confused about actual readings and the schematic call outs

[stingray_65]

I have several old PT's I've pulled. A few of them I have schematics for the original circuits they were in.

I'm looking at 1 now, pulled from a '57 Wurlitzer organ.

I measure 372-0-374 unloaded and 120VAC on the mains.

The schematic says I should measure 720V between pins 4 and 6 on the recto tube which suggests its 360-0-360, close enough to what I measure.

Thing is, after the recto tube (5U4GB) the schematic says I should be reading 390V.

But my math says : 360 * 1.41 = 507V and then -50V for 5U4 loss would be 457V

What am I missing here? why would the schematic give a lower voltage, like -70V!

I run the Duncan PSUD and it spout out 449 rms.

Ray

Darned thing is I usually take a few readings when I can before I tear down things, I had the schemo in hand when I did and I don't recall my reading not jibing withe the schemo.

[stingray_65]

OK,
Here's a second one, pulled from a Capitol record player built in '57.

I KNOW I took readings before teardown  and they were close to whats called out on the schematic.

But again its a 360-0-360 PT with 5U4GB recto and it had 415V after the 5U4GB.

is all these losses from load?

[jjasilli]

Yes, load is the issue.  See:  "Electric Guitar Amplifier Handbook", by Jack Darr.  Expect UNloaded voltages to be 10 - 15% higher than Loaded.  Organs used lots of tubes so let's go with 15%. 457VDC - 15% = 388.45 -- essentially dead-on to the schematic's 390.

[stingray_65]

  Expect UNloaded voltages to be 10 - 15% higher than Loaded.  Organs used lots of tubes so let's go with 15%. 457VDC - 15% = 388.45 -- essentially dead-on to the schematic's 390.

That's a handy rule of thumb!

Thanks!

[jjasilli]

he's got that figured in:  "But my math says : 360 * 1.41 = 507V and then -50V for 5U4 loss would be 457V"

[stingray_65]

I guess I wasn't being thorough with my math JJ.

If I had actually been thinking that with big numbers, tolerances of 20% are big too.

These old transformers look beefy, I know some of it is because of the extreme 6.3v current demands of an organ, I was just going to try and see if I could figure out the current capabilities of the HT windings as was immediately disappointed.

I used to just generalize and hope with these old irons, I figure if they pushed a pair of 6L6's and half a dozen 9 pins they would be good for something with 3 preamps and and a PP 6L6 power section.

But using the little bit of math I picked up and retained, I was going to give the old ohms law a try and get some firm(er) numbers to help me decide where they might be most useful (besides e-bay)

I think that'll be my math homework tonight, maybe some answers tommorow.

"anyone?.....any one?....Beuller??"

[HotBluePlates]

If you use smallish-value filter caps, the rectified voltage will fall away from the x1.414 peak faster. Your meter measures d.c., and averages (or computes true-RMS) of the sawtooth a.c. ripple.

If the ripple voltage falls farther/faster from the peak, the d.c. meter reads a lower d.c. voltage.

The Duncan PSU program may lie to you, unless you know how to properly spec the "non-ideal-ness" of your parts, including lower cap values, high-ish ESR, etc.

So that means a supply with 8-10uF filter caps will have a lower d.c. voltage than the same supply, same secondary voltage but using 40uF filter caps.

[stingray_65]

The Duncan PSU program may lie to you, unless you know how to properly spec the "non-ideal-ness" of your parts, including lower cap values, high-ish ESR, etc.

Boy, I've always wanted to know how to REALLY put in all those variable in the PSU.

I've been able to ballpark a few simple supplies with the PSU, but I know it can do more than I know how to do with it

[PRR]

> figure if they pushed a pair of 6L6's and half a dozen 9 pins they would be good for something with 3 preamps and and a PP 6L6 power section.

If your truck was made for 2 elephants and half-dozen dogs, sold to a church, yeah it might manage 2 similar elephants and 3 dogs a few hours a night.

[HotBluePlates]

Boy, I've always wanted to know how to REALLY put in all those variable in the PSU.

You and me both!!