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

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 for cold-solders, no-solders, cracked solders and so forth.

b) https://www.stereo.net.au/forums/top...d-ms-10d-mkii/ With that in mind, it is unlikely that the tubes within that beast have much to do with your issues, so now go to 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 auto-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 functional.

Good luck with it - you will likely need it.

Peter Wieck
Melrose Park, PA

Is it a mono or stereo amp? If stereo, is the noise identical in both channels? Can you hear suspicious noises coming directly from the electronics (like crackling from the transformers)? In the absence of any diagnostic linformation, my knee-jerk reflex is to suspect the coupling condensers, followed by the cathode bypass and power supply filters. When you have the noise, 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 conquer.

On Mon, 30 Dec 2019 06:52:40 -0800 (PST), "
wrote:

Is it a mono or stereo amp? If stereo, is the noise identical in both channels? Can you hear suspicious noises coming directly from the electronics (like crackling from the transformers)? In the absence of any diagnostic linformation, my knee-jerk reflex is to suspect the coupling condensers, followed by the cathode bypass and power supply filters. When you have the noise, 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 conquer.
The amp is stereo. The noise is coming from one side only. I'll post
more info in another post.
Thanks,
Eric

On Mon, 30 Dec 2019 05:01:07 -0800 (PST), "
wrote:

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 for cold-solders, no-solders, cracked solders and so forth.

b) https://www.stereo.net.au/forums/top...d-ms-10d-mkii/ With that in mind, it is unlikely that the tubes within that beast have much to do with your issues, so now go to 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 auto-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 functional.

Good luck with it - you will likely need it.

Peter Wieck
Melrose Park, PA
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:
https://www.doukaudio.com/el34-tube-...025-p0025.html
This amp is similar but does not have the bluetooth module.
Eric

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

On Mon, 30 Dec 2019 13:33:24 -0800 (PST), "
wrote:

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
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

On 2019/12/29 11:29 a.m., wrote:
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


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 :-#)#

On Sunday, 29 December 2019 19:29:49 UTC, wrote:
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

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

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


NT

On 12/30/19 9:58 PM, wrote:
Most likely cause is a bad connection somewhere.

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
WA6FWi
http:foxsmercantile.com

On Mon, 30 Dec 2019 13:56:06 -0800, wrote:

isolated auto transformer

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.

http://www.ipernity.com/doc/777361/45315684

Two components in one box. One of several examples.

Peter Wieck
Melrose Park, PA

In article ,
says...

On Mon, 30 Dec 2019 13:56:06 -0800, wrote:

isolated auto transformer

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.

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.

On Tuesday, 31 December 2019 04:14:24 UTC, Fox's Mercantile wrote:
On 12/30/19 9:58 PM, tabbypurr wrote:
Most likely cause is a bad connection somewhere.

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.

that info came later.

On Wed, 01 Jan 2020 17:00:03 +0100, HW wrote:

On Mon, 30 Dec 2019 13:56:06 -0800, wrote:

isolated auto transformer

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.
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

On Thursday, January 2, 2020 at 12:17:25 PM UTC-5, wrote:

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.


This gets down to basic bench safety. I do have some "expensive" instruments, 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 can 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 fairly 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 confidence, ultimately leading to a better (and safer) outcome. No wonky power-supply as one example disguising a more critical condition down-line.

Peter Wieck
Melrose Park, PA

On Thu, 02 Jan 2020 09:17:25 -0800, wrote:

The auto transformer is isolated by another transformer all in the
same case.

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?

In article ,
says...

The auto transformer is isolated by another transformer all in the
same case.

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?



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.