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Author Topic: BC-610 sending audio to 100th tubes from 500 0hm output on PA amp  (Read 3294 times)
K1YTG
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Posts: 225




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« on: March 08, 2013, 10:31:02 AM »

I am driving the audio to my BC610 from a Stromberg Carlson PA amp with a pair of 6L6's in the output.  It has a 500 ohm output.
There is a stage of amplification on the BC610 audio with 2A3's in it.  But the 6L6's have enough power to drive the 100th grids without the 2A3's.
So I want to connect a line to grid transformer to connect the grids up and leave out the 2A3's.  They are costly to replace, as well as not needed.
I am confused as to the impedance and turns ratios. 
The Hallicrafters transformer in the bc610 that drives the 100th grids is labeled as 2750 ohms on the grid side.  But I am not sure if that is 1/2 secondary or all of the secondary.
I have a Merit A-3121 Transformer that is multi-tap for from 1:1.75 to 1:3  turns ratio primary to 1/2 secondary.
But I have yet to find a wiring diagram for the Merit transformer so connections may be difficult.
I can feed it some low voltages and measure the output to find turns ratios.
Based on 500 ohm input to 2750 ohm to the grids it is an impedance ratio of 5.5  and a turns ratio of 1: 2.345
But I think that is primary to all of the secondary.  so to half the secondary it would be 1: 1.17
There are many things here that may be wrong.
What is the grid impedance of a pair of 100th tubes?  Is it 2750 ohms?
What is the ideal turns ratio match to drive that from a 500 ohm line in?
And when is it expressed as 1/2 secondary and why.
In testing the Merit transformer do I measure 1/2 secondary or all of the secondary?
Given a multi tap transformer, is it possible to just try different ratios and compare them to see which is the best match?
I would assume that the best match is achieved when for a given input the output from the 100th tubes is the greatest.
Thanks in advance for helping me sort out this connection confusion.
Norm

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KB4QAA
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« Reply #1 on: March 08, 2013, 01:52:01 PM »

w5jgv.com/downloads/Eimac-1948.PDF

Above is the Eimac 1948 catalog and data sheets.

Page 72 is the 100TH data sheet
Page 12 Has the formulas with examples for calculating the inputs for tubes.

'Grid Impedance' is not a given characteristic.  You want to be concerned with not exceeding the grid voltage and current ratings.  As the example shows, this is a case where you work backwards from your desired 100TH output back to 100TH input and the driver output.

If you can't find data for your transformer, then yes, you will need to unhook leads, feed a low voltage in and measure the output.   bill
« Last Edit: March 08, 2013, 01:54:54 PM by KB4QAA » Logged
K1YTG
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Posts: 225




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« Reply #2 on: March 08, 2013, 02:28:14 PM »

I do see the calculations for an RF amp there but not for an audio modulator.
I assume the calculations would be different.
Where are the audio calculations?
Norm
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KB4QAA
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« Reply #3 on: March 08, 2013, 04:45:49 PM »

If you don't have a  piano, don't worry about it.  Smiley

Good luck.

Edit:  I don't see what the difference is.  A signal being amplified is a signal being amplified as long as it's within the rated frequency range of the tube....
« Last Edit: March 08, 2013, 04:47:53 PM by KB4QAA » Logged
KB4QAA
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« Reply #4 on: March 08, 2013, 05:10:19 PM »

Good news!

Bill Orr, in his west coast handbook has the step by step procedure for determining the impedance ratios and turns ratios for the transformer.  It is three short paragraphs. 

I will scan it in and then we can figure how to post it.   bill.
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K1YTG
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« Reply #5 on: March 08, 2013, 06:01:25 PM »

I look forward to seeing it.
The Eimac book has the calculations for the RF amp and it says it is for class C amplifier
The eimac sheet on the 100th tube has the audio specs listed as class B
I was assuming different parameters for running in different classes.
Norm
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KB4QAA
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« Reply #6 on: March 08, 2013, 06:22:24 PM »


http://www.flickr.com/photos/93888532@N07/8541132972/in/ph.otostream

Ok, I posted this as a jpeg and highlighted the starting paragraph.  Looks straightforward.  I think the key is matching 'plate impedances', which makes more sense to me now.

I searched the Radiotron Designers Handbook, 1953, and just couldn't find a relevant entry.

Let me know if you can't get this from Flickr.    cheers, bill
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K1YTG
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Posts: 225




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« Reply #7 on: March 08, 2013, 08:43:19 PM »

The page came through fine and is a very good discussion to calculate the ratio of a modulation transformer to couple modulator tubes to the RF final.
Can you tell me how to use it to calculate the turns ratio coupling 500 ohm line in to the grids of a pair of 100th modulator tubes?
Norm
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KG8LB
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Posts: 243




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« Reply #8 on: April 29, 2013, 11:24:35 AM »

  No time like the present to learn this . You can get off to a good start with this article . It's all in here :
http://www.amwindow.org/tech/pdf/rcaclassB.pdf
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