I recently acquired a Dynaco PAS 3 that was built as a kit. I brought it up slowly on a variac but none of the 12AX7's lit up, only the 12X4. I tested all of the tubes on my B&K tube tester and they all tested strong with no shorts. I also pulled the 2000 mfd capacitors out of circuit, one tested good, the other not so good. After doing some googling, it seems as though the selenium rectifier should also be replaced.
While I am in there, I'd like to replace the 4-section can capacitor which is 30/20/20/10 mfd @ 450V. The only recommended replacement I can find is 20/20/20/20 @ 475V from dynakitparts.com. Is this a good option?
I want to keep the preamp as designed, with no mods other than replacing the selenium rectifier with the 1N4006 diodes as recommended by Curcio Audio. I will also rebuild it from the ground up since the wiring and soldering of this kit was poorly done.
Thanks in advance.
While I am in there, I'd like to replace the 4-section can capacitor which is 30/20/20/10 mfd @ 450V. The only recommended replacement I can find is 20/20/20/20 @ 475V from dynakitparts.com. Is this a good option?
I want to keep the preamp as designed, with no mods other than replacing the selenium rectifier with the 1N4006 diodes as recommended by Curcio Audio. I will also rebuild it from the ground up since the wiring and soldering of this kit was poorly done.
Thanks in advance.
That quad cap is fine. Make sure the height will fit inside without touching the lid.
It won't hurt to also increase the filament capacitors to 3300uF or 4700uF.
The line output electrolytic coupling capacitors (1uF 50V) could be refreshed also,
or with a film type instead.
It won't hurt to also increase the filament capacitors to 3300uF or 4700uF.
The line output electrolytic coupling capacitors (1uF 50V) could be refreshed also,
or with a film type instead.
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Thanks. Yes, the height is correct. Dynakitparts has a can cap with the same values as the original but is much taller.
That one may be for the ST-70, etc. I'd replace the dropping resistors on the quad cap
with new ones, metal film works well here.
I have no electronics training, but I'm curious why it is often recommended to increase capacitor values yet the 4-section can capacitor replacement has a 20 mfd value rather than the original value of 30 mfd ?
The 20uF will work ok, with some extra ripple, but most will be filtered out.
If you could get a 30/20/20/20 quad to fit, that would be better, but you go with what you can get.
At this point in time the options are somewhat limited. The 12X4 can take up to 40uF.
I've used 10,000uF caps for the filament circuit with good results, but then use 3A diodes instead.
This has the benefit of raising the DC voltage to more like 12.2VDC, which the tubes like more,
especially the RIAA stage.
If you could get a 30/20/20/20 quad to fit, that would be better, but you go with what you can get.
At this point in time the options are somewhat limited. The 12X4 can take up to 40uF.
I've used 10,000uF caps for the filament circuit with good results, but then use 3A diodes instead.
This has the benefit of raising the DC voltage to more like 12.2VDC, which the tubes like more,
especially the RIAA stage.
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I'm considering using individual capacitors with the correct values to replicate the original can cap, just need to see if they fit. Someone suggested mounting the transformer on the outside of the chassis, that may be an option.
If you're looking at that, there are pcbs available for the PAS to use standard through hole caps.
There are several, here are some examples. Or you could use perf board and easily make your own.
SDS Labs Capacitor Board for Dynaco PAS, with parts & instructions
Dyna PAS-3 Power Supply
Antique Electronic Supply has a 40/20/20/20 mfd @ 525V can cap that fits, would this be better than the Dynakitparts 20/20/20/20 mfd @ 475V ?
The original is 30/20/20/10 mfd @ 450V
The original is 30/20/20/10 mfd @ 450V
Any voltage 450V and up is fine, but the 525V caps are mainly for power amps.
Are you sure that 525V one will fit inside the PAS? I'd be surprised if it did.
From the photo it appears their listing of 2.5" height is wrong in proportion to 1.375" diameter.
Either version should work electrically fine, and appear to be new production.
Are you sure that 525V one will fit inside the PAS? I'd be surprised if it did.
From the photo it appears their listing of 2.5" height is wrong in proportion to 1.375" diameter.
Either version should work electrically fine, and appear to be new production.
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Good catch, yes it does appear to be taller, more like the ST 70 can cap.
The can cap I removed from the PAS 3 is 2-7/16" tall and 1-3/8" dia. at the base and looks more squat than the one in the photo.
The 4 ECC83 tube filaments are powered by a se-rectifier acting as voltage
doubler. If the rectifier or any of the 2 caps surrounding it you won't have
filament.
You also stated that the can cap might be replaced. All together it makes
me recommend a "power supply board" that will fix all of the above.
There is several sources , see rayma'a post above.
A 20uf 450v capacitor isn’t so big - couldn’t that be fitted in parallel to one of the can cap sections under the chassis?
Thanks for all of the replies so far.
On page 11 of the Dynaco PAS 3 assembly manual there is a voltage chart, if the capacitor values of the can cap and or filament supply caps are altered, do the values in the voltage chart still apply?
On page 11 of the Dynaco PAS 3 assembly manual there is a voltage chart, if the capacitor values of the can cap and or filament supply caps are altered, do the values in the voltage chart still apply?
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The HV readings shouldn't change as long as the resistors are the same values.
You may need to burn in the capacitor in circuit for a few hours, to reform it and reduce leakage.
The filament voltages will go up somewhat, because of the silicon diodes and larger capacitance,
but the positive and negative filament readings should be about the same, around 12.0VDC each.
You may need to burn in the capacitor in circuit for a few hours, to reform it and reduce leakage.
The filament voltages will go up somewhat, because of the silicon diodes and larger capacitance,
but the positive and negative filament readings should be about the same, around 12.0VDC each.
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Thanks. The PAS has the original Dynaco labeled (West Germany) 12AX7's, I'll probably get some JJ's to test the pre-amp for a few weeks after the restoration.