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 - any advice appreciated.
Russell
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 - any advice appreciated.
Russell
Attachments
The LM317 circuit schematic (not hand-written but shown as "Tumbnails") seems in OK to me and even with some intelligence in the design.
R3 will generate a 12mA current that will bias the zener-diodes (replacing resistors) and leave a loss of a good watt in each zener-diode (they should be 5W types. D4/Q1 serves, as you already know, as voltage clamp before the LM317 input, controlled by the LM317 output, preventing the LM317 to exceed 37V input-output.
Your questions:
1) I find no 350V or 250V capacitors after the bridge rectifier. In the hand-written schematic, I find such capacitor ratings but that circuit is non-sense.
2) I find no 250V capacitor. In the hand-written schematic, I find such capacitor rating but that circuit is non-sense.
3) The resistor (R3; 100Ohm) needs to be there to generate a reference current. The reference current may be sent through a resistance to ground or through another circuit (like zeners) that will generate a voltage to control the LM317 reference pin. If you mean the proposal in the hand-written schematic, it is a flawed use of an LM317.
4) I know virtually nothing about vacuum tube amplifiers.
5) True, the voltage will only be as stable as the voltage across the zeners.
6) The JungHV circuit seems fine. This is what I appreciate here.
R3 will generate a 12mA current that will bias the zener-diodes (replacing resistors) and leave a loss of a good watt in each zener-diode (they should be 5W types. D4/Q1 serves, as you already know, as voltage clamp before the LM317 input, controlled by the LM317 output, preventing the LM317 to exceed 37V input-output.
Your questions:
1) I find no 350V or 250V capacitors after the bridge rectifier. In the hand-written schematic, I find such capacitor ratings but that circuit is non-sense.
2) I find no 250V capacitor. In the hand-written schematic, I find such capacitor rating but that circuit is non-sense.
3) The resistor (R3; 100Ohm) needs to be there to generate a reference current. The reference current may be sent through a resistance to ground or through another circuit (like zeners) that will generate a voltage to control the LM317 reference pin. If you mean the proposal in the hand-written schematic, it is a flawed use of an LM317.
4) I know virtually nothing about vacuum tube amplifiers.
5) True, the voltage will only be as stable as the voltage across the zeners.
6) The JungHV circuit seems fine. This is what I appreciate here.
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Hi Russell,
Yes, for a start I looked only at the "Thumbnails" schematics. Then I realized that at least some of your comments and questions related to another circuit. I then found the hand-written schematic.
In short, the hand-written circuit has many flaws and I would replace it with the JungHV. If you prefer, you can modify the hand-written circuit to something that may work, but it will be quite some work. Perhaps as much work as doing it right from the start.
Yes, for a start I looked only at the "Thumbnails" schematics. Then I realized that at least some of your comments and questions related to another circuit. I then found the hand-written schematic.
In short, the hand-written circuit has many flaws and I would replace it with the JungHV. If you prefer, you can modify the hand-written circuit to something that may work, but it will be quite some work. Perhaps as much work as doing it right from the start.
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