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Author Topic: simple RF amp using 12AX7 tubes  (Read 7420 times)
AC5UP
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Posts: 3866




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« Reply #15 on: February 10, 2013, 01:19:48 PM »

The 12AX7s I have, I got in 2007 for about $15 each from guitar center (the big chain guitar shop, for the unfamiliar), and all the power tubes were about $40.

What a coincidence.......... I remember cleaning out the garage around 2007 and I'm pretty sure I threw out a big box of........

If you really want to build a small-ish tube amplifier for your HT you'll ether spend way too much on the parts or take forever to acquire all the pieces through the miracle of scrounging.  Places like RF Parts and others can hook you up with RF power modules that are cheap, reliable and require no support parts containing 15% unobtanium... AKA:  U-15

Or, you could research a vintage schematic using the venerable 2E26.  That's the mid power version of a 6146 that's good up to 175 MHz.  Figure 25 watts Class C for ICAS duty on 2 Meters. Most repeaters have a three minute timeout so you shouldn't smoke the tube on your first Q...........
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G8HQP
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« Reply #16 on: February 10, 2013, 01:35:11 PM »

If you want to play at low and fairly safe power level you could try the 12AU7/ECC82 - sort of a cousin to the 12AX7. It is actually two 6C4/EC90 power triodes in the same envelope, with a slightly reduced maximum anode dissipation.
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G3RZP
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« Reply #17 on: February 10, 2013, 02:22:29 PM »

Any of the twin triodes ( 12AT7, 12AU7, 12AX7, 6J6) are limited to around two to three watts or so output. So unless your HT runs less than about 300mW out, it is just not worth the effort. The worthwhile step up is with a 6360 to around 12 watts input. If you have  5 watt HT, then the tube step up is to either a 5894 at around 100 watts input or even a 4CX250B and 500 or so watts input.

73

Peter G3RZP
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G8HQP
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Posts: 123




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« Reply #18 on: February 11, 2013, 03:47:18 AM »

If someone's knowledge and experience with valves and RF is such that he considers using a 12AX7 for a VHF PA do you think he will be safe using a 4CX250B and the associated PSU?

The 6360 (QQV03/10) suggestion is good, but even there RF burns and widespread harmonics are a possibility.
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G3RZP
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« Reply #19 on: February 11, 2013, 10:16:42 AM »

There's always a QV04-7 or a TT11 or even a 5763.....

But harmonics and so on can be a problem if you don't know what you're doing.
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WB6BYU
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Posts: 13243




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« Reply #20 on: February 12, 2013, 08:12:55 AM »

Quote from: G8HQP
If someone's knowledge and experience with valves and RF is such that he considers using a 12AX7 for a VHF PA do you think he will be safe using a 4CX250B and the associated PSU?


That's a good point, but he doesn't need to run the 4CX250B at full power or voltage. 
They are available used at reasonable prices (especially compared to guitar amp tubes)
and would be virtually indestructible at 20W output.  I see more of them at hamfests than
some of the other VHF tubes mentioned.
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RADMANCF
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Posts: 12




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« Reply #21 on: February 12, 2013, 09:31:18 AM »

If someone's knowledge and experience with valves and RF is such that he considers using a 12AX7 for a VHF PA do you think he will be safe using a 4CX250B and the associated PSU?

The 6360 (QQV03/10) suggestion is good, but even there RF burns and widespread harmonics are a possibility.
You're correct in that I have no hands on experience with tubes, but I have worked with flourescent lighting, replacing ballasts and fixtures. I'm no stranger to high voltage safety. I have worked around RF before, mounting signs on a 3kw FM broadcast tower. In terms of electronics experience, I've taken a basic electronics class, as well as a linear integrated circuits class. In both classes, I worked extensively with op amps, and a bit with transistors.

KD0UFC
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VE3LYX
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« Reply #22 on: February 15, 2013, 07:34:04 PM »

I wouldnt be so quick to say no. A 12ax7 can put out as much as 7 watts (3.5 per section) I often run them paired. Parallel pair in one tube. They make a pretty decent little amp. I have one in a am transmitter project and its brother is the recvr in the same chassis. No it wouln't put out 100 watts but it certainly will give some stomp. Unless the HT has more then that to start with.And then there is the 12bh7. Big brother to the 12ax7. Fits in the same hole , no changes. It will make you sit up and take notice.  My personal advice is if you have them and feel like it it will only take an hour or so to build . Why not. The more you build the more you learn.
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G3RZP
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« Reply #23 on: February 16, 2013, 02:58:15 AM »

You have 1 watt per section maximum plate dissipation. The OP wanted an amplifier to be driven by a HT. Max output at RF (the tube isn't rated for RF) will be about 4 watts. A grounded grid amplifier would probably be best, since it's a high mu triode. So a 1 watt HT will get 6dB gain, and you are unlikely once you've matched the input and tuned the output to do much better than 10dB gain at the most. Most modern HTs do at least a watt, so it becomes an 'is it worth it?' exercise. Other tubes would be much better and give more 'bang for the buck'
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RADMANCF
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« Reply #24 on: February 16, 2013, 08:01:46 AM »

Well thanks for the replies everyone. My HT is a Yaesu FT-60, with a max output of five watts. After reading whats been posted, I figure I'll use the 12AX7s for somthing other than RF amplification. I have more prior experience with AF circuitry, so I'd figure an AF tube amp would be a better way to learn.
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N0NZG
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Posts: 122




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« Reply #25 on: February 28, 2013, 08:12:06 PM »

The Good news is the high end audio market is still driving the production of  813 tubes. This is good news for those of use brave enough to build a home brew HF amp on such an old bottle.
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WB6BYU
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Posts: 13243




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« Reply #26 on: March 01, 2013, 10:02:18 AM »

But the 813 probably isn't a very good choice for 2m, either, due to the
long lead lengths.
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