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eHam Forums => HomeBrew => Topic started by: RADMANCF on February 07, 2013, 02:03:29 PM



Title: simple RF amp using 12AX7 tubes
Post by: RADMANCF on February 07, 2013, 02:03:29 PM
Hi, I've just recently started out in amateur radio, and am interested in homebrewing a tube power amp to go with my HT. I have a couple of brand new 12AX7 tubes in my junk box, and was wondering wheather these would be a good starting point.
Thanks,
KD0UFC


Title: RE: simple RF amp using 12AX7 tubes
Post by: N2EY on February 07, 2013, 02:04:27 PM
No

73 de Jim, N2EY


Title: RE: simple RF amp using 12AX7 tubes
Post by: RADMANCF on February 07, 2013, 02:08:44 PM
No

73 de Jim, N2EY
Ok, I realize that the 12AX7 is typically used as an audio preamp tube; is it not suitable for use with RF currents?


Title: RE: simple RF amp using 12AX7 tubes
Post by: KB1GMX on February 07, 2013, 03:47:31 PM
As a power amp your ht puts out more power than they can.  It works fine as a amplifier
but not a high power one.

The idea of a power amp it to use devices that can take the available power and deliver more
not all tubes can do that and 12AX7 is just a lightweight sometimes used in transmitters for
the microphone amplifier or other low level work.


Allison


Title: RE: simple RF amp using 12AX7 tubes
Post by: WB6BYU on February 07, 2013, 04:21:01 PM
The 12AX7 will work up to 2m (though not designed specifically for such high
frequencies.)  However, the plate dissipation of each section is 1.2W.  Using both
sides in parallel you might get 3 watts maximum out of the tube
with an optimum circuit.  And it might be difficult to get an optimum circuit
at VHF.

So it only would be practical if your HT was limited to 250mW or less.  However
if you want the experience of building a tube final, it could be done.


Title: RE: simple RF amp using 12AX7 tubes
Post by: RADMANCF on February 07, 2013, 08:22:57 PM
[Tquote author=WB6BYU link=topic=87995.msg652245#msg652245 date=1360282861]
The 12AX7 will work up to 2m (though not designed specifically for such high
frequencies.)  However, the plate dissipation of each section is 1.2W.  Using both
sides in parallel you might get 3 watts maximum out of the tube
with an optimum circuit.  And it might be difficult to get an optimum circuit
at VHF.

So it only would be practical if your HT was limited to 250mW or less.  However
if you want the experience of building a tube final, it could be done.
[/quote]
Ok, that makes sense. Among the reasons I found the 12AX7 to be attractive is the fact that it is widely used in guitar amps, and thus, should be available well into the future. Could somebody suggest a power tube that is common in guitar amps, but suitible for use in RF amps?


Title: RE: simple RF amp using 12AX7 tubes
Post by: K5LXP on February 08, 2013, 06:37:19 AM
Quote from:  link=topic=87995.msg652273#msg652273 date=1360297377
Could somebody suggest a power tube that is common in guitar amps, but suitible for use in RF amps?

I believe the 807 was an audio amp many hams made into power oscillators and low power amps.

Here's an audio amp made from an RF tube:

http://www.wavac-audio.jp/sh833_e.shtml (http://www.wavac-audio.jp/sh833_e.shtml)


Mark K5LXP
Albuquerque, MN


Title: RE: simple RF amp using 12AX7 tubes
Post by: KE3WD on February 08, 2013, 06:57:13 AM
Your HT is very likely in the VHF or UHF band or both. 

While there are audio power tubes that can be used in the HF spectrum and RF tubes that can be used in the Audio spectrum, you are not likely to find an easy way to use an Audio tube as an RF Power Amp for the bands your HT operates on. 

Then there is the issue of tuning the tube for each frequency as well.  While it is possible to design a tube driven RF amplifier as a broadband notune amplifier, such designs are problematic and not really the better approach. 

For use with the VHF/UHF HT, a solid state broadband amplifier approach would make much more sense.  But building one may end up costing more in time and components than the cost of one of the manufactured solid state amplifiers marketed for the task. 

If you want to experiment with RF and tubes, time well spent in upgrading the ham license if necessary in order to get HF privileges might be the more appropriate path to take.  At the lower HF frquencies, a lot more can be done with the type of tubes you are talking about. 

The 6L6, a rather common guitar amp output tube, for example, can be used as the final in an HF CW amplifier and yield about 7 to 10 watts of RF on the HF SW bands. 

The world of RF design is a lot different from the world of AF amplifier design.  That said, a good understanding of the AF amp can be a good head start to finding out what it takes to amplify RF with tubes.  "two pi LC" and forward.  (that's jargon, for all you self-inflicted Internet Police...)


73


Title: RE: simple RF amp using 12AX7 tubes
Post by: WB6BYU on February 08, 2013, 08:51:14 AM
Quote from: RADMANCF

Could somebody suggest a power tube that is common in guitar amps, but suitible for use in RF amps?



Actually, tubes suitable for guitar amps are often in high demand and overpriced
compared to RF tubes designed for RF service.  The tube in a well-designed
RF amp should last 20+ years if you take reasonable care of it.  Since many
tubes aren't being made any more, I think you'll get better performance at a
lower price if you choose a good RF tube to start with.

The 6V6 / 6L6 series of tubes were commonly used in HF transmitters, along with
the 6AQ5, etc.  But the performance of many such tubes starts dropping off
around 20 - 30 MHz as the stray inductance of the lead lengths becomes more
critical.  A couple inches of wire from the pin to the cathode is no problem at
audio or lower HF, but it is a significant portion of a 1/4 wave on VHF.

Tubes commonly used in 150 MHz commercial equipment included the 2E26 or
6146 (and relatives such as the 6883, 8032, etc.)  Even these couldn't be run
at full ratings compared to what they could do at HF.  One of the better choices
might be a 4CX250 - it is designed for VHF/UHF operation, there are lots of them
around, and you can let it loaf because it is capable of far more output than you
need.  (You can get 200+ watts out from one tube.)  You can find a number of
power amplifier circuits designed for this tube.


Title: RE: simple RF amp using 12AX7 tubes
Post by: RADMANCF on February 08, 2013, 09:31:49 AM
Well, thanks for the suggestions. I had no idea that the tubes commonly used by guitarists were considered to be over priced. 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.
KD0UFC


Title: RE: simple RF amp using 12AX7 tubes
Post by: W9GB on February 08, 2013, 10:05:55 AM
Quote from: KD0UFC
I had no idea that the tubes commonly used by guitarists were considered to be over priced. 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.
Yes, many radio amateurs are aware of the Guitar Center retail chain.

I remember, not that long ago (early 1980s), when the 12AX7 sold for $2 or less as NOS / surplus.
The glass vacuum tube production in USA began to close down in early 1980s.  
Jack Welch, new CEO of GE (known as Neutron Jack in that era) shutdown the GE/RCA tube production as well as much of the GE manufacturing (or shipped offshore) by end of 1980s in favor of GE Capital (finance) and Broadcast (NBC) divisions.


RCA's HAM TIPS (from the Golden Age)
http://n4trb.com/AmateurRadio/RCA_Ham_Tips/rca_ham_tips.htm

Most of the glass vacuum tubes, that are now imported to USA, are mfg. in Russia, China, old Balkan countries, Czech Republic, or Slovakia.  The markup by these importers is substantial.  
New Sensor Corp. in Queens (NYC) has locked up ($$) much of the USA distribution for minature audio / guitar tubes from these mfg.
http://www.newsensor.com/

A new Chinese mfg. 807 tube (late 1930s RCA design) now costs > $30.
http://w5jgv.com/downloads/RCA%20807.pdf

The reason that the AUDIO / GUITAR Vacuum tube market exists is due to professional musicians ($$$), who invested their profits to keep American component mfg. (parts) in business to mfg. the parts for classic guitar amplifiers, such as Marshall, Fender, etc.
The Eastern Asian market (10x population of USA) has maintained the audiophile vacuum tube market.
===
FOR VACUUM TUBE RADIOS -- Look at the Classic American Mfg. from 1940 to 1970 period, as well as GE and MOTOROLA for VHF/UHF Land Mobile radios.

RCA Notes for 807 in 1939
http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips0202.pdf

===
Only GLASS TUBES still mfg. that can be used for higher power HF amplifiers are:  811A, 572B, and 3-500Z.


Title: RE: simple RF amp using 12AX7 tubes
Post by: KA5N on February 08, 2013, 11:13:00 AM

     "YOU can't make a silk purse out of a sows ear,  especially if sows ears are not
       sold anymore,"

       A restatement of an old addage.  Just substitute RF tubes usable for low power
       VHF/ UHF  amplifiers  for sows ears.    Tubes that might be considered are the
       2E26 and maybe 5763.    Audio tubes are generally poor choices, what is good
       for a guitar amp just won't do the job for VHF/UHF duty. 
       Then there are all the other parts and circuitry.  Also if you use tubes you will
        need power supplies for the filament power and high voltages.   Power supplies
        will be a problem for portable operation.  Battery power would require some sort   
        sort of convertor. 
        A small amp using transistors is much easier to accomplish and less costly.

       Allen KA5N




Title: RE: simple RF amp using 12AX7 tubes
Post by: KD0REQ on February 08, 2013, 11:45:29 AM
807 was the 6L6 with the plate moved out of the stem and up to a cap on top of the bulb.  that got the frequency band of the tube past 20 meters.


Title: RE: simple RF amp using 12AX7 tubes
Post by: RADMANCF on February 08, 2013, 11:58:09 AM
[quote author=W9GB link=topic=87995.msg652367#msg652367 date=1360346755
Snip
The reason that the AUDIO / GUITAR Vacuum tube market exists is due to professional musicians ($$$), who invested their profits to keep American component mfg. (parts) in business to mfg. the parts for classic guitar amplifiers, such as Marshall, Fender, etc.
The Eastern Asian market (10x population of USA) has maintained the audiophile vacuum tube market.
[/quote]
It's not just classic guitar amps that generate the demand for tubes in the music world. Marshall, Fender, Vox, Peavey, Mesa et. al. keep developing tube amps, and reissuing the old models from time to time. I remember when Vox reissued the AC-30 a few years ago, IIRC it uses a mass of 12AX7s. There are also a number of stompboxes that use tubes these days also. Anyway, I'm actually surprised at the markups on tubes. About how much do they cost to produce? And I suppose service life in a guitar amp isn't a good indicator of service life in RF applications, since the normal operating conditions in a guitar amp would be considered abuse by most other standards (making the tubes clip 'cuz it sounds good), so typically, how long do tubes last in RF power amps, and do they start to sound different as they age?


Title: RE: simple RF amp using 12AX7 tubes
Post by: KE3WD on February 08, 2013, 04:31:24 PM
In modern manufacturing, it isn't how much it costs to produce, it is how many units one can produce and sell. 

When a market drops off, the single price of a unit must rise.  Supply and demand. 

When the majority of customers had tube driven gear, whether radios, tvs, audio, or what have you, the sheer proliferation of devices meant huge sales in vacuum tubes.  So much so that even drugstores used to carry them.  12AX7's once sold for less than a dollar each. 

Today the vacuum tube is a niche market, regardless of what that market may be. 

73


Title: RE: simple RF amp using 12AX7 tubes
Post by: AC5UP 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...........


Title: RE: simple RF amp using 12AX7 tubes
Post by: G8HQP 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.


Title: RE: simple RF amp using 12AX7 tubes
Post by: G3RZP 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


Title: RE: simple RF amp using 12AX7 tubes
Post by: G8HQP 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.


Title: RE: simple RF amp using 12AX7 tubes
Post by: G3RZP 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.


Title: RE: simple RF amp using 12AX7 tubes
Post by: WB6BYU 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.


Title: RE: simple RF amp using 12AX7 tubes
Post by: RADMANCF 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


Title: RE: simple RF amp using 12AX7 tubes
Post by: VE3LYX 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.


Title: RE: simple RF amp using 12AX7 tubes
Post by: G3RZP 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'


Title: RE: simple RF amp using 12AX7 tubes
Post by: RADMANCF 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.


Title: RE: simple RF amp using 12AX7 tubes
Post by: N0NZG 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.


Title: RE: simple RF amp using 12AX7 tubes
Post by: WB6BYU 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.