Adding Filament Transformer to Sovtek Mig-50. How to place it?

[Gabriel E.]

Hi,

So I am in the process of converting my old Sovtek Mig-50 to accept el34s. I already replaced the screen resistors and am upping the bias trimmer to 22k. I did note that the PT doesn’t provide enough heater current so I am installing an additional filament transformer that will service the power tubes only. My question is how to place it? The PT and OT are already 90 degrees from each other. Should the filament transformer be placed close to the PT? In line or 90 degrees off? Will it make a difference for such a small tranny?

Thanks

G

[Enzo]

Wire it in temporarily, and move it around. That will tell you if it matters, and if so, what is the best orientation.

[Chuck H]

If that were a 4 power tube amp I would probably assume to use an auxiliary filament transformer. Since it's a two tube amp I would probably just check the filament voltage with the 6l6's, swap in the el34's and recheck. If the filament voltage doesn't drop more than a couple of 10'ths, and the transformer doesn't get untouchably hot, I wouldn't sweat it. The added filament draw will be about .8A total. The winding spec is probably good for that. Presuming to add the auxiliary transformer, with it's corresponding solving for hum and location, is fixing something that may not be broken IMHE. Things that you will certainly need to deal with when changing to el34's are:

Check the socket wiring. Pins 1 and 8 need to wired together for the cathode circuit and any corresponding component relocation managed.

Check that the bias supply voltage will support the new requirements (you already noted this).

Change the screen resistors to 1k (you already noted this)

Make sure the power tube grid loads (tied to the bias supply) aren't any higher than 220k. Some guys feel that 150k is better for el34's if you plan to crank the amp a lot. Some 6l6 amps use 330k or 470k even with fixed bias. So do check.