Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on July 09, 2020, 08:59:20 am
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Hola amigos, I m having a hard time trying to fix the f...french amplifier someone brought to me.
What would help me is finding a schematics with voltages at different points so I could eventually pinpoint the issue. All voltages seem good at first glance but something in that amp is laughing at me and I really want to take my revenge thnx
Colas
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Is that you are looking for ?
https://robrobinette.com/AB763_Modifications.htm#AB763_Annotated_Schematic
(https://live.staticflickr.com/65535/50094311762_02d466accf_h.jpg) (https://flic.kr/p/2jjErCo) (https://flic.kr/p/2jjErCo)
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did you follow the links from your other post, reply #8?
Nowadays the MI60 has also its own forum (in French) : https://www.projetg5.com/phpbb3/viewforum.php?f=11
Hope that helps.
something in that amp is laughing at me and I really want to take my revenge
yup, typical French :icon_biggrin:
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Sure I did shooter ! I am also a member of that forum. But I don t speak quite well fremch :laugh:
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Your just jealous of french canadians
It's required to speak frauncee' otherwise you couldn't make fun of Americans :icon_biggrin:
just turn it into a Fender and call it whatever bumper is called in French :laugh:
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Omg I wish I could turn it into a Fender.....
Hey wait a minute, you re giving me an idea ! Nobody will know (but you and I ) that I have modded his garbage can to that point, the important to my client is the way it sounds now, not how it was sounding before !!!!!!
Ok what model should I mod it into ?
Bassman, 2204, TopHat, name it I ll do it.
50 watter without rev. nor trem.
Yeah I think I found the right path for me to get rid of this shit :icon_biggrin:
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Hi Colas
Is this the amp?
http://www.projetg5.com/Projets/MI60/MI60_original.pdf (http://www.projetg5.com/Projets/MI60/MI60_original.pdf)
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Yes it is !
What I did up to now is :
new bias circuit with adjustment pot
result : max 60 mA @ 370V each tube
swapped El34 for known good ones
installed 1 Ohm resistors from tied 1-8 to gnd
swapped 12ax7's for good ones
disconnected temporarily the tone stack by hooking up a .047 from first preamp tube direct to the PI tube
replaced signal wires by shielded ones
replaced the 2 coupling caps from PI to power tubes
replaced the 2 grid stoppers to control grids of power tubes
re soldered half a dozen of wires between tubes sockets and pc board
measured resistance between OPT center tap and plate supply +/- 50 ohm each side
checked ground connections
Weird thing: To obtain 60mA through the tubes,
I gotta raise the control grids voltages up to -23V which is very positive according to my experience
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max 60 mA @ 370V each tube
the datasheet I'm seeing appears to want 30mA per tube :dontknow:
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max 60 mA @ 370V each tube
the datasheet I'm seeing appears to want 30mA per tube :dontknow:
60 ma may damaged tubes
With 370 volts at 70 % power dissipation tube bias should be less than 47 ma .
Close to 30 ma could be better if sound is good to ears
http://www.tedweber.com/webervst/tubes1/calcbias.htm
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max 60 mA @ 370V each tube
the datasheet I'm seeing appears to want 30mA per tube :dontknow:
34.4mA if you add in screen current
‘Lord Valve‘ (quoted in an article on Duncan’s Amp site) suggests rating EL34s as a 20W tube- which would line up with the suggestion on that Mullard datasheet.
http://www.duncanamps.com/technical/lvbias.html (http://www.duncanamps.com/technical/lvbias.html)
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I think your calculation is too low
At 370V, an EL34 at max diss of 17.5W calls for 48 mA . When I mentioned 60mA, that was the max the adjust. pot could deliver and was for test purpose only. I still find bizarre the so positive voltage on the grids but I can live with it.
Anyway, I got rid of the bias issue now. Before I put a bias pot and replaced the couplimg caps, one of the tubes was passing 0mA.
The main issue is poor sound lack of volume and definition, and hum. I suspect poor PCB design leading to oscillation. I already told yousomething in that amp is laughing at me .
:cussing:
I ll try to learn more from the frenchmen in that musique industrie forum, but I have doubts they will help me. Anyway, one beer at a time (I wish I d smoke pot, but now that it s legalized, I think.I would find that boring....)
Thnx guys
.
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you need to drink french wine before working on frog crap. j/k :icon_biggrin:
what's bugging me is why the B+ is 70V lower than it should be... IF that PT is the original, it should be ~340VAC to a FWB - what do you measure for AC at the bridge AC terminals?
if that PS is sucking off 70V +/- then you likely have some main filters that need replacing.
--pete
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I only drink ultra cold german blonde beer when it s hot
like today: 80 deg or more. In winter I dont drink but water. French wine contains sulfites that make me feel bad.
(my life is so boring ...) :laugh:
Explanation : the power tranny had been replaced in the past, Mr. Sherlock Holmes.
Anyway, with 450 or 350V, this amp should work fine.
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Good point Pete, tomorrow I will measure the AC voltage at the pt out. That voltage X 1.4 should be read in DC at the rectifier bridge. I ll do whatever has to be done with the power caps acordingly.
Colas
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I think.I would find that boring....)
ah, it's fun, get a lawn chair, some ledZepplin, good buzz, wear underwear only wave to the good people :icon_biggrin:
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My brain can t support drugs, my lungs smoke
and my stomach too much alcohol. I tried LSD when teenager but my anxiety was multiplied by 10 .
Good music, motorbike ride in the nature , guitar and songs composing( right word? the frog in me doesn t know :laugh: )
are all it takes !!!