to a stock PAS, and I was quite disappointed. I added a couple of 1N4007s and two 7500 µF caps to the filament supply, and it sounded great. The reason I was disappointed was because it actually sounded better than my Audio Research SP3-A-1 which had, at one time been considered by some to be the best preamp made. I couldn't believe that a "lowly" Dyna PAS with a few dollars in parts could surpass it.
"You won't come back from Fletcher-Munson curve"-Jan and Dean
I did the full Curcio PAS upgrade / rebuild. It was quite good. I mean REAL nice sounding. However it did have one nasty problem. The power supply board would blow out the voltage regulator and take out a my very expensive and rare NIB 12AX7 tubes. You do know that now you can't get those tubes - GONE! This would happen about every three to four months. You could mark it on the calendar.
The pas can be upgraded into a very nice preamp. Search and ask and it will happen.
This is from memory but the 35 is a sweet little amp. Do it up nicely.
charles
...but the ST-35 with cathode bias is a sweet sounder. Some of that sweetness might be lost when converting to fixed bias. Converting to dual mono power supplies, requiring a larger chassis with two power trannies, plus independent bias for each channel could be a ticket toward sonic bliss...
The PAS has so much potential. First challenge is replacing that awful volume control. The tone controls, other than the PAS-3X versions, also leave something to be desired. As 1973shovel mentioned, an upgraded PAS can beat an upgraded ARC SP-3A. The simplicity of the four tube circuit in the PAS, plus no cathode followers in the PAS make the resto job well worthwhile. A sensibly upgraded PAS will trounce many "high end" preamps of any vintage...
If the OP can find the early 1980s Audio Dimensions lit, including the Trevor Lees cascode phono stage circuit, that one deserves mention; but it had no line stage. With some resourceful mods, including a more robust power supply, the PAS can fit a simple line stage.
One of the cool features about the PAS is that Blend Control; great with vintage discs. Unfortunately, there is no Stereo Reverse option built in. If the OP leaves out the tone controls, there are some interesting faceplates to be had for the PAS...
So I should mention that the PAS I referenced also had the tone controls disabled, so it wasn't just a filament supply upgrade which took it beyond my stock ARC SP3-A-1.Regarding the bias supply for the ST-35, you could be right regarding fixed vs. cathode biasing. I do a lot of searching of the Asylum archives, and there was a post on that very subject. I'll start by saying that I can't remember the member's moniker, so take this with the proverbial grain of salt. I will say that he had his own web site of tube history, schematics, etc. and seemed to be well thought of in the tube community. Gary somebody, maybe?
He posted that he converted a ST-35 from cathode to fixed bias, four individual pots, one for each tube. He stated that the sound got worse than with the cathode bias, but he wasn't sure why. Several years ago I read the David Gillespie article on EFB, which seemed to explain why the sound likely got worse after going to fixed bias. I sent a link to his email address at his web site, but he never responded.
As I have mentioned, my experience with the SCA-35 (same output stage and transformers as the ST-35) was positive after installing the EFB. A nice sonic upgrade for next to nothing, and increasing output tube life is a bonus.
Edited to insert omitted word.
"You won't come back from Fletcher-Munson curve"-Jan and Dean
Edits: 07/27/15
I have owned three PAS Preamps, all were worked on by me and all sounded superb after some basic work.
The ST35 I owned was used for year all stock but the boards were charred and cracked and developed noises and hum.
On the PAS3 I try to stay close to the original design, respecting the heritage and original designer's intent.
Restoring Vintage amps in my mind is akin to restoring an ancient painting. I strive for maximum originality, but with the goal to extract the maximum performance from the original design as in better quality parts that measure correctly.
PAS2, 3/3X.
Replace the plate resistors on all the tubes with high quality Tantalum, Carbon or in a pinch Metal Oxide. I have a crush on Riken, Kiwame and Ohmite Little Devils
Replace the stock coupling and tone caps with quality Jupiter Red's or something that will keep a similar tone. The Jupiter yellows work well in a pinch. I find that these Black Catties measure all over the place and can be a source of buzz and a somewhat smeared midrange. I try to keep the one's that are on the Tone Controls and Loudness for last. These are the .02uF/400v, .002 uF/400v and 0.1 uF/200V
If you can spend the extra, install a pair of 0,22uF TFTP Vcaps on the 0.22uF output cap, you will not regret it. Try to use something small, not alot of room there. Maybe try to keep the originals if they are not noisy, sometimes they work just fine, other times they will just not give you the pop you may want.
On the PAS3X install a PIO 1uF/100V quality cap in place of the Electrolytics that block the DC from the special pots. Or a good quality 1uF Solen, Axon, etc. nothing fancy, but keep it small.
In the Power Supply keep it stock, put a fresh FP cap and replace the selenium rectifier with Fast Recovery low noise rectifiers. Also replace the voltage dropping resistors and make sure they measure well. Any good quality set of resistors will do.
Use fresh Electrolytic caps on the 12V supply. The SDS labs board is a great way to upgrade the power supply and keep close to original.
Lots of great good power supply upgrades out there for the Dyna. I have not tried the Regulated offerings so cannot comment.
Install a fuse.
Replace the 1500pf and 33pf with quality Silver Mica or if you have the green Vishay caps.
This will make the PAS3 sing. Try to keep the 12x4 rectifier, but if you prefer solid state, you will need to adjust the values of the voltage dropping resistors due to the deleveraging of the secondary loading on the stock power transformer, the boost will change the character of the sound.
ST35
Replace the stock electrolytic. Install the quad ST35 Adjustable Cathode Bias Board and install a Dyna Choke in place of the series dropping power supply resistor.
I prefer the heavy duty Russian 7189 equivalent power tubes, that way you have a wider biasing range.
Regular EL84/6BQ5 will need to be biased conservatively so as not to exceed the plate dissipation.
The stock Z565 outputs ROCK......
I replace the stock ST35 PC boards with good quality boards (lots to choose from) and stuff with Prime Cut Top Sirloin parts. Keep the old boards in case you are ever accused of amp molestation.
This has been my recipe for a great musical Dyna Dinner.
The bitterness of poor quality remains long after the sweetness of low price is forgotten
I had the 35 MANY years ago. Until my brother in law stole it! Miss that little amp.
The PAS is also a fine little preamp if upgraded and restored correctly. Just don't go TOOOOO crazy upgrading.
charles
So much of a stock PAS preamp sucks . :> ( It can be made into something good, but the AVA upgrades, which are just fine, can easily be regarded as not being cost effective.
Eli D.
I restored a couple of PAS-3's and was pleasantly surprised at the outcome. Sound was no slouch and better than what I had to led to expect. Of course all the caps had to changed and the voltage doubler filament supply had to be redone ( I get 24,1 volts now with theuse of UF diodes)
Of course YMMV and FWIW
FWIW, I focus on the tone controls and the MISERABLE drive capability of 12AX7s. IMO/IME stock needs a lot of work.
Undoubtedly, potential lies inside the box. I had a PAS rebuilt by AVA decades ago and it now resides in my son's rig. That rebuilt "puppy" is good. I now have better, but ...
Eli D.
Agree Eli..
There are lots of designs from the classic days that suck, I agree
Yet they remain statements from another age.
Take the A700 Harman Kardon Award Series amp...
It is just impossible to make the crappy stock Output Transformers sound good, no matter what is done to the amp. The music just seems to stop cold.
Yet the A300's and A500's have wonderful Freed's, I don't think the A700 used Freed Outputs, the part number does not start with an FT.
I have repaired several A700's and the best alternative I found was to replace the Outputs with Hammond 1620's or even 1650F if one can squeeze them in.
It makes a huge improvement in the sound.
Dyna's are great for transforming them into better sounding gear.
A properly restored PAS2/PAS3 (except the Phono Stage that needs to be corrected) can sound pretty good.
As a starter Preamplifier up to the inherent birth defects and limitations a great beginner Preamp for 2 channel enjoyment.
But Dyna PAS have an annoying bleed over of music between Inputs.
One can hear music playing across them making it a poor AV Centerpiece where often there are several concurrent active line levels.
This can be fixed with a new selector switch and quality wiring.. but to me I would rather build something new.
There are so many cool transformations one can make on Dyna gear it is a fantastic opportunity for people to learn and enjoy.
The bitterness of poor quality remains long after the sweetness of low price is forgotten
Site also has other articles about EFB.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Maybe sell the PAS, as is. The ST-35 is arguably the best of the Dyna amps, courtesy of the Z565 O/P "iron", and is a "keeper".
A ST-35 change that should be all but automatic is installing a RC bias network in each channel, instead of a single network shared by all 4 O/P tubes. Matched quads of O/P tubes are a PITA, in more than 1 way.
Eli D.
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