Nobsound 300b Monoblocks - HELP NEEDED!

[Wakefield Turntables]

A pair of these monoblocks landed in the listening room. They each have three valves to each monobloc. 300b x1 6H8C ×1 Soviet Union 5U4C ×1. Now the problems start. Douk audio in their wisdom left no markings for the position of the 6H8C or the 5U4C, so I could very easily plug them in the wrong way around. There are no markings on the amp. To add insult to injury my allen keys are worn down and useless and I don't have the right drill bit to have a look at the PCB. You couldn't make it up! :lol:. So my question dear AOSer's, do any of you own a pair or know the correct position of the 6H8C or 5U4C valves? The 300b just sits in the middle, that's the easy bit. So, basically, HELP!

[Edward]

Judging from the ebay listing (https://www.ebay.co.uk/itm/Hi-Fi-300B-Mono-Valve-Tube-Power-Amplifier-Split-Class-Home-Stereo-Audio-Amp-/283781971629) the taller valves (the 5U4C) sit at the rear and the shorter valves (the 6H8C) sit to the front of the 300b. Download the 7th image of the listing so you can zoom in on the valve type. The other images show the orientation.

I hasten to add I don't have this amp nor direct knowledge - just going on the ebay listing.

[Wakefield Turntables]

I did just as you said several hours ago. They are now working, nothing has blown up yet. Thanks for your reply.

[Edward]

Interested to know what your impressions are once you have tuned into them.

§

[willbewill]

Interested to know what your impressions are once you have tuned into them.

§

+1

[Wakefield Turntables]

Might write a few words tonight if I get time from decorating today. :(

[Wakefield Turntables]

These 300b are off back to China. These amps hum like a complete bugger. So, I've got the valves in the correct positions. The amps exhibit no hum from the Tx when fired up but as hum like a complete bugger when you attach them to a passive speaker, this is with no pre-amp connected. I've had several different mains leads from cheap walwart jobbies to some expensive Furutech foo shielded monsters. All hum. Both units work, I've had them playing music. This is a HUGE disappointment. Any ideas why they should hum so badly? I'm going to wait to see if any forum members can come up a quick fix, else, it's reboxing them and back to nobsound. I'm very very annoyed.

[Pharos]

Short the I/P to make sure that you are not putting hum into them. (Short = 0V.)

[Wakefield Turntables]

I've powered one amp up (altough this phenomen happens with both units) and just attached the speaker cable from the speaker to the amp. No other cables. I've just tried powering up from the wall socket and still same old hum, I thought it might be the extension I was using.I'm not sure what I/P means Dennis.

[Wakefield Turntables]

Here's a little video demonstrating the problems. I'm beginning to think it might be an internal grounding problem, but on BOTH amps :steam:. The hum is a hell of a lot louder than on the video. I'm certain it's not Tx hum.



https://www.youtube.com/watch?v=vJBcNsF2W80

[Jazid]

What do Douk Audio say? They used to be very responsive (but several years ago was the last time I had reason). They're good 'uns in my book..

Sent from my BLA-L09 using Tapatalk

[Wakefield Turntables]

Sent message, still waiting.

[Pharos]

I've powered one amp up (altough this phenomen happens with both units) and just attached the speaker cable from the speaker to the amp. No other cables. I've just tried powering up from the wall socket and still same old hum, I thought it might be the extension I was using.I'm not sure what I/P means Dennis.

I'm sorry, I/P is standard for input in telecoms. Short the input to make sure no hum voltage is going into the amps, driving them.

[Lawrence001]

To be clear you mean connect the + and - inputs together on each mono amp?

[Jazid]

He means short out the signal input. Easiest way is to stick an interconnect cable into it and short out the other end with a paper clip or something conducting. If the hum ceases, that's a good start. If not back to the hunt.

Sent from my BLA-L09 using Tapatalk

[Pharos]

Yes, on each amp connect the + and - together, thus ensuring no input voltage is fed into each amp.

Eliminating this, if the hum persists, it is inherent to the amp circuitry.

[Lawrence001]

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.

[walpurgis]

Perhaps there's a mains related issue. Trying a DC blocker might be useful.

[Edward]

I wonder if having the amp so close to the (IMF) speaker is part of the problem?

Do I see a PS Audio powerplant in the background? Try that. If you have a DC issue the conditioner should cure it.

But yeah having a source connected to the input seems the obvious first step to try.

§

[Wakefield Turntables]

Right I've had a few beers so I'll try all your recommendations tomorrow chaps. Can I say thanks for the input.

[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 :whistle:

[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.

[Wakefield Turntables]

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.

Yes, of course, I don't make a habit of just plugging a single monoblock into a speaker and listening to it all night.

[Lawrence001]

Ok it's just the only two statements referring to hum are these:

"The amps exhibit no hum from the Tx when fired up but as hum like a complete bugger when you attach them to a passive speaker, this is with no pre-amp connected."

"just attached the speaker cable from the speaker to the amp. No other cables. I've just tried powering up from the wall socket and still same old hum"

Personally I'd expect them to hum a bit connected as you describe, from my own experience with valve amps. At no point do you refer to any hum when they are connected into the full system and playing music. So I'm just asking if they also hum in that situation?

[rcarruthers]

https://www.ebay.co.uk/itm/284646515786?hash=item424641244a:g:HCEAAOSwhMFh-TCk

"Note: This 300B tube amplifier is second-hand, and there is some noise because the tube filament voltage is not adjusted well, we will send you a potentiometer to solve this problem.”

I was hoping to try these out with my ESL 57s & see if they had enough power to handle the impedance swings. SETs are usually not recommended for electrostats, but someone tried 300Bs & said they worked great.

They are affordable & good looking, so prob too good to be true!

[Lawrence001]

Is it really wise to bung in a potentiometer to the average punter to solve a heater filament voltage issue??

Sent from my BKL-L09 using Tapatalk

[Firebottle]

I think they are suggesting fitment of a hum-bucking potentiometer, it may reduce the hum, it may not.

I don't think the heater voltage is the issue.

[Lawrence001]

Either way I'm not sure eBay would approve of them saying this in the description "we will send you a potentiometer to solve this problem.”

Sent from my BKL-L09 using Tapatalk

[guy]

Strange that if it would be easy for potential purchaser to fix that the seller doesn't just do it before listing :scratch:

[rcarruthers]

Strange that if it would be easy for potential purchaser to fix that the seller doesn't just do it before listing :scratch:

That was my question to Douk. No reply yet.

[Wakefield Turntables]

You'll not get one either!