300B Hum

Hi, this is my first posting; needless to say I 'm hoping for some help. One of my monoblocs has developed a larger-than-usual hum and, after swopping valves, input and output leads, replacing a split insulation washer on the output transformer and checking all connections visible, the hum still persists. Can anyone suggest what I should do next? Although I assembled the amps myself, I must admit I don't know anything about electronics.

[andrew ivimey]

What kind of 300b amp is it?

You are going to have to go inside if all the other things you have done haven't altered / cured the hum.

Have you a multimeter to check what ought to be?

This hum is very likely to be a dry / broken solder / joint.

Check your joints on the phono inputs, wiggle wires to see if it makes it better or worse. Is teh coupling cap loose? Look hard at your joints. Poke with a screwdriver.

You have to do this with your amp switched on so be careful. You aren't likely to kill yourself but a shock is nasty.

Thanks Andrew. The amp is a 300B PSE. I shall have another look at the areas you mention and do it first without going live. I do have a meter, though when I built the amps the readings didn't make sense either to me or the lads in Chelmsford. Both amps were faulty and they rectified them for me. I'm going on holiday for a few days and so won't get around to it until next week.

[Greg.]

Hi Jeff,

Considering your reply I think first you need to re-do a voltage check as specified by the instructions and then report back here your findings, particularly any readings you get that step outside of 15% of the original spec target. I'm sure there will be more help thereafter.

Best wishes,

Greg

[Ianm2]

redo the earthing with thick wire, input cables with semi balanced, and lift the mains/chassis.

if that doens't fix it, go solid state

[NickG]

Not the most helpful post there Ianm2..

[John Caswell]

Hi Jeff, no don't do any rewiring as that won't help and may compound the felony. As one amp doesn't hum then it is likely that a component or joint has failed in the one that does.
Is the hum 50 Hz ie electrostatic pickup or 100Hz ie rectified HT type pickup.

John aka Dr John

[Ianm2]

hums are not my forte, Nick, I have lost patience with valve amps that hum and **** transformers. I, perhaps erroneously, expect things to work out of the box from day 1.

I am afraid I have little tolerance for cost cutting causing poor diy'ers intractable problems, sadly, nice as they are, a lot of world audio amps have some design issues causing hums/reliability problems that should've been sorted b4 they ever were marketed. I am sure they were designed within half an hour.

I hope pjc gives more attention to quality control and revamps wads glorious heritage and makes it a success to be envied as it deserves to be.

could be a leaky cap, too....

p.s, did you know even border patrols £6k amps use sp components ( I unwound one, the insulation is partly paper masking tape, and the layering minimal, the stacking wasn't very good, lots of uneveness!!) el cheapo tx's? talk about cheapskate gary jews, I have seen better made tx's from china.

I have even heard, but don't know if this is true, that Nick Lucas was the chief designer, not AG.

best wishes

[Greg.]

Hi Ian,

Don't blame SP Wound. They build to a price and having bought 7 lumps of copper and iron off them, I'd say their Tx's are better made than the later WAD ones. Remember, SPW made the Tx's for the original WAD 300B PP. I like the sound of mine and I think you've heard it. I know what I paid for my set, buying directly from SPW and it was very reasonable. You're being a bit unfair to suggest SPW's construction is worse than that available from China. SPW are not good at cosmetics. The Chinese hi-fi manufacturers apparently are. Good cosmetics can hide numerous flaws and inadequacies. Now look at what BP sell their SPW made Tx's for. Maybe that's where the criticism should be levelled. IMHO, it's blatant immoral profiteering. Of course, unless you're in the know, the average punter will be none the wiser.

Best wishes,

Greg

[NickG]

I am afraid I have little tolerance for cost cutting causing poor diy'ers intractable problems, sadly, nice as they are, a lot of world audio amps have some design issues causing hums/reliability problems that should've been sorted b4 they ever were marketed. I am sure they were designed within half an hour.

I will ignore most of that post I think, but given the BB is hosted by WD, I think the above deserves a little clarification.

AFAIK, and am happy to be corrected the only such problems I know of were the PreII which partly dur to huge gain, and partly down the TX location problems could hum, and the Kit88 which I think (before my time) Richard and others fixed a few problems. And I think there were problems with the kt88 suppliers, IN know NK has mentioned this a few times.

Far from a lot IMHO.

I have only built the one WAD kit from parts, but I have seen many others, and they all seem to have happy owners. Maybe your experence of building WAD kits differs from that, which kits have you built? Instead of buying ready made, and then proceeded to modify to the point of unusability.