Sovtek Mig 100H help Re: no sound

[tubenoob]

Thanks to tubenit for bringing me to the forum.

Turned on the head one day, and there was zero sound. No pops, buzz, or hum. I have no to little experience working on amps, and don't know where to start. I hope this is a fix I can do on my own. Here is what I can tell the amp does have/do as of now

-Fuses are fine tested them with a multimeter
-Tubes light up

I found the schematic here at the site, and I'm willing to open 'er up.

[plexi50]

Well i am a bit hesitant to have you sticking your hands in a 500+ volt amp but here are the first things you need to know. Do not ever rest your arm or hand on the chassis while probing with the red probe to test any voltages.

If the amp is not blowing the fuse do the following below to start with.

First thing: Pull the power tubes. You can leave the preamp tubes in the amp for now.

Turn the amp on all the way. Power and standby switches in the on position

Set your meter to handle 1000 volts DC if it is not a high quality automatic voltage reading DVM meter.

With the black meter lead to the chassis ground, take the red meter probe and tell me what you find for DC voltage on the standby switch.

Im online for a while /

[spacelabstudio]

Troubleshooting is a matter of understanding how the circuit works in normal operation, reading voltages looking for anomalies, and then reasoning about what would cause the observed anomalies.  You really have to have some measurements and a basic idea of what's going on.  And these things are dangerous, so I'd be hesitant to advise someone with no practical experience to open it up and start poking.  What would be ideal is if you had someone local with a little more experience who could help you out in person.  If you're anywhere near Durham, NC USA, I'm happy to help.

[tubenit]

OK, the fuses are good and the tubes "light up".  That's a start.

Tubes could still be bad, so if you have some good ones .............. try the known good ones.

Use your voltmeter to make sure the speaker cable wire to the amp head is good.

Use a known good speaker hooked up to the head.

Next is to check voltages.  You can get killed working around lethal voltages, so you MUST know how to do this safely.  Check out some You Tubes on working on amps:

PLEASE note that I have NOT watched these You Tubes and don't have time to today to check them out. Hopefully, these will be useful to you.  IF they advocate using a "shunt method" to measure plate voltages,  don't do that.  It's dangerous and foolish in my opinion.

MORE Tube Amps 101 - Draining Filter Caps - DVD on Repairs, maintenance and more!

Tube Amp Survival Guide #1

Tube Amp Survival Guide #2

Discharging.wmv

With respect, Tubenit

[tubenit]

The way I check voltages is like this:

1) make sure the speaker is good and the wires to speaker are good before getting started

2) I put the volume knobs on the amp closer to "2" instead of "10".  The reason I do this is when you touch a tube plate pin, it can make
    a loud pop with the amp volume up high.  I don't want to be startled and  accidently slip off and touch something I shouldn't.

3) I set my multimeter to DC 600V.  I use the black probe clipped to the chassis.

4) With one hand behind my back (being careful to not touch the chassis anywhere or anything else grounded),  I will place my red
     probe  on a plate pin.  For example on a V1 preamp tube. I would touch the red probe to pin 1 & read the voltage.  I'd write the voltage   
     down.

5) You will have to know which pins are for plates and for cathodes.  On 12AX7, you will have pins 1 & 6 be plate voltages. and pins
     3 & 8 be cathode voltages.  

(You can look up what the plate and cathode pinouts are on this website:  http://tdsl.duncanamps.com/tubesearch.php)

6) On the 6L6 tubes, pins 3 will go to the plates.  These will have VERY high and VERY dangerous voltages on them in excess of 300v to
    500v (for example).  

Get on line and read about how to safely measure voltages on tube plates.

http://www.freestompboxes.org/viewtopic.php?f=54&t=5665

With respect, Tubenit  

When I first got started, I wore a rubber glove on my right hand when I did this and kept my left hand behind my back.  I don't wear a rubber glove anymore but it was one more layer of protection when I did.

IF YOU ARE NERVOUS and lack confidence that you can do this ..................... DON'T.  Just take it to a tech.

[tubenit]

The voltages will tell you if the tubes are working properly.  You can reference voltages you are getting and compare those to voltages in Hoffman volt chart library and to the voltages posted on the Sovtek schematic for the plates.

Let's say you measure the volts and things look somewhat correct within reason.  When you're measuring the volts, let's say you hear a noise and a pop measuring the power tube plates.  And then you measure the plates on the LTPI (long tail phase invertor) and those look good and when you measure them, you hear noise and a pop thru the speaker also.

BUT ................ then you measure the next tube V2 plates and while the voltages are good,  you don't hear any pop or noise coming thru the speaker when you measure them.   That may mean that there is a break in continuity in the signal path between V2 (the second preamp tube) and V3 (the long tail phase invertor tube).

With respect, Tubenit

[tubenit]

You are more like to have a bad connection at the input jack or output jack (where the speaker cable plugs in) before you would have a PT or OT go out.   Jacks do go out and become defective.

You are more likely to have a bad tube before the PT or OT goes out.

You are more like to have a bad connection in the signal path before a PT or OT goes out.

BE SAFE!  Again, if you aren't confident that you can do this safely, take it to a tech.

With respect, Tubenit

[tubenit]

OK,  I took the Sovtek schematic and made some notes on it with red so you can understand what is going on.

A 12AX7 is a dual "triode".  What that means is that V1 is the tube.  It has two possible gain stages  V1a (a triode) & V1b  (a triode).

So a single 12AX7 can have two gain stages like on your amp in V1.

Signal travels from  V1a to V1b to V2a to V2b into the V3 which is a LTPI.  The LTPI sends the signal into the power tubes in a manner that is called  "push/pull".

You will need to magnify the GIF schematic to see it well.

Be safe!!    Post any questions that you have on the forum & we will try to help you.  Don't take unnecessary risks.

With respect, Tubenit

[tubenit]

OK, here is a photo of a 6L6 or 5881 tube socket on one of my amps.   I labeled it for you.

My voltmeter is set on DC for 600V.    My black probe has an alligator clip on the end of it.  I attach this to the chassis side OR a very good chassis ground.

My left hand is behind my back.  

My right hand has the red probe (plastic handle only ) and I touch (the metal tip) it to pin 3 and record volts.  I touch it to pin 4 and I record volts.  

I then will reset the voltmeter for DC 200v.  I then touch the red probe to pin 8  on my cathode biased amps & record volts.

ON fixed biased amps like the Sovtek 100, you would have negative volts on pin 5.

My left hand never goes anywhere near any ground or chassis or guitar that is plugged into the amp (if that is the case).

With respect, Tubenit

*  The angle of the photo gives the mistaken appearance that the pins are close to touching one another or are touching.  That is NOT the case, they are separated from one another with plenty of distance.

[tubenit]

Looking down on a 9 pin socket,  the two pins which have the greatest distance between them are pins 1 & pin 9.

So, this photo is showing the pin out for a 12AX7     (or 12A_7) type tube.

You might typically expect voltages between 100-200 volts on pins 1 or 6.

You might typically expect voltages under 5 volts on pins 3 or pin 8.         

(exception would be the cathode pin of a cathode follower which will have voltages similar to plate voltages)


With respect, Tubenit

[Willabe]

V2a (&V2b cathode follower) into the V3 which is a LTPI. 

V2b has the TS taking it's feed from the plate.

But, look how they hooked up V2b's grid leak R to the top of the K R/cathode of V2a. I think that might be NFB? Which could give compression effect to V2b?

The more signal that V2a's plate generates the more signal it's K generates and is fed to V2b's grid. K is 180 degrees out of phase with it's plate. So K signal fed to V2b's grid phase cancels with the signal taken from V2a's plate? The more signal the more it cancels, so automatic volume = compression?


                  Brad        

[plexi50]

Also the tone of the Mig amps i believe is due largely to all the heavy filtering from the PS throughout the preamp section. 200uf's & 100uf's

[tubenit]

V2b has the TS taking it's feed from the plate.

Good catch!   I was in a hurry typing and posting this stuff as I had afternoon projects to do.

V2b is simply another gain stage and not a cathode follower.   At first glance,  I saw Marshall/Bassman topology (using a cathode follower) that wasn't there.

Thanks!  Tubenit

[tubenit]

Something that would reveal if the input jacks are a problem would be to test the voltage on plate of V1 pin 1 (V1-1).  IF there is a popping noise thru the speaker when testing that plate voltage,  that may mean the problem exists prior to V1 which would be the input jack.

with respect, Tubenit

[Willabe]

I was in a hurry typing and posting this stuff as I had afternoon projects to do.

You posted all that including the highlighted drawing and pictures and did afternoon projects?

Wow, very nice. 


              Brad