Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bluemolded85 on March 22, 2025, 12:03:21 am
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Hello Guys,
I'm working on a Sovtek MiG 50. The amp is such a familiar design but also totally weird in other aspects. Like the fact that it has dual secondary 6.3V filament windings. One set for the power tubes and another set for the preamp tubes. Never seen that before !
Anyhow, I was hoping to disconnect the US/European voltage selector switch and hardwire it just for 120v.
Is it as simple as only using the 120v primary leads (Green and white) and relocating them to the power switch, and then insulating the other primary leads (yellow and blue) and not using them ?
Or would I actually need to tie wires together ? Looking at the Mercury magnetic schematic, it looks like I need to tie the white and yellow leads together, and then tie the Green and Blue together for 120v. Can anyone offer some advice ? I've never done this operation of disconnecting the euro voltage on dual primary transformers
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I need to tie the white and yellow leads together, and then tie the Green and Blue together for 120v
yes
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Anyhow, I was hoping to disconnect the US/European voltage selector switch and hardwire it just for 120v.
Why do you want to do that?
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Anyhow, I was hoping to disconnect the US/European voltage selector switch and hardwire it just for 120v.
Why do you want to do that?
Because the owner wants it that way . I can see why. The switch is a toggle in the back that's very exposed.
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Thanks for the help on disconnecting the voltage selector switch. Worked fine.
I have a new problem though. Major actually...The heater voltage on V1 and V2 is 28v !I'm not sure what the problem is. I listed schematic earlier above.
As mentioned before, this transformer has dual 6.3V heater secondaries. One set for v3 and the power tubes and another set for v1 and V2.
Looking at the schematic, there is a tap in the middle of the filament winding for v1 and v2 . It's a white lead and it's also measuring 28v. The weird thing is I disconnected the heater leads from v1 and V2 to measure and they are measuring 6.7v across.Seems perfectly fine but then when soldered at V1 and V2 , I'm getting 28v at the heater pins . Must be being fed by that white lead in the middle of the filament winding somehow ? This obviously can't be good for the tubes.
Any ideas ?
(Please go easy on me, I'm still learning)
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Are you measuring AC or DC? The heater center tap is DC-elevated for noise and/or cathode follower max heater-cathode voltage reasons.
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Are you measuring AC or DC? The heater center tap is DC-elevated for noise and/or cathode follower max heater-cathode voltage reasons.
I was measuring in auto and it was 28v DC ! I had never worked on an amp with the elevated DC heater supply. Crazy how you can measure in DC and it measures 28v and 6.7v when you measure in AC.
I read up and studied quite a bit. I never encountered this setup.
Thanks for the help !