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Old 17th February 2007, 12:49 PM   #1
GK is offline GK  Australia
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Default Now wouldn’t this make an awesome monoblock.

G’day guys. Here is a project I’m currently working on:

http://homepages.picknowl.com.au/glenk/ctm2k.htm

Looking at the schematic for the RF deck…..

http://homepages.picknowl.com.au/glenk/rfdeck.jpg

…it is clear that a conversion to audio amplification would be as easy as pie.
Just substitute 2TR1 with an audio transformer, disconnect the AGC and tie the secondary CT to ground. Get rid of 2L1 and 2L2, re-bias and increase the values of the plate resistors for 2V1 to 2V4. Increase the value of all the inter-stage coupling caps for low reactance at the low end of the audio spectrum. Get rid of all the output stage RF stuff and replace 2L3 with a freaking big mutha of a custom audio transformer, and that’s about it.

Four 4CX1000A Tetrodes in class AB1 push-pull!

Hmmmmmm…….What do you fellas reckon? Maybe I should try getting my hands on a another couple of CTM-2K’s for my lounge room stereo system. With a combined weight of nearly 3000 pounds I might need to get my floorboards re-enforced a little though......

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Old 17th February 2007, 01:39 PM   #2
kmtang is offline kmtang  Canada
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Hummm, easy as Pie?? It depends how much money and efforts you want to put it this project.

First of all, it is hard to find someone who can make the big Output Transformer handling such high voltage and power with sufficient band width and flat response.

The driver circuit need to be redesign to pass audio signal and change it's bias to class A operation.

I would say it's better to send this transmitter to the Radio Museum......

Anyway, it brings me the memory of my good old days as Radio Officer on board ships.


Johnny
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Old 17th February 2007, 11:16 PM   #3
GK is offline GK  Australia
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Actually, I’m not really seriously considering turning a pair of these into audio amplifiers, I just though is would be interesting to contemplate, from a technical point of view. My unit is destined for resurrection on the 160m band, as outlined on my web page.

That being said, the 4CX1000A is a tube with rather good linearity and Eimac actually lists audio amplification as an application for this valve (such as a plate modulator amplifier for class C AM transmitters).

The 4CX1000A is a popular valve in commercial ‘desk top’ linear amplifiers manufactured for the HAM radio market. Picking one of these up for a reasonable price at a HAM swap meet or similar wouldn’t be too hard. Even a lower power unit, such as one with a pair of 4CX250A’s in push-pull would be a good start.

That would be an excellent way of getting into really high power tube audio. I’ll be keeping by eyes open at the next HAMfest.

Cheers,
Glen
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Old 20th February 2007, 07:46 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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Better get your own substation installed too.

I'd hate to accidentally create an oscillation in an amp with that kind of power, you might turn it into a transmitter!
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Old 20th February 2007, 11:32 AM   #5
SY is offline SY  United States
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The main issues for the 4CX1000 in audio are the noise from the forced air cooling and the rather high required plate-to-plate impedance necessary for linearity.

Why the 160 meter band? Because it's closest to AM broadcast frequency? It's usually rather empty, but a California Kilowatt booming in might shake things up.
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Old 20th February 2007, 09:50 PM   #6
GK is offline GK  Australia
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Quote:
Originally posted by SY
Why the 160 meter band? Because it's closest to AM broadcast frequency? It's usually rather empty, but a California Kilowatt booming in might shake things up.

I already have legal limit TX's for all the other HF bands!
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Old 22nd February 2007, 04:44 PM   #7
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There is a small group of folks dedicated to the 160 meter band, usually using SSB, but sometimes, for old times' sake, using AM. I have a friend with a military surplus TMC 10,000 watt linear amplifier that he may retune to the 160 meter band. See picture below. He of course always attenuates the output below the legal limit.

The blowers in this thing are deafening, as is the two-phase to three-phase motor converter he had to buy to feed the transmitter supply. We're forced to wear communications headphones and noise-cancelling mikes when jabbering on the air waves.

These huge transmitters always seem to suggest audio amplification. As has been stated many times, the challenges are many, include mating the high output resistance of typical RF power tubes to the speaker, requiring a high-turns ratio OPT that can also stand off thousands of volts of B+, not to mention hundreds if not thousands of watts being transferred to the load. It can be, and has been, done. It isn’t easy.
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Old 23rd February 2007, 09:08 AM   #8
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10kW - WOW! I take it that's PEP.

My TX is 8kW PEP, 2kW continuous carrier. According to the manual, when pumping full power MCW (AM), it's total power consumption is 8.9kW.

That'll get ones HAM shack nice and toasty on a cold winters morning!

Cheers,
Glen
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Old 24th February 2007, 02:59 PM   #9
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Quote:
Originally posted by G.Kleinschmidt
That'll get ones HAM shack nice and toasty on a cold winters morning!
Yes, indeed, and my friend lives in Florida where space heaters are not usually welcomed. His large "lab" uses three window-style AC units. But he fires up this beast only on "ceremonial" occasions.
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