How best to run a pair of 115vac filament transformers across 240vac

[KK7UV]

Referring to the attached drawings:

1.  Primaries in series, returning the "center" to neutral with a rheo.
2.  Primaries each with a rheo, returned to neutral.
3.  Primaries in series with end of leg connecting to L2 with a rheo.

By the way this is same 240 line that will run the plate transformer.

Thoughts?

Steve, KK7UV
 

[AB2EZ]

Steve

I don’t think any of these will work, unless the loads across the two secondaries are identical. None of these configurations will cause the input voltage to split 50-50 across the two primaries (again, except when the two secondary loads are equal.

Stu

[kb3ouk]

Just run both from L1, I've seen a lot of 240 stuff that all the 115 loads are on L1 and only the other side of the 240 is on L2.

[KK7UV]

The loads on the filament transformers are the same (pairs of 813s).

[Opcom]

The top right one lets you individually adjust the fil's. Will you individually meter them in that case?

But yes either side of the 240V, L1 or L2, to neutral would be 120V.

[AB2EZ]

I stand corrected. If you assume that the voltage between L1 and the neutral wire is
0.5 x (the voltage between L1 and L2), then the upper right design will work fine.

Stu

[w4bfs]

I stand corrected. If you assume that the voltage between L1 and the neutral wire is
0.5 x (the voltage between L1 and L2), then the upper right design will work fine.

Stu

agreed if you will adjust the rheos separately .... you should also select 813's for equal current draw at same fil V ... the state of the vacuum will affect this ... I have found that about 30% of randomly selected 813's will be outside spec

[kb2vxa]

There you go thinking like electrical engineers analyzing it to death. Think like an electrician, with a neutral conductor the same diameter as the current carrying conductors it doesn't matter. The current imbalance is carried by the neutral, so with one on each side the neutral current is the difference between the two, very little to none when the loads are the same.

Then if you follow the RCA Transmitting & Industrial Tubes manual you go back to thinking like an electrical engineer and over tax your brain with circuit design. The filament should be operated at 10V +/- 5% which in itself isn't a problem since mains voltage is +/- 8% maximum and with a 10:1 reduction the secondary easily remains well within +/- 5%. Unless you're in an old neighborhood with outside wiring that hasn't been updated since 1950 it remains fairly static. Here's where the fun begins, during standby periods in ICAS operation it is recommended that filament voltage be reduced to 80% of normal when periods are less than 15 minutes. For longer periods the filament should be turned off. Well, you can add a step start circuit to your PTT, add manual rheostats and switches that increase the uh oh factor, use Eimac tubes instead of RCA, (;->) or best yet use tubes that aren't so fussy.