Sovtek repair question

[Geezer]

  Hoffman Amplifiers
    > Tube Amp Building - Tweaks - Repairs
        > Sovtek repair question      
 
<< Prev Topic | Next Topic >>  
Author  Comment  
remow
Junior tube assistant
Posts: 1
(4/7/04 8:04 am)
 Sovtek repair question
--------------------------------------------------------------------------------
 Back Story - I bought a Sovtek which at the time had a bad prescence pot, but everything else worked great. During a recording session, durring which the amp was left on a good part of the day, the amp would experience huge amounts of breakup at low volume (around 3-4). The harder I played the more distortion I would get. This was not the way the amp had played for months so I turned it off and took it to my amp guy.

He said he thought it was a bad volume pot causing the problem and recommended switching it out. THe previous owner had messed with the pots before and had superglued the knobs down. Long story short all the pots have been replaced and work great. I played the amp last night and it sounded great for about an hour or so, then the horrible break up at low volume (2-3 this time). Again the harder I play the more distortion I get.

Anyone have any thoughts on what is causing this?


 
Funkalicousgroove
Junior tube assistant
Posts: 42
(4/7/04 10:27 am)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 You may have a power tube that's arcing, I would change them if you haven't. I had a similar problem in one of my amps where the grid resistors were mis-matched from the factoryand causing one of the tubes to arc and fail .
 
remow
Junior tube assistant
Posts: 2
(4/7/04 10:30 am)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 I have never replaced them. Is there any way to check for that, or should I just replace them and be done with it?
 
6G6
Junior tube assistant
Posts: 154
(4/7/04 11:07 am)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 If you have never replaced them, and don't know for a fact that they are "low mileage" tubes, then it's time.
If you can't read the markings on them, it's time.
If nothing else seems to correct your problem it's time.
 
Funkalicousgroove
Junior tube assistant
Posts: 44
(4/7/04 11:33 am)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 Replace them, make sure to re-bias. an easy way to do this is to put a 3 watt 1ohm 1% resistor between each cathode and ground and then measure the dc milivolts on the socket side of the resistor. 6l6 should be between 30 and 35 milivolts DC, whatever sounds best to your ears. It's also a good idea to change your phase inverter whenever you do your power tubes and for that matter all of the preamp tubes. BE SURE TO DRAIN YOUR FILTER CAPS BEFORE DOING ANY WORK ON YOUR AMP. just my little safety speech--Have fun
 
jimf
Junior tube assistant
Posts: 12
(4/7/04 11:04 pm)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 
Quote:
--------------------------------------------------------------------------------
Anyone have any thoughts on what is causing this?
--------------------------------------------------------------------------------



No... sorry. :)

But I was thinking that maybe if we had some more info someone might be able to think of something.

First off, which model of Sovtek is it? I've found schematics for the Mig-50, Mig-100, and Mig-50H. Is it any of these?

Second, what else happens when it gets into a funny mode. Is the output volume reduced in addition to the increased distortion? The more details you can give the better.

Also, are you interested in doing this work yourself? The last post about rebiasing is asking a lot if you're not interested in going inside the amp. For example, you may not have a volt meter or a soldering iron, both of which are required to follow those instructions.

My next thought is that maybe you should get a second opinion from another amp tech. Anyway, maybe this guy or another tech can leave the amp running for a while and actually probe the amp with test equipment and figure out exactly where the problem is.

My last thought is that you *can* easily test your tubes to see if they are bad, but you need to find someone with a tube tester (which may not be easy). Also, it's not necessarily true that you need to replace the phase inverter (or preamp tubes) when you replace the power tubes. If they test good, why replace them?

Jim
 
remow
Posts: 3
(4/8/04 9:55 am)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 It is a MIG-50 (standard not the H)

Its pretty much just more distortion, I don't notice any volume change. However, I played it again last night and I do believe there is more volume now (meaning the volume I used to get at 3, I can get at 1.5 - 2) This may just be my ears but it seems like it is louder now.

Yes, I am interested in doing this myself. I replaced all the pots and they work fine. I have a multimeter and a soldering iron.

All the stores in town take their tube amps to my guy, he's the only one I got.


 
jimf
Junior tube assistant
Posts: 13
(4/8/04 7:49 pm)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 Hmmm....

Well, here is a copy of the schematic if you don't have one already....

www1.korksoft.com/~schem/..._mig50.pdf

Does your amp guy have a tube tester? Maybe you could pop the tubes out and take them down and test them all just to see how they rate.

Looking at the schematic, it looks like the two inputs (and two volumes) are in parallel. Does it behave the same regardless of which input you're using? This would at least eliminate a problem with one volume pot, and maybe rule out the whole first stage (unless both sides of the first tube were broken in the same way).

I seems like, in order to get more distortion for a given input, you either need more gain or a reduction in headroom... that is HT voltage. You could try to measure HT voltage when you first fire it up and when it starts to act up to see if it's some fault in the power supply. I guess I would measure at the anode of the power tubes. What tubes do you have in there? Anyway, I think it would be pin 3... that's for a 6L6 or 5881. While you're at it measure the rest of the HT voltages. And the bias voltage as well.... Do all of these things stay the same? I'm not sure how familiar you are with these things. Should I be very specific as to where to measure? Also, I should say please be very careful. These are leathal voltages you're measuring.

If the power supply doesn't change, then somehow you're getting extra gain. The fact that you say the volume is the same at 1.5-2 as it was at 3 is intersting. This suggests you *are* getting more gain. Not that I know why this might be... Um. Does the tone of the signal change (if you try to get the same tone by fiddling the volume)? I'm looking at the tone stack and wondering what might happen if something in there wasn't well connected....

I guess for now I'd try to test the tubes, then test the power supply, because those things are pretty easy. If that all checks out, some signal path investigation will be necessary. Or maybe someone smarter than me will just know the answer... :)

Jim
 
Funkalicousgroove
Junior tube assistant
Posts: 52
(4/9/04 6:25 pm)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 My first inclination is always to swap tubes when there is a problem. Tube testers do not supply anywhere near the voltage that your amp does, so tubes that test OK on a tube tester may not be OK in your amp. I personally see no reason to hang on to Russian tubes if they don't sound right-----that is the ultimate goal isn't it.....sound??? I have a question about what you're playing through the amp: If you're not experiencing a volume drop, but you are getting more "Distortion" are you using the same guitar and cable each time? you will experience more signal loss with a 20ft cable than a 10ft, moreover, a 10ft cheapo cable will generally have more signal loss than a George L or proco or (INsert brand name here). Just a thought.
 
remow
Posts: 4
(4/12/04 8:35 am)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 Yes, same guitar/cables each time. I have a borrowed Marshall that I have been using, and I don't get any problems when using it. Its definitely the Sovtek. I am going to pull the tubes and switch their positions tonight and play through it and see what happens. Someone suggested I do this before replacing them. I will try it and post the results.
 
Funkalicousgroove
Junior tube assistant
Posts: 59
(4/12/04 5:07 pm)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 I had a thought while helping someone else, Did you check the values of your plate resistors on your preamp tubes vs the schematic? if these resistors have drifted toward more resistance then Your tubes will break up earlier with a more "Brown sound" just a thought-
 
remow
Posts: 5
(4/14/04 3:38 pm)
 Re: Sovtek repair question
--------------------------------------------------------------------------------
 OK, at the suggestion of a friend, I switched the existing power tubes locataions and made sure they were well seated. I fired it up and everything worked fine for about an hour and the distortion started up again. I went over to look behind the amp and I hit my input cable and all went dead, no sound. I jiggle the c