Sovtek MIG 60 - diagnosing problem

[vintagekiki]

To reduce the hum from input tubes, heaters V1/ V2 use rectified DC voltage (D11/ 14, C24/ 25, R46), attached to elevated* + HV voltage (R47/ R48). Elevated heater used in early HiFi amps.
Heater input tubes (V1/ V2) must not have contact with gnd, because tube heaters still operate at 6.3V, which floats on some elevation +HV voltage.
* The most probable values for (R47/ R48) should be 100k not 100 ohms.

https://el34world.com/Forum/index.php?topic=15932.0

https://el34world.com/Forum/index.php?topic=18005.0

https://www.marshallforum.com/threads/elevated-heaters-question.93903/

http://www.valvewizard.co.uk/heater.html

[Mr_bibbles]

If the 1R was in the place of the 5R, they probably found it was dropping too much voltage and adjusted the value.
Yes, the heater filter caps (C24 & C25) could be killing the DC heaters, I've seen it happen in other amps. But also check for any bad solder if the bridge is mounted to a circuit board (and any other solder connections in the DC heater circuit).

Thanks G1! This is excellent information. Checking the board was the first thing I did, it seems pretty solid. This is a "Wednesday amp" if I've ever seen one, whoever was putting it together was very on point, clean leads and joints.

To reduce the hum from input tubes, heaters V1/ V2 use rectified DC voltage (D11/ 14, C24/ 25, R46), attached to elevated* + HV voltage (R47/ R48). Elevated heater used in early HiFi amps.
Heater input tubes (V1/ V2) must not have contact with gnd, because tube heaters still operate at 6.3V, which floats on some elevation +HV voltage.
* The most probable values for (R47/ R48) should be 100k not 100 ohms.

Thanks vintagekiki! I will read through these to increase my understanding. I've measured everything connected to the circuit and only the 5.6R was off, r47/r48 were both pretty spot on 99k. If they were lighting up when the standby was "off", that means the ground connection scenario would probably happen through a damaged component somewhere in our B+ line or beyond... leaky caps seem to still be top billing for our villains.


Thanks y'all, I'll keep you updated!

[Helmholtz]

What DC voltages do you measure across (not to ground!) C25 and C24 with V1/V2 in and out?

[Mr_bibbles]

What DC voltages do you measure across (not to ground!) C25 and C24 with V1/V2 in and out?

I assume you meant with the bad capacitors, but by the time I read this I had already replaced them... either way without is 7.9VDC / 7.8 VDC and WITH tubes installed it's 6.8VDC / 6.1 VDC across c24/c25

[Helmholtz]

Looks good. So everything is fine now?

What value is your R46 now? As the voltage drop is 0.7V, I guess 1R. Should be at least 1W with higher surge current rating. Best would be a wire wound type.