Welcome Guest! Be sure you know and follow the Phorum Rules before posting. Thank you and Enjoy! (January 12) x

Thread Closed
Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5

Replacing an Output Transformer with tapped secondary
#1

My 40-216 has a defective output transformer. I doubt I'll find a GOOD replacement any time soon (both halves of the primary were open, and the connections are in the middle so I cannot get to them without removing the secondary!) Likely many potential donor chassis out there ALSO have bad transformers.

Anyway, this set, as well as other large Philcos, use a transformer with a tapped secondary. The tap feeds the tone and volume control circuit - I assume some sort of feedback arrangement.

The secondary was good, so I applied about 2 volts from my HP 200A oscillator at 500 hz and determined that the tap is about 10% of the total.

Can I get away with just shunting the secondary with a couple of resistors to provide the 10% tap? For example, 100 ohms in series with 10 ohms. Perhaps they can even be higher such as 1K and 100 ohms. I realize any parallel resistive load will be reflected back into the primary, so the higher the better.

I calculated that the ratio of the replacement transformer should be about 41. So a 110 ohm load will be reflected as 184K in parallel with the output tubes plate resistance. That should not hurt too much. The tube manuals calls for 10K plate to plate load impedance.

Opinions? Will this work? Is there any limit on the values of the shunt resistors?

Thanks!
Dave




Users browsing this thread: 1 Guest(s)
[-]
Recent Posts
Philco 39-7C Restore
A little more work today. I got the first IF transformer back together after replacing the wiring going to the 78 tube ...klondike98 — 12:06 AM
Two small radios from France. Restoration and review.
And here they are, the French beauties ready: In summary, the suspicion that radio transmits the national cha...Vlad95 — 10:46 PM
Two small radios from France. Restoration and review.
One is ready for washing, the other for painting: ATTENTION Faint-hearted and heart patients, please d...Vlad95 — 10:36 PM
Two small radios from France. Restoration and review.
Range Switch Block: I thought that mica capacitors of this type died out in the 30s, but no, they just moved ...Vlad95 — 10:29 PM
Two small radios from France. Restoration and review.
Chassis bottom view: Again, like twins, especially in the range switch part. Front panels: And...Vlad95 — 10:25 PM
Two small radios from France. Restoration and review.
Chassis removed, front view: Chassis rear view: It will be enough for the first device to prescribe a bath. Bu...Vlad95 — 10:11 PM
Two small radios from France. Restoration and review.
France Peculiarities of national character. Excelsior-52 threw me off track. It was too far outside my established...Vlad95 — 10:06 PM
Philco 16b no shortwave reception
First let thank everyone who helped ID this radio a few weeks ago! I have replaced all the capacitors and was able to r...Ken D. — 08:07 PM
Philco model 38 code 121 not receiving signal.
Ah yes, the Philco "Green Death" strikes again, someone suspected that cellulose nitrate based insulation migh...Arran — 06:23 PM
Philco model 38 code 121 not receiving signal.
Not sure if it matters much, but measure where the winding sits on the tube and try to rewind as close as possible. Tak...GarySP — 05:45 PM

[-]
Who's Online
There are currently no members online.

>