Phonoland

Hi,

I have an AMi Continenal (1) with a mono amp.

After about 30 minutes play, if I play a song with a lot of bass, it gets very bassy with blurred sound.

I assume capacitors may be out of range? Would I be correct?

The amp was recapped a few years back.

Any ideas where I might look first?

Kind Regards

Gordon.

Gordon, there are several possibilities here:

-Acoustic feedback. Does this still happen if the volume is turned way down. If it goes away then, I suggest making sure the mech is as isolated as possible from the cabinet as practical. make sure any mach tiedown bolts are not touching the mechanism chassis.

-Electrical overload. What pickup cart are you using? The original magnetic had a typical output of about 4-6 mv. I replaced these with Pickering DAT series with no problems.

-Is the AGC ckt working? AMI used a lot of extra gain in these amps and a loud or bassy record will overload the tone control stages if the AGC isn't attenuating it correctly -or at all.

Capacitors, if quality caps were used in the rebuild they should not be leaking for decades to come. Leakage was mostly associated with the old style paper dielectric types. Still, if this -only- happens after warmup it is possible and the only way to find out is to remove the amp enough so as to keep it connected while you test for the presence of DC on the tube grids. If you have a signal generator you can put the amp on a bench and work more comfortably.

The output of the preamp-tone-AGC module is available at the RCA plug. Hook that up to an external amp, if the distortion is still there, the problem is in the preamp module.

Rob-NYC

Thanks Rob,

I will let you know how I go.

Regards

Gordon.

Hi,

The problem appears to be resolved.

To provide an update...

I checked what I could, but was unable to find a problem (but I have only limited electronics experience).
As I live in Australia, jukebox technicians are not available, so I contacted a guitar amp technician who did some repair work for me previously. I provided him with a copy of your advice.

He tested the AGC circuit and that appeared to be working perfectly.

He found that the first gain tube, a 12AX7 was a bit noisy and causing some vibration in the chassis. He suggested that this may cause some acoustic feedback of sorts. He advised me that a noisy tube is not necessarily faulty, so he just swapped it around with another one in the amp. He also cleaned as many contacts as possible. It appeared to sort out the problem.

I have test run the amp and it does appear to be working well.

I guess I will keep hoping that it was just a minor issue, perhaps just dirty contacts or the noisy tube.

Thanks for your help.

Regards

Gordon..