Dynaco pas 3x for use with a st-35

blankenberge

blankenberge

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Dynaco pas 3x for use with an st-35

I have a stock pas 3x that I want to upgrade because it has a river rushing sound in the speakers and is kind of gutless sounding. I will be using it with an st-35 that is in fine shape and needs no mods. I do not want to spend a ton of money on all kinds of extras but just want the pas to sound good! It sounds real bad right now so anything will be an improvement. Does anyone know where to get quality caps to replace the old black cats? What else should i do besides that to make this thing soud good without turning it ito a completely different amp and spending more than neccessary?
 
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A re-cap and power supply rebuild will be a good first step. Check/replace any resistors that are out of spec, or just do them all. Noisy carbon comps tend to have a windy sort of noise. Caps are your choice but plain old orange drops or Soviet PIO caps do fine. You might also want to try different tubes. I have a pair of 12AX7's that sound like a day at the beach if you put them in a preamp. These do use a selenium rectifier in the heater power supply. If that is weak and producing low heater voltage it might sound more gutless than it should.
 
I have tried 2 different sets of tubes. Still windy. I am considering a tubes4hifi prebuilt power supply board. Any experience with them? They seem easy to install which is perfect because I have a guy who is gonna give some pointers but this will be my first foray into installing anything in an electronic device.
 
I used a classicvalves.ca power supply regulator board in mine. None of the original stuff was there and I didn't have the tube either. The board gave me all that I was missing and regulated output voltage to boot. Its kind of a DIY thing though. You buy the board from them and assemble your parts from wherever you prefer, then solder the whole thing together. One nice thing about it, its fully SS, so it removes the load of the rectifier from the marginal power transformer. Mine runs quite cool now, and they're not really known for doing that.
 
In tube equipment it is usually not tubes or capacitors that will cause white noise (wind) sounds. A lot of times it is a bad resistor in the power supply. In my experience, it is usually a power resistor getting ready to let go or (technically speaking) open.
 
Low filament voltage will cause odd tube noise and "a...kind of gutless sound." Very common in the Dynaco PAS, developing over the years as the selenium rectifier in the filament supply goes high-resistance. You may be operating those 12 volt filaments on eight, seven volts or less. You could bridge across the selenium rectifier with silicon diodes but the capacitors are very aged, too; a proper power supply rebuild kit, as others have described, will eliminate all those problems.
 
IMHO, the PAS sounds significantly better with silicon diodes in the heater supply.

Even when the selenium rectifier is working correctly, you're only getting 11v of heater voltage. OK for new 12AX7s, and even many used ones- but it can lead to "cathode starvation" on some.

A silicon diode will raise the voltage up to right at 12.25v per tube. That's within "nominal" spec for the 12AX7 (12.6v +- 10%). Any decent tube will work fine under those conditions.

Regards,
Gordon.
 
crispycircuit said:
Make sure your power amp is high impedance. These will not drive a low impedance amp (gutless)....
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True, in stock form.

This is why I tend to do a total rebuild (mod) of the line board, to convert it from dual-gain stage, to single-gain-stage plus output buffer (and minus the tone controls, too). This reduces the output impedance from something like 10000 ohms to about 500 ohms... it'll drive almost anything after that...

Regards,
Gordon.
 
I would take it in steps.

Step 1 is get the power supply right. Either approach mentioned here would work well for you. Your objectives should include replace the selenium rectifiers with solid state ones, replace the tube rectifier with solid state ones and replace all the power supply caps (low and high voltage) with new, modern ones. Brand of parts chosen is not critical here as long as the parts meet or exceed the original specs (voltage rating and size [i.e. current capacity for diodes, wattage for resistors, microfarads for capacitors]) and are from a reputable source. The kits mentioned fit that description.

Step 2 is to then listen to the now more reliable unit to see if it is still "gutless" and still noisy. If so, you then want to address the coupling caps in the circuit. Here, some folks say they can hear differences between various caps of the same size and voltage, I can't. Since I can't I would use and quality film cap that meets the microfarad value and meets or exceeds the voltage requirement. Orange Drops do well, I've used Panasonic films from Mouser cause they are inexpensive (I always like that feature a lot!). Any will do as long as they again come from a reputable source like the big electronics supply houses, Digikey, Allied, Newark, Mouser, MCMElectronics. I've ordered from all of those and they are all reputable.

Step 3 is to listen again. Still noisy, you now have your work cut out for you. There will be one or more parts in the circuit that are noisy and or out of spec and you will have to hunt that down and replace.


That's enough work for the moment.

Good Luck

Shelly_D
 
Thank you folks. I ordered a power supply today and will learn to install that this weekend. As for caps, I will probably order those this weekend to do either way. Has anyone merely replaced power supply and caps in one of these and been pleased with the sound by just fixing what immediately needed fixing?
 
blankenberge said:
Thank you folks. I ordered a power supply today and will learn to install that this weekend. As for caps, I will probably order those this weekend to do either way. Has anyone merely replaced power supply and caps in one of these and been pleased with the sound by just fixing what immediately needed fixing?
Click to expand...

Yes.
 
Don't overlook the volume control, they can be very noisy, much like you describe. That said, PAS preamps are real giant killers when rebuilt, I did a lot of them in the late 70's, when rebuilt they are quiet with a lot of gain and a lot of dynamic range. If my memory serves me correctly, the tubes that came in them were Dynaco branded Telefunkens, and with a filament supply of only 11 volts, most of tubes will test as almost new.
 
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