[MelonHead]

I've made further evaluations regarding my quiet but constant (30dB/650Hz) noise problem.

Today I disconnected the upgraded fans, but the noise remained, so I can exclude the fans as a possible source of the problem.
After this I switched off the amp and removed all the power tubes which resulted a silent amp. The noise has gone away.
As I put the tubes one after the other back I noticed a very-very quiet noise after the first tube, which has been developed further as I installed the others back.
After the four power tubes the noise level is the same as before.

I contacted a good technician nearby me who is specialized on tube amps. He will check it the next week.
Is there any PCB layout blueprint or other schematics available for the Little Dot MK VI+?

Thanks for any additional help.

 

[ViperGeek]

I contacted a good technician nearby me who is specialized on tube amps. He will check it the next week.
Is there any PCB layout blueprint or other schematics available for the Little Dot MK VI+?

The only official Little Dot reference I'm aware of is:

http://www.littledot.net/forum/viewtopic.php?f=9&t=831

I recall someone had a reverse-engineered schematic at one point. Maybe in the Little Dot Super-Mods thread?

https://www.head-fi.org/threads/lit...s-all-verified-mods-are-on-first-page.782183/

- Dave

 

[SonicTrance]

Today I disconnected the upgraded fans, but the noise remained, so I can exclude the fans as a possible source of the problem.
After this I switched off the amp and removed all the power tubes which resulted a silent amp. The noise has gone away.
As I put the tubes one after the other back I noticed a very-very quiet noise after the first tube, which has been developed further as I installed the others back.
After the four power tubes the noise level is the same as before.

I don't understand. Does the noise come through the headphones or does the actual components hum?

You should not run the amp without tubes as the power supply doesn't like it.

Is there any PCB layout blueprint or other schematics available for the Little Dot MK VI+?

Here's the psu part. It's the same for mk6 excluding the tube circuits.

Audio circuit

 

[SonicTrance]

R2 and R3 are actually 220k in the original schematic.

 

[MelonHead]

Many thanks for the schematics @SonicTrance . I think it will make the task of my technician easier.

The headphones and the speakers are completely silent. The hum is coming from the area of the transformers or the tubes I was not able to locate it even when my ears were 3 cm close to the running tubes.

Thanks for the notice as well. I read this tube removing process somewhere, and the driver tubes remained in their position, but I don't want to risk the amp. I was lucky enough and no problem occurred.

 

[MelonHead]

To save my ears from excessive heat, I used my kids' toy stethoscope to locate the problem better.
I can't be sure, as an audible buzz comes from the transformer cubes over the stethoscope as well, but the tubes seems to be more suspicious concerning the aforementioned "hum" noise.

 

[SonicTrance]

To save my ears from excessive heat, I used my kids' toy stethoscope to locate the problem better.
I can't be sure, as an audible buzz comes from the transformer cubes over the stethoscope as well, but the tubes seems to be more suspicious concerning the aforementioned "hum" noise.

I've never heard a tube hum or buzz, except through the audio circuit. Tubes only make "pings" or "pops" while heating up. Transformers can hum though. If it's not loud it's probably fine.

 

[MelonHead]

I've never heard a tube hum or buzz, except through the audio circuit. Tubes only make "pings" or "pops" while heating up. Transformers can hum though. If it's not loud it's probably fine.

I don't have major problem with it in daytime, but I often use my system on low volume levels (55-65dB) during nights. In this scenario this hum(?) is somewhat annoying.
This is the frequency what comes from the power cubes or tubes:

Turning down the volume knob is mandatory as the SPL is not more than 32-38dB depending on the distance from the amp.

 

[SonicTrance]

This is the frequency what comes from the power cubes or tubes:

Ok, now that's weird. That's neither a hum or buzz but a clean 630 Hz signal. I have no idea of what might be causing that.

 

[MelonHead]

Yep...
That's why I linked the 630Hz sine wave (it is a bit less, maybe 615Hz in reality) as this sound is not the standard transformer buzz or hum.
Anyway, I'm going to hand it over to the technician on Monday, and hopefully he will be able to solve this problem.

 

[SonicTrance]

Yep...
That's why I linked the 630Hz sine wave (it is a bit less, maybe 615Hz in reality) as this sound is not the standard transformer buzz or hum.
Anyway, I'm going to hand it over to the technician on Monday, and hopefully he will be able to solve this problem.

Keep us updated. Gonna be interesting to hear whats causing the noise.

 

[MelonHead]

Keep us updated. Gonna be interesting to hear whats causing the noise.

I will.
Unfortunately my man went to sick leave today. Maybe tomorrow.

Meanwhile I made some post-processing on your photo for better visibility. If you don't mind, I share it here.

 

[SonicTrance]

Meanwhile I made some post-processing on your photo for better visibility. If you don't mind, I share it here.

That's fine. And to clarify, its user @Redge78 that has drawn that psu schematic. Its posted on the first page in the mk6/8 modding thread.

 

[MelonHead]

That's fine. And to clarify, its user @Redge78 that has drawn that psu schematic. Its posted on the first page in the mk6/8 modding thread.

Thanks for mentioning the author SonicTrance!

Annoying noise:
I was a bit unsuccessful with the technician.
At least I know, this is a buzz which comes from the power transformer cubes and the two metal cover boxes emphasize and transform it to 35dB/600Hz.
Unfortunately we were unable to unscrew all the bolts, as the cover of the rectification part between the two metal cubes blocks the way to the inner bolts. He refused to spend more time with it, as the disassembling was too tricky. He a is well known and very busy guitar tube amplifier service expert here, who got used to much bigger devices. At least, he was honest and didn't accept any payment for his 30 minutes long task.

As I arrived back, I was playing a bit with the transformer cover boxes. After unscrewing the outer bolts the intensity of the noise was remarkably lower. Pressing the boxes at some random points, and moving them a little, I was able to change the frequency and the intensity of the annoying sound to almost zero. So maybe some insulation between the cover boxes and the inner metal parts can totally solve my problem.

Is there any uncomplicated way available to remove the power transformer cover boxes?
Thanks in advance for any hints or help.

By the way, he mentioned, turning the amplifier on without the tubes means absolutely no problem. According to him there are some awkward designs where it may lead to troubles, but this is a well planned and assembled amplifier except the illogical placement of transformer cube screws. He demonstrated it by pulling out the power tubes, and testing the noise without them.

Tube rolling:
My 50th anniversary, matched Shuguang Black Treasures are on the way from Spain.
As a first, inexpensive batch of power tubes I ordered a quad of matched NOS Svetlana 6N13S "winged C" from the '70-s as well. As I've read the general consensus is positive, so any improvement compared to the stock tubes -which produces more than acceptable sound to me- is a good thing. They should arrive next week as well.
Meanwhile I tried two Shuguang 6AS7G-s (spare tubes for my Darkvoice 336SE) switching out the first and third stock power tubes. I know they don't have a good reputation, but using them I received a bit more holographic and clearer sound. Not a bad start.

 

[SonicTrance]

As I arrived back, I was playing a bit with the transformer cover boxes. After unscrewing the outer bolts the intensity of the noise was remarkably lower. Pressing the boxes at some random points, and moving them a little, I was able to change the frequency and the intensity of the annoying sound to almost zero. So maybe some insulation between the cover boxes and the inner metal parts can totally solve my problem.

Ok, now it sounds like you have vibrating power transformers. Don't remember if there's any anti-vibration pads under the transformers (don't have my mk6 anymore).

Is it an easy way to remove the power transformer cover boxes?

If you can't get to the screws without removing the plate between the transformers you need to remove the pcb to get access to the screws for that plate. Removing the pcb is hard the first time you do it. Lots of info in the mod thread.

By the way, he mentioned, turning the amplifier on without the tubes means absolutely no problem. According to him there are some awkward designs where it may lead to troubles, but this is a well planned and assembled amplifier except the illogical placement of transformer cube screws. He demonstrated it by pulling out the power tubes, and testing the noise without them.

If you power on without output tubes, the only load the psu sees is the 56k drain resistors. Then the zeners will see a much higher voltage and could get damaged or brake. The resistors that are in series with the zeners sets the current for the zeners. Those resistors will also burn at much higher voltage since they're already working very near their 0.5W rating. The voltage drop will be much higher across the series resistors causing more current to flow through them (and the zeners). Which in turn could brake the power transistors after the zeners.