Terrible tube amp noise

A little over a year ago, or was it two, I bought a Nobsound tube amp. The amp is a type A and operates in the ultra linear mode. It uses 1 5U4C Russian rectifier tube, two Chinese 6N9P tubes and two EL34B tubes. I don't kinow if they are Russian or Chinese. When I first powered it up everything went smoothly but after maybe an hour or so it made this loud static like noise. I promptly turned it off. Deciphering the directions I realized the amp was made to operate on

110 volts, not the 125 typical in my house. Oops. So I wired up a transformer in buck configuration to get 110 volts and tried the amp again. And it has operated fine since. Until a few days ago. Once again the loud static noise. Changing the volume changed nothing and I shut the amp off. After it cooled I pulled each tube out and then re-inserted it thinking that maybe a bad connection may have been the problem. After again powering the amp up it is once again performing flawlessly. So what gives? I have read about red plating, how it damages tubes pretty fast, and am thinking that maybe I damaged the tubes running them at the too high voltage for a while. But why does the amp sound great after it has a chance to cool off? Since the latest incident I have run the amp for about 4 hours straight so I don't think the problem is heat related. I mean just because the amp is hot it doesn't necessarily mean it is going to act up. Both times the amp made this noise it was getting its input from a built in bluetooth reciever. I thought this might be the problem but since the noise is volume control insensitive it seems to me that the problem is probably unrelated to the input. The noise is LOUD too. I mean hurt your ears loud. I never turn the volume up as loud as the static noise was. When I say static I mean scratchy crackling noises, not white noise. Kinda like if someone was dragging one wire across another. Unfortunately I could not tell if the noise was coming from both speakers. I just ran over and turned it off the first time it happened. The second time I tried turning down the volume and then turned it off. Should I be shopping for tubes? And could it be any of the tubes? There are no tube testers local to me. The closest I know of is an hour and a half drive away. Thanks, Eric
Reply to
etpm
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Chinese amp with Chinese transformers. So:

a) Obtain a dental-tool, one of those devices with a sharp, pointy hook on either end. With this tool, check _EVERY_ connection inside the amp. Look f or cold-solders, no-solders, cracked solders and so forth.

b)

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With that in mind, it is unlikely tha t the tubes within that beast have much to do with your issues, so now go t o the boards and look for any faulty connections, over heating components ( freeze-spray is your friend), raised traces and so forth.

c) Once done with the physical examination, do obtain a metered variable au to-transformer and determine two things: i) At what voltage does B+ kick in? Given a 5U4, should be about 80 V

+/- ii) And there should be a definite hard rise in current at that onset . NOTE: You should be drawing within 20% of +/- 50 watts or so, quiescent. If you are drawing substantially less than that, the tubes are eyewash, not f unctional.

Good luck with it - you will likely need it.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

Is it a mono or stereo amp? If stereo, is the noise identical in both chan nels? Can you hear suspicious noises coming directly from the electronics (like crackling from the transformers)? In the absence of any diagnostic l information, my knee-jerk reflex is to suspect the coupling condensers, fol lowed by the cathode bypass and power supply filters. When you have the no ise, try freezing one component at a time using freon spray. If you have an oscilloscope, does the noise show up on the power supply b+? These things do not have very many components, and you ought to be able to divide and c onquer.

Reply to
jfeng

The amp is stereo. The noise is coming from one side only. I'll post more info in another post. Thanks, Eric

Reply to
etpm

The amp you linked to is not my amp. My amp has no solid state devices except for the bluetooth module. But it is obviously separate from the amplifying electronics as it only connects to the input through a relay. Check out the link:

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This amp is similar but does not have the bluetooth module. Eric

Reply to
etpm

Same suggestions, although a bad Russian tube is no surprise whatsoever. Jeff is linking you to a reasonable site, however.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

I already have a really nice isolated auto transformer with a meter. The amp does draw a little more than 50 watts. I measured it when I made up the buck xmfr power supply for it. If the 5u4 tube starts to conduct much different than 80 volts what does that mean? Eric

Reply to
etpm

Hi Eric,

Look for flashes of light in any of the tubes - that would be serious and that tube(s) needs immediate replacement.

When your amp again misbehaves try unplugging the inputs to it. Still have loud noise?

Then unplug the two 6N9 tubes, one at a time to hear if that makes the noise go away. These are the phase splitter tubes for the four output tubes if stereo. Two EL84Bs if mono.

At least that is what I would do...

John :-#)#

Reply to
John Robertson

Most likely cause is a bad connection somewhere. The fault is most likely a fter the volume ctrl, though it might be on a secondary supply line. Connec tions can be prodded with something insulating. A resistor in series with t he speaker would keep the volume & Pdiss down. Keep an eye on supply curren t that it doesn't get excessive during fault.

A failing tube, cap or carbon resistor are also possible. Less likely deter iorating insulation. Comparing the voltages in the 2 channels while it faul ts might help home in on where it's going awry.

NT

Reply to
tabbypurr

Thank you Captain Obvious. If you had read the entire thread, you would have seen that it was a defective EL34. Please try to keep up.

--
"I am a river to my people." 
Jeff-1.0 
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Reply to
Fox's Mercantile

I always thought an autotransformer was a transformer with only one winding, and input and output partially share that one winding.

But if that is true, an autotransformer cannot possibly be isolated.

Reply to
HW

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Two components in one box. One of several examples.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

Agreed. I once wrote to a magazine who had mentioned "autotransformer" in the context of safety isolation on the basis that anyone who just bought verbatim would be putting themselves in danger! They published the correction...

Mike.

Reply to
Mike Coon

that info came later.

Reply to
tabbypurr

Yeah, it seems odd to have an isolated auto transformer. But it is. The auto transformer is isolated by another transformer all in the same case. Eric

Reply to
etpm

This gets down to basic bench safety. I do have some "expensive" instrument s, but my very first line of 'defense' is a metered iso-variac - the Heath IP-5220 pictured. On first introduction, any new-to-me item goes through it for preliminary screening and testing. And after many years of using this device, and finding all sorts of potentially fatal and/or dangerous faults thereby, I purchased a second unit for my traveling kit.

As a diagnostic tool, it is quite useful - I can "see" B+ at onset, and I c an measure current draw as compared to what it should be in theory. For the most part, those things we see and service are simple devices with a fairl y limited set of common failure modes. Getting past these conditions allows us to get to the heart of the matter much faster and with much more confid ence, ultimately leading to a better (and safer) outcome. No wonky power-su pply as one example disguising a more critical condition down-line.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

Do they really need two transformers? Sounds like a lot of redundant iron and copper.

Could they not use one normal (isolated) transformer and then have contact wiper move along the secondary winding?

Reply to
HW

I am sure they could. The problem is just geting one made. They probably just took 2 transformers that are already on the market and put them together. Less expensive than making just a few thousand of a new design.

Reply to
Ralph Mowery

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