Yaqin MC-5881A amplifier improvements - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th September 2007, 02:12 PM   #21
DanDini is offline DanDini  United Kingdom
diyAudio Member
 
DanDini's Avatar
 
Join Date: Nov 2006
One other thing I didn't note before (and should have). The noise is present from switch on - and doesn't get any worse (or less) when the valves warm up. In other words, it must be some ground loop hum or similar from the supply I guess?..
  Reply With Quote
Old 8th September 2007, 03:34 PM   #22
Salas is offline Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Quote:
Originally posted by DanDini


OK, I've done that and get 41.7Vdc down to 3.9mV on one side and 40.8Vdc down to 4.1mV on the other.

Any idea where this voltage inconsistency comes from as the transformers appear to deliver an even supply. Is it enough to be an issue?

---- No its not an issue, its just that one pair draws just a little differently because of valve tolerances. In fact its a triumph that we got practically on target for heater voltage and bias mods just from watching a photo! Now each valve draws 41mA and your heaters are averaging 6.3VAC. Heat is normal and bias too. Super!

Quote:
Originally posted by DanDini
One other thing I didn't note before (and should have). The noise is present from switch on - and doesn't get any worse (or less) when the valves warm up. In other words, it must be some ground loop hum or similar from the supply I guess?..
Its a ground loop by any bookmaker's favorite bet, and I would experiment braking it using a 10R 5-10W resistor. I would leave a cable go to chassis from mains ground terminal on the IEC socket, but I would insert the resistor between the ground wires common point and mains ground terminal. So mains ground directly to chassis, but ground cables common point is broken by the resistor to chassis. Keep in mind that chasing a ground loop for a given construction, layout and ground scheme from a pic its just betting on issues and it will be a miracle if we solve it or just attenuate it. If I had the amp before me, tweaking it, I could be much more positive.

But we can try ideas one after another. Just do that with the resistor for now so we see what it does.

P.S. As you practically saw, such cheap amps look good but they are not only using 3rd class components but they can be so badly cathode heated and biased out of spec that they can enter fire hazard zone. Having a strong ground loop in a finished commercial product is childish at least.

The chosen circuit is not bad at all though, and you can benefit from Auricaps, =C= 6L6GC output valves, and a small PSU choke if you can fit it. But small chokes have a lot of resistance and if common for channels it will eat much B+. Another idea is to use an extra 100uF 450V cap before the 470uF that is already there, and link them with a 10R 10W. CRC will cut somehow on your PSU ripple, and you will lose just 10V B+. A no space for 2 good chokes situation can benefit somehow from the CRC alternative suggestion. Appreciably better than the single cap you have there now.
  Reply With Quote
Old 8th September 2007, 03:45 PM   #23
Salas is offline Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Here is what the simulator shows for your single 470uF / 450V cap.
Attached Images
File Type: gif screenshot2.gif (5.0 KB, 1660 views)
  Reply With Quote
Old 8th September 2007, 03:47 PM   #24
Salas is offline Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
And here is what it shows for the 100uF - 10R - 470uF CRC. Notice the leaner horizontal line. A good reduction for ripple given the tight space that may not allow for chokes. Economic and straightforward too.
Attached Images
File Type: gif screenshot1.gif (4.9 KB, 1622 views)
  Reply With Quote
Old 9th September 2007, 03:17 PM   #25
DanDini is offline DanDini  United Kingdom
diyAudio Member
 
DanDini's Avatar
 
Join Date: Nov 2006
Right, after some fiddling with the grounds I discovered that most of the noise is coming from the input select and volume module. When this is disconnected from earth - it pretty much shuts up. You can see in the picture that there is (was) a wire soldered to this module and screwed to the chassis, but it is also earthed through the bodies of the switch and pot where they physically touch the chassis.

Click the image to open in full size.

If you look at the scope pic with the earth connected:

Click the image to open in full size.

...and without:

Click the image to open in full size.

..you'll see the difference. There is still some noise (as shown in the pic!) but a LOT less.

I think I'll try it like this for a while and consider the other mods as a next step. Overall it really doens't sound bad and now runs much cooler. It's interesting that the manufacturers specifically earthed this if it creates this problem.

Thanks for all the help!
  Reply With Quote
Old 9th September 2007, 04:01 PM   #26
Salas is offline Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Bravo Dan! They have created a loop within the GND circuit and you have broken it. If you try breaking with a resistor in other places like the one I mentioned earlier, you may probably kill hum even more. But you don't run extra sensitive speakers so maybe its futile. I am happy that we did a fruitful step by step problem solving with no back and forth. Well worthed the effort.

Regards

Salas.
  Reply With Quote
Old 10th September 2007, 09:33 AM   #27
DanDini is offline DanDini  United Kingdom
diyAudio Member
 
DanDini's Avatar
 
Join Date: Nov 2006
The big problem now is isolating the thing. Unfortunately the body of the input switcher is grounded and as the chassis is metal it's hard to get it rights. Is there not any way of resolving these things without breaking the connection to earth? Can you not 'equalise' the gnds so no loops occur?

Thanks,

Dan
  Reply With Quote
Old 10th September 2007, 10:00 AM   #28
DanDini is offline DanDini  United Kingdom
diyAudio Member
 
DanDini's Avatar
 
Join Date: Nov 2006
Another thing regarding the current we adjusted by lifting one of the 500ohm resistors... according to this data sheet, we are safe with 88ma, didn't I only have 70 or so per tube in the first place?

http://www.retrovox.com.au/tsol5881.pdf

Thanks,

Dan
  Reply With Quote
Old 10th September 2007, 04:14 PM   #29
Salas is offline Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
1. Yes, to 'equalize' the GND currents is what a competent layout in any commercial amp should have done in the first place. See about that 10R break suggestion I have proposed earlier.

2. That data sheet says 'values are for two tubes'. Was overheated, wasn't it? Each tube was at max dissipation before. Devouring itself.
  Reply With Quote
Old 10th September 2007, 05:19 PM   #30
DanDini is offline DanDini  United Kingdom
diyAudio Member
 
DanDini's Avatar
 
Join Date: Nov 2006
Quote:
1. Yes, to 'equalize' the GND currents is what a competent layout in any commercial amp should have done in the first place. See about that 10R break suggestion I have proposed earlier.
I'm looking into this next

Quote:
2. That data sheet says 'values are for two tubes'. Was overheated, wasn't it? Each tube was at max dissipation before. Devouring itself
Sorry - it was a misreading. I thought it was for one valve. My mistake.

Dan
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Yaqin mc10L gasman Tubes / Valves 385 Today 04:43 AM
Yaqin MC10K ramallo Tubes / Valves 0 22nd January 2009 05:42 PM
Yaqin MC-10L cruzrman Tubes / Valves 0 23rd July 2008 03:37 AM
Yaqin Valve Amplifier UK Sales & Service marktravuk Vendor's Bazaar 3 16th November 2007 05:55 AM
yaqin mc10k inquisitive Tubes / Valves 33 8th October 2007 01:56 PM


New To Site? Need Help?

All times are GMT. The time now is 06:56 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2