Globe Champ

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Steve - K4HX:
Almost any tube output audio tranny would work to match 8 Ohms to the grids of the 811(A)s. I used 20 watt unit and it was overkill. But it's what I had and it worked FB.


Quote from: K9ACT on June 19, 2018, 02:01:07 AM

I am thinking that all those serious mods to the audio are sort of like making a silk purse out of a sow's ear.  It's probably not fair to the Champ to call it a sow's ear as it is pretty nice as is.  Making all those mods to it seems like a lot of work when there may be a much simpler way to upgrade the audio.

Both my 810 and 2 X 8000 transmitter modulators were driven by an inexpensive audio amp board using a TDQ2030 and some kind of transformer driving the 811s or 813's.

The audio chain could be switched to either transmitter as desired and it all worked very well.

Problem is, all my files were lost in the fire and I don't remember what the transformer was.  I think it was a Hammond but I don't see anything in their list that would match  8 ohms to whatever the 811 grids would be.

Can anyone suggest a readily available transformer?

Thanks,

js

K9ACT:
My grand plans for an outboard audio chain for the Champ seem to have flopped.

I have: dynamic mic>mic preamp>Behringer 2946>8W speech amp>Hammond 125C transformer>811 grids.

Seems functional but I can't seem, to get more than 50% modulation.

To  minimize the variables for testing, I slimmed it down to a 4V p-p 1Kc sine wave into the speech amp>transformer>grids.

In this config, I can get up to 200V p-p on the grids which is more than the data sheet says is needed but still only 50% modulation.

I was getting something like 80% modulation before I "fixed it".

The audio section in the Champ is turned off and the transformer leads snipped.  I brought out the grid pins and chassis ground to the outboard transformer.

What did I do wrong here?

js

w4bfs:
Quote from: K9ACT on July 03, 2018, 09:21:57 PM

My grand plans for an outboard audio chain for the Champ seem to have flopped.

I have: dynamic mic>mic preamp>Behringer 2946>8W speech amp>Hammond 125C transformer>811 grids.

Seems functional but I can't seem, to get more than 50% modulation.

To  minimize the variables for testing, I slimmed it down to a 4V p-p 1Kc sine wave into the speech amp>transformer>grids.

In this config, I can get up to 200V p-p on the grids which is more than the data sheet says is needed but still only 50% modulation.

I was getting something like 80% modulation before I "fixed it".

The audio section in the Champ is turned off and the transformer leads snipped.  I brought out the grid pins and chassis ground to the outboard transformer.

What did I do wrong here?

js


need to see some oscope info .... positive swing vs neg swing at each grid .... exoect pos swing to be rolled off and smaller with neg swing looking like half sine wave (obviously with sine wave excitation) .... also look back at amp output to see if distorted .... measurements please

look at whats going on .... 4V pp drive yielding 200V pp is a voltage transformation ratio of 1:50 and an impedance transformation ratio of 1:2500... this means for a 8 Ohm output Z the the grid load should be 20kOhm ... this is too high of a transform ratio as the minimum grid impedance can be roughly estimated as Z=ExE/P however the positive going peak is much less than the negative going peak .... due to grid current .... guessing 100V peak positive gives Z= 100 x 100 /10 = 1kOhm minimum .... lower the turns ratio and increase drive power and see what happens

K9ACT:

>need to see some oscope info .... positive swing vs neg swing at each grid .... exoect pos swing to be rolled off and smaller with neg swing looking like half sine wave (obviously with sine wave excitation) .... also look back at amp output to see if distorted .... measurements please

Seems like I am over my head here. Not sure what or how to measure it.

What I have described is measuring grid to grid as prescribed in the data sheet. I presume this means what it sounds like, not from grid to CT. I see no difference in the size, shape or position relative to 0 voltage no matter which way I measure it.

>look at whats going on .... 4V pp drive yielding 200V pp is a voltage transformation ratio of 1:50 and an impedance transformation ratio of 1:2500... this means for a 8 Ohm output Z the the grid load should be 20kOhm ... this is too high of a transform ratio as the minimum grid impedance can be roughly estimated as Z=ExE/P however the positive going peak is much less than the negative going peak .... due to grid current .... guessing 100V peak positive gives Z= 100 x 100 /10 = 1kOhm minimum .... lower the turns ratio and increase drive power and see what happens

By this you mean increase the speech amp gain?

I don't understand why lowering the turns ratio would increase the voltage at the output.

I have tried a number of different ratios and none get me where I need to go. Here is a link to a hook up table for the tranny...
http://www.hammondmfg.com/125.htm
I have tried 1-2, 4-5, 1-6

Can you suggest an appropriate one?

I think I did something dumb with the basic connection to the grids. I assumed these levels were high enough to be able to get by with the unshielded wire I had on hand but that was probably a bad assumption. Don't know what the wire is called and can't find what I want so I ordered a 10ft XLR cable that I will cut off the connectors and use what I need. The shield would be grounded to the chassis and the other end goes to the CT.

Thanks for your help,

Jack
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w4bfs:
ok Jack .... starting with 811 data sheet .... the required peak to peak grid swing and power required to do it properly will define the minimum grid impedance and therefore the target transformer secondary impedance .... do the calculation

your initial setup with 4Vpp excitation is not the required drive power but just some milliwatts ... so calculate what the 8 Watts available can provide for a voltage swing at its favored load impedance (usually 4 or 8 Ohm)  remembering the rms to Vpp conversion

now you know the voltage ratio primary to secondary so choose the taps that will give you that transformation .... looks like 1-6 offhand

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