Arcing is more common in older amplifiers due to chemical degradation of insulators, insulator cracks and component warping from thermal and electronic stress, and the oxidation and burning of the contacts in physical switches. Although arcing is rare in solid state amplifiers since they operate at relatively low voltages that is no panacea since they suffer from other ills. But this article is about tube amplifiers.
Not long after putting my recently acquired vintage Drake L7 to work it suffered from intermittent arcing at the high end of its power range. Because the high DC plate voltage is independent of power level the arcing was almost certainly at RF. Although the arcing has been eliminated I expect it to return because the faulty part requires replacement. It's on my very long to-do list and it'll take a while until I get around to a implementing a long term solution.
If you've never worked on a high power tube amplifier my relating of how I investigated and resolved the arcing may be of interest. Despite the large size and lethal operating voltages and currents tube amplifiers are actually quite uncomplicated. With attention to safety and the peculiarities of dealing with high voltage, current and power they are not too difficult to work on.
Locating the arcs
The case of the L7 has no opening except on the bottom and rear for forced air cooling. These are useless as windows into the amplifier's interior. The case must be removed. This is the first problem in any properly designed amplifier. There are interlocks to prevent accidental electrocution and burns to budding technicians who are out of their depth.
The Drake L7 has two interlocks: one shorts the 3000 VDC plate supply and other (well hidden) prevents the amplifier from being turned on. Both interlocks must be disabled to operate the amplifier without the case. Before you try this, or even consider doing so, you must educate yourself about what you're attempting to do. Better still, have a knowledgable friend help you out. If you get yourself killed don't say I didn't warn you.
Yes, that's really a brick! I needed something flat, heavy and non-conducting to safely disable the high voltage interlock. The mains interlock (not visible) on the bottom is disabled with a chunk of plastic wedged under it. In the picture the amp is on and idling with a plastic (non-conducting) LED desk lamp for added illumination. Arcs are so bright that no ordinary lighting will wash them out.
I mentioned my suspicion about the loading capacitor in an earlier article. I dutifully straightened the multitude of plates until they maintained a decent gap for their full rotation. Unfortunately that repair resulted in no improvement. Hence the deep dive into the amp's innards.
As I increased drive the amp arced as expected but not where I expected. The loading capacitor sat quietly when the fireworks began.
It was the plate capacitor that was arcing. Although the spacing was generally good (and much wider than the loading capacitor) a number of rotor plates didn't track well. I straightened them without needing to remove the capacitor from the amp. There is an unrelated problem that will require its eventual removal for repair but that can wait.
Straightening the plates did not fix the problem. Worse, the arc location was seemingly random. Each one occurred in a different location. The arcs were evidently due to a fault elsewhere that caused an excess voltage condition beyond the rating of the capacitor. The capacitor is working just fine.
This is interesting so let's take a detour to review the design of a tube amplifier's output network.
Amplifier pi-network
The simplified schematic below is that of a typical pi-network found in many tube amplifiers. It transforms the high impedance of the tube output to the low impedance of the antenna system. The operator adjusts the plate capacitor to resonate the plate circuit and the loading capacitor for a high efficiency match to the impedance presented by the antenna system.
The resonance condition is a typical feature of an impedance transformation network, as previously covered in this blog. The circuit also attenuates harmonics since it is a low pass filter.
The blocking capacitor keeps DC out of the antenna circuit and the choke keeps the RF out of the power supply. Band switching (not shown) alters the range of the variable capacitors and the inductor value. The T/R switch (input side not show) bypasses the amplifier during receive.
RF voltage is determined by the power and impedance in accord with Ohm's Law: E = SQRT(PZ). From the data sheet for a pair of 3-500Z tubes a little arithmetic suggests there is approximately 3000 volts across the plate capacitor at 1000 watts RF output. The recommended capacitor rating is 4.5 kV because the impedance, and therefore voltage, can be higher depending on operating parameters.
On the high bands the L7 places a fixed capacitor in series with the variable capacitor to reduce the capacitance. This also lowers the voltage across each capacitor since capacitors in series act as a voltage divider. Because of this arcing incidence is greater on the low bands. However it is not eliminated. Something more dire is going on to cause arcing when the RF voltage is low.
The voltage across the loading capacitor is easier to calculate since the antenna system impedance is nominally 50 Ω. At 1000 watts the potential is a less than 300 volts. The voltage will often be higher when the SWR is greater than 1, which is very common for most hams. If the SWR is too high the loading capacitor with its smaller plate spacing can arc.
Sequencing
Since the capacitors appear to be in good shape and the output impedance is well within the acceptable range the problem must originate elsewhere. Most likely is a fault that affects the impedance at the output port. The antennas, external switches and transmission lines were ruled out by additional testing. High power can aggravate weak components and loose connections in an antenna system to create intermittent and permanent impedance changes.
By this process of elimination I focussed my attention on the T/R relay. It is common for slow or faulty amplifier relays to cause plate capacitor arcing when transmitter power appears at the amplifier input before the output relay contacts have settled. Unsettled contacts cause a momentary high impedance (open condition) at the output port. Once an arc starts it can continue after the relay stabilizes since the path to ground is always lower impedance than the antenna system.
The solution is sequencing to ensure the amplifier relays settle before power is applied to the input port. This can be coordinated with the transmitter, signal source (e.g. PTT in advance of transmit) and even with the amplifier itself by having the output port relay close faster than the input port relay.
The open frame relay in the L7 is typical of many vintage amplifiers. It iss slow at best, and with age the contacts are suspect. Replacements can be found but better solutions are available.
While arcing at turn on was occurring its incidence was less than that of arcing during a transmission. That is sufficient evidence to rule out sequencing as the cause of my problem. It does not mean sequencing isn't a concern, just that it isn't responsible for the observed behaviour.
Relay woes
I had good reason to suspect the relay. I earlier had to clean the contacts on the input and output of the bypass side due to intermittent signal attenuation on receive. With an ohmmeter connected to the centre pins of the input and output SO239 jacks you clean the contacts until you reliably read 0 Ω. Since the relay arms can shift laterally it is important to test for this by manipulating the relay arms.
The bad contact can be isolated by connecting one ohmmeter probe to the bypass bridge seen on the right. The output port is at the bottom of the picture and the input port is at the top. The centre arm applies tube cutoff bias during receive. Just my luck that both sides of the relay were corroded.
I had to resort to aggressive cleaning with an abrasive when a deoxidizing contact cleaner and non-abrasive buffing were insufficient. Do this only when absolutely necessary since abrasives can easily damage the thin contact coating, assuming there is any left (usually silver) after several decades of use. This is discussed in more detail by W8JI. I used a thin strip of 3000x sandpaper to be as gentle as possible. It worked.
Checking the through amplifier relay contacts is more difficult since an ohmmeter cannot be easily employed to measure resistance. Deoxidizing cleaner and buffing didn't resolve the arcing problem, but it did seem to reduce its frequency. Having gone that far I resorted once more to the sandpaper. After thoroughly clearing the contacts of debris I did another test. This time the arcing vanished entirely.
Permanent solution
I don't know how long the repair will last. The relay needs to be replaced. In any case it is slow and loud. Not only is that very annoying it is cause for ongoing worry. I don't want to take the risk of it failing during a contest.
Designs and even kits to replace T/R relays in vintage amplifiers are available. Some are fast enough to enable QSK operation. I don't need QSK just a solution fast, reliable and quiet.
Another problem with the existing T/R switching is the long lead lengths along the bypass path. On 6 meters it is enough (almost 0.03λ) to significantly raise the SWR. With 6 meter season in full swing I manually bypass the amplifier when I am not using it. Any T/R relay replacement will need to address this issue.
Vintage amplifiers are cost effective assuming you have the time and motivation to repair aging equipment and add modern features. My next amplifier will likely be a new purchase, one with a stiff power supply, silent operation and 6 meters. That way I can tolerate the quirks and faults of its older cousin at the secondary operating position.
Thanks for posting. That is exactly the problem I'm having. Will attempt to clean first with solvent and proceed to light sandpaper if needed. Best regards.
ReplyDelete