Sovtek Mig-50 redplating

[Markorock37]

Had a Mig 50 come in to the shop, with a blown 5881. Showing 562V on the plates. Replaced tubes with Tung Sol 6L6GC-STR and can't get the bias down enough with the pot all the way down. At this point I get very slight redplating after its warmed up 5-10 minutes. Bad filter caps? Just wondering in case anyone else has encountered this problem with these amps. Thanks!

[Enzo]

So is there any ripple on the bias supply? What voltage is available for bias, how high will it go at the grid pins? A poorly filtered bias supply would cause hum.

562v seems very high. Was that reading taken with the tubes removed? Voltage readings are only legitimate with all tubes installed and running.

And you wouldn;t be the first person who ever had to change a resistor or two to get adjustable bias back in range. On any amp, not just SOvteks.

[Markorock37]

I'll get all that info up when I get out of work today...I'm leaning towards a possible drifted resistor. Trying to find a schematic but no luck yet as its hard to read all the Russian.

[Enzo]

Schematics are around. Here are several.

Post 70's Amp Heaven Schematics - Acoustic Bogner Kettner HiWatt Matchless Orange Peavey Rickenbacker Rivera Soldano

[Markorock37]

This is what I got today with tubes in and ON:
510V on Pin 3
-54 on Pin 5
67.5ma across a 1 ohm resistor installed on pin 8 to ground
470V at standby switch

Biased down to 47ma (end of trim pot travel) but Weber says it should be 41.1ma
Turned off and remeasured in ON position a few minutes later bias had went back to 68ma
Screen resistors check OK
220K resistors before the 1K5 for Pin 5 test OK
1K5 resistors measure 2K which is out of line over 5%
470K bias resistor tests OK
20K bias resistor reads 68K then fluctuates higher *is being in circuit affecting this)

My guess is its the 20K and maybe the 1K5's but I'm still learning.

[Enzo]

Don't focus too hard on resistors being out of tolerance.

You have a 1 ohm resistor to ground at each socket? Or are both pin 8 wired together and the 1 ohm resistor serves them all? In other words, how many tubes does that 67ma represent?

20k resistor? What value is printed on the resistor? AMps do not always exactly match schematics. Yes, reading in circuit can cause odd readings. If the reading is funny, try reversing your meter leads. If you then get a different reading or even a "negative resistance" reading, then you have some small residual voltage in the circuit, and/or your meter is trying to charge up a cap somewhere.

470k bias resistor? DO you mean 470 ohm?


And just for grins, have you tried some other pair of tubes? Your Tung Sol pair might just be hotter than average.

[Markorock37]

Just one socket has the 1 ohm so one tube.
the schem shows its a 20K, no markings on it make any sense. I found the 470 ohm (yes that is what it is not 470K, sorry), the diode and the trim pot all in the same location so I'm guessing the resistor is the 20k.
I'll try another set of tubes just in case.

[Enzo]

The question remains for me. The one pin 8 has a 1 ohm to ground. The other socket pin 8, is it wired to ground direct or to the other pin 8?

That 20k serves to limit how low you can turn the bias voltage. Because the first resistor is only 470 ohms, changing the 20k will not increase the maximum voltage in the bias supply, all it will do is alter the range of the control. In other words if the thing sweeps from -55 to -35 with 20k, then changing it to 40k might mean the sweep becomes -55 to -40. I made up the numbers, but they illustrate the point. In fact, if that 20k went open, all it would do would be max the bias voltage, and that max you already have.

[Markorock37]

Yes It is grounded to pin 8 of the other tube, so I take it that is the reading for both tubes?
Thank you for explaining the bias resistor, that makes sense to me.
I tried two other sets of tubes:
JJ 6L6GC - 56.8ma
520V-pin 3

Sylvania 6L6GC - 95.8ma!
498V-pin 3

I tested the power transformer and got this:
Standby w /tubes installed 582V & 531V (one positive from transformer, other lead to chassis ground)
ON w/tubes installed 539V & 506V

One side consistently higher no matter what tubes
469-473V consistent at standby switch

So is it all about the right tubes in this beast?

[Enzo]

Well then, you have 67ma for two tubes, meaning 34ma per tube, and at 510v across the tube, that is about 17 watts dissipation per tube. Nothing wrong with that.

The more current you draw through the tubes the lower the B+ will sag. That is normal. That's why your JJs let the B+ rise and your SYlvanias dragged it down more. Again, normal.

Tubes have natural variation, as you are finding. KNowing now that your current readings are for two tubes, divide them in half for a per-tube figure and voila... current levels are perfectly acceptable as they sit.

As to the transformer being a trifle lopsided, I doubt it is an issue. But I am confused as to what you are measuring. If you have roughly 500vDC B+, then you must have roughly 350vAC across the transformer. There is no center tap, at least not on my schematic, so AC readings to chassis don't mean much of anything. And DC readings won't mean anything taken at transformer leads. In fact even at the + corner of the bridge, with the standby off, there is no filtration and your DC would read really low there. Turn standby ON and the filters are present and B+ comes up to normal.

Then 469-473 at standby? DC? I am not sure what you mean here either. How can you have 469-473v at the standby switch, yet 500v or more at the tubes. Voltage can;t grow 40 volts from one end of a wire to the other.

What matters is that the amp isn;t blowing fuses, and you have a nice smooth DC voltage after the standby switch of around 500vDC. That is a pretty common B+ area for 6L6s.


Not sure I should bring it up here, but... Looking at the schematic, I am looking at the one Bruce drew now, it appears they used a transformer winding with an off-center tap for heaters and bias. It APPEARS they wired the heaters and grounded one side - the side that is that tap. Depending upon how they ran the wires, that is, if they just twisted a pair and wired tube to tube and grounded that tap. IT seems that we could move that ground to the other end of the heater winding. That would have the effect of adding about 6 volts AC to the winding the bias supply sees. That would run your bias supply about 8 volts higher DC. At thgis point I don't reallt think your amp has a bias problem, but if the chassis wiring cooperates as I described, then we could get enough extra volots to make a difference.

I actually own three of these amps, but they are all in my warehouse. Two are still new in the box.

[Markorock37]

It does have one of the heater wires grounded, pin 7 to 8 on one of the power tubes.
I'll give it a go biasing it again tonight with the Tung Sol's since nothing is out of line. I think the slight glow in one of the tubes got to me.

[Markorock37]

Got it all biased up yesterday morning with no problems. And the one power tube isn't showing any signs of redplating. Thanks for your help!