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

Ronno25

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I'm looking into this power transformer that I got out of a 1950's Capehart-Farnsworth radio / record player. I was initially considering it for a tweed champ or 18w Marshall build but am now thinking of trying the power transformer in my tweed deluxe clone (it fits the cutout in the chassis exactly). I have a classic tone in there now and voltages seem okay. But curiosity will be the death of me and I can't help but wonder about how it will sound with lower voltages. It wouldn't be too much hassle to swap it out but I want to make sure I don't blow up my amp in the process.

I wired up the primary of this 50's PT to the wall @ 119VAC and took AC readings off the high voltage line, which came out to 330VAC to center tap. I guess I'm not sure what that will result in when under load in an actual amp circuit. Ideas?

I'm also unsure of the current the transformer puts out, especially on the high voltage secondary. Can this be calculated from the tube chart of the original radio it powered? I'll attach it here as well as a grainy schematic of the original radio.

Thanks guys!
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King Fan

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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+?
 

gigante

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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?
 

Lowerleftcoast

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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.
 

Ronno25

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I will take the VDC reading as a typo.

Did you take an AC or DC measurement?

Yep, you guys are 100% correct. It's 330 VAC.

Which ClassicTone is in there now, and what’s your B+?

I have a Classic Tone 40-18021. I just measured B+1 of 370 and Plates of 360 and 363. Wall voltage was 119.5 VAC at time of measurement. The B+ seems to vary between about 375 and 365 when I measure it at different times... must be due to fluctuation in wall voltage throughout the day.
 
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King Fan

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I have a Classic Tone 40-18021. I just measured B+1 of 370 and Plates of 360 and 363. Wall voltage was 119.5 VAC at time of measurement. The B+ seems to vary between about 375 and 365 when I measure it at different times... must be due to fluctuating in wall voltage throughout the day.

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.
 

Ronno25

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If your old PT has tons of current available, your B+ may not drop as much due to load, but why not find out.

I've gathered that having excessive current from a PT can also direct the sound away from sag and note "bloom" to a stiffer kind of response. I definitely prefer the saggy feel. So aside from getting the B+ where I want it I was concerned about this PT having too much current. Is there a way to calculate for certain what the current is?
 

Lowerleftcoast

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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.
 
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Ronno25

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Very interesting stuff. I'm gonna switch it out and will report back with my voltages!
 

dan40

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@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.
 

Ronno25

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@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've been measuring to ground. This is good to know! Thank you!!
 

printer2

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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.
 

Ronno25

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Okay transformer is installed. Powered it up with a lightbulb limiter. Everything looked good, took a couple preliminary voltage readings. Unplugged lightbulb limiter. Turned back on. Noticed some concerning arcing or some type of spark happening at the rectifier. Immediately turned off. Poked around with a chopstick with the lightbulb limiter. Didn't notice any problems. Powered up again without lightbulb limiter, same issue. Turned off. Tried a third time and everything seemed fine.

Notable problem: The amp is humming LOUDLY. When I first built this amp it had some fairly bad hum, which I was able to mostly remedy with a floating heater center tap and grounding the high voltage center tap to the board. I didn't wire this one up that way but will tinker if I decide I like the sound and want to keep it in.

Took voltages:

VAC at wall = 120.4

b+1 340
b+2 292.4
b+3 213

V4p3 328.5
V4p4 291.4
V4p8 17.97

V3p3 328
V3p4 292
V3p8 17.95

V2p1 135.5
V2p3 1.19
V2p6 170.5
V2p7 14.7
V2p8 40.9

V3p1 113.5
V3p3 1.67
V3p6 118.3
V3p8 1.66

In some ways the amp sounds better: warmer lows, smoother breakup. In some worse: The high end became more brittle and there is a harshness to the higher frequencies in the breakup that I don't like. I'm going to put some different rectifiers in to see if I can find a middle ground.
 

Ronno25

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Wow, with the Sovtek the B+ is 375!

and........... The amp sounds nothing like it did with the original PT and B+ 370. It sounds really harsh, not good at all.

I tried an old Tung Sol 5y3, which brought the B+ to 345... Sounds the same as at 340.

I don't have any other 5y3 that will give a different B+ so that's about all I can try. My conclusion is that the transformer just doesn't sound that great regardless of B+.
 
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Ronno25

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I'm going to put the ClassicTone back in and call this a good lesson for me in understanding that B+ isn't everything. This was a pretty fun little experiment and I'm glad I did it.
 

Nickfl

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Wow, with the Sovtek the B+ is 375!

and........... The amp sounds nothing like it did with the original PT and B+ 370. It sounds really harsh, not good at all.

I tried an old Tung Sol 5y3, which brought the B+ to 345... Sounds the same as at 340.

I don't have any other 5y3 that will give a different B+ so that's about all I can try. My conclusion is that the transformer just doesn't sound that great regardless of B+.

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.
 

dougsta

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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.
 

King Fan

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I'm going to put the ClassicTone back in and call this a good lesson for me in understanding that B+ isn't everything. This was a pretty fun little experiment and I'm glad I did it.

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...

58C2F293-8DAB-4260-883A-1B4211991245.jpeg
 

Ronno25

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

If I recall correctly unloaded I got about 5.4 VAC for the rectifier and 6.6 VAC or so for the heater filaments. I may be remembering this wrong for the heater filaments.

Right now under load I'm getting 5.18VAC on rectifier and 6.9VAC on filaments.

Here are my calculations for how the amp is biased. Hopefully I did the calculation right.

Plate voltage measured ~330 on each tube. Cathode voltage ~18.

So I'm at 80%, which is considered cold. It looks like a 200Ohm cathode resistor will do the trick to get me at 100%. I'll try that, unless you guys think it's a bad idea....

Now to figure out this incredibly loud 120hz hum.

Screen Shot 2021-02-25 at 5.18.17 PM.png
 
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