Hissing problems Sovtek mig 50

[pyo]

Hey Everybody,

I just got a sovtek mig 50 directly from russia. I live in France, and there's not a lot of people who can help me out. I have some hissing problems on the way 1, and a lot more on the second one. Could it come from the pre amp tubes? or the caps?? I don't know....
Could you tell me a little bit more about what can be checked out. I'd like to understand how this little beast works because it's an old guitar amp, and I'd like to keep it working for a long time!
Other question, is it normal that the output the way 2 is a lot higher than the first one??
Thanks for answering
pyo

[PRNDL]

The cascode V2 has a fair amount of gain, which makes this amp susceptible to hiss.
Changing the preamp tubes is usually the first step.

Depending on the age, it may also be time to replace the electrolytics, since there's a lot of filtering to reduce hum.

Channel 1 and 2 are voiced differently - 1 has a flat response, 2 emphasizes the mids and highs (similar to a Marshall). The volumes may be different depending on the speaker.

It looks like a great little amp!

[pyo]

Thanks for answering.
I may change the pre amps tubes first.
Do you have any advise about the tubes I should buy. I know that there are 12ax7 but i don't know which brand I should choose!! If you know a little bit more about changing tubes, let me know. And what about rebiasing the tubes, is it necessary??
Thanks
pyo

[PRNDL]

It's hard to tell which manufacturers have the best tubes, plus each one sounds a bit different. Sovtek and EH are good choices. I buy from a local tube amp tech (Larry Rodgers) - he can mail tubes.

Checking bias is important when changing power tubes.

[pyo]

Hey,

Do you know if the three 121X7 have to be similar?? Cuz, i just opened the amp, and the tubes that were inside don't look exactly the same. There are writings on them but most of it has dissapeared with the time passing by!!Difficult to tell what brand it is or which model!!
I'll try to replace them and i'll let you know how things go!!
pyo

[PRNDL]

>> Do you know if the three 121X7 have to be similar?

No. It's common to mix different types of preamp tubes, which can change the amps tone.

>> the tubes that were inside don't look exactly the same.

Most likely they are not original.

My guess is it had ... Sovteks!

[pyo]

Hey,

I managed to get some info about th tubes inside. It wasn't easy but I translated everythhing from russian, I finally ended on a web site decribing every logos of russian tubes.
The power tubes are : Reflector Corp 6P3S-E
The pre amps tubes : Voskhod 6N1P-EV, Voshkod 6N2P, and the last one I couldn't find out.
Do you think I should try to get the same, or some equivalents. Just like 12AX7 for pre amp??
pyo

[PRNDL]

I'm not an expert in Russian tubes, but 6N1P and 6N2P appear to be variants of the 12A?7 family.

The simplest way is to start with three 12AX7's.
Some put a 12AT7 for V3 (phase inverter), because it can handle more current.
A 12AY7 for V1 or V2 can help lower the gain.

It's not usually possible to tell how each of these combinations will sound beforehand.

[Bruce / Mission Amps]

The cascode V2 has a fair amount of gain, which makes this amp susceptible to hiss.
Changing the preamp tubes is usually the first step.

Depending on the age, it may also be time to replace the electrolytics, since there's a lot of filtering to reduce hum.

Channel 1 and 2 are voiced differently - 1 has a flat response, 2 emphasizes the mids and highs (similar to a Marshall). The volumes may be different depending on the speaker.

It looks like a great little amp!

Uhh... ha ha, where is a cascode preamp in the Mig 50?
V2 is a just a standard triode with a directly coupled cathode follower (low zed) driving the tone stack.
As far as I know there is none... there isn't even the extra gain stage of the Mig 60 or a two input Marshal, both of which can be very hissy.
But yes , it is very similar to a tweed Bassman, JTM45, small box Marshall, etc.

Hum is not hiss and I think abnormal hiss, from the semi-mild gain of a Mig 50, is probably from very high plate voltages and or a noisy tube.

Do the Voskhod 6N1P-EV, Voshkod 6N2P tubes have the same pinout as a 12AX7?
I think a 12AX7 is a 12v tube.
With the lugs 4 and 5 tied together and feeding the other side at lug/pin 9, it makes it able to use 6v3.
What about the 6N1P-EV and 6N2P tubes though?

[PRNDL]

Sorry about that ... I took a brief look at the schematics and made a wrong assumption.

Does this circuit really need all that power supply filtering?

[Bruce / Mission Amps]

Sorry about that ... I took a brief look at the schematics and made a wrong assumption.

Does this circuit really need all that power supply filtering?

No problem... as far as the power supply goes, I don't know if it really needs all that filtering but it doesn't seem to hurt, unless a player considers the extra tightness of the B+ rail to be a problem. I don't.
I'd be more concerned with whether or not someone rewired the sockets to allow the use of the 6v3 6n1P series tubes.

[pyo]

Hey,

I'm back...
I replaced the preamp and the power tubes. I also changed all the sockets and rewire everything the tubes. My mig 50 sounds a little bit better. The sound is more clean but there still this hissing problem. Could it come from the pots?or the caps?or some ground problem?
What do you think the next step is, to get my mig 50 in shape.

Thanks

Pyo

[pdf64]

Can the hiss be turned down with the volume controls? Do the tone control settings affect it?
How loud is the hiss compared to a guitar?
If different 12AX7s are put in the V1 socket, does the hiss level change?
Peter.

[pyo]

Thank for answering!!

Well yes, when I turn the volume pots it affects the hissing. Higher is the volume, louder will be the hissing sound
And also the tone pots change the hissing!
For the low input, the guitar sounds louder than the hissing, it's just when I stop playing that the hiss is really annoying.
And for the bright input, the hiss is pretty loud.Much louder than the previous input.

I'll try to put a different pre amp tube in V1. Maybe I could put the same that V2 and V3.

Thank again
Pyo

[Wakculloch]

The 6N2P/6N1P has a slightly different pinout requiring one to rewire the socket if using 12AX7 etc. Heater is on 4 and 5, and 9 is used as a screen.
The original russian valves are easily available on the interweb.