The AM Forum

I have a 750 vct transformer in my possession with the proper filament winding. The stock low voltage is 850 vct. As I see it the thing will wind up about 50 volts low. I am asking the panel of experts if this will fly or should I wait to have one made.

John N8QPC

If you took a defunct 5Z4 tube, or one with a similar base, and installed solid state rectifiers in it, you'd gain back some of the "lost" voltage, at the cost of the rig not being virgin stock.  A plug-in modification would be easy to reverse and relatively painless.
73 de Norm W1ITT

I intend to solid state it all. I am not going to use the 5 volt winding. I wonder how much I will gain by that.

750Vac/2 = 375Vac.

!.4 X 375Vac = 525Vdc to first choke input no load with say 1N5408 diodes.

The 5Z4 has a per plate resistance of ~ 160 ohms which dropped voltages across them at about 20 volts at 0.125A so you not be losing that voltage with SS rectifiers.

What was the previously measured or manual stated voltage across the bleeder resistors under no-load and loaded conditions?

Phil

Phil

You may have missed that the filter network is L input. So you would be looking at average voltage I believe that would be .637 x peak. the filter caps don't give a voltage rise just get rid of ripple. Not the wine variety either. So either way I would pic up about 20 volts.

John N8QPC

No I didnd't miss it as you have L301 and L302 and each filter inductor has an internal resistance which drops a dc voltage across it.

How much resistance does each choke have?

If we knew that fact we could predict a voltage at each point in the LV power supply at a current of 125 mA!

Phil

Collins says there is 240 vdc at the junction of r310 and L302. Any way the voltage will always be average in a properly designed inductive input filter. R 310 and R 309 provide a 12.5 k load to draw 19.2 ma thru the two chokes so they work.

John

Predicted voltages:


Phil

Don't forget, those voltages were from yesteryear. With today's line voltages and using an older transformer, you might indeed end up with what you're looking for.
Regards,
Gary

I said screw it ordered a new one 200 plus shipping.

John

If Solid Stated you might need a 180 ohm 5 Watt resistor before L301 to bring the LV down to 245V or so.


Phil

I was going to solid state it,but I may not I have total confidence in the new transformer. I have never had anything fail from Heybor. https://www.heyboertransformers.com/. I figure that the rig was made in 1949 if this one lasts half as long I will be long dead and won't care. Hammond has a off the shelf one for 200.00 an old Peter Dahl design. In fact they are using his name.

John N8QPC

If it is similar to this one http://www.hammondmfg.com/pdf/EDB279X.pdf then I don't see any problems.

Heyboer also makes good transformers.

Phil

More like this one http://pwdahl.com/pdf/EDBPWDP13005.pdf. The transformer you listed only has one 5 volt tap @ 3 amps. I need 4 amps on each of two 5 volt taps. The 6 volt tap also is 5 amps it needs to be 9 amps.

John N8QPC

Have your bad one, rewound.

I believe that would be more expensive. As well as it was a bad design in the first place. I wouldn't know where to have it done either. New one is on it's way so it's a moot point.

John

I was just offering some alternative that wasn't mentioned. Sorry it was moot.

I said my example was, "...similar to..." in terms of secondary voltage.

In the time period of this design this was an effective and efficient design.

Replacing the tube with SS rectifiers, and adding a series power resistor that simulates the former tube's internal plate resistance, should bring the new voltage close to the original voltage.  


Phil

-And don't forget the possibility of using a bucking transformer in line to get today's 120 to 125VAC down
to 110 to 115 for any of these rigs.  I've got one for the 32V2, 75A2A combination using a 220v to 10v @ 20 amps
 Raytheon fil. xfor.   Buck wired with only half primary voltage, etc.,  it drops my 120 VAC down to 115 with plenty of current to spare.

Also "go easy" on the hi/low switch for HV on the 32V.   
I know it's hard to resist, especially when something is dragging down the plate
voltage slightly such as a dying screen bypass capacitor.  It'll only get worse so
it pays to find out what is going on.

This is my master plan. I had the new transformer wound with 125 volt primary. I intend to run the rig in the 600 volt position to offset the higher voltage on the plate side. In the meantime I have the rig on the bench. I am going thru it and replacing anything out of tolerance. Resistors that have drifted all electrolytic caps are getting replaced whether good or bad.  I have already replaced most of the silver mica's. As I had one short about 30 years ago. The dendrites just keep growing no matter whether you like it or not.   http://amfone.net/Amforum/index.php?topic=4906.0  The Huntron is my friend it is right up there with a desoldering station. I  have been thinking about getting an ESR meter any comments would be appreciated. Don't laugh but I have an old Cornell Dubilier BF-60 capacitance bridge I use out of circuit. I also have my homemade leakage tester that tests up to 2.5 k volts and reads actual current. I have it current limited or it will supply a full amp of current. I like it for testing didoes caps etc. anything that I want to know the real deal. I put an isolation transformer in it so as to keep from killing myself..

John N8QPC