Melos SHA Gold Issue - 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 5th March 2008, 02:37 PM   #1
john65b is offline john65b  United States
diyAudio Member
 
john65b's Avatar
 
Join Date: Jan 2005
Location: Chicago
Default Melos SHA Gold Issue

Hello all,

I was given a non-functioning Melos SHA-Gold Preamp by a friend the other day to see what is wrong. I finally got into it and noticed the heaters were not firing. I pulled the 6922 and went around the horn and saw the plates appear OK at 90V.

But the heaters pins (4 and 5 referenced to ground and no heater center tap on 6922) were +6.5VDC and +7.3VDC. There's only a .8V potential between them to fire the filament. I looked up the schematic (took me forever to find it) and noticed that the heater schematic shows connecting a dual supply of +6.3VDC and -6.3VDC on the heater pins. Does this seem odd to have 12V across the heater pins and not 6.3V on the 6922? I was expecting 6.3V and ground on the other heater pin.

Anyway, the +7.33V should be a -6.3V if schematic is correct, so maybe the LM337 regulator is toast...


And it got me thinkin' (uh oh...) I pulled my 12B4 pre I built a while back before I knew what I was doing (please don't laugh) and I measured the heater pins on the 12B4 - and noticed they both are 9.3VDC (I put DC on heaters after too much AC hum) referenced to the heater center-tap. This seems too high by 3V each side, correct?

I understand I am most likely sucking some life out of the tubes, but does too much voltage on heaters affect sound quality?

Should all these heater voltages be measured loaded - i.e. - with tube installed?

Sorry for the lame-o - tyro questions -
Thanks in advanced.
__________________
All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian
  Reply With Quote
Old 5th March 2008, 04:56 PM   #2
john65b is offline john65b  United States
diyAudio Member
 
john65b's Avatar
 
Join Date: Jan 2005
Location: Chicago
I realized after I posted the above that the whole shebang needs to be loaded - heater voltages taken with tubes in - duh

Temporary insanity here...

My question still stands on over voltage of heaters make a difference in sound quality...
__________________
All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian
  Reply With Quote
Old 6th March 2008, 04:29 AM   #3
john65b is offline john65b  United States
diyAudio Member
 
john65b's Avatar
 
Join Date: Jan 2005
Location: Chicago
OK, so after pulling those voltages with the tube back in, I got 6.5VDC on each side wrt heater centertap on the 12B4, so all good here..

But those voltages on the Melos are +8vdc on both heater pins 4 and 5.

So either the LM317 and/or the LM337 are bad or the heater rail caps are toast...

Still don't know why there's +6.3VDC and -6.3VDC on the heaters in the first place...anyone?
__________________
All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian
  Reply With Quote
Old 7th March 2008, 01:40 AM   #4
john65b is offline john65b  United States
diyAudio Member
 
john65b's Avatar
 
Join Date: Jan 2005
Location: Chicago
OK, I thought since I had DC, thought the rectifier was good to go, but if no negative rail, then it has to be a bad rectifier...

The LM337 has nothing to do with not having a negative rail...
__________________
All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian
  Reply With Quote
Old 7th March 2008, 04:54 AM   #5
john65b is offline john65b  United States
diyAudio Member
 
john65b's Avatar
 
Join Date: Jan 2005
Location: Chicago
Well replaced the rectifier and all is good. Heaters are all lit and working fine. Works great.

I told my friend that if I fix it, I was going to keep it for a week to play with it. Sounds pretty nice.

Still don't understand the whole heater thingy, but, well, she works.

Thanks anyway...
__________________
All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian
  Reply With Quote
Old 7th March 2008, 10:04 PM   #6
diyAudio Member
 
Join Date: Oct 2001
Location: .
Quote:
Originally posted by john65b


.....Still don't understand the whole heater thingy, .....

The heater is the load for the solid state output stage. +6v3 to the darlington, -6v3 to one end of the heater and the other connected to the emitter of the darlington, forming the output. A servo keeps the output at 0v. 6v3 across the heater.
  Reply With Quote
Old 8th March 2008, 02:09 AM   #7
john65b is offline john65b  United States
diyAudio Member
 
john65b's Avatar
 
Join Date: Jan 2005
Location: Chicago
OK,

I can see this on the schematics now...kinda glad I didn't disconnect the neg rail and connect other heater pin to ground...

I have seen something kinda similar where a heater filament is powered by another adjacent tubes cathode bias voltage...

So my friend thinks I ROCK since I was the only one who could not only find a schematic, but fix his preamp. He got shot down a few times by a few local shops that would have charged him an arm and a leg to replace the rectifier had they been able to locate a schematic.
__________________
All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian
  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
Info on Melos MA-111? neddyboy Tubes / Valves 7 8th June 2011 05:27 PM
Melos Sha Gold ManUtd Tubes / Valves 12 30th March 2006 01:42 AM
Help with Melos SHA repair dsavitsk Tubes / Valves 2 15th July 2005 08:37 PM
FS: Melos MAT-180 175 wpc tube amp Kip Swap Meet 0 31st July 2003 06:09 PM
FS: Melos SHA-1 Tomo Swap Meet 1 13th October 2002 05:12 AM


New To Site? Need Help?

All times are GMT. The time now is 10:10 AM.


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