Hi : I would like to thank two members for assisting me regarding my pas-3 120 hz hum. Connecting the return on the filament with the B+ return fixed the problem. I am using an alps dual ganged 100K pot. when i turn the pot to 9 o'clock my system is way to loud. Being that it is a linear pot I tried ESP log taper trick but i still reach full volume at 10 o'clock. Do I need to go to a 250K pot or is it practical to lower the gain of the pas high level stage. the high level circiit is modified as per David Vorhis 2/76 article from the Audio Amateur. Thanks much!!
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Hi,
Going to a 250k pot wouldn't change anything; you'd still have the same level at 10 o'clock.
One option would be a series resistor between your signal input and the top of the pot. Using 100k there halves the signal at the top of the pot and thus at all pot settings. It may not be enough though and then you could, in addition to the 100k series resistor, put a parallel resistor in parallel to the pot (from pot top to ground) of say 33k, that will make your signal 1/4 of the original. If it is still not enough, experiment with the 33k parallel resistor, make it even smaller.
Have fun!
jan didden
But You can adjust plate current, changing Plate and cathode resistors.
Current of 12AX is about 1-2 ma. but for good sound 12AT needs current about 6-8 ma.
/Plate res. will be about 20-30 kohm, and cathode res - about 300-500 ohm/.
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Yes, but reducing the input signal is the worst possible sollution i can immagine.
I have a large stock of 12ax7 NOS from al over the world including Germany, Great britain and holland so I would perfer to stay with 12ax7. I am able to change the plate and cathode resistance. I do have two 12at7 but would perfer to save them. Local feedback sounds like a good option.However this would require specific instructions to make these changes because I do not know what values to use and how to connect the local feedback. Woul any one be able to help me. many thanks to you all.
It's a complete redesign. Problem is, 12AX7 is not a great tube for modern line amps (you don't really want much gain out of them) and has lousy output impedance. You can only get the gain down by increasing feedback and that will make keeping things stable something of an exercise. The tube swap suggested won't help much- the gain is mostly set by the feedback loop, not by the open loop gain. Going from a 12AX7 to a 12AT7 mught drop the gain a dB, hardly worth doing.
If it were me, I'd just pull out the whole line stage board and replace it with a circuit that's happy at between zero and 10dB gain and with a low source impedance.
A redesign of the line stage would be the best approach as Sy suggests. On the other hand, Jan's answer -- like the approach or not -- addresses the issue that caused the problem to begin with.
When the OP changed out the original level control, it's quite likely that there is no loudness circuit associated with the new level control anymore. Besides the original control being of a log taper, the loudness circuit provided significant attenuation of the input signal -- up to the point where the loudness tap occurred on the track. Besides the log action itself, that's why the volume increased so significantly after that point was reached.
Granted, Jan's approach is permanent, as the signal lost through his input divider is lost for good. But if there is plenty of gain anyway, his approach should perform at least as good as the original, since any series input resistance will only build out the new 100K level control to nearly equal that of the original control.
Dave
When the OP changed out the original level control, it's quite likely that there is no loudness circuit associated with the new level control anymore. Besides the original control being of a log taper, the loudness circuit provided significant attenuation of the input signal -- up to the point where the loudness tap occurred on the track. Besides the log action itself, that's why the volume increased so significantly after that point was reached.
Granted, Jan's approach is permanent, as the signal lost through his input divider is lost for good. But if there is plenty of gain anyway, his approach should perform at least as good as the original, since any series input resistance will only build out the new 100K level control to nearly equal that of the original control.
Dave
Could anyone suggest a high level circuit with low gain with a low source impedance. tubes that I have to select from which I have in stock are, Sylvania 6sn7gtb brown base,amperex 12au7,Rayathon 5687,and 5965 and 6922 from a company called Beckman. I can also purchase 12b4's. I have circuit diagrams by N. Koren that uses 12au7 and a circuit that that uses a 12au7 obtained from the RCA tube manual but I do not know if they are low gain or have low source impedance. thanks
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