300B PSE Loss of Volume, Bass, and distortion

[Richard]

Very thorough David and let's hope it has fixed it.

If not,

"When I rebuilt these, I reinstated the feedback loop, which a previous owner had cut out. Having experimented with both settings, I ended up leaving the feedback off, and that is how the amps were when the fault first appeared."

I had problems when rebuilding my 300BPP with switchable fb from a poor build so I removed it as it sounded better without fb anyway.

It would be easy to hardwire the junction of R5/C3 to signal ground to eliminate a bad switch or connection in what is a sensitive part of the circuit which would affect volume/cause noise if there is a problem.

[David Roberts]

Update 3

Having reinstated the components removed for testing, I thought the problem might be caused by a component (most likely, a resistor), changing its electrical property when heating-up during use. I decided to soak-test the amplifier using an old set of JJ 300b, rather than the Gold Lion PX300B that I have been using recently.

Firstly, I found that the hum figure was far better (5.2mA) with the older tubes. Has anyone else noticed that different manufacturer’s output tubes have varying effects on hum levels? Anyway, very pleased to report that the amp’ performed as well as it used to, and with no hint of a problem, for more than 70 minutes after which, I switched it off.

Next day, switching on again, the fault was evident immediately: low volume, and lacking in tonal range - as though the speaker had been wrapped in a blanket. This continued for about 20 minutes before some intermittent ‘popping’ occurred after which, the volume suddenly jumped back up to ‘normal’. So, the problem doesn’t appear to be dependent on the thermal properties of the amplifier, its components, or anything else, as far as I can see!

Next, I tried Richard’s suggestion, and shorted the feedback switch (as though FB was permanently ‘off’), but to no improvement. The more I think about it, the more the fault suggests a dry joint making an intermittent circuit. But, try as I might, I cannot locate the culprit. As Mark pointed out, the amp’ is hard wired, so a dry joint should be relatively easy to spot by tap-testing during a thorough inspection.

Thinking about Alastair’s comment re voltages, my next task is to measure them again, to see if any have drifted. Apart from that, I am at a loss what to do next, and seem to have run into a dead end.

[bob orbell]

David, apologies if this has been mentioned to you or you have tried it, but turn the amps on their side with the bottom cover off, connect to speakers and switch on, then wiggle each valve holder pin in turn ( I use plastic pliers ) it may show something. Bob

[Richard]

Hi David,

Component-wise, bypass caps C1,5,6 going open circuit would cause a loss of volume.

Re. R’s changing value; R3 1W dissipates 0.4W and is probably carbon film which can break down after a while. Then R9,10, 3W dissipating 0.76W which is perhaps less likely. Then R11,14, 7W dissipating 2.6W but probably wire-wound so less likely still, but these are all free-air ratings and it can get warm inside a valve amp so over-rate them.

[David Roberts]

Update 4

Thank you Bob and Richard for your input. I like the idea of plastic pliers to wiggle socket pins from the inside, and will keep an eye open for such a useful bit of kit. I eventually found a suitably fine 70’s plastic orange peeler (Tupperware, if I’m any judge) to wiggle them with – not quite as good as pliers, but did the job. However, all sockets proved to be as tight as a drum.

The next step I suppose, would be to inject a suitable wave form into the input, and trace its progress throughout the amp using a ‘scope. A procedure not recommended for the faint of heart, especially if you’ve never done anything like it before. There are high voltage issues of which to be aware, and I read something many moons ago about measuring through a capacitor attached to the probe, but cannot remember any details being given as to how to do this safely. A step or two above my rather rudimentary skills, I think.

It’s all very disappointing, really. When both amps were working, it was without a doubt the best audio system I have ever owned.

[bob orbell]

If you are not sure about the tests and repair David, it would be worth getting in touch with our John Caswell, I think he still does repairs for forum members but, you will need to send the amp to him, unless you could take it to him, he is in Berkshire, but contact him first. Bob