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Thread: Checking BIAS on a Sovtek Tube Midget 50H

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    Forum Member Michael Smith's Avatar
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    Checking BIAS on a Sovtek Tube Midget 50H

    From the blurry schematic I could find for this amp, it appears that the cathodes are grounded, thus meaning this amp is grid biased. Does anyone know the test points I should be looking for to measure the output transformer center tap resistance to pin 3 of the output tubes, and then measuring the voltage drop? I tried the input to the HT fuse, and got some very mismatched readings. Perhaps they are correct and my 5881's are very mismatched, but I'm in over my head a bit on this one.

    My measurements are as follows: Left tube (Sovtek 5881WXT) Resistance 69.7 ohms, Voltage drop -2.6 volts, plate voltage 505 volts. Right tube Resistance 69.3 ohms, Voltage drop -1.36 volts, plate voltage 507 volts. From these measurements, I get a plate dissipation of 18.84 watts for the left tube and 9.94 watts for the right tube (facing the chassis from the power tube side).

    Why did I buy a Russian amp, lol?
    "When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top

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    Forum Member phantomman's Avatar
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    Re: Checking BIAS on a Sovtek Tube Midget 50H

    Quote Originally Posted by Michael Smith View Post
    Why did I buy a Russian amp, lol?
    You're a Leninist at heart?

    "When injustice becomes law then rebellion becomes duty."

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    Forum Member Michael Smith's Avatar
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    Re: Checking BIAS on a Sovtek Tube Midget 50H

    Quote Originally Posted by phantomman View Post
    You're a Leninist at heart?

    I think I am more of a "McCartneyist", lol.

    Attached is a marked up partial schematic for the Tube Midget, along with my bias calculations. When using brand new 5881's they appear to be very mismatched. Trying old Sovtek 6L6GC's appear to give better results, IF I am connecting my probe to the correct place on the amp. ( I connected the neutral probe to fuse 2 as shown on the schematic). I am sure about pin 3 of the power tubes, but guessing a bit on the Output Transformer Center Tap. I don't have bias probes, but a friend has offered to let me try his, I just have to make the time and sync up with his schedule.

    I'm a little concerned with the plate voltage in excess of 500 volts, but the schematic doesn't indicate any voltages. I believe that is too high for 5881's, even though that's what the schematic calls for.

    Ignore the bias calculations for the Fender Super Reverb, they were just on the same spreadsheet.

    "When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top

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    Forum Member phantomman's Avatar
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    Re: Checking BIAS on a Sovtek Tube Midget 50H

    I concur with you re the plate-voltage specs for the 5881's -- I wouldn't push the hardiest of them beyond about 465 VDC.

    That said I'm going to offer a conjecture: if the tubes are not red-plating and the amp otherwise sounds fine then it's possible that the output stage was designed to operate in such a mis-matched condition. A trained engineer might be able so shed additional light on these performance parameters but I am not that person.
    "When injustice becomes law then rebellion becomes duty."

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