[Jake Blake]

Has anyone ever used a power transformer as a substitute in a radio, that had a little higher high voltage secondary than the original, and replaced a few key resistors to bring the d.c. supply voltages back in line to what they're supposed to be?

I'm curious about doing this with the 39-6 C I, and replace  resistors, (part numbers) 19, 23 and 24.  Also maybe insert some resistance in the   output transformer / output plate connections for the 41 tube?   Has anyone done this successfully?

Jake

[Radioroslyn]

There's a couple of ways to lower the dc output of the power  supply. One way is to lower the value of #26 and increase the value of #25. Or you could add a series resistor to the field coil.

[morzh]

...or use a small (it's a small radio) bucking transformer.

[35Z5]

OR use a rectifier that has greater voltage drop. Sometimes takes a bit of experimentation.

[morzh]

Tom

A rectifier that has greater voltage drop is what Terry just suggested - increasing the FC resistance (placing a resistor in series).
Not many other options: using a tube with higher plate resistance won't do much plus it will run hotter. Using a selenium rectifier - like peeing against the wind as everyone is doing the opposite - they are getting bad.
Inserting diodes with resistors - see Terry's suggestion again.

[35Z5]

Voltage drop between tube rectifiers vary between different tubes.

I replaced a transformer in a zenith amp that B+ was 360v using a 5V4. With replacement transformer voltage was approx 20v high. Subbing a 5Y3 brought voltage within a couple volts of original.

[Radioroslyn]

Tom,
If you need more of a drop use a 5R4. Has a 2a filament but it's the same size as a 5U4G or there is a mil spec version. [Image: https://proxy.duckduckgo.com/iu/?u=https...nt.jpg&f=1]

[morzh]

Yes. Again, they run hotter.

[Jake Blake]

Terry ,

The adjusting filter capacitance change is most-interesting.  I didn't know anything about that.
I suppose the simplest route is the added resistance in the dc supply. The filter caps I've bought will handle the extra voltage, ( 450 volts), so I don't think changing them for fear of them being underrated is necessary.   I would assume it best to add the resistance after the field coil.    

Not sure it will be necessary...... I'm trying to buy another transformer from someone on here.  He may have one more-like the original.  there's one on Ebay that's a little high.  I'm not rushing into either choice yet.

I appreciate the help.    

morzh ,

An additional transformer sounds expensive, and a little complicated.  I'll probably stick with resistance if I have to do it at all .  I'm hoping to get a better replacement  from someone I came across after I posted my original post.  Thanks for the suggestion.    

Tom,
 
Your suggestion of a different rectifier sounds wonderfully the easiest to implement, ( as long as the pin arrangement is the same...not that I'm not capable of re-wiring it).   This thing has an 84 rectifier though, and I'm feeling any newer rectifier ( like the 5Y3),  may go in the other direction. I don't know things like plate resistances, so I need to study that more in the online tube manuals.    


Jake

[morzh]

An additional small transformer (I recently bought 2 larger ones, 3A 12V, for $6 each) is electrically ideal way: there is no power dropout, except for parasitics. All other ways result in power dissipation.
Expensive it is not, if bought on eBay.

[35Z5]

Tom,
If you need more of a drop use a 5R4. Has a 2a filament but it's the same size as a 5U4G or there is a mil spec version. [Image: https://proxy.duckduckgo.com/iu/?u=https...nt.jpg&f=1]

The 5R4 has 67v drop at 250ma, meaning in real world, it drops approx half voltage of the 5Y3 that's rated 60v drop at 125ma. In said amp, that draws roughly 80ma, the 5R4 & 5V4 gave similar B+. A 5V4 is rated 25v drop at 175 ma.

As far as heat, will be similar in any 5v, 2A filmant that handling same amount of current.

The 84 has a 20v drop at 60 ma. Being a big pin and 6.3v tube vs 5v, are less choices for substitutes. Almost any workable choice will increase voltage.

[Radioroslyn]

Not sure abt how all the math works out but my recollection is that the 5R4 is able to rectify much higher voltage (1800vac) but the max current is abt 150ma. Swapped out a pair of 5U4's w/5R4's in a transmitter I had and there was some drop in the hv by comparison. Maybe 50v or so. I wasn't suggesting that the 5R4 would be a suitable replacement for the 84 I only mentioned it to illustrate that that some have a higher voltage drop than others.

[Jake Blake]

Radioroslyn

An excellent point, well-taken bud.  Thanks.

Jake

[35Z5]

The 5R4 has a maximum current rating of 250ma, generally make dandy subs for 5U4, but watch current as 5U4GB can handle 275ma. Probably not a issue, most any equipment that draws over 200-225ma likely uses two rectifiers. The 5U4GA is rated 250ma, orig 5U4G only 225ma.

[morzh]

As it is with solid state devices, same size type with higher collector/drain voltage has lower current limits and higher resistance in the fully open state.