RadioBanter - RF amp help

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I am trying to build a small class C RF amp for 160 AM. The amp is to be
plated modulated and I have about 12 watts of audio available. I have built
amps before(30 years ago) but I have never built one around a modulator,
previously I built the modulator to suit the amp. Any help with tube and
voltage selections would be greatly appreciated.

Jimmie

Jimmie D wrote:
I am trying to build a small class C RF amp for 160 AM. The amp is to be
plated modulated and I have about 12 watts of audio available. I have built
amps before(30 years ago) but I have never built one around a modulator,
previously I built the modulator to suit the amp. Any help with tube and
voltage selections would be greatly appreciated.

With only 12 watts of audio available, I would be
tempted to use grid modulation on a pair of 6146's.
--
73, Cecil http://www.w5dxp.com

"Jimmie D" wrote in message
...
I am trying to build a small class C RF amp for 160 AM. The amp is to be
plated modulated and I have about 12 watts of audio available. I have built
amps before(30 years ago) but I have never built one around a modulator,
previously I built the modulator to suit the amp. Any help with tube and
voltage selections would be greatly appreciated.

Jimmie

I presume you mean 160 KHz, not MHz.

Almost any of the N channel power FETs intended for switching power supply
use will work at 160 KHz.. Forget about the 12W amp; you would need a low
impedance modulation transformer for that. I would start with a non
regulated power supply at about 30 volts, and run that to a buck regulator.
Set the quiescent voltage to 12 V. Now, AC couple the audio ( a few
milliwatts) into the regulator reference input, and adjust the audio
amplitude so that the regulator output swings between 0 and 24V. That is the
supply for the power FET.

To operate in class C, you will want to drive the gate with a rectangular
waveform. I would start with about 25% duty cycle. There will be a tuned
circuit at the output of the FET; that has to have a fairly low loaded Q,
or you will cut off the sidebands.

Actually, It should be possible to do this without even having a separate RF
amplifier. Run the buck regulator at 160 KHz with almost no filtering at the
output. The regulator output will be AM modulated 160 KHz. Filter it in an
LPF.

If you meant 160 MHz, the first paragraph still applies. Use the regulator
output to power an RF transistor operating at 160 MHz.

Tam

Tam/WB2TT wrote:
I presume you mean 160 KHz, not MHz.

Probably neither - probably 160m.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
et...
Tam/WB2TT wrote:
I presume you mean 160 KHz, not MHz.

Probably neither - probably 160m.
--
73, Cecil http://www.w5dxp.com

Makes sense. Forgot this was a ham site. He would need an RF transistor.

Tam

On Apr 9, 11:07 am, "Ralph Mowery" wrote:
"Jimmie D" wrote in message

...

I am trying to build a small class C RF amp for 160 AM. The amp is to be
plated modulated and I have about 12 watts of audio available. I have built
amps before(30 years ago) but I have never built one around a modulator,
previously I built the modulator to suit the amp. Any help with tube and
voltage selections would be greatly appreciated.

Jimmie

You have about enough audio power to plate modulate a 25 watt input amp.
Maybe something using a 2E26 or a 6DQ6 tube.

As Cecil mentioned you may want to modulate the screen of a pair of 6146
tubes or similar to get enough power out to make it worthwhile.

Thanks, I have a couple each of 2E26s and 6DQ6s I think I can come
pretty close to the impedance values I need by driving one of my LS-55
transformers witth a solidstate amp using one of the low impedance
connections as the primary of a modulation transformer.

Jimmie

"JIMMIE" wrote in
oups.com:

On Apr 9, 11:07 am, "Ralph Mowery" wrote:
"Jimmie D" wrote in message

...

I am trying to build a small class C RF amp for 160 AM. The amp is
to be plated modulated and I have about 12 watts of audio available.
I have built amps before(30 years ago) but I have never built one
around a modulator, previously I built the modulator to suit the
amp. Any help with tube and voltage selections would be greatly
appreciated.

Jimmie

You have about enough audio power to plate modulate a 25 watt input
amp. Maybe something using a 2E26 or a 6DQ6 tube.

As Cecil mentioned you may want to modulate the screen of a pair of
6146 tubes or similar to get enough power out to make it worthwhile.

Thanks, I have a couple each of 2E26s and 6DQ6s I think I can come
pretty close to the impedance values I need by driving one of my LS-55
transformers witth a solidstate amp using one of the low impedance
connections as the primary of a modulation transformer.

Some thoughts:

If you have 12W of audio and you want to use plate and screen modulation,
you have enough to cover 24W of DC input at 100% modulation, but it
wouldn't hurt to have a slightly higher powered final with 60% to 80%
modulation. That would suggest up to about 36W input, which at 66%
efficiency would give you a 24W carrier, 66% modulated. In that case, you
would need a PA tube with more than 12W of anode dissipation. The 2E26
has an ICAS plate dissipation of 12W.

One approach is to choose a tube that is likely to remain available due
to its widespread use and the consequent replacement demand. I don't
think the ones you nominate would be high on that list. Although a 6146B
might seem overkill, it is cheap, can run comfortably from a 500V supply
at good efficiency, and will idle along at about 60% of CCS ratings.

Important factors will be the supply voltage and anode current
compatibility with your modulation transformer... otherwise your 12W of
audio might not go very far at all. The 2e26 has less room to move in
accomodating your mod tranny than the 6146B.

Owen