French amplifier issue

[Colas LeGrippa]

Hi buddies, I hope you're doin' well.
I have on my bench a MUSIQUE INDUSTRIE MI60 french amplifier (supposedly they have one in abbey road studio...hoping that this particular one works better).
The problem is the bias. I measure 350V on the plates and screens,  minus 35 on the control grids. The k's are pulling only 3mA, and of course very poor guitar sound. Swapped the 2 X EL34, same problem. Never biased so positive with so poor current. This amp has been tweaked badly . They used pin no. 1 of the power tubes as tied point for the grid stoppers.
Always tied 1 to 8  on el34's and  I m not sure if the  bias issue comes from there. Anyway  it is always the same: after a couple of months without tweaking amplifiers, my brain cpu is overheating trying to get it s memory goin'.
Colas

[shooter]

pin 1 is G3, usually tied to pin 8, no clue why grid stoppers are there, I'd put them where they belong.

The k's are pulling only 3mA

self biased then?

[tubeswell]

Could be a screen supply issue. What are your screen voltages? (if g2 voltage is insufficient, the tubes will barely conduct even if bias voltage is 'normal')

[Colas LeGrippa]

350 v screen grid, as first written

[DummyLoad]

seems to me with -30-ish volts applied to suppressor grid you wouldn't flow much plate current. undo the grid stoppers from pin1 and tie them to pins 5 & 6 where they belong.

--pete

[DummyLoad]

if you have a pair of 6L6GB/GC handy, install those and see what K current draw is. that way you wouldn't have to undo the grid stoppers to pin1 - load won't care.
 
--pete

[sluckey]

They used pin no. 1 of the power tubes as tied point for the grid stoppers.

Well that works fine for 6L6 type amps such as fender AB763, but you absolutely cannot do that with EL34s. Maybe this amp originally used 6L6s or KT66s. But if you insist on using EL34s, you better wire pin 1 properly.

[Colas LeGrippa]

Yeah that s what I thought. I will do the mod and get back to you.
Thnx

[archaos]

The MI60 indeed used a pair of EL34s : https://www.projetg5.com/Projets/MI60/MI60_original.pdf

Bias should be around -37V ; around 430V on the plates ; OT = 4k2

One guy said he had a model different than the above schematic : g1 & g3 tied together with a 700R resistor (a bit burnt  yet), and no control grid resistors : https://www.projetg5.com/phpbb3/viewtopic.php?p=42813#p42813
Unfortunately his schematic has gone.

It seems the former models used either EL34s, 6550s or even 6146s.
6550 model :
http://stickyducks.free.fr/files/divers/laurent/MI_1968/PICT0010.jpg
http://stickyducks.free.fr/files/divers/laurent/MI_1968/PICT0009.jpg

Nowadays the MI60 has also its own forum (in French) : https://www.projetg5.com/phpbb3/viewforum.php?f=11
Hope that helps.

[sluckey]

They used pin no. 1 of the power tubes as tied point for the grid stoppers.

.... you absolutely cannot do that with EL34s....

I'm gonna back off that statement. I've never seen G3 connected like that but I have seen G3 connected to the bias supply. Maybe this strange wiring works. Could you post a hi-rez pic showing exactly how those resistors are wired to the sockets?

[Colas LeGrippa]

No. Already removed them . The grid stopper was placed right in the middle of the socket, making a bridge between pin 1 and 5, on each power tube. I m up to the point where I ll redo the bias suplly. I just cant understand why all voltages bein good, and the tubes too, there is nocurrent through the tube.
Geewhizzz.....lemme think...my Fluke would be bad ?????

[shooter]

my Fluke would be bad ?????

 

Tech101  - verify test equipment before testing equipment 

[Colas LeGrippa]

yep. Fluke is gone. Thrill is gone. (try to imagine BB King singin' : Fluke is gone, fluke is gone away !!! )


I redid all the solders I could , battery was good, fuses too, leads too. I think one of the two master controlers out


dunno how much time will last my hardware store meter , pure china, pure crap, pure scrap but real bargain at 9.95 $


Sluckey: I won't rely on you anymore from now on I ll double check your statements


I ll forgive you if you find me a used Fluke in good condition….

[sluckey]

Aw come on Colas. You know I've always treated you special!   

[Colas LeGrippa]

...the kind of treatment nobody's seeking after

[Colas LeGrippa]

I d spend 250 bucks for a new Fluke the prob is that there are a lot to choose from.  Any suggestion, amigos ? Don t need the temperature or UFO measuring options. Don t need to monitor my fridge from another city in the world .
Thanks


Colas

[shooter]

Don t need

don't use 

I'd eat bean n rice for a week to get the extra $100 and buy the 87 V

Anything Fluke has is quality - or was, my meters 10+ yrs old

[Colas LeGrippa]

That s the model number, 87V  ?

[shooter]

actually I believe it's Roman numeral 5, but most just call it V, same with my EVO X,  the tenth version, language, shoulda never built that tesla tower 

[DummyLoad]

from what i've been reading - the best currently available DMMs for those on a tight budget are extech and amprobe models. YMMV - 90% are made in asian countries with cheapest components it seems.

i had a Fry's sourced B&K DMM that gave up the ghost measuring 650AC across a PT: it WAS rated for 1000 VAC/DC - which apparently = 100V  by chinese mfg? kind'a pissed me off as i paid 150bux for the damn thing. now it's just junk that keeps getting in the way... cap and ohms still work and why i haven't canned it.

--pete

[DummyLoad]

did you get this amp sounding any better?


--pete

[Colas LeGrippa]

Yep. I silicone glued a 50k mini pot on the PCB  to adjust the bias.
Now one tube draws 90 mA while the other 0.
I suspect the  corresponding coupling cap from the LTP that I have already removed.  I m lacking time to work full time on it : I promised my daughter to finish her patio, I m having troubles bleeding the rear caliper on my Harley, one of my friend asked me to repair one of his apt before renting it , all of that between rehearsals and girlfriend who always comes back for.more, shit, leave me alone for a while !! But I ll be a rock star very soon and will afford to pay a carpenter, mecanic, plumber, electrician  and amp tech. I.pray the.lord every.night , I feel my wishes will become reality soon ! !   
Pray for me please !!!!!!!!!

[Colas LeGrippa]

Even you Sluckey, pray for me. I know you love me in the bottom of your heart
Your just jealous of french canadians.but lord forgives you and I love you too !
( wouldn't you lend me five thou ?.)

[Colas LeGrippa]

Yes shootet :


V = 5 ,  X = 10   C = one hundred  and so on

[DummyLoad]

I m having troubles bleeding the rear caliper on my Harley,

oh, i.c., a two-wheeled tractor. can't imagine why you're having any difficulty there?!? i used to do my own maintenance on my harley's. sold a dyna super sport (FXDX) that i regret doing. the rest, meh! i'm guessing the 5K you're hitting steve up for is to keep the harley in spare parts and the oil filled?

Q: what's difference between a harley and hoover vacuum?
A: location of the dirt-bag! 

did you check OT winding continuity from pin3 of tube that draws 0 mA to OT CT?

--pete 

[sluckey]

i'm guessing the 5K you're hitting steve up for is to keep the harley in spare parts and the oil filled?

Actually he needs to hire someone to kiss his girlfriend so he can get all that other shit done.   

[Colas LeGrippa]

Dummy,yes I did check the continuity. I get 39.2 ohm from.one side and 42.3 from the other.


Sluckey: no comments about the different abilities of my girlfriend. I wouldn't dare talk about that particular subject with a moderator in a tube amp forum......


It seems that air bubbles are trapped somewhere in the brake line between the master cylinder and the bleeder nipple.
Oops I apologize for posting  this in the wrong forum.


Thanx for help. I ll keep you informed about
the developments concerning the MI amplifier.


Colas

[DummyLoad]

i have one of these: they are indispensable when you need to evacuate a system to rid them of air pockets. rear brakes on harley's are notorious for getting trapped air in the caliper.

remove the caliper, attach that bleeder tool and hang the caliper below the level of the master cylinder, evacuate as per the instructions. that should get rid of that pesky air bubble. another possible issue is that air is entering the master cylinder or slave cylinder, usually there's a fluid leak when that happens though.

good luck with amp and scooter!


--pete 

[shooter]

afford to pay a carpenter, mechanic, plumber, electrician  and amp tech

 
when I could afford them, I loved doing it myself, now that I can't afford them, I really hate looking at another broke thing....

[Colas LeGrippa]

Thanx Pete, you're a bro. Btw my bike is a dyna superglide FXD from 2002.


Right Shooter, it is always a pleasure trying to fix things without any obligation of results.
When you re broke, you gotta succeed and that causes unwanted stress.