Hello. I just got my first tube system set up. The left channel seems to be playing at a lower volume during phono playback. I have to turn the gain down pretty significantly on the right channel of my ST-70 to compensate. In turn I have to increase the volume on my PAS-2 to about the 1o’clock posish to get a medium listening level which I think is creating a little more fuzz than I’d like. I switched the input cables going to the power amp and the problem switched sides so I think I’ve narrowed it down to the PAS2. Also, when using the “special” input the left channel is distorted. It sounds like someone’s singing into a wet bag. So I hooked my iPhone up to radio input and the right and left channel seem balanced. Any thoughts as to what could be going on? Pls halp.
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On a side note, should I be able to tell if the 12ax7 tubes are lit up in the PAS2? The 12x4 is definitely lit up and one of the 12ax7’s. The only reason I can tell that one 12ax7 is lit is because the mirrored finish on the top off the tube is off center. Not sure what I’m supposed to be looking at and if the tubes could be a potential issue.
The way they are wired, if one is lit, both are lit on the board. They run in series. You'd have no sound at all if one was bad. Try swapping the tubes to see if the problem switches sides. If it doesn't move, its not the tubes.
"Special" is a low level input with no EQ. Basically its for a microphone. You can modify it so it works as a phono input or a line level but thats not how it is wired originally.
"Special" is a low level input with no EQ. Basically its for a microphone. You can modify it so it works as a phono input or a line level but thats not how it is wired originally.
If the channels are unequal it's most likely some resistor that has drifted in value. ( yes it could be
one half of a 12ax7 but that is rather uncommon).
Get the schematics . Start with measuring the voltages mentioned in the manual. If they are within
+-15% you are probably ok.
Continue with ( after power off and cooling down) measuring all the resistors on the phono board. I
don't think you need to unsolder anyone just make sure the power cap has discharged completly.
Resistors that differs in value more then 10% should be replaced.
one half of a 12ax7 but that is rather uncommon).
Get the schematics . Start with measuring the voltages mentioned in the manual. If they are within
+-15% you are probably ok.
Continue with ( after power off and cooling down) measuring all the resistors on the phono board. I
don't think you need to unsolder anyone just make sure the power cap has discharged completly.
Resistors that differs in value more then 10% should be replaced.
Thanks. Switched the tubes on the phono board. Seems like the problem switched from the left side to the right side. Do you happen to have a recommendation for replacement 12ax7’s and should I get them matched?
JJ E83CC is a new framegrid tube with low noise and decent price. Similar to telefunken ECC803S and the
famous tesla.
I second this, and yes you should pay a little extra for matching, IMO.
just before we get too far along here, is this an all original condition PAS ? If so, you might want to rebuild the heater supply at least before replacing the tubes. If the heater voltage is considerably too low, the tubes will not work well. Slightly marginal tubes are more affected by low voltage.
That's why a voltage check is imperative. 11 Volt pin 4 to 5 is suggested in the manual, which is at the
lower side of the tube spec, but it is what the PAS is built for.
Some says that somewhat cooler cathods reduces noise and keeps the tubes for much longer.
The E83CC works fine at this voltage.
I’m pretty sure it’s all original. The guy said he had it serviced about a year ago and that it’s in perfect condition and recommended not doing any updates. I will check voltages here soon tho just to be sure. I just have to do a little more research first because I have no idea what I’m doing.
So I’m having a little bit of a problem finding two 12ax7 tubes on both the pc5 and pc6 board that sound good. Some sound better than others with less fuzz and are more clear. I think the guy I bought it from gave me some tired tubes. I was going to try to move two of the best tubes to the phono board to get by for now, but I think I will just replace them all which kind of sucks because I think I already paid too much for the unit, but I digress. I want to run over the board to check voltages just to double check everything before I install fresh tubes. I’ve looked over schematics and it’s so foreign to me. It doesn’t resemble the actual bird all that much. Would anyone mind putting arrows to the resistors I should be checking or anything else that is of importance? It would be a huge help. Thanks in advance.
Forget the resistors for now, just measure the voltages the manual has listed. If some voltage is way off
then it is time to start checking resistors that may affect the affected voltage.
Yes 4 JJ-E83CC is a good choice.
The figures ( 1 to 9 ) and ( 1 to 7 ) refers to the tube pins. Easiest to reach from the underside.
From the underside they are numbered clockwise, with the space between 1 and 9 marking where to start.
All voltages are with the chassies as ground, and sheet or screw will do.
Meter on "DC auto" black to ground or chassies and red on the tube pins.
The voltages will be affected by the tube at that position, so a wildly differing
voltage could be checked using a different tube in this position.
From the underside they are numbered clockwise, with the space between 1 and 9 marking where to start.
All voltages are with the chassies as ground, and sheet or screw will do.
Meter on "DC auto" black to ground or chassies and red on the tube pins.
The voltages will be affected by the tube at that position, so a wildly differing
voltage could be checked using a different tube in this position.
Should tubes be in? Power on?
Front PC6
- 129.1
- 0
- 0.65
- 0
- -7.85
- 159.9
- 0
- 0.62
- -3.76
- 118.3
- 0
- 0.72
- 0
- 8.8
- 155.7
- 0
- 0.68
- 4.19
- 180.8
- 0
- 1.6
- -0.38
- 8.8
- 208.9
- 0
- 1.16
- 4.1
- 214.6
- 0
- 1.13
- -0.38
- -7.9
- 307.8
- 0
- 0.245
- -4.21
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