Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ansel on January 05, 2019, 01:19:56 pm
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i'm interested in changing the power transformer in the MIG 50 with VOX AC30 type power transformer for lower voltage on B+, any other suggestion?
MIG 50 :
The plate sits at around +535V and the screen at +530V. The bias voltage is about –58V and the input impedance for the OT is 4.75K Ohms
the AC30 type power transformer gives a 290vac secondary , and with the MIG 50 bridge rectifier it will give about 290/0.71= 408vdc which is very fair on the B+ and ofcourse it will be less on the screen
my question , if i do this change , do i need to change any resistor or capacitor value? or just tweaking the bias knob for the grid?
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Welcome.
Just changing the main B+ down about 24%, no radical change needed. G1 bias should be approximately 408/535 of present 58V, around 44V. That straight-line interpolation is a little off, but all tubes vary, so you will dial-in from there while observing cathode currents and plate dissipation.
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the AC30 type power transformer gives a 290vac secondary
Some AC30s use a PT meant for a FWB, others use a PT meant for a conventional FW rectifier. Be careful to pick the correct PT to use with your Mig FWB, or you may end up with twice as much B+ as you want! Also, you'll probably need to modify the bias circuit since the AC30 PTs don't have a bias winding. Fairly simple but it is a big deal.
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I didn't know that we have the polling option,
My poll would be something like, I have re-wired and changed every component:
a. Do you think that little voltage issue took out my OT
b. Should I stick to just playing guita