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RE: TL-404 , more than one version for impedance taps?

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In Reply to: RE: TL-404 , more than one version for impedance taps? posted by Ian L on January 13, 2023 at 12:33:05

Ian,
I feel your pain. There were two TL-404 core sizes and a variety of tap, insulation and wire options. And given the requests for tap info for these I'll assume most have nothing written on the core like mine. So yes there was more than one tap scheme for these.

The short answer is to tap hop until you get the best sound. Or if you have the ability determine the tap impedance more empirically. One additional issue is the MGM studio spec many were built to used three different gauges of wire, so use of DCR to help needs that warning.

Here is the info Mike sent me for mine which appears to be how a few of these were made.
#1 black ground
#3 orange 4 ohms
#4 yellow 10 ohms
#5 white 50 ohms
#6 green 200 ohms
#7 red 500 ohms
#8 blue 5000 ohms (goes to plate of tube)

Not all units have the four ohm tap.
PRI (1-8) dcr is approx 88 ohms
MOL 100 vrms @ 25 hz
MOL 125 vrms @ 30 hz

Nominal impedance ratios (with respect to the full winding) of the various taps are as follows;
terminal #3 is 1250:1
terminal #4 is 500:1
terminal #5 is 100:1
terminal #6 is 25:1
terminal #7 is 10:1

I use mine with the EXO-010 plate choke.
Good luck they are worth sorting out.
Matt

Follow Ups:

RE: TL-404 , more than one version for impedance taps? - Ian L 13:04:43 01/25/23 (2)
Matt, Thank you so much for taking the time to write all that.

You got me measuring the resistances. Posting them here for possible future searches.

Clipping to the ground tap (#1) and running up to #8 , the resistances are :

1-3 = 0.2Ω
1-4 = 0.3Ω
1-5 = 2.5Ω
1-6 = 6.4Ω
1-7 = 10.8Ω

The problem showed itself at the top of the winding.

1-8 = 90.5Ω on one and 450Ω on the other.

(One note that on the high resistance unit, Tap #8 was marked P , not 8.)

Thinking that there might have been a little lacquer left on the winding leadout I heated the tab up well with solder and it came down to 170.

A number of times more brought it down to 130Ω but additional repeats were not able to move it from there.
I don't know how separate the taps are from the rest of the coil and worry if I overdo it hot solder might get to the lacquer on the coil proper, so now the task of cutting the paper to see if I can arrange a solution.

Will post results if there's anything that might be useful for others.
Without a page meter it's impossible to tell how many come for a look at this forum as there isn't much in the way of new posts. I was very pleasantly surprised that you visited with a reply. Thanks again!

And if Mr. Mike is out there anywhere, howdy and good wishes.
- RE: TL-404 , more than one version for impedance taps? - mwiebe 16:46:16 01/27/23 (1)
  Ian,
  For what it is worth here are my TL-404 DCR readings from one autoformer.
  1-1 0.7 ohms
  1-3 0.9
  1-4 0.9
  1-5 3.1
  1-6 6.9
  1-7 11.2
  1-8 88.7
  
  So we are essentially the same on our units. The "P" marking means plate which is where #8 is going. I do not think Mike made these with an impedance of more than 5K but maybe he did.
  
  Take a close look at your errant #8 tap and see if the coil wire is wrapped well around the tab. You can always re-wet it will good solder and flux just not too much heat. Heat will change the DCR so always measure cold.
  
  Sometimes depending on how much tab flexing there has been the magnet wire can fatigue. You can repair it but work in the morning without coffee and know exactly what you want to do.
  
  Usually 10% plus or minus is not much to worry about and you are getting close to that. You can always wire it in and if it sounds good forget your DCR reading.
  Matt
  - RE: TL-404 , more than one version for impedance taps? - Ian L 15:50:52 01/30/23 (0)
    Hi Matt,
    
    Thanks for your help,
    
    I cut through the top layer of brown paper, opening up a very small flap for a look-see and found that the tabs are mounted on small pieces of thin flexible fibreglass board that would likely help protect the windings below from the heat above so gave a little more heat and solder but no change from 130Ω.
    
    130Ω is close to 50% greater than nominal so at first seemed large but thinking again, with no DC drop to consider and the coil's AC impedance far larger, I thought maybe now it's not an issue.
    
    Putting a probe across tabs 1 and 4 and a sine across the whole thing the response was ruler flat from 10 to 50Khz. I don't think I've ever seen another transformer that flat do that!
    
    It's "good" sibling tested exactly the same.
    
    So they went into the circuit again this evening and so far so good. The sound is very distinctly different from the 5KΩ output transformers that they are displacing and I think I'll reserve judgement for a while until I get past the novelty and start to take in more than the obvious plusses.
    
    Thanks again!

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