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I recent had one of these in my system for a month. The tubes were marked "Dynaco" 12AX7.The only special thing I heard was more distortion. Compared to my other preamp (Panar/Dynaco PAS4) it was a little noisier (not by much) and less refined, but there was no large dollop of creamy, classic tube sound, harmonic richness, dimensionality or any of the virtues extolled by the collectors of classic tubes gear. Note that the Panor is fairly neutral, closer to Sonic Frontiers without the sterile highs.
Any comments? Something I should have done? Remarks welcomed.
Follow Ups:
If ya wanna try a stock vintage pre,on the cheap,with a tube rectifier,get an Eico HF85.Rebuild it!,and it`ll wipe out the Dyna.Totally.It`s not marginal.Good Luck scrooie looie
Hi -I'd use a 12AU7 with one half for gain, direct-coupled to the other half as a cathode follower - no loop negative feedback - the 12AU7 sounded nice in Bruce Moore's preamps - to 'properly' do this it would pull about 6mA - think that's too much for the PAS transformer (?) anyhow :
6mA version: (B+ 260-300 vdc)
Rp (plate load)=47K, Rk1 (cathode resistor on 1st stage)=2.2K
Rk2 (cathode resistor on follower)=47K
Low Current version (about 3mA total)
Rp =100K, Rk1= 4.7K, Rk2 = 100K
You can use a piece of perfboard or perhpas a PAS blank line board if any are available. Cheap Resistors MO 2w in the 6mA version will suffice -1watt in the low current version. Output caps could be modest polypropylene - maybe paper/oil - might depend on 12AU7Use a voltage divider to float the filaments about 70-90 vdc above ground - you may have to cut a trace on the phono board - I haven't done this for many years
You could carefully convert the line board but sometimes the foil would lift.
Freddy
ps - I think this sounds 'better' in most cases then the feedback pair -
my 2 cents
Also try replacing all of the cathode and plate resistors in the preamp stage with Bulk Foils (Vishay-$$$, Caddock-not too pricy)
I never liked the sound of the Dynaco pre. If you really want to try a vintage tube pre, look elsewhere. I use a Scott 355 tuner/pre. Not quite as good as the 130, but not too bad. Nice phono stage. Tubes make a difference - I went to Mullards in the phono stage to warm up a cartridge, now I've changed carts and it's too warm - back to Tele's.
Tell us how you are using it - What amp is it driving, especially tell us what the input impedance is. If I recall, this one ( and yes I own one) does not have a CF output, so you need a 500k-1M input impedance on your amp. I like the phono stage on the pas3, which I think sounds best to me with 60's Amperex Holland shortplates.
You need to make sure the selenium in the heater supply is not gone south, otherwise your heaters are running at 5 or 6 volts instead of 11 (they ran lower than spec on purpose). This might spoil the sound. I would also replace all the coupling caps, or at least check for leakage after well warmed up. Eico HF85, one on EBAY currently, is also well regarded vintage BTW. I use a VTV 6SN7, which you have to build yourself.
BF
The version of the preamp was the PAS-3A, which had been designed by Dynaco to work with transistor amps. At the time, I used a pair of Antique Sounds 108 mono blocks and a Sunfire. The only difference was more hum with the Antiques.
if it was stock, then you have unbiased ears. that's pretty much how it can sound.
This is one bland & subliminally ugly sounding preamp. It doesn't shriek or particularly flatten dynamics, and it is rather clean, neutral & distortion-free. If mated to too low an impedance, say below 100K ohms, the bass gets up & walks away, along with output gain. Distortion only occurs if you are WAY below that value.
I have over a dozen of the darn things, and I'm keeping one or two stock for posterity sake.
I gut mine & turn them into other things. In deference to the poster who says he likes the phono stage, I agree that it isn't all that bad.
It's the line stage that really sucks an egg...
Probably took me at least ten years to figure out what I really thought about it myself. Congrats on figuring it out right away!
Exactly. The unit I had was unmodified.
I've replaced every capacitor & resistor in them in the past, whilst keeping the circuitry stock.
Guess what?
After all that work, it did sound cleaner with noticeably better frequency extremes & more detail.
It also sounded even more bland & hands-off ugly...
Then I looked into mods I could do whilst keeping the original tube configuration. In twenty years, I've come up donuts.
I'm still fiddling with them, I own about 15(!) of the darn things.
I like the way they're built & how easy they are to play with/service.
I've designed an entirely new & different line stage onto the PCB, and I'm working on a way to convert the phono stage into something that lives up to its (considerable) potential. Looks for the moment like the end result is going to have 6 tubes, no tone controls, no balance or blend controls & an external power supply with a proper filament supply.
Why do I even bother? Definitely masochism.
Definitely.
Isn't audio fun?
Take a poke over at the Bottlehead forum. The Foreplay linestage with active current sources on a 12au7 with CF output would be a good bet for a retrofit. Using a 12AX7 with all that gain in a linestage makes no sense to me. Everyone runs these PAS# at like 8 or 9 o'clock - you barely get it off the stop and it's too LOUD. Seems like the 12au7 Foreplay circuit could easily fit in a pas3 and use existing heater /power supplies. If that was your goal.
As I said earlier, you could do much worse for 100. than this phono stage. Not an ultimate, but a damn good 100 buck stage. The trick is NOT using the existing line stage, but using a better outboard one. Since this defies logic for most, almost nobody ever listens to just the phono stage.
BF
Funny thing, but I'm in SPOOKILY TOTAL agreement with you!!!
My line stage is a mu-follower using a 12AU7, and some would describe such a circuit as having "an active current source" (although you usually put the current source in the cathode of the amplifying tube circuit, whereas in a Mu-Follower you have a cathode-follower for a plate load).
Yep, my luck with the phono stage is this:
It sounds good at a lower B+, thus robbing it of enough gain to properly drive its RIAA network below 100c/s. I end up with this fantastic sound, but a -3db point of 50c/s. OK for apartment dwelling (which is when I ran it, so it was fine), not so good if you've got a full-range system & you like subterranean bass. Which I must say I kind of do!
Another problem is that the 12AX7 has a very high anode impedance & poor slewing capabilities, and an active RIAA network loads the poor final stage down to death at higher frequencies. Remember that feedback continues to increase ad infinitum in an active network like the one in the PAS, so that the feedback resistance basically drops to zero as you charge on up into the ultrasonic frequency range!
Putting a short circuit load on a 12AX7, even at very high frequencies where there isn't much signal, is a recipe for trouble.
And sure enough, I thought this phono stage sounded far less strained & capable once I inserted a passive network in between the first & second stages & ditched the feedback EQ/RIAA network.
As I've said in other posts, I LIKE feedback. I'm an active & vociferous proponent of its use. But if it doesn't give the result you want, I'm the first guy to throw it in the bin.
So funny thing, my amplifiers tend to use huge amounts of feedback (never less than 30db, and my reference uses about 60db!), whilst my preamps almost always have NONE.
The phono stage in the PAS-3 is a paradox: You can hear the benefits of active RIAA, and it's a great sounding phono stage that readily responds to parts upgrades & power supply improvements. But you can also hear & measure the problems of using a great sounding tube like the 12AX7 to run in such a configuration, and you realize that the tube is unsuitable, despite the fab mids & highs. You're limited to two stages because of phase shift considerations, so you can't just go & use a lower mu tube with better slewing capability & tighten up the feedback. I tested the idea of slewing ability on this design by experimenting with the overall impedance of the network (lowered it to try to increase the LF feedback to get my sub-100c/s response back to flat) with rather disasterous results. I ended up concluding that the original EQ impedance was the optimal comprimise...
An unsolvable Rubik's cube!
Joe-
Allow me to pester you with something completely different. Every time I solder, I end up with bad oxydation of the wire. I heat some speaker cable (18 to 12 guage) until it's hot enought to melt the solder (with rosen, high quality). Within a month, the speaker wire is ugly green at the soldering joint. The oxidation extends about an inch or two.What, oh what, to do?
Thanks in advance,
NT
Rosen is always high quality.
But I think you meant "rosin", as in flux.
So that begs the question, are you using a separate, paste flux?
Are you SURE you didn't stumble across a roll of daddy's acid core solder that he used for plumbing repairs? Never use non-flux core solder for electrical work!!
Yeesh! Green is copper oxide, and I can't see any rosin core solder doing that to it, unless you normally run your speaker cable ends in salt water!
I'd try a fresh roll of known electrical solder, insist on Kester brand. Otherwise, what are you using for a soldering iron?
Something veddy strange is going on here...
Are there any particular modifications that you recommend?
Wayne
I too have folled with pas preamps over the years. best mod I have encountered, is the Van Alstein super pas kit. Have owned two, still do and I like them fine. It also has a phono stage and the kit is reasonable in price
The basic design is ok and is sensitive to the tubes used.I would replace all of the caps and resistors to start. Then upgrade the volume and balance pots. I would pay attention to the length of the wireing- channel to channel. I would also remove the filter, loudness and tone control circuits.
Yes a lot of work, but you will end up with a fine sounding pre-amp.
As I said the basic design is good, it is let down by execution and secondary parts.
Best,
Ross
Mine is still a work in progress, altought the PAS is not my
primary preamp not even my secondary preamp. My pirmary is
is a Scott 130, second is a modified version of a Sansui 500A.
But my PAS 3 has Gold RCA's for Audio outs back to Solid Silver
wire to the main board, Oil caps for Outputs, and Angela Tin Films
for other coupling caps.
What I wanted to do was brighten the sound a bit. It's still not
as crisp as my Scott 130 nor does it has the drive of my Sansui,
but it's starting to sound pretty good.
Next move is to replace some of the resistors with upgraded parts.
Have fun!
Willie
hi wayne, this is a tough one because i can't think of one particular one that makes a tremendous difference. people usually end up changing MANY things by the time they start getting happy, and some end up gutting it to start from ground zero. i would start with changing ALL the caps to better ones first, and i see no reason not to mix up the brands involved- for instance, using axon f&f's in the first stage and paper in oil for the next. just salt and spice to taste...
Thanks for the advice!
Wayne
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