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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Melos SHA Gold Issue

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
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.
 

john65b

Member
Paid Member
2005-01-09 2:32 am
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?
 

john65b

Member
Paid Member
2005-01-09 2:32 am
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...
 

john65b

Member
Paid Member
2005-01-09 2:32 am
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.