McMurdo-Silver MP6 Custom T10 Audio Interstage Transformer

[Felix-A. Gravel]

Hi there folks,

About 1.5 year ago I took a little break in restoring my McMurdo-Silver Masterpiece VI because it is missing its T10 audio interstage transformer. Thanks to members of this forum, I now have most of the specifications I need in order to find a sub. Its primary winding measures 1500ohms, each of the secondaries measure 4000 ohms, and the ratio between the primary and each secondary is about 1:2. Using this data, a friend of mine tried to wind a custom transformer from a random core he had kicking around, however the results were not good.

I finally decided to get in touch with Heyboer Transformers and the owner asked me some additional questions I cannot answer. He needs to know the following :

1- Wattage, or wire size the transformer is wound with
2- Physical dimensions

As far as wattage, I suppose we could calculate it? The transformer is fed by the plate of a 6J5, which has a maximum plate dissipation of 2.5W, and drives two 6L6 tubes. Is 2.5W the number I'm looking for?

As far as size, if someone has their MP6 power supply on the bench right now, I'd very much appreciate it if you could give me some rough measurements.

Thank you so much for helping me complete this important project of mine.

[Robert Meader]

Actually you can buy a new replacement of the shelf.  As to which one depends on how the secondary winding is connected.  If the two secondaries connect to a common point ie ground or negative supply then a standard one winding primary and center tapped secondary will do.  A Stancor A53-C (Now available as P-T156 from antique radio supply)  is a drop in replacement.  If however the secondaries don't have a common point then you will need a "dual secondary" winding.  A good choice is hammond 124E  (available from Antique radio supply as P-T124E).

This transformer is designed to be driven buy push-pull 6j5's to push-pull 6l6's.  Now since we have only one 6j5 we will connected to one side of the center tapped primary and resistor connected to the other side of the primary to ground.  The value chosen is to match the current of the 6j5 tube balancing out the current through the transformer.  Now looking at the transformer specifications the ratio across the entire primary to secondary is 1:2.  Now  since we are using only one half of the primary the step up ratio is 1:4.  We need to cut it half it half.  Connect two 100k resistor is series across the each of the two dual secondary windings. Connect the center point of the series resistor to the grid of the  6l6.  Now the step up ratio between the plate of the 6j5 to the grid of the 6l6 is 1:2.  The response the transformer is  basically flat to 15khz.  This is too good.  One screwed up in reverse.  The solution is to connect a capacitor from the plate of 6j5 to ground or the center tap of the transformer to cause the response to roll off starting of 5khz.  See easy as pie!

[Felix-A. Gravel]

Good morning, Bob!

Is that what you had in mind [minus the missing resistor which I forgot to draw on the primary].

The problem here is that the original transformer's split secondaries are connected in a certain way to provide negative feedback.

I'm not exactly sure how I would make the setup you suggested work in that particular arrangement.

[Alex R. Whitaker]

Felix,

I have two MPVIs......I just saw that the interstage transformer is on the amp chassis.  I'll get to one of the amp chassis that I have, and get you the physical dimensions of the transformer.  I don't know how hard the 1st audio 6J5 is driven on that set, but I would bet that it's not that hard.  The turns ratio on the transformer is 4/1.5 which would be around 2.67 to 1.  I wouldn't think they would have the 6J5 dissipate 2.5 watts...that would shorten tube life, and make an already complex radio even more troublesome.  Here's the data sheet on a 6L6G:  https://frank.pocnet.net/sheets/137/6/6L6G.pdf, and a 6J5G:  https://frank.pocnet.net/sheets/049/6/6J5.pdf.  I feel dumb, as I can't remember offhand how to calculate plate current, given the plate voltage, cathode voltage, and grid bias.  You can look it up, and figure out how much the 6J5 is dissipating.  Knowing you, you've probably already looked at the data sheets, thoroughly.   I would also imagine you could look up how much grid drive that a pair of 6L6s, running class AB1, as pentodes, would take (I think that's the amplifier class it's running....plain old push-pull).  That would also give you an idea of what size wire the secondary would take.  

[Robert Meader]

Looking at your diagram on the top winding connect the joint of  the bottom 100k resistor and transformer winding to the feedback network.  On the bottom winding connect the joint of the top 100k resistor and transformer winding to the feedback network.  Be sure to check the Hammond 124E phasing diagram to properly connect the secondary windings in series or else all hell breaks out!

One side note the Hammond 124F is the correct replacement for the driver transformer on the Scott Philharmonic and Phantom.

[Stephen Scheel]

could you just send the original transformer to get rewound?   That way he should have all information he needs to create a new core?

[Felix-A. Gravel]

Said transformer is missing.

Stephen Scheel said:

could you just send the original transformer to get rewound?   That way he should have all information he needs to create a new core?