northpaw
Super Member
I have a couple of questions about bias pots on a Nakamichi PA-5 amp.
This PA-5 is in daily use, and I pull it once or twice a year to check it over and, in particular, to check and reset the bias if it has drifted. It uses single-turn 150Ω bias pots, and as is common, they are very touchy. I have cleaned them and they behave smoothly and their resistance varies smoothly, but it is just that even very small rotation has a too-large effect for my tastes. I’ve decided to replace them with a multi-turn.
First issue I ran into is that multiturn pots in 150Ω are hard to find, much less one with an appropriate configuration. But after looking at the schematic, it really doesn’t need to be 150Ω, as the bias circuit connects the wiper to one end of the track, effectively placing the “unused” part of the track in parallel with 0Ω wire (see partial schematic image below; bias pot is VR102, near the screw head symbol). I’ve checked the resistance of the “used” part of the track for each channel at their present settings, and I get 39Ω for one channel, 65Ω for the other. So it seems I can safely go with 100Ω pots, which are widely available.
First question: Is the fact that I get a sizable difference between the ohm settings on the bias pots for the two channels (39Ω for one, vs. 65Ω for the other) a problem? That is, is this within the expected range of variation for an amplifier circuit (such as due to typical sample-to-sample performance variations among the output transistors - 5 in each channel on the PA-5), or does it indicate there is some issue I should look into?
Second question: Is a Bourns 3296 series 25-turn pot a good choice for a replacement, or are there others that might be better?
https://www.mouser.com/ProductDetail/Bourns/3296W-1-101LF?qs=sGAEpiMZZMvygUB3GLcD7n4W/LGLppVQEdi8TtOM2aU=
The 3296 a cermet pot with good TC (100ppm/°) and is available in a range of configurations, but none are a perfect match for the vertical board arrangement in the PA-5. I need a pot with “face-mounted” pins in an offset pattern, and a top adjust screw. The 3296P is that, but due to its particular offset pattern, it would have to be mounted with the adjust screw pointing downward in the PA-5. The 3296W (link above) seems the best option: it has 3 in-line pins mounted on an edge and a “top adjust” on the opposing edge, and I will need to bend the pins to (1) match the offset pattern and (2) tilt the pot such that adjust screw points upward.
Anyone aware of an alternative or better pot I should consider? A 25-turn pot isn’t necessary; a 5-turn or 10-turn would be more than adequate.
This PA-5 is in daily use, and I pull it once or twice a year to check it over and, in particular, to check and reset the bias if it has drifted. It uses single-turn 150Ω bias pots, and as is common, they are very touchy. I have cleaned them and they behave smoothly and their resistance varies smoothly, but it is just that even very small rotation has a too-large effect for my tastes. I’ve decided to replace them with a multi-turn.
First issue I ran into is that multiturn pots in 150Ω are hard to find, much less one with an appropriate configuration. But after looking at the schematic, it really doesn’t need to be 150Ω, as the bias circuit connects the wiper to one end of the track, effectively placing the “unused” part of the track in parallel with 0Ω wire (see partial schematic image below; bias pot is VR102, near the screw head symbol). I’ve checked the resistance of the “used” part of the track for each channel at their present settings, and I get 39Ω for one channel, 65Ω for the other. So it seems I can safely go with 100Ω pots, which are widely available.
First question: Is the fact that I get a sizable difference between the ohm settings on the bias pots for the two channels (39Ω for one, vs. 65Ω for the other) a problem? That is, is this within the expected range of variation for an amplifier circuit (such as due to typical sample-to-sample performance variations among the output transistors - 5 in each channel on the PA-5), or does it indicate there is some issue I should look into?
Second question: Is a Bourns 3296 series 25-turn pot a good choice for a replacement, or are there others that might be better?
https://www.mouser.com/ProductDetail/Bourns/3296W-1-101LF?qs=sGAEpiMZZMvygUB3GLcD7n4W/LGLppVQEdi8TtOM2aU=
The 3296 a cermet pot with good TC (100ppm/°) and is available in a range of configurations, but none are a perfect match for the vertical board arrangement in the PA-5. I need a pot with “face-mounted” pins in an offset pattern, and a top adjust screw. The 3296P is that, but due to its particular offset pattern, it would have to be mounted with the adjust screw pointing downward in the PA-5. The 3296W (link above) seems the best option: it has 3 in-line pins mounted on an edge and a “top adjust” on the opposing edge, and I will need to bend the pins to (1) match the offset pattern and (2) tilt the pot such that adjust screw points upward.
Anyone aware of an alternative or better pot I should consider? A 25-turn pot isn’t necessary; a 5-turn or 10-turn would be more than adequate.