Mig100 with 550v on the plate trying to fry my 6L6's

[pi-amp]

Hello!
This is my first post on this great forum
I'm trying ti bring a sovtek mig 100 back to normal operation
i found it with the screen resistors fried
It's basicaly a jcm800 with 6L6's and a odd power schematics (tension doubler)
The beast puts 550V on the plate and the screen of the 6L6's... Currently it has new screen resistors and brand new 6L6GC.
But i'm afraid they won't last long with that voltage...
So i have 2 questions for you guys:
1/ is there a 6L6 equivalent that would stand that voltage? i heard of sovtek 5881WXT?
2/ is there a way to reduce my B+? I know the zener trick on the CT but the schematic of this one is odd...
Thanks!

[2deaf]

1/ is there a 6L6 equivalent that would stand that voltage? i heard of sovtek 5881WXT?

The Sovtek 5881WXT is the same as the 5881/6L6WGC that originally came in those amps.  Supposedly.  Those were some tough tubes with a mean bite when overdriven.

[sluckey]

I would just replace those fried 750Ω/1W screen resistors with 1K/5W and let her rip for a while. Watch for red plates!

[PRR]

6L6GC (any modern "6L6") will take 550V on plates.

The 540V on Screens is over-the-top.

So do you want to PLAY LOUD!! ? Or live forever?

Tubes today are cheaper than any time in my life. When they go bad, replace them, just like drummers. <G>

OTOH, the screen feed could be moved to the 275V tap. I do not think maximum power will be down much. Obviously the G1 bias supply has to be quite different- this is major hackery on an amplifier that mostly works fine stock.

[shooter]

If you're ordering n soldering and keeping, this might be a good candidate for 1ohmers on the power tubes cathodes 

[Ritchie200]

What Sluckey and PRR said. Switch out all those 750 with 1K FIVE watt and those tubes will be fine. Someone probably lost a tube and kept playing till the fuse cooked. They are LOUD boogers for sure.  My buddy's music store used to sell them.  If I remember, he did not have any quality issues with them as they were built like tanks.  Great sounding amps.

"When they go bad, replace them, just like drummers. <G>" Or keyboard players!  Or bass players for that matter....

Jim

[2deaf]

If you're ordering n soldering and keeping, this might be a good candidate for 1ohmers on the power tubes cathodes

It can be done, but it is not as easy as it sounds.  The cathodes are daisy-chained together and then wired to a turret over on the power supply board.

[2deaf]

What Sluckey and PRR said. Switch out all those 750 with 1K FIVE watt and those tubes will be fine. Someone probably lost a tube and kept playing till the fuse cooked. They are LOUD boogers for sure.  My buddy's music store used to sell them.  If I remember, he did not have any quality issues with them as they were built like tanks.  Great sounding amps.

"When they go bad, replace them, just like drummers. <G>" Or keyboard players!  Or bass players for that matter....

A lot of people seem to be down on Sovteks, but it was just like you said.  They were built like tanks, even using cold-war era military parts left over after the fall.  Barely even got warm playing wide open.  It was like the wild west days of amplifiers all over again.  Each one had a unique set of components and a unique sound.

Bass players?  Now wait a minute.  All of the singers flaked-out years ago, the drummer can't hit two beats in a row on time, the guitar player is too jacked-up on anti-psychotics to have any inspiration, and the keyboard player is dead.  The bass player (me) is the only one that can still play.

[pi-amp]

Thanks for your replies!
I already wired 4 1 ohm bias resistors and biased on the conservative side, i got no red plating.
I'd better keep the original screen grid resistors value, they are 470R 5w
Don't know What to think with feeding the screen at 275v... Could loose that beasty sound !

What about putting some zeners between the diodes and ground to reduce b+ ?

[alerich]

I have a MIG60 that runs the EL34 outputs at 640VDC on the plates but because it has a stacked power supply with two independent secondary windings and rectifiers the screens run at about 350VDC. As Lord Valve from Usenet once told me "If you take care of the screens you can murder the plates". The majority of catastrophic tube failures are due to excessive screen current. Your fried screen resistors are a testament to that. I like the idea of moving the screen supply tap. It's worth a try and would give you the safety margin you need. If you don't like the way it impacts the tone (you might even like it more that way), restore it and move on to plan B. The B+ voltage is on the high side for a 6L6GC but good quality current production tubes should operate okay. The 1K 5W screen resistor suggestion sounds good, also.

[Willabe]

As Lord Valve from Usenet once told me "If you take care of the screens you can murder the plates". The majority of catastrophic tube failures are due to excessive screen current.

That's why you at least want to do this if you don't take the screen dcv from the center of the B+ A node cap stack;

The 1K 5W screen resistor suggestion sounds good, also.

The grids in a tube are just a thin wire, with little mass. With the screen grid, too much current heats them up, and they fail. The screen grid R helps limit the current.

[pi-amp]

I tried to feed the screens at 2nd node. I still get 400v there !
But now i can't bias the tubes... At full bias i'm at 20mA. Don't want to alter the bias circuit... So i'll leave it at 540 plate and screen and let's rock !

[Willabe]

I tried to feed the screens at 2nd node. I still get 400v there !
But now i can't bias the tubes... At full bias i'm at 20mA. Don't want to alter the bias circuit...

Did you turn the -bias pot both ways? 'Full -bias' is the most negative setting, least mA setting. You need less negative dcv bias to run the tubes hotter.   

Your probably getting 400dcv at the B+ filter cap stack mid point because it also changed the power tubes current draw. Less load on the B+ PT wind.

Lower current through the power tubes the B+ dcv rises, higher current, it drops.

You'd have to measure the B+ at the power tube plates to see how much it rose.

You'd only have to change 1, maybe 2 R's to decrease the -bias so the tubes run hotter. I'd probably try dropping the 47K to maybe 20K or 15K?

You might not even have take the 47K out, just alligator jumper cable in another 47K in parallel with that 47K. That would give you 23.5K to test it with.

Take the power tubes out, keep them out, measure the max and min. -dcv bias at the power tubes grid pin, write it down. Then clip in the 47K and measure max/min. -dcv again, write it down. They should now both be lower, less negative. With the -dcv bias still set on max, most/highest negative setting, pop the power tubes back in, then try to bias them hotter, less negative dcv and see where your at.

If still not enough, and this might be better anyway, double or triple the value of the 15K that's between the 2 @ 50uF -bias filter caps. And leave the 47K alone. You might have to do both? It just easier to clip in across that 47K for a test, so no soldering.   

[chocopower]

It's basicaly a jcm800 with 6L6's and a odd power schematics (tension doubler)

I think that´s not a doubler.
Power transformer´s center tap is balancing the operation in the stacked e-caps.


And as PRR said, "the screen feed could be moved to the 275V tap".

[2deaf]

Did you connect the screens to the center tap on the PT or to node C?  The 15K 2W dropping resistor tends to fry in stock MIG-100's and if the screens are hooked to node C, that should finish that resistor off.

The 10K bias dropping resistor is mounted on turrets and it isn't always 10K.  Kind of suggests that they weren't sure what was going to come out of that bias winding until they fired it up.  In any event, it makes it easy to change that resistor to get whatever bias you want.

[chocopower]

Hi. this way.

[pi-amp]

Hi chocopower,
This way what ?

2deaf : thanks for pointing out the stress on the dropping resistor... You're right!
Anyway it's going back to stock, i'll see if it resists!

[davidwpack]

I just happened to notice that EHX has a Mig 50. Same amp?

[2deaf]

I just happened to notice that EHX has a Mig 50. Same amp?

No.

[pi-amp]

Not the same but same insane voltage on the 5881 :D