Pacer 100 hum

This forum is for talking about all kinds of Peavey guitar amplifiers.
Bartman
Member
Posts: 600
Joined: Fri Sep 19, 2008 11:15 am

Re: Pacer 100 hum

Post by Bartman » Fri Sep 16, 2022 9:26 am

Signal breakup at very low volume could be the biasing diode for the main output transistors. Transistors don't "turn on" until the input voltage is about 0.7v, and if you have a transistor (or bank of transistors) on the + side and another set on the - side, when the input voltage goes from + to -, there is a brief period from +0.7 to -0.7 that the transistors are completely off. A biasing diode is therefore put into the circuit to always supply 0.7 volts, just enough to keep the transistors in an active state so that there's no noise, a slight buzz sound, when the output is switching between the + and - transistors. I repaired an amp that had this problem one time, and it was simply the bias diode wasn't soldered in good. The diode itself was fine, so I didn't have to replace any parts.

User avatar
dak
Member
Posts: 428
Joined: Fri Dec 24, 2021 7:20 am
Location: Germany

Re: Pacer 100 hum

Post by dak » Fri Sep 16, 2022 2:02 pm

Bartman wrote:
Fri Sep 16, 2022 9:26 am
Signal breakup at very low volume could be the biasing diode for the main output transistors. Transistors don't "turn on" until the input voltage is about 0.7v, and if you have a transistor (or bank of transistors) on the + side and another set on the - side, when the input voltage goes from + to -, there is a brief period from +0.7 to -0.7 that the transistors are completely off. A biasing diode is therefore put into the circuit to always supply 0.7 volts, just enough to keep the transistors in an active state so that there's no noise, a slight buzz sound, when the output is switching between the + and - transistors. I repaired an amp that had this problem one time, and it was simply the bias diode wasn't soldered in good. The diode itself was fine, so I didn't have to replace any parts.
Huh, with the class B designs I know, the reference voltage is taken from the output, so that gap is just "skipped", of course except for the time it takes to discharge the base capacitance of the to-be-switched-on transistor and for the finite amplification factor of the feedback circuit.

But I have a class B 100W amp from the 70s here which unashamedly uses no biasing diode at all, and it has to traverse 1.8V (1.2V for a Darlington stage and 0.6V for a Sziklai pair since both power BJTs are NPN, classic RCA 2N3055). I've thought of putting some in, but it actually sounds good enough.

The Peaveys I've seen seem comparatively carefully biased class B or maybe slightly AB. Of course if the biasing diode is not as much shortcircuited rather than open, all bets are off what the circuit might actually be doing.

User avatar
Pappy B
Member
Posts: 767
Joined: Sun Jan 31, 2016 2:30 pm

Re: Pacer 100 hum

Post by Pappy B » Sat Sep 17, 2022 11:52 am

I spent yesterday afternoon doing some maintenance on my Pacer. Couple of the 10K pots where getting noisy and erratic. I actually pulled almost all of the pots off of the board and opened them up. Cleaned a lot of old sticky dark grease out of most of them. I think spraying them through the small openings was just moving that junk around and it wasn't ever going to completely flush them out. And what did come out had to be cleaned off of the circuit board.

Also cleaned the reverb connections, jacks and speaker connections and reflowed several solder points. I also pulled the square magnet Eminence speaker out and put a Sheffield 1230 in it. Double checked my work then put it all back together. Didn't bother to test it 1st. When I Plugged the guitar in and flipped the switch It made the usual low pop sound but then I thought it was dead. I made my settings then picked up the guitar and it sound better than its sounded in a long time. My barely audible low hum was gone.

I have 4 different speakers I like to move around between my Pacer and Backstage Plus (It is rehoused as a small 112 stack). The amp sounds so much better now that I want to put the Eminence back in it in a week or so. I like the old Sheffield in everything I've used it in but I always end up with the old square magnet Eminence in my Pacer.
The more I learn the more I realize how much I don't know.

The_Chris
Member
Posts: 10
Joined: Fri Jan 02, 2015 7:25 pm

Re: Pacer 100 hum

Post by The_Chris » Sat Sep 17, 2022 2:22 pm

I used mine for a gig last night. First time in 30 years. Absolutely killed. At stage volume, I didn't have any issue with low-volume thing. Can't believe how great it sounded. Image

https://www.dropbox.com/s/cigsd6u12y88f ... g.jpg?dl=0

Post Reply