Modified Power supply for Dynaco PAS3 - question

Russell Sher

New Member
Hi all - A friend gave me a PAS3 preamp to sort out. It is a modified PAS 3 based on the Article "The last PAS3" By David Vorhis (The Audio Amateur - 4/1982) - but with some changes by a previous builder.
One thing that interests me is the PSU -
The original article HV PSU is attached as PSU2.pdf below.
The actual PSU used is based on the schematic PSU.pdf - file attached below, which was traced out and drawn by someone and I have a couple of questions/comments:

Regarding the HV section
1. If I measure the HV out of the power transformer I get 300 Vac, so after rectifying and smoothing, the DC voltage is going to be about 424V. The 100uf 350V cap and 2.2uf 250V caps at the bridge rectifier out seem woefully too small a voltage rating.

2. The 250V cap at the regulated output also seems too small - Dynaco suggest 285 VDC is the supply required.

3. The configuration of the LM317 seems unusual - there are no voltage setting resistors (ignore that squigggle below the LM317 - it means not connected/cross-out). I can't see what the intention is here. I don’t see any app note suggesting this configuration. My theory is that perhaps this should have been one of the fixed 3 terminal regulators, lifted by the zeners to give the HV output. (?) Checking on the board confirms no other resistors.
4. Question – Is this not an overkill having both separate filament supplies as well as separate HV supplies?
5. Question – isn’t the concept of high voltage zeners a bad idea since these higher voltage types aren’t particularly stable. The regulation will only be as good as the reference provided above ground by the zeners.
5. The zeners were blown as was the pass transistor and the LM317 seems shorted too (on both boards)
6. So final question -- Should I rather build the supplies? -it seems like a better idea. Perhaps using the JungHV image schematic attached.
(Apparently this amp did work but had a long period of neglection afterwards - and maybe it was hacked since then.)
Thanks all
 

Attachments

  • PSU.pdf
    285 KB · Views: 77
  • PSU2.pdf
    656.5 KB · Views: 59
  • JungHV.jpg
    JungHV.jpg
    31.4 KB · Views: 75
Hello Russ, welcome to AK. This preamp is a real mess. I have a PAS-3x and I don't see why all of this is necessary. The preamp draws so little current that the power supply just needs to be well filtered. I did modify mine to use the SDS power supply board from Triode Electronics. One caveat here though, if you retain the 12X4 rectifier, you should change the first filter cap from the 47 uF to a 10 uF as the rectifier can only handle that much capacitance directly. I fudged it a bit by using a 22uF, which is working fine.
 
Maybe the 317 is being used as a soft start or a ripple reducer, but I've never seen it done that way.
 
If I remember correctly, this article was illustrated with a photo of the unit with two enormous capacitors outside. I believe he said the sound of the unit improved with the heater supply capacitance, "apparently without limit." Since you've got two windings, I guess he figured the cross-talk between channels could be slightly reduced. I also remember Frank Van Alstine's PAS modification article saying that the filament supply in the original design could be hampered by the pilot lamp bulb, which could be replaced by an LED. I have used other gear with SDS Labs circuits and recommend them enthusiastically.
 
If you wish to use the original power transformer, then I recommend rebuilding the B+ regulator following Jung's schematic closely. Walt Jung is one of the finest analog circuit designers to practice the art. Note that Jung specifies an LM317HV, which allows greater input voltage than the standard version of this part. Also note the correction data in TAA issue 1/1983 (page 54) changing R33 to 39K.

HV Zener diodes aren't terrible with respect to stability, but you can get some improvement by selecting lower voltage types and stacking more of them.

David Vorhis acknowledges that higher B+ is likely to improve PAS performance, and outboard PSUs are probably the best way to get it. I designed a (mostly) tube outboard regulated high-voltage that was published here on AK somewhere. Let me know if you want to see the schematic.
 
from what I remember the can cap for the PAS2/3 were rated a lot higher than 350v, I remember
measuring well past 450v on initial startup surge. I would then adjust the dropping resistors
to bring the elevated voltages (due to line AC being a lot higher than what it was in the 60's)
back to the schematic listed numbers.
 
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