Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: stingray_65 on January 29, 2012, 05:47:00 pm
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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.
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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?
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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.
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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!
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he's got that figured in: "But my math says : 360 * 1.41 = 507V and then -50V for 5U4 loss would be 457V"
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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??"
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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.
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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 :laugh:
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> 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.
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Boy, I've always wanted to know how to REALLY put in all those variable in the PSU.
You and me both!!