Greetings from FixitLand!
Quote:
Originally Posted by VintagePC
The pot has a 4th lug which appears to be some sort of center tap- resistance goes from 300K to 0 and back up again as the knob is turned.
The schematic just shows the wiper going straight to the 6AU6, but here, the wiper goes through a 220K to ground, as well as to the 6AU6. One channel is shielded, the other isn't, so that may well be the source of some hum.
In addition, the audio input from the function switch runs through a cap to the 4th lead of the pot, then to a 47K resistor, through another cap, and to ground. The same circuit exists on both channels.
Does anyone have an idea what purpose this might serve? The mounting for the volume pot is different than the others, so I wonder if someone changed it and added a hack to make it fit/convert a linear to a log pot? My meter doesn't go to 3 megs, but afaict, it is a 3 meg pot.
Does someone have an idea what this might be?
|
The fourth lug on the pot usually means it has a loudness tap. Components connect to it in such a manner that bass is accentuated at lower volume levels, making the sound (supposedly) more "full-bodied." Anyway, as you note, the schematic doesn't show this. No harm in leaving it as is, at least for the time being. It could've been a factory circuit change.
Having one channel shielded (I assume from the function switch to the 6AU6 grid) and the other not shielded is definitely a hum issue. Does the hum you hear come from the unshielded channel? Or both, equally? If the latter, we're talking a power-supply issue most likely.
From the volume-control wiper to the 6AU6 grid, you say you have a 220K resistor to ground as well. That's a good idea, actually; should the wiper go open-circuit, without that 220K resistor the grid would float and cause the tube to operate erratically at best (go into runaway at worst).
Take care,
--
J. E. Knox 'The Victor Freak'
07-17-2010, 07:47 PM
Threaded Mode