Newbie with WAD 300b PSE monoblock hum

[sonic fiend]

Hi All,

Newbie with WAD 300b PSE monoblock hum. Really like these units and would like to use them more often, otherwise they may have to be sold to someone who has the time. I have looked through the previous message boards and found this to be useful but a hit and miss affair with not too many success stories. I do have some knowledge in electronics/repair and have had valve equipment for many years, i have also built my own diy amplifiers and the like with various levels of success and disaster.
I Bought the PSE monoblocks a few years ago, already used and they always had hum (same on both). I put a new set of valves in but nothing changed and actually got worse so put them away as i didn't have the time to look into it or any more££.
I got them out again a few weeks ago, blew off the dust had a look inside, The kit looks like a standard build and has been made well with good solder joints etc. I found some wiring that had come away from their original position. With the amp on so i could hear the change as I pushed the wiring back into position which helped a lot but still have a small amount of hum. Checked all valves on a tester and the power supply caps with ESR and capacitance meters and all good.
Any advice would be greatly appreciated and or possible upgrade with better components/circuit from the original.

Thanks in advance

[Richard]

Hi, first thing, if you didn't build them, is to check the build against the circuit. (In particular check these are 300BPSE as the 300BPP were very similar in looks and used the same chassis.)

The PSE did hum relatively more than many amps and the PSE quoted 4mV compared with 0.5mV from the PP for example. They used a similar HT set up (the PSE was launched first iirc) but the PP had the advantage of hum cancelling in the OPTX and the small tubes having their heater voltage raised.

Next, check mV AC at the speaker terminals to get an idea of hum level to see if it is in spec or is a fault.

[Ali Tait]

Wot Richard said. Also, are your speakers particularly sensitive?

[sonic fiend]

Thanks Richard, it is the PSE as per schematic. The speaker ac o/p measures 3mA on both so within spec, just noticed the hum balance on one unit has stopped working but luckily its where it needs to be. i take it was not a popular amp as it was changed from PSE to PP.
Speaker wise i'm using some Klipisch RF7 ii around 97-100db rating so can expect some hiss/hum.
They also suffer from ground loop noise so have to lift the earth and let the interconnects do the work, not ideal, was thinking a 10R resistor on the ground lift might help?.

[Ali Tait]

The PSE was pretty popular, and AFAIK sold more than the later PP.

The amps measure pretty well, you'll struggle to get the hum to lower levels than that, the sensitivity of your speakers not helping in this respect I'm afraid.

[Richard]

The PSE came out in 2000 and the PP in 2003. They were both available together for a while and the PSE was well regarded as a high quality amp for moderately efficient speakers. Certainly many members on here at the time built them and the only real drawback was the cost! Happy days, Ali brought his down to mine back then and they sounded great even into my old power hungry transmission line speakers.

Yes lift the ground;

Fit/check the phono RCA input socket is the isolated type.

Fit/check the chassis has a solidly connected earth post. Run the yellow/green mains earth wire solidly to it and also any electrostatic shield if there is one on the mains TX.

Then detach the signal 0V grounds and check with a meter that the circuit ground is completely isolated from the chassis/earth post.

Now re-attach the signal grounds (including the negs of C9, C10) to the earth post via a 10R w/w resistor paralleled with a 0.1uF film cap (to provide a path for any RF).

Check again with the meter that all circuit ground 0V points to chassis earth post now measure 10R.

Whilst in there you might want to check the circuit signal grounding priorities. If you do nothing else, isolate then connect the high current 300B cathode resistors R11, R14, directly to the most earthy part, the neg of C9, C10. This is to prevent large currents modulating small ones. (Whilst the circuit is correctly drawn, we would not construct it that way. As drawn, the 300B cathode resistors share the same ground wire as the 6AU6; separate them and take each directly to ground.) We then end up with a proritised grounding somewhere between full star grounding and a bus-bar which helps stop modulation

[sonic fiend]

Yes, build is found as you describe, i/p is isolated, so a quick fix would be to isolate sig 0V@6au6 & C9,10, R11,14 from the neg bus and reconnect them to the 10R w/w paralleled with a cap to the earth post.