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Old 26th November 2004, 06:29 AM   #1
lpd is offline lpd  Canada
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Default New Pas Question no photos

I have taken voltage readings on my Pas and have different readings for tubes on linestage. Can anyone help me here.

Tube 1 Tube 2 on PC-5

Pin1 149/148
Pin2 0/0
Pin3 1.5/1.5
Pin4 9.9/9.9
Pin5 1.5/20???
Pin6 198/200
Pin7 0/0
Pin8 1.5/1.3
Pin9 4.2/2.4

Manuals readings are

Pin1 175
Pin2 0
Pin3 1.45
Pin4 -1
Pin5 -/=11
Pin6 200
Pin7 0
Pin8 1.25
Pin9 5.5

This is a modded unit, and was not done by me so I'm trying to decipher the mods and am mostly concerned about pins 4 and 5 as they are really out of spec. The board has been simplified with only 3 capacitors and 7 resistors per channel. Very purist but without a design hard to decipher. Any help is appreciated.
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Old 26th November 2004, 07:16 AM   #2
PRR is offline PRR  United States
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> am mostly concerned about pins 4 and 5 as they are really out of spec.

Maybe some folks here have 12AX7 pinout memorized, but I don't, and not noting the pin functions forced me to look it up to even have a clue what you are telling us.

And once I saw what 4 and 5 are, it was clear.

These are the heaters. 4 and 5 should nominally be 12.6V apart. In this very low-current low-noise application, it was common to run tubes with lower heater power: Dyna used 11V, wired as +11 and -11. Someone has re/de-wired so both heaters are positive (or floating?). Nominally it should run 25.2V to 0V (12.6V per heater), this one is running with about 10V per heater. That's low, but it will work. It might upset the plate voltage, except plate voltage should rise with less heater and it looks like yours dropped a bit.

Is that antique selenium rectifier still in the heater supply? As long as you have your hands in there, replace it with silicon, replace the funky old electrolytic, and add a little series resistance to get 11-12V across each heater. Nothing really wrong with these pioneering old parts, until they fail, which happens a lot.

Personally I'd siliconize the B+ rectifier too. Higher B+ makes cleaner sound. And less heat. But this is very much a matter of taste. Some folks can't dig music unless the power comes from glass rectification. And low-volting a 12AX7 does have a certain flavor.

> without a design hard to decipher.

PAS schematics and board layouts are in the manual, or available on the web. While I dimly recall how a PAS was wired, only you can know how yours has been (radically) re-wired. Xerox a schematic, then look at your unit and see what parts are omitted, what parts have been changed from Dyna specs, and pencil it up.

Nothing looks "wrong" to me. Triode circuits are pretty tolerant of variations. The low voltage on pins 1 may be a different value plate resistor, or just a different supply voltage (which you did not report).

Another thing I don't recall you mentioning: does it work? does it sound OK?

If it sounds OK, it probably is OK, or as OK as a Dyna tube-PAS ever gets. It was wicked good for its time and price, but isn't the pinnacle of audio perfection. Losing the tone-network helped some PAS models (getting gain and EQ in the same 2-triode stage was a stretch), and that seems to have been done to yours. The 2-triode phono stage is as good as it gets, but is far from perfection. I have a tube-PAS around, but I use my transistor-PAS because funky as it is, it is less colorful and I'm not into arty enhancement.
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Old 26th November 2004, 03:12 PM   #3
lpd is offline lpd  Canada
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This Pas I picked up has diodes swapped in for selenium rectifier, seperate transformer for heaters, chokes, no tone controls or other switches, stepped attenuator for volume, and power supply is outboard. I'm more cncerned about why pin 5 on tube #2 as the heater voltages for 4 add up to 12 volts on the other tube (1.5/9.9 ok close though) but on the 2nd tube it is 9.9 and 20.
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Old 26th November 2004, 06:31 PM   #4
PRR is offline PRR  United States
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Pin1 149/148
Pin2 0/0
Pin3 1.5/1.5
Pin4 9.9/9.9
Pin5 1.5/20???
Pin6 198/200
Pin7 0/0
Pin8 1.5/1.3
Pin9 4.2/2.4


> I'm more cncerned about why pin 5 on tube #2 as the heater voltages for 4 add up to 12 volts on the other tube (1.5/9.9 ok close though) but on the 2nd tube it is 9.9 and 20.

This is basic electricity, and you seem confused.

You don't add, you take the difference.

Tube 2 is showing 20V-9.9V= 10.1V. In a high-power stage, or some directly-heated cathodes, or under very high plate voltage, that would be wrong; in the tame PAS it won't hurt anything and will run fine. You can play with 9V to 13V and see if it changes the sound; in a PAS probably not.

On Tube 1: ***uming you read +1.5 and +9.9 (and that +1.5V is right: I suspect you mis-read it), then 9.9-1.5= 8.4V across the "12.6V" heater. This is a little like feeding 120V household lamps with 80V: they won't make full brightness, will be pretty dim, but will "work". However it is very odd that the two tubes in series don't show the same voltage drop. And the 1.5V where I would suspect zero is odd. But I can't guess what is really happening from here, since I can't see it, smell it, or put a voltmeter on it.

Does it work? If those heater voltages were really wrong, it would not work at all well. The other voltages look fine, within component tolerances, with possible replaced parts.

Wait a second... your pin 9 voltages don't look at all right. They should be midway between the pin 4 and pin 5 voltages on each tube. Assuming the T1P9 voltage is really 4.2V, then the T1P5 voltage makes sense as -1.5V. 9.9v-4.2V=5.7V, 4.2V-(-1.5V)=5.7V. BUt then we'd expect T2P9 to be somewhere around +15V, and you claim 2.4V. Is the amp wrong, or is it the measurement?
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Old 26th November 2004, 07:49 PM   #5
lpd is offline lpd  Canada
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T2P9 is 15V actually. What I don't understand is why both tubes are different in heater currents. The pre-amp sounds great!!! I was just concerned the difference in the two tube voltages would be a problem.
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Old 27th November 2004, 07:06 AM   #6
lpd is offline lpd  Canada
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Ok so I have remeasured voltages across pins 4 and 5 (filament) and I've got an even 12.5 volts.
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