Ampeg B25B
Moderator: jingle_jangle
Ampeg B25B
Im still working on this amp . I've cleaned the pots replaced 2 of the filter caps and put in new preamp tubes . When I turn either volume pot about 1/4 of the way up I get a big Pop and the speaker moves alot . Is this the pots or what they are connected to ?
It's too early in the morning to talk about our relationship !
Re: Ampeg B25B
I can't say for sure but it sounds like some of the coupling caps need replacing. You are putting a lot of DC into the speakers when you turn the volume up.
Re: Ampeg B25B
Hi Mr Scott . Are the coupling caps in the preamp section ? Any colors or identifying marks ? Thanks .
It's too early in the morning to talk about our relationship !
Re: Ampeg B25B
Don't Mr. Scott me!!! 
They would be in the preamp section between the various sections of the circuitry, but there are far more knowledgeable here than me concerning these things. Greg Simon and John Allgaier are two that come to mind.
They would be in the preamp section between the various sections of the circuitry, but there are far more knowledgeable here than me concerning these things. Greg Simon and John Allgaier are two that come to mind.
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soundmasterg
- RRF Consultant
- Posts: 1923
- Joined: Tue Oct 01, 2002 1:06 pm
Re: Ampeg B25B
If that amp hasn't had the electrolytics replaced aside from thw two that you did, then you should replace all of the other electrolytics in the amp. They're old and past their expected life date and they amp will sound and perform better with new electrolytics.
The coupling caps function is to block DC and pass AC, and they also can affect the frequency response based on their value. If they pass DC in addition to AC, then they affect the stages following them by altering their bias point. If can sound cool or bad, or it can cause a stage to become an oscillator instead of an amplifier. There is a way to test coupling caps for function but you have to be very careful because it requires you to have your hands inside a live amp and you can die if you don't follow safety protocols and accidentally touch the wrong thing. Please be very careful or take it to a tech to troubleshoot if you are not completely comfortable with working in a live chassis.
To test a coupling cap for leakage, you make sure the amp is off and unplugged, with the electolytic caps drained, then you unsolder the grid end of a coupling cap. Clip your meter lead to this end that is now hanging up in the air and the other meter lead to ground. Turn amp on and watch the voltage on the meter. It will spike and then steady out at a particular voltage. If it is any more than .5v you should replace the cap. Good replacement choices are Mallory 150's and/or Sprague Orange Drop 716P or 6PS series in addition to many other types and brands. Coupling caps are in the preamp and poweramp too btw.
Your problem could be many things. What should really be done is that the amp needs to be troubleshot stage by stage until you find the problem. Try to isolate where the problem is occuring. Check voltages at the different stages and compare with the schematic. Short out one stage and see if the problem continues. The fact that your volume control causes it to come and go is a clue that the problem is most likely before the volume control somewhere.
Greg
The coupling caps function is to block DC and pass AC, and they also can affect the frequency response based on their value. If they pass DC in addition to AC, then they affect the stages following them by altering their bias point. If can sound cool or bad, or it can cause a stage to become an oscillator instead of an amplifier. There is a way to test coupling caps for function but you have to be very careful because it requires you to have your hands inside a live amp and you can die if you don't follow safety protocols and accidentally touch the wrong thing. Please be very careful or take it to a tech to troubleshoot if you are not completely comfortable with working in a live chassis.
To test a coupling cap for leakage, you make sure the amp is off and unplugged, with the electolytic caps drained, then you unsolder the grid end of a coupling cap. Clip your meter lead to this end that is now hanging up in the air and the other meter lead to ground. Turn amp on and watch the voltage on the meter. It will spike and then steady out at a particular voltage. If it is any more than .5v you should replace the cap. Good replacement choices are Mallory 150's and/or Sprague Orange Drop 716P or 6PS series in addition to many other types and brands. Coupling caps are in the preamp and poweramp too btw.
Your problem could be many things. What should really be done is that the amp needs to be troubleshot stage by stage until you find the problem. Try to isolate where the problem is occuring. Check voltages at the different stages and compare with the schematic. Short out one stage and see if the problem continues. The fact that your volume control causes it to come and go is a clue that the problem is most likely before the volume control somewhere.
Greg
Re: Ampeg B25B
Hi Greg . Sounds like I should pack it well and ship it your way before I kills myself . Do you do repair work ? Would it cost $$$$$ ? Thanks . Rick .
It's too early in the morning to talk about our relationship !
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soundmasterg
- RRF Consultant
- Posts: 1923
- Joined: Tue Oct 01, 2002 1:06 pm
Re: Ampeg B25B
Rick, I do indeed do repair work, though I have a M-F full time job so it has to fit in the spaces of my free time.
Send me a PM with your email addy or phone # and we can go from there.
Greg
Send me a PM with your email addy or phone # and we can go from there.
Greg