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1-6 of 6 messages
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Newbie Amplifier question...
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by KE7FXO on October 19, 2009
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I've tried the search feature, but didn't find what I was looking for, so for the sake of brevity, I just wanted to ask two quick questions here. I'm new to tube amplifiers, but not new to radio in general. I finally got my station set up, but haven't purchaed an amp, because I have been concentrating on the antenna installation, and making sure everything is copacetic.
Quick station rundown:
Kenwood TS-480SAT
ZeroFive multiband 43 ft. vertical, mounted above rooftop, with tuned radials for several bands (80/40/20/10)
58 feet of LMR-400 coax
Now, after MUCH tweaking and pruning, I have managed to obtain a very low SWR on all of the above bands. I do not have an antenna tuner, but I have the built-in tuner in the radio, for times I might be haning out on the edge of the bandwidth, and want to be able to still put full power to the antenna (minus matching and coax loss or course)
Believe it or not, this setup has gotten me some fairly impressive results. People are having a hard time believing that I do not have an amp of some kind, even on 75/80. I do intend to add a relatively small amplifier, something like an Ameritron 811H perhaps.
So my questions are this:
What is the highest SWR a person can safely run a tube type amp into, without stressing it on moderate length key ups? I've had solid state amps, and pretty much know the scoop on how the SWR's should absolutely be held to a MINIMUM, and I also realize that a tube amp should be feeding into a good SWR too. But is it as crucial with a tube amp, as it is with a solid state amp?
The reason I am asking, is because I do not intend to get an antenna tuner (for awhile anyway), and as I said, my SWR's are very good on all the bands I intend to operate. I have plenty of bandwidth, as provided by the matching setup at the antenna. I was just wondering about the band edges, and what is an acceptable SWR, that will not harm the amp.
I will only be running 500 watts tops, out of an amp that will do more than 700.
The second question is, what s the minimum wire gauge (radials on my vertical) that will tolerate 500 watts P.E.P. ?
The reason that I am asking, is because when I put up this antenna, I hadn't planned on any amplification (at the time), and I happened to have a huge spool of 14 gauge insulated wire, so that is what I used.
I will upgrade to 12 gauge sometime later on, but was wondering... Until then, will 14 gauge wire radials handle moderate power like 500 watts?
Any help is much appreciated... 73's!
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RE: Newbie Amplifier question...
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by K2MK on October 20, 2009
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Current in radials is divided between all of your radials. There is no problem with 14 gauge radials even at full legal limit.
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RE: Newbie Amplifier question...
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by VR2AX on October 20, 2009
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The 811H is specified as ok for up to a 3:1 VSWR output into 50 ohm at the output. Moins an ATU, that means your amp tank circuit should cope with roughly what your 480SAT internal tuner can match barefoot into the line, no more no less.
If it was my call, as this is an amp forum and we care more about the health of amps than radios,I would use a matching unit between the amp and line ;-)
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RE: Newbie Amplifier question...
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by WX7G on October 20, 2009
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500 watts and two 1/4 wavelength radials? At less than 2 amps per radial #18 wire will work. At 30 MHz it will run 0.7 watts/foot of wire, heating the wire roughly 10 deg C.
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RE: Newbie Amplifier question...
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by K4DPK on October 20, 2009
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All tube-type amplifiers, with the exception of rare no-tune or pre-set types, have adjustable tuned output circuits. For the most part, these are either pi-network or pi-L networks.
These tuned circuits are able to match the plate load impedance of the tube(s) with the antenna system impedance, and do so over a fairly broad range of impedances.
This is quite unlike, and much more forgiving than, solid state amps with fixed-ratio output transformers.
In general, if you are able to tune and load the antenna at a satisfactory drive vs output power level, the tubes won't really care because they are, at that point, matched to the load.
Phil C. Sr.
k4dpk
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RE: Newbie Amplifier question...
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by W8JI on October 24, 2009
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Any amplifier does not give a rat's behind what the load SWR is provided:
1.) The amplifier has an adjustable matching network.
2.) That matching network is operated within it's design limits.
3.) You adjust that matching network to get the output devices at the load line. This means the right efficiency and operating currents.
There is a commonly held MYTH that reflected power goes back into an amp and heats components up, or heats the tubes or transistors up. All a high SWR does is increase your cable losses if the above three points were all satisfied.
An 811 amp will handle various amounts of SWR depending on the band, power output, and the exact impedance of the mismatched load so there is no easy way to say how much SWR it will really take, but 3:1 is a reasonable assumption on most bands. It will take LESS SWR mismatch on 160, and MORE on 40 and higher. Unfortunately your antenna has the worse imaginable SWR on 160 (the antenna will fail at the base if you really ever manage to get 300-500 watts into it on 160) and SWR excursions will get progressively better because of coaxial cable and other losses on higher bands.
Tom
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