Typical voltages in a PP amp

[Colas LeGrippa]

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

[Latole]

Is that you are looking for ?

https://robrobinette.com/AB763_Modifications.htm#AB763_Annotated_Schematic

[shooter]

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 

[Colas LeGrippa]

Sure I did shooter ! I am also a member of that forum. But I don t speak quite well fremch

[shooter]

Your just jealous of french canadians

It's required to speak frauncee' otherwise you couldn't make fun of Americans 

just turn it into a Fender and call it whatever bumper is called in French 

[Colas LeGrippa]

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 

[tubeswell]

Hi Colas


Is this the amp?


http://www.projetg5.com/Projets/MI60/MI60_original.pdf

[Colas LeGrippa]

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

[shooter]

max 60 mA @ 370V each tube

the datasheet I'm seeing appears to want 30mA per tube 

[Latole]

max 60 mA @ 370V each tube

the datasheet I'm seeing appears to want 30mA per tube 

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

[tubeswell]

max 60 mA @ 370V each tube

the datasheet I'm seeing appears to want 30mA per tube 

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

[Colas LeGrippa]

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 .


 
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

.

[DummyLoad]

you need to drink french wine before working on frog crap. j/k   

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

[Colas LeGrippa]

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 ...)


Explanation : the power tranny had been replaced in the past, Mr. Sherlock Holmes.


Anyway, with 450 or 350V, this amp should work fine.

[Colas LeGrippa]

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

[shooter]

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 

[Colas LeGrippa]

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 )   


are all it takes !!!