Ampeg B15N sound cuts off

[boroman]

I have strange problem with my B15N (I think it's a 74 - silverace w/blacklining, chrome switches, CTS speaker) - amp sounds great, no hum, awesome sound, no scratchy pots etc. Bias and all the tubes were checked (I even changed all the tubes to be sure). Sometimes when I push amp really hard it just gets loud scratchy and next super quiet and distorted (like 5% of overall volume + sound is not clear). Its stays like that until I switch it off and cool down. Then again, sounds good. I thought it was a speaker issue, hooked up to another B15N speaker and its the same. What should I check?

Amp did not have a cap job ever (caps looks like old ones) so this is my first thought, but is it my case really a "cap" issue?

 

[coreyfyfe]

Does it happen on both channels?

Doesn't sound like a power supply cap issue off hand, sounds like something going bad with heat or an oscillation issue since you say it happens after a loud/hard note and goes away after shut down.

 

[boroman]

Yes, both channels. I will try to record it, maybe that helps.

 

[boroman]

By the way, maybe bias caps?

 

[BassmanPaul]

Your amp needs to see a tube tech.

Check the wiring and contacts of the Extension speaker jack. These switches change the impedance the speakers see from 8Ω to 16Ω when an extension is connected. With no extension the jack shorts to connect the internal driver. Internal driver operates in series with the 8Ω extension when it’s connected.

 

[agedhorse]

By the way, maybe bias caps?

Usually, bias cap failure results in either very loud hum (open), total failure of the bias supply and runaway of the output tubes, (shorted) or unpredictable distortion (partially open, high ESR).

This applies to fixed bias amps, cathode biased amps (which this is not) can exhibit different symptoms, the primary symptom is the loss of low frequencies as the ESR increases and the effective capacitance falls. Rather than guessing, measurements are used to identify this condition.

 

[beans-on-toast]

I would check the components in the bias circuit to ensure that they are within spec when hot. The larger wattage (bigger) resistor in the fixed bias circuit can drift out of spec. The bias cap(s) could have an issue.

You might have a bad coupling cap.


Take the preamp out (ext amp) and feed it into another amp with an instrument cable. Push it so that the amp cuts off as you mentioned. See if the preamp signal going to the other amp is affected. If no, your issue is probably in the B-15’s power amp. When doing this, keep the speaker connected to the B-15.

 

[boroman]

Will do. Thank you all for the replies. Will come back soon with details

 

[boroman]

This problem was diagnosed today, it was in the preamp, one of the resistors was out of specs when amp was put to continous high level signal

 

[whitelines]

hi @boroman, did the resistor replacement fix your issue? I believe my b15n is having a similar issue. It works great until running for a while, then a loud hum starts fading in and the bass sound fades to become very weak. thanks
W

 

[boroman]

hi @boroman, did the resistor replacement fix your issue? I believe my b15n is having a similar issue. It works great until running for a while, then a loud hum starts fading in and the bass sound fades to become very weak. thanks
W

Yes, but I quite don't remember which resistor it was. I gave the amp to the tech back then..

 

[whitelines]

Ok. Thanks. I was able to capture it on video, so I think my tech should be able to fix it.