Low frequency distortion

[JDA99]

I have an original 300b pp amplifier, that has started to produce very high levels of distortion along with bass notes - it sound like the effect you hear when a car boot is driven into resonance by a sub woofer.
It only occurs on one channel.

Where do I start looking?

Thanks in anticipation.

JDA

[Cobblers]

Originally Posted by JDA99

I have an original 300b pp amplifier, that has started to produce very high levels of distortion along with bass notes - it sound like the effect you hear when a car boot is driven into resonance by a sub woofer.
It only occurs on one channel.

Where do I start looking?

Thanks in anticipation.

JDA

1 make sure that it is not the speaker at fault then
2 Try different output tubes.

you could try switching the output tubes over (when the amp's cold!) to check the channels. then try the input valves.

It would help if you could tell the group what the amp actually is as some advanced members might have experience of that particular model.

[JDA99]

It is an original WAD 300B push pull.
Have swapped over speakers, power valves, input from pre-amp and replaced both the 5687 and 6072s. The problem stays on RH channel.


JDA

[Richard]

Hi,

I use this amp. I'd check simple stuff first such as good connections for the OPTX earth and signal. Pay close attention (or go over again with big iron and solder) the earth connections in partic as some are quite heavy and a dry joint may be the cause. Meter back into the OP binding posts looking for a similar reading on both channels of a fraction of an ohm. Is feedback fitted and does switching it in/out affect the bass? Past that it could be a failing 300B cathode bypass cap or a PS cap going down. Take careful voltage measurements and compare bad channel to good looking for a difference that may locate the position of the fault.

Rich

[Ianm2]

Its a long shot, chances are its something straightforward, but an optx may have gone down and be shorting. It happened to me on this amp.

[JDA99]

After posting, the channel died completely. Eventually I discovered that what had been a dry joint, where the signal feeds into the output transformer, had failed completely.
Remade the joint, and am now back in business.

Thanks for all the input.

JDA