Nobsound 300b Monoblocks - HELP NEEDED!

[Pharos]

I'm sure I've had amps before that hum if there's no input, doesn't the input provide a ground that might get rid of the hum? Plug in a pre (also plugged into the mains and switched on before powering these up) and see what the hum is like them.

You could well have; the connection of a pre will put a lowish impedance across the input, tending towards a short circuit.

It is not possible to develop a voltage across a short circuit.

[Wakefield Turntables]

OK, update, these are going back. I just can't be bothered trying to get these fixed, they should have worked from the beginning. 300b journey over before began.

[philv]

That's a shame.
Deserve better after taking a punt.

[Wakefield Turntables]

Shit happens! Lets move on to the next adventure....

[Haselsh1]

My only experience with 300B's was a WAD 300B PP directly heated triode setup and guess what..? It hummed like a bugger. Bloody lovely sound quality but I just couldn't get over the low frequency tinnitus.

[AD Audio]

A WAD 300b PP properly built and sorted should have no audible hum. I built four clones and hum was inaudible, even with 93db sensitive speakers. DC heaters on the input stages helped, but it was even possible to get hum levels really low with AC heaters. The input stage as standard is very sensitive though at around 300mv for full output. I presume you tried reducing the hum on the output valves with the humbucker adjusters?
David

[Haselsh1]

A WAD 300b PP properly built and sorted should have no audible hum. I built four clones and hum was inaudible, even with 93db sensitive speakers. DC heaters on the input stages helped, but it was even possible to get hum levels really low with AC heaters. The input stage as standard is very sensitive though at around 300mv for full output. I presume you tried reducing the hum on the output valves with the humbucker adjusters?
David

Yep, the humbucker knobs reduced the hum but never got rid of it. The amp was built by World Audio themselves as was the accompanying KLP1 preamp which was faultless but the power amp, even though the tonal sound quality was stunning never stopped humming. Also, the stereo imaging was pretty suspect. The amp replaced a Musical Fidelity A100X which was staggeringly good at imaging; the 300B was poor in comparison.

[AD Audio]

Yep, the humbucker knobs reduced the hum but never got rid of it. The amp was built by World Audio themselves as was the accompanying KLP1 preamp which was faultless but the power amp, even though the tonal sound quality was stunning never stopped humming. Also, the stereo imaging was pretty suspect. The amp replaced a Musical Fidelity A100X which was staggeringly good at imaging; the 300B was poor in comparison.

That's a real shame. With a pair of sensitive speakers a properly sorted WAD 300b pp is among the very best around. Really sorry yours never was, despite being built by them, and that you never experienced its magic. Of course, I built the clones point to point which meant I could improve the layout over the pcbs WAD used.
David

[Haselsh1]

That's a real shame. With a pair of sensitive speakers a properly sorted WAD 300b pp is among the very best around. Really sorry yours never was, despite being built by them, and that you never experienced its magic. Of course, I built the clones point to point which meant I could improve the layout over the pcbs WAD used.
David

Yes David with regard to the very best around, the sound of space and air along with the liquid purity was stunning and so much better than I have now so many years later but it was a hugely flawed amp. Yeah OK so it was connected to wholly unsuitable speakers but still gave its best. Had I kept it a year or two longer it would have ended up driving Audio Note AZ-2's but after the A100X I just couldn't forgive its shortcomings. The lack of a huge soundstage was just not good enough and the hum was a big problem but I kept it for a few years before saying goodbye. Years later I used an Audion 300B SET which was every bit as good and better in some respects. These days I have given up on valves

[Lawrence001]

OK, update, these are going back. I just can't be bothered trying to get these fixed, they should have worked from the beginning. 300b journey over before began.

When you say they should have worked from the beginning, you did start off by plugging in a source and pre and playing some music didn't you? As you only mention running then with no input and you can't judge a valve amp that way as it's not designed to be used with no input.