Sovtek MIG 60

[wui223]

I got this from a friend with missing output trans. The problem is i can't find a schematic which show the working voltages of each stage.

I google around and found out that the B+ for this amp is quite high ~600VDC. I am not sure how to connect the power trans to form the power supply sections as shown in the schematic i got from Schematic Heaven.

Anyone has any experience or idea to share?

Thanks in advance.

[Enzo]

What you have is all the drawings there are. It is common for no voltage readings in schematics.

With no output transformer, your B+ WILL be very high - the majoruty of power drain in a tube amp is in the power tubes, which of course cannot be powered when teh OT is missing.

What is the difficulty you are having matching the drawing to reality? The circuit? Or that your voltages areso high?

Isn't the power transformer already connected?

[wui223]

Ermm. Because the power tram has multiple tappings which provide different output voltages. But from the schematic i have, it doesnt show which tapping connect to which power section.

Well, i guess the highest output is for B+, second highest is for OT center tap, and the lowest is for negative bias supply, i omit the heater parts coz that one is well recognized. Pls correct me if i am wrong.

[Amp Kat]

Not all but most transformers are color coded. Red and red are the B+ wires and the Red/yellow is the center tap that gets chassis grounded. The bias tap wire is usually either blue or purple,heaters are green and low voltage usually orange maybe yellow. I don't know why they disconnected the power tranny wires to the filter caps. The OT primary wires go to the plates and the CT should have it's own connection to the B+ supply on the board.

[wui223]

The biggest problem is: no wire from the PT at all, all wires had been dissolder from the PT. After running some tests, i recognize following facts: 250V, 6.3V, 50V, 210V , these are the output voltage i can get from the PT by feeding 220VAC line voltage.

Anyone could point me a direction ?

[Satamax]

Hi Wui!

Well there's no two equal taps? Sounds like a blown trany! Even the voltages given seem a tad low. Any picture???

Bye.

Max.

[soundmasterg]

My brother used to have one of these and the plate voltage was over 600v, and it would eat EL34's on a regular basis. I can't help you about which wires to hook up where on the PT, but if the PT i bad, I'd suggest to get something else that will give you a lower B+.

[Amp Kat]

Originally Posted by wui223

: 250V, 6.3V, 50V, 210V , these are the output voltage i can get from the PT by feeding 220VAC line voltage.

Anyone could point me a direction ?

.

210 volts could be center tap to one leg of the HV supply 210 X 2 = 420 x 1.414 =593 volts. 6.3 = heaters 50 volts = bias supply. fwiw these are called windings not taps with the exception of the single bias tap. I do agree with Soundmaster that at 600 volts it will run them pretty hot if the screen voltage is high also they will pull alot of current. Lower plate voltage would be a good thing. Anyway looking at that schematic I'm seeing 3 windings and a bias tap. There is a HV winding that is full waved then a winding that forms a center tap that is also full waved to the middle of the two HV filter caps in series. Then the heater winding and the bias tap. The screen voltage is derived from the HV supply across the 10 watt resistor.

[wui223]

Originally Posted by Amp Kat

.

210 volts could be center tap to one leg of the HV supply 210 X 2 = 420 x 1.414 =593 volts. 6.3 = heaters 50 volts = bias supply. fwiw these are called windings not taps with the exception of the single bias tap. I do agree with Soundmaster that at 600 volts it will run them pretty hot if the screen voltage is high also they will pull alot of current. Lower plate voltage would be a good thing. Anyway looking at that schematic I'm seeing 3 windings and a bias tap. There is a HV winding that is full waved then a winding that forms a center tap that is also full waved to the middle of the two HV filter caps in series. Then the heater winding and the bias tap. The screen voltage is derived from the HV supply across the 10 watt resistor.

Yes, there are not single bias tap, thanks for correcting my mistake. 210VAC is from one winding so as the other voltage reading i got.

Is 250V = HV winding and 210V = center tap winding ?

If i want to lower the plate voltage, how to make it? zener trick?

What is the suitable plate voltage to give an output of 50W using push-pull?

[Enzo]

Look at the schematic. There are no center taps. It shows two HV windings, a separate bias winding, and a heater winding, four windings, and you have four windings also. This is exactly what you have measured. There are two full bridges, one stacked over the other, that is to say the upper one's negative end is connceted to the positive end of the lower one. So if the lower one makes 350V, then the upper one will add on its own 300 volts or so. I must assume the voltages on the transformer now will drop a good amount once the circuit is operating, so those DC examples will be lower.

Wire the 250VAC winding to the lower bridge. (lower in the schematic drawing) That will put less than 350 volts of DC on C20 which serves the screens and all the smaller tubes. Then add the 210VAC winding to the upper bridge. This will put less than 300 volts on C19. That voltage adds to the other, and it goes only one place - the plates of the power tubes. So they should have less than 650 volts. I thought EL34s were good to 800 volts.

WHen you have a mystery transformer with its wires detatched, if you want to know if a wire taps into another wire's winding, power down and use an ohm meter. A tap will have continuity to the rest of the winding. Your 250 and 210 windings I think do not have continuity to each other, they are separate windings.

[wui223]

Thanks alot. You guys really helpful.

The ori filter caps are big can type with 2 legs, from schematic, seems that it is a 100+30 uF can, but i wonder where is the ground leg on the can cap?

Alternatively, can i replace the filter cap with 2x40uF cap? since high voltage caps are hardly available in my area. Or should i paraller or series them to get close to original values ?

[tubeswell]

Works the opposite way to resistors

Caps in parallel = the sum of the cap values. E.g. two 330pF caps in parallel would equal 660pF.

Caps in series = less than the lowest value of any of the caps. There's a formula for working it out, namely; C total = (C1xC2xCn...)/(C1+C2+Cn...)
where C is the capacitance value of the cap(s). E.g. two 330pF caps in series = (330x330)/(330+330) = 165pF

[Amp Kat]

The ground on the can cap is the can itself. You will see a lug on the outside attached to the can. That is the ground which makes running them in series difficult because the grounds on the cans are all tied together. You can use the existing cans if they are good or you can use individual caps and parallel them. Note that one of the can caps is not grounded but is parralled so do not let that can hit the chassis or any ground as it is isolated somehow from chassis ground. The bottom cap is grounded.

[soundmasterg]

EL34's are good to 800v for the B+, at least OS were...who knows about new stuff. The screen rating was never that high though and these things run the screens a couple volts lower than the plate, which is why they like to eat EL34's. They're a similar circuit design to a Plexi.....if the voltages were lower they may hold up long enough to sound like one....

[Amp Kat]

Originally Posted by soundmasterg

EL34's are good to 800v for the B+, at least OS were...who knows about new stuff. ..

I wouldn't put 800 on any current production tubes including KT-88's and feel good about it. I've been working on lots of old, old amps and it's amazing how tough those old tubes are. Still pegging my tube tester out and have unbelibable tone. Your lucky if you get 6 months out of current production tubes at moderate voltage & bias.

[Satamax]

Hey guys!

Well, if there's enough bias voltage, the waffer base 6P3S-E or more comonly known sovtek 5881 are the tubes to use in a sovtek head. They stand the voltage and overdissipation with no problems. I've had some in a laney biassed for EL34 (about -25 volts or so) dissipating 32 watts on the plates at iddle, and they're still in there a year later.

HTH.

Max.

[Enzo]

Look at his schematic, the screens are on the lower B+, 300 and some volts. The 650v ONLY goes to the power plates. The screens are NOT at the plate voltage.

[wui223]

The EL34s i have now are EH tubes. So is it alot easier if i use the zener trick to lower the B+ and power the tubes at "safe" voltage ?

How much the bias voltage should be ?

[Enzo]

I suppose it could be done, but just where will you put that zener in this stacked supply?

[wui223]

This is the question i wanna ask. Btw, i am thinking of modifying the original power section.

I tried series the 250V and 210V windings results a 450VAC from PT unloaded. But this voltage is too high for the caps i have. So if i want to put zener after rectifiers, is it ok? or should i put it before rectifiers?

Is zener trick capable of handling AC voltage?

[Satamax]

Originally Posted by wui223

This is the question i wanna ask. Btw, i am thinking of modifying the original power section.

I tried series the 250V and 210V windings results a 450VAC from PT unloaded. But this voltage is too high for the caps i have. So if i want to put zener after rectifiers, is it ok? or should i put it before rectifiers?

Is zener trick capable of handling AC voltage?

Wui, i think i'd use the PT as designed. Or may be put a zener on one leg of the 250vac winding, a 40 volts zener and use this as a centertap along with the other end of the 210v winding and use them as a full wave rectifier for real low voltages. Otherwise, i'd rather use the PT as it is and find tubes which can wistand the 630 or 650 volts you have ! That amp as well, is suposed to sound very good with the high voltages and all. You could use the old marshall trick! and stuff 6550 or KT88 in there. But i'd go cheap myself, wafer base 5881!

http://cgi.ebay.fr/Pair-Sovtek-5881-...QQcmdZViewItem

http://cgi.ebay.fr/Set-of-2-Sovtek-5...QQcmdZViewItem

This set could be good if you know how to match tubes in your amp.

http://cgi.ebay.fr/RARE-5881-6L6GC-6...QQcmdZViewItem
and then resell the rest that you don't need, even may be matched; and make money on them!

If you tell me they're not EL34, who cares, they're not 5881 nor 6L6 either!

They are 6P3C-E

http://cgi.ebay.fr/Sovtek-5881-6L6WG...QQcmdZViewItem

And most certainly what the mig 60 has been designed for!

http://search.ebay.fr/ws/search/Sale...&fcl=3&frpp=50

[Amp Kat]

I think what Max and Enzo are telling you is the zener trick is designed for transformers that have center taps. Yours doesn't have a center tap which is also why it uses full wave bridges instead of just full waves. Another good point that Enzo made is with the lower screen voltage doesn't dissipate as much wattage as at just below plate voltage so I agree with Max that that's what the amp is designed for. I didn't take into consideration of the lower screen voltage so my bad on that but I would leave it as is. The 5199 tube will take the voltage if your worried about that but it's tone IMO is not as good as the EL-34. These amps do sound awesome and are loud as hell so I'd keep it like is. $.002

[Enzo]

If you just rectify 450VAC, you will get well over 600 volts, and that is higher than everyones caps. You would have to do what anyone else would do, wire caps in series. But in this dual power supply, they are NOT wiring the windings in series to add their voltages, they are making instead two separate high voltage supplies and then wiring THOSE in series. The caps only have to withstand what ONE of those suplies puts out.

I really think the best path to take is to wire the transformer as origianal, as the schematic is drawn.

[soundmasterg]

These MIG 60 amps, at least when they were first offered for sale in the US came with Sovtek EL34G's, which are so terrible they don't even deserve the EL34 designation. The Sovtek 5881 would definitly sound better than those in this amp and hold up better to boot. These amps sound pretty good, but compared to the real Plexi that they are modelled after, they sound kind of buzzy and harsh. They're a fun amp though if you get some tubes that will hold up in them.

[wui223]

Well i am back with more questions.

The OT i am using now is actually for Plexi 50, so if i restore the circuit to Sovtek MIG60 and run the B+ for around 600V, do i need to change the OT?

Or any idea i can stick with the OT but change the circuit here and there?

[soundmasterg]

I think that OT should be fine and will sound better than what was in there. I'd just mod the amp here and there to change the sound. Maybe reduce gain and voltages in some places to make the tone less buzzy.

[wui223]

I wired up the transformer as described by Enzo. When i power up, nothing happend, but once i flick the Standby switch, both fuses blown (250mA and 500mA).

Anybody got idea whats went wrong?

PS: I didnt put in OT and tubes yet. I just want test the power supply section.