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Old 14th October 2009, 08:33 PM   #1
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Default 6550A HF Amp

I'm developing a 500V at a few dozen watts variable frequency modulator for someone. It will need to operate 0 to 20MHZ at a minimum. The wave will be a variable PWM variable square wave input from a function generator. My friend has a 6550A tube and I'm wondering if this has sufficient frequency handling for HF use. From what I've read it's generally used for audio amplification but would it work for this?
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Old 14th October 2009, 10:11 PM   #2
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I can see squeezing 25 W. from a Class "C" 6550A. I don't know if that type specifically developed for high power musical instrument service will work at 20 MHz. OTOH, the very similar KT88 definitely has RF capability.

If you want more power than 25 W. or require better linearity than Class "C" delivers, I suspect you'll have go to PP.
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Old 14th October 2009, 10:21 PM   #3
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I'll go for a PP setup in that case since I'd like to shoot for having at least 50-75 watts available. Do you have any tube suggestions to do this? Keeping the price as low as possible is a high priority.

Also would solid-state be more appropriate? This seems to be a bit outside the frequency range of power MOSFET's so I was thinking tubes.
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Old 15th October 2009, 12:46 AM   #4
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That's a wicked range for a tube to push a few watts out at. 10MHz to 100MHz would actually be easier to pull off with consistent output than 0-20MHz.

I'd suggest 807's if you want to use tubes... lots available still and they are made for RF and have the internal precautions.

Otherwise *dons asbestos suit* I think you're in the realm of.. MOSFETS

Cheers!
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Old 15th October 2009, 12:49 AM   #5
kevinkr is offline kevinkr  United States
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6550 were commonly used in the broadband (untuned) linear amplifiers used in early phasing type SSB exciters for use on Ham and Mars bands. I had one of these boat anchors and IIRC it operated up to 28MHz at reduced output power - it had a couple of 6550 in the final PA, and on the lower bands was capable of > 150W PEP on SSB. The company that made it was snapped up by Collins at some point in the early 1960s - wish I still remembered the name. The exciter was very nicely made (on a par with Collins) and took only a couple of hours to get tuned up and working.
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Old 15th October 2009, 12:53 AM   #6
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Quote:
Originally Posted by kevinkr View Post
6550 were commonly used in the broadband (untuned) linear amplifiers used in early phasing type SSB exciters for use on Ham and Mars bands.
Whale oil be.... I've only worked on/seen schematics of TV sweep tube ones (which many also came with the RF precautions).

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Old 15th October 2009, 01:00 AM   #7
kevinkr is offline kevinkr  United States
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Hi Gregg,
I had a Drake TR-4CW transceiver with three sweep tubes in its PA and there were plenty of warnings that went with that rig, but it was a great radio.. It could really crank on CW about 270Wpep.. Very stable VFO as well, probably one of the best at the time.

I have long since retired from the ham world and let my license lapse in favor of focusing on audio.
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Old 15th October 2009, 01:17 AM   #8
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I'd think most of the "usual" audio power tubes,and darn near all of the sweeps should work up to at least few dozen Mhz without too much trouble,just keep an eye on the inter-electrode capacitance.
I've even seen 6L6's,EL34's,6550,etc. all used in various Ham TX circuits,up to the 6meter band.(~50Mhz)
Take a gander at some schematics of older HAM equipment for ideas/input.
BAMA has loads of schematics and manuals.
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Old 15th October 2009, 02:14 AM   #9
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Heya Kevin,

Quote:
Originally Posted by kevinkr View Post
Hi Gregg,
I had a Drake TR-4CW transceiver with three sweep tubes in its PA and there were plenty of warnings that went with that rig, but it was a great radio.. It could really crank on CW about 270Wpep.. Very stable VFO as well, probably one of the best at the time.

I have long since retired from the ham world and let my license lapse in favor of focusing on audio.
Ah yes, memories of the good ol' fun days of playing with the VE4's in Winterpeg

Actually, the ham guys could take care of themselves... it was the CB'ers showing up at the back door of the shop with smoke stained boxes that had sweep TV sweeps in them asking.. "It broke, make go again"

Cheers!
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Old 15th October 2009, 03:12 AM   #10
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I'll go for a PP setup in that case since I'd like to shoot for having at least 50-75 watts available. Do you have any tube suggestions to do this? Keeping the price as low as possible is a high priority.

PP Class AB1 KT88s can easily produce that kind of power. AB2 operation is safe. So, power O/P is not a problem. The ElectroHarmonix KT88 is not particularly expensive, while proving to be well made.

BTW, other than the top cap plate connection, which allows a "taller" B+ rail to be used, MOV made TT21s and KT88s are (sic) identical. TT in this case stands for transmitting tetrode. I don't think 20 MHz. is at all difficult for KT88s.
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