Sovtek MIG 60

[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]

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.

[alerich]

Yes, necro thread... I know. But folks still refer to these threads through searches (like I did) and I wanted to clear up a little misinformation. The MIG60 is not a Plexi variant. It is more closely based on the cascaded master volume JCM800 (2203/2204) circuit. My MIG60 has about 640VDC on the plates and 350VDC on the screens due to the stacked supply. Most catastrophic tube failures occur due to excessive screen current. As one tech who I trust told me "If you manage the screens, you can murder the plates". I run Tung Sol EL34B tubes in my MIG60 and they handle the task just fine. Scott Splawn did a few mods to this one back when he was still modding amps (swapped the iron for Mercury Magnetics, added a bit of gain to the first stage and converted the cathode follower to a gain stage) and it's a real fire breather. Sounds very close to my early 90s Soldano Hot Rod 50.

[SoulFetish]

.

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.

the calculation for the HT is incorrect. The peak voltage calculation would be taken from the center to one side of the secondary. so if you were reading 210-0-210V RMS, the rectified DC voltage would be calculated 210*1.414=296.94V (under light load).
Or 420*0.707=296.94V

Edit: Fahey brought up a good point so I wanted to clarify that my calculation above would apply only when using a traditional 2-phase rectifier where both windings feed into the same load. See his comments below for a different way to configure this where you would calculate the rectified DC from the full 420V CT, much like you would for a bridge rectifier

[SoulFetish]

Yes, necro thread... I know. But folks still refer to these threads through searches (like I did) and I wanted to clear up a little misinformation. The MIG60 is not a Plexi variant. It is more closely based on the cascaded master volume JCM800 (2203/2204) circuit. My MIG60 has about 640VDC on the plates and 350VDC on the screens due to the stacked supply. Most catastrophic tube failures occur due to excessive screen current. As one tech who I trust told me "If you manage the screens, you can murder the plates". I run Tung Sol EL34B tubes in my MIG60 and they handle the task just fine. Scott Splawn did a few mods to this one back when he was still modding amps (swapped the iron for Mercury Magnetics, added a bit of gain to the first stage and converted the cathode follower to a gain stage) and it's a real fire breather. Sounds very close to my early 90s Soldano Hot Rod 50.

I just worked on an very early MIG 60 and the input was not cascaded.