50's Power Transformer for Tweed Champ, 18w Marshall, or Deluxe?

I haven’t had a second cup of coffee, but my animal hindbrain is trying to say 330-0-330 nominal, with enough current to drive all those original tubes, isn’t gonna rectify and load down to 'lower voltage' in a 5E3. Could be wrong, for sure. Which ClassicTone is in there now, and what’s your B+?

 

I'm still learning about power supply design, but I agree with @King Fan that I would expect the voltage to be higher in the 5e3 than in the original radio.

Additionally, you specified (in two places) that you were measuring the output with a DC reading. Did you take an AC or DC measurement? Did you have it hooked up to a rectifier?

 

I will take the VDC reading as a typo.

This transformer will handle the voltage and current demands of a 5E3 ime. The original circuit has two 6V6 in cathode bias just like a 5E3. Plus it has several more tubes than the 5E3.

The 330 VAC reading without load will result in an unknown B+ at this point. I bet this PT will be in the ballpark but... even if the B+ is low a different rectifier tube can be installed to bring the B+ up.

I like using old iron in builds. Go for it.

 

OK good, you're right. IIRC, that was their 'hot' 5E3 PT -- 710V nominal and 100mA -- it may indeed have been "reverse engineered" from an original (which ran on 110, after all), but itss B+ was notoriously "toasty." So I feel more hopeful about your 330-0-330. That's what my 5E3 has (with ClassicTone's 'chill' 5E3 PT, the 40-18016), and my B+ comes in nicely around 370VDC. Must be other factors going on, since you're getting 370-ish with your 18021, but YMMV in any case.

I agree with @Lowerleftcoast -- you should try it. If your old PT has tons of current available, your B+ may not drop as much due to load, but why not find out. If you do get your B+ down, FWIW, my 5E3 running on my 'vintage voltage adapter' at ~114V sounds great at B+ ~350.

 

Sorry, we're going above my paygrade. Maybe @Lowerleftcoast can tell us.

 

The 5E3 ime does not sag much. The cathode bias of the 5E3 works against current and voltage swings. It does not sag like the fixed bias counterparts that are firmly in class AB.

The Classic Tone 40-18021 was rated for 710 CT @ 100mA
@King Fan alluded to another favored 5E3 PT... The Classic Tone 40-18016 which was rated for 660 CT @ 120mA, and came in with a B+ of about 370.

To compare two different PTs for current, a fixed load could be applied and the resulting voltage measured.

The current capability has to do with wire size, length of wire, and the winding itself with the associated metal core, Henries, etc.

 

@Ronno25 ...are you measuring your plate voltage from plate to ground or plate to cathode? In a cathode bias amp like your, your actual plate voltage is determined when measuring from plate (pin 3) to cathode (pin 8). This number will be about 20v lower than measuring to ground.

 

I'm feeling lazy so I'll just say if you want to know what the old circuit used for current, each tube has the cathode resistor value and the voltage at that pin. Use Ohm's Law to determine the current through the resistors. Then just add them up.

Then there is taking an unloaded voltage reading of the primary and then (with no voltage) take the resistance reading across the winding. You can compare the readings between the two transformers and if inclined use the readings in Duncan's Power supply calculator.

http://www.duncanamps.com/psud2/

If the voltage is low the tube rectifier can be replaced with solid state diodes. Usually that gets hifi transformers up to guitar voltage levels.

 

You should be doing a bias check with each of these changes and probably adjusting your cathode resistor value to get similar dissipation in order to make a fair comparison. That transformer probably sounds bad because at those lower voltages the bias is colder than it was with the original PT.

 

Just out of interest did you check what ac voltage you got for the 6.3v filament supply under load and no load?

Running 4 tubes instead of the 10 it was designed for would half the current draw so reduce the voltage drop. Should still be within 10% even with the 120v ac supply.

 

I know a guy who has a huge collection of different rectifier tubes and swaps 'em around to change B+ and bias. Being lazy and liking instant gratification, I built a version of Rob's bucking transformer... it can switch my supply voltage, on the fly, between ~120, ~114, and ~108. My tweeds really like the 114, and the slightly lower B+ does indeed make 'em warmer and sweeter. Oh, and you say your 5E3 gets raspy in the highs? World's simplest mod, a 470K PI grid stop, often fixes that fast...