Discussion in 'Amateur Radio Amplifiers' started by N8FVJ, May 19, 2019.
It could be a bit of both...
One problem with that statement. Why do the band switches do not arc over in my recommended HF amplifiers? My recommended HF amplifiers are never mis-tuned or slowly tuned? Plus, mistuned HF amps apply less voltage on the band switches anyways (less RF output). The tubes take the abuse, not the band switch.
Bandswitch arching is actually pretty common and has been for decades. Anti-corona washers installed in the right places helps, and Kenwood (and others!) should have done that, if they didn't.
One problem is creep distances which can get really short when you have any sharp corners on anything, like bandswitch contact.
The only amps I've ever owned (commercial ones) that never had such a problem ever are Henry amps which didn't use a rotary switch in the plate tank, only in the cathode circuit. They used a camshaft and plungers which are coin silver and big with huge spacing everywhere. That was all a bit Rube Goldberg, with a chain drive and sprockets to interconnect the bandswitch shaft to parts that had to move, but usually they worked fine and lasted many years.
My darned 3K Ultra could be "bandswitched" while driving it. That wasn't good for the tube but the plate tank contacts would BANG and SPARK and nothing else would happen. No damage other than some tarnish on the big silver contacts. Of course, that's always an "oops" and usually happens at multi-op contests when the operator is excited or tired and doesn't know much about amps.
Who says they don't?!
In my opinion, I have certainly noticed an inherent bias from several people on this Forum against ANY equipment not made in the USA !
I have worked on a lions share of Kenwood TL-922's. Seldom do I see the band switch portion arced and damaged but I do see the auxiliary contacts burned up that bring in the additional capacitance on 80 and 160. Sometimes one or the other or both, The band switch in the 922 is beefier than that in the SB-220. I approximate that 7 out of 10 band switches in SB-220's today have at least one or more burned contacts.. That is not a good ratio. I don't buy into the TL-922 being unstable as designed. One's that may be unstable I am sure can be traced to a fault other than design. The 922 outshines the SB-220 for operation on the higher bands, especially better on 15 and 10 and quite a bit better on the lower bands. The main problem with the 922 is the switching circuitry. Two clunky relays for RF and bias switching is not a very good idea. When they decided to use two relays they would have been better off making the antenna relay a 3 pole and have the 3rd pole actuate the bias relay. This way the antenna (load) would be positively connected BEFORE the tube was biased on. If you do see some instability in this amp, it could occur due to the improper timing of the relays. Many amps could oscillate if the tubes a biased on with no load connected. Using a 922 with older radios that also used relay switching within them gave the relays in the amp a bit more time to settle in before RF was present at the contacts. Newer radios will almost certainly hot switch the amp. This problem is worst on CW. SB-220 amps that are known for band switch arcing that have been converted over to the W7RY QSK eliminates the hot switching issues. Once again, the timing of the load being present to the tank before the bias is turned on is paramount. In fact ANY vintage amp with an older open frame relay will benefit from "timing the relay". This a simple and very effective method to help prevent hot switching.
I own several TL-922 amps and I have become more favorable to them over the years than SB-220 amps. Once the grids are properly grounded on these amps they produce a bit more power and perform much better than the 220 with their grids grounded. It is not unusual to see legal limit from a 922 after it has been worked on with sensible grid and plate currents well within the confines of the tubes.
The facts do not support your post that the recommended HF amps have a band switch arcing issue. Why do you continue to report such nonsense.
The auxiliary switch is part of the band switch, is it not? It fails. The recommended HF amps do not fail. When using a modern transceiver, use the delay xmit setting to stop slow antenna relay hot switching issues. For CW, the recommended Amp Supply HF amps did have an optional antenna vacuum relay. Look for that option.
I personally like the relays in the TL-922, as they are very reliable. (unlike many used by Ameritron)
The speed of their switching isn't an issue, as I have ALWAYS used a footswitch on CW, and PTT on SSB.
For those that want to use break-in, there are very simple mods (including complete kits) to fit faster relays (as with any older amps)
As W1QJ posted, I've never experienced ANY instability on any band with my TL-922 . . . and none of the owners I spoke to over here who bought them new and still use them have ever had any bandswitch arcing.
I imagine that people who damage bandswitches are accidentally changing them while transmitting . . . or else using very badly mismatched antennas.
When I bought my TL-922 it was totally unmodified . . . the only mod I have done is to directly ground the grids (which is very simple on this amp). The only other thing I do is feed the AC mains via a 20 ohm resistor with a switch across it (in the mains lead just after the plug), as a very simple manual soft-start.
Ameritron do not have any reported antenna relay issues. Where do you get that? The relay is a standard 600 volt rated relay rated at 20 amps current and the output voltage & current is less than 600 volts and 20 amps. I think you just want to 'win' a discussion and will state anything to do so.
Again folks, my recommended HF amps will provide years of trouble free service. The HF amps I recommended are proven over many years.
Although my first choice in amplifiers would not be any amplifier using 3-500 tubes I wouldn't say that the 922 is one to "stay away from". In my shack I only use amps that have 8877 tubes or 3cx800 tubes. Right now in my shack I have the following hooked up. One Ameritron Al-800H, One Titan 425, and One Al-1500.