300B PSE Loss of Volume, Bass, and distortion

[David Roberts]

During the first lockdown, I undertook a full ‘nut and bolt’ restoration of a pair of WAD 300B PSE that had been waiting patiently in my workshop for about seven years. The rebuild went well and, apart from some minor issues, which were quickly sorted, with one of the amps when first switched on, I was very pleased with the final result and immensely impressed by the sound. I didn’t manage to hit the hum level quoted in the literature with no feedback (4mV), the best I could manage was 7.6mV, but it isn’t intrusive, and is perfectly acceptable for my listening. Both amps ran perfectly for a year or so, but now one has started to misbehave…

The fault manifests itself as a drop in volume, lack of bass, and an occasional tendency to distort. At first, this occurred after the amp had been on for about 30 mins, but more recently the fault manifests itself at switch-on, which sometimes 'clears itself' and produces full volume, which can be, er, ‘surprising’ to say the least!

I have checked the voltages of both amps: they are similar and in accordance with the instruction manual - certainly well within the quoted 15% tolerance. I have successively swapped the valves with the good amp, switched input cables, switched output cables, inspected, measured and tested various critical components (which resulted in changing C3 and C6 in both amps), but all to no avail. I am now at a loss at what to try next, and any suggestions from more experienced engineers who might have encountered a similar problem would be most welcome.

Many thanks.

[John Caswell]

Hi David,
Well initially you have done all the things that I would suggest. Soo.. it may be a dry joint, quite where I wouldn't like to guess, so as a beginning I suggest that you look at all the joints but especially the ones where you have replaced components, and any that look suspect re-solder.
Do you have an oscilloscope and audio generator for testing purposes?

John

[A Stuart]

I am sure others will come in with advice, but I am sure it would help them to know whether the voltages were measured while the amp was "behaving" or "misbehaving".
Questions that jump to mind are:
Are all the heaters lighting up the same when both behaving and misbehaving?
Have you cleaned the valve pins/sockets, given the years in storage?

Just noticed John has answered since I started writing - listen to him, not me !

Alastair

My thinking behind this: Something could be pushing a valve outside its operating area (think of the graph of volts/current) to give lack of amplification and distortion. Reminiscent of when I built mine, not understanding what heater CT meant (common tap) so I was running two heaters in series which should have been in parallel, so the valve was not emitting enough electrons - lack of gain and distortion, wrong current so wrong voltages. Listen to John. This just about exhausts my fund of repair experience.

[John Caswell]

Ali wrote
"Just noticed John has answered since I started writing - listen to him, not me !."

Rubbish, listen to everyone you never know!

John

[Richard]

Hi David,

I'd suspect mechanical things first, 6AU6 pins and sockets especially oxidised NOS ones seem prone to poor connection and noise, then feedback switch try bypassing it, and humbucker perhaps a better quality one.

Then coupling caps C2 especially if they are paper swap for high voltage plastic film.

Check the earth points, a poor connection may cause varying gain via the feedback loop or a cathode connection.

On the PSU side high voltage electrolytics C7/8 will take hammer, and the choke, I've had one go intermittently short turns which happened when it became warm, the circuit suggests the choke should measure around 55R see if it changes.

[David Roberts]

Thank you gentlemen for your suggestions. I will work through them systematically, and report back.
Regards,
David

[Mark]

Dave,

John has pointed to possible dry joints and others have pointed to something mechanical. I find dry joints hard to spot visually. You could try gently and methodically prodding all the mechanical and soldered joints and components with the end of a chopstick with the equipment on (and I suggest with your other hand in your pocket !) and see if you get an intermittent connection. Gently deflecting pcbs to check for cracked tracks and bad joints can also be helpful, but I guess your amps are all point-to-point wiring.

Mark

[David Roberts]

Update 1

Just to confirm that I started to investigate for dry joints and, with amp on and fault in evidence, began gently moving wires and tapping components. Nothing untoward until I tapped C4. There was a slight micro-phonic ‘thump’ and the faint buzzing stopped, but low volume/distortion fault still present. Despite tapping C4 and its near neighbours R3 and R4 again, I could not effect any further change. I am going to focus on this area first and re-flow all solder joints. If that doesn’t work, I will unsolder each component and test them individually before replacing/renewing.

Also, measured the resistance across L1 = 62.1r

Will update again when I have completed the above.

[Richard]

Hi David,

Just looking over the posts again,
how are you using the amps when the problem happens, with feedback switched on or off, or are they built without the fb circuit and switch wired in at all?

John asked if test equipment is available.

Another fault I've had in the past was an intermittent short from connections/wires/tags on the hidden side of the tag strips.

Just ideas, hope you're eliminating some of the possibilities

[David Roberts]

Update 2

I have checked for dry joints, and also investigated C4 and its adjacent components, but did not find any problems. I de-soldered, and measured the following: R3, R4, R6/7, C1, C2, C4. All are within spec: C2 & 4 are plastic and measure 0.01mA leakage at their specified voltage. C1 is an electrolytic, and measures 0.02mA.

I also checked the power supply caps, C7 & 8, and the balancing resistors R17 & 18. Both resistors are good, and the caps leakage measured approx’ 0.07mA and 0.09mA respectively. Virtually all passive components were replaced during the rebuild.

All the valves light up, and the heater supplies measure correctly. The pins of V1 & V2 were all cleaned with a fibre pencil when the fault first appeared. Prior to that, the sockets had been soaked in isopropyl alcohol, washed and cleaned thoroughly before fitting. I don’t think that corrosion/oxidisation is causing the problem, but I will clean pins and sockets with Deoxit prior to testing again.

All earth points have continuity back to chassis (circuit earth being 10.2r above chassis).

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.

A curiosity then! Both amps have worked perfectly since completing the rebuild, and one of them still is, but there seems to be nothing that would explain the cause of the fault with the other. I do have access to a scope and function generator, but would need some guidance on how to set up the test, and what I should be looking for. Having said that, I am more than happy to work under supervision!

Regards,

David