A couple of weeks ago I blue some output transistors in one of my two PA-7 amps. I admit I was playing it very loudly for an extended amount of time with 4 ohm load.
Here's my question. I had it fixed already which cost me 400 dollars. The repair tech said he set the biasing to 20mv rather than 40 as a more conservative setting and lowering heat in an aging amplifier. He said I won't be able to hear the difference.
I pick it up tomorrow so I have not heard it yet but does anyone else have any thoughts on this? I'd like to know because I kind of like the idea since these amps are approaching 30 years old and I'd like to keep them for awhile yet if possible. If it sounds good I'd like to do the same with my 2nd PA-7.
Thanks ahead for any opinions, Mark
Here's my question. I had it fixed already which cost me 400 dollars. The repair tech said he set the biasing to 20mv rather than 40 as a more conservative setting and lowering heat in an aging amplifier. He said I won't be able to hear the difference.
I pick it up tomorrow so I have not heard it yet but does anyone else have any thoughts on this? I'd like to know because I kind of like the idea since these amps are approaching 30 years old and I'd like to keep them for awhile yet if possible. If it sounds good I'd like to do the same with my 2nd PA-7.
Thanks ahead for any opinions, Mark
Mark,
By reducing the bias, he is effectively reducing the quiscent (idle) current in the amp.
This does make the amp run much cooler, but if lowered too much, can introduce crossover distortion at low volumes. However, it seems like you should be okay on that front.
Lowering the bias current on the other amp is a good idea, especially if it is the same age and no parts have been replaced.
I would, at the very least, replace any and all electrolytic capacitors in the amplifiers. This is relatively inexpensive, and can increase the performance and lifespan of your amp, as well as reducing the chances of something going wrong in the future.
Make sure you use capacitors of the SAME value (measured in farads (F), microfarads (uF), etc.) You CAN use higher-voltage rated capacitors if you so choose.
In the power supply section, you CAN use higher-value caps (more total capacitance), which will increase the power reserve, reduce hum (if any is present), and MAY (not guaranteed) give you more control in the bass. However, I recommend that you bypass the power supply caps with a 4.7uF or 2.2uF cap to lower the impedance of the power supply at high frequencies. (Bypassing is simply placing a smaller cap in parallel with the larger one).
I recently repaired my old Threshold 400A, which is also a Nelson Pass design, so there should be some similarities.
I ended up replacing ALL the transistors and electrolytic capacitors, adding new thermal compound on the back of each transistor, adding more power supply capacitance, replacing the rectifier bridge with a new, 45amp Fast-Recovery bridge, and replacing the fuse holders.
If you really want this amp to last, I highly recommend sending it to Jon Soderberg of Vintage Amp Repair out in California for preventative maintenance. He's very reasonably priced, and knows the Nelson Pass designs extremely well (he used to work for Threshold/Nelson Pass).
By reducing the bias, he is effectively reducing the quiscent (idle) current in the amp.
This does make the amp run much cooler, but if lowered too much, can introduce crossover distortion at low volumes. However, it seems like you should be okay on that front.
Lowering the bias current on the other amp is a good idea, especially if it is the same age and no parts have been replaced.
I would, at the very least, replace any and all electrolytic capacitors in the amplifiers. This is relatively inexpensive, and can increase the performance and lifespan of your amp, as well as reducing the chances of something going wrong in the future.
Make sure you use capacitors of the SAME value (measured in farads (F), microfarads (uF), etc.) You CAN use higher-voltage rated capacitors if you so choose.
In the power supply section, you CAN use higher-value caps (more total capacitance), which will increase the power reserve, reduce hum (if any is present), and MAY (not guaranteed) give you more control in the bass. However, I recommend that you bypass the power supply caps with a 4.7uF or 2.2uF cap to lower the impedance of the power supply at high frequencies. (Bypassing is simply placing a smaller cap in parallel with the larger one).
I recently repaired my old Threshold 400A, which is also a Nelson Pass design, so there should be some similarities.
I ended up replacing ALL the transistors and electrolytic capacitors, adding new thermal compound on the back of each transistor, adding more power supply capacitance, replacing the rectifier bridge with a new, 45amp Fast-Recovery bridge, and replacing the fuse holders.
If you really want this amp to last, I highly recommend sending it to Jon Soderberg of Vintage Amp Repair out in California for preventative maintenance. He's very reasonably priced, and knows the Nelson Pass designs extremely well (he used to work for Threshold/Nelson Pass).
Thanks for the reply,
Yes this amp was already upgraded by Jon Soderberg at Vintage Amp Repair. I should probably get around to having my other one upgraded as well. Sadly it's the upgraded one that blew on me or I'd just have it all done to the other one.
Thanks for the biasing info! I'll see how it sounds compared with my other one tomorrow or next day. Biasing aside this upgraded one did already sound a little better than the other. All Jon did was replace all the smaller caps and the biasing and offset pots. The larger caps are getting hard to find anymore I think but he said those were okay still.
Mark
Upgrade or not to upgrade.
"Yes this amp was already upgraded by Jon Soderberg at Vintage Amp Repair. I should probably get around to having my other one upgraded as well. Sadly it's the upgraded one that blew on me or I'd just have it all done to the other one".
My argument entirely.
Leave the highly successful designers choices alone. The designers job is too take all aspects of performance into consideration.
"Yes this amp was already upgraded by Jon Soderberg at Vintage Amp Repair. I should probably get around to having my other one upgraded as well. Sadly it's the upgraded one that blew on me or I'd just have it all done to the other one".
My argument entirely.
Leave the highly successful designers choices alone. The designers job is too take all aspects of performance into consideration.
Sounds more like it was serviced, not upgraded.
Wouldn't replacing caps and pots (of the same or similar value) be classed as maintenance or servicing.
To me an upgrade would be changing values beyond standard tolerance to achieve a different performance.
40mv is the factory biasing.
I'd be looking at the heatsink grease being the reason for its failure.
Old grease dried out, lost thermal conduction, outputs ran hotter than designed for, poof, dead outputs.
Just my thoughts anyway 🙂
Wouldn't replacing caps and pots (of the same or similar value) be classed as maintenance or servicing.
To me an upgrade would be changing values beyond standard tolerance to achieve a different performance.
40mv is the factory biasing.
I'd be looking at the heatsink grease being the reason for its failure.
Old grease dried out, lost thermal conduction, outputs ran hotter than designed for, poof, dead outputs.
Just my thoughts anyway 🙂
Thanks for the replies.
Yes "servicing" would be correct. Except for the upgraded multi-turn pots.
I thought of that about the grease too. The heat sinks did get hot so it was conducting heat but at this age maybe not optimally anymore. Maybe something to put on my list when I have more time.
I got the amp back today and have been comparing with my older one on my Acoustat 2+2 electrostatic speakers and I don't think I can tell any difference even at low volumes. These speakers are normally VERY revealing of differences in power amps so I feel satisfied I hear none. Thus after listening I decided to lower my other one as well. When I checked that one one channel was at 70mv and the other about 45.
Matched them up at 20mv and got the DC offset dang near zero on both amps.
I think I'm good to go.
Thanks everyone for the thoughts.
Yes "servicing" would be correct. Except for the upgraded multi-turn pots.
I thought of that about the grease too. The heat sinks did get hot so it was conducting heat but at this age maybe not optimally anymore. Maybe something to put on my list when I have more time.
I got the amp back today and have been comparing with my older one on my Acoustat 2+2 electrostatic speakers and I don't think I can tell any difference even at low volumes. These speakers are normally VERY revealing of differences in power amps so I feel satisfied I hear none. Thus after listening I decided to lower my other one as well. When I checked that one one channel was at 70mv and the other about 45.
Matched them up at 20mv and got the DC offset dang near zero on both amps.
I think I'm good to go.
Thanks everyone for the thoughts.
Last edited:
- Status
- Not open for further replies.