Tube-type HF amplifier Frequently Asked Questions.

HF amplifier FAQ's

By Matt Erickson KK5DR

This page should help the less experienced users solve common problems and questions about HF tube type amplifiers.

Q "Why must I "tune" the amplifier?"

A For those who are not familiar with tube-type HF amplifiers, I will explain the reason. Why do you tune a guitar? So that the strings produce sound in the proper frequency. Very much the same in an amp. The process of "tuning" the amp causes the output circuit to resonate, and also matches the impedance of the tube to that of the output/antenna. The signal that drives the amp will not be amplified if the output circuit of the amp is not resonated and matched.

Q "What amplifier should I buy?"

A What are your needs? How much power output do you wish to use most of the time? These are questions that the user needs to answer before making a choice. For example, if the user wishes to only use about 500 watts output, there are a good number of units that will fit this requirement, the exact model will mainly be up to the users personal choice. There are a large group of amps in the 1kW output class. Each model has a given duty cycle and range of bands. The larger the list of user requirements, the smaller the list of units that will fit into those requirements. Narrow your list to only the units that fit best. For more insight into what makes a good amplifier read my article.

Q "I'm afraid of those expensive tubes, which ones should I get?"

A At this point in time, there are still several types of glass enveloped tubes available, but this is soon to change as tube manufacturers phase-out glass tubes. There are a few types of metal/ceramic tubes that are soon to be phased-out too. All is not lost, if you want a type that is soon to go the way of the dinosaurs, it would be a good idea to buy several sets of "spares", this way the amp/tubes will last the life of the user, or enhance a resale later. Research the type of tube in a given model amp that you are interested in, find out what gear aside of ham gear it is used in, if there is a large commercial market for the tube, chances are that it will be around for some time. For more insight on vacuum tubes read this article.

Q "Should I run my amp on 220vac instead of 120vac?"

A For the 1kW output amp, 120vac is not a very good option, because most homes are not wired to handle the heavy current needs at 120v. If the amp pulls 15Amps on 220v, it will then pull 30Amps on 120v. Since most houses are wired with 12ga. wire, which can handle 20Amps, the voltage drop at 30Amps would seriously degrade the performance of the unit. On 220v, the amp will run cooler, with better high voltage regulation, and less "flickering lights" in the home. A dedicated circuit with 12ga. wire will handle 5kW continuous current draw. Be sure to use a circuit breaker that is near the upper limit rating of the amp being used, i.e. 20Amps. Good high voltage regulation will mean that the amplifier's RF output will be more linear with less waveform distortions.

Q "How can I know if my amp is prone to "parasitic oscillations"?"

A A quick and easy test to detect parasitic tendencies is as follows: Place the amp on the highest band it can operate on, be sure the unit is ON, and in STANDBY mode. Place the TUNE control at or near the setting it would normally be for that band. Slowly rotate the LOAD control from one end of it's rotation to the other, observe the GRID current meter during the control rotation. If there is any movement in the meter, it is likely that the amp might be prone to parasitics. Steps should be taken to suppress these parasitics. My technical article on HF tube type amps should help you understand much more.

Q "Can I key my "older" HF amp, with my "new" radio?"

A Don't bet on it! Many of the late model radios now use small low voltage, low current relays or transistors to key outboard amplifiers. As a general rule it would be prudent to use an outboard heavy duty relay that is keyed by the radio, and then keys the amp. This isolates the radio from the amp keying voltage/current. These relays are cheap and easy to build, and can be bought ready made too. I have designed a relay that will work on most radios and amps.

Q "How can I control the RF drive, when my radio does not have an RF power control?"

A Use the ALC on the amplifier! Regardless of whether the radio can control it's RF drive level or not. The ALC circuit in the amp will send a voltage back to the radio that will reduce the RF output level automatically. This will prevent chronic over-drive, and help prevent the "oops" factor. Here is how to set the ALC level: Tune the amp up as normal, apply enough RF drive to reach full rated output of the amp, now adjust the ALC level control (usually on the back of the amp), until a slight reduction in RF output is noted. That's it, all done. Now the RF drive will be limited to the same output on the amp regardless of the band used.

Q "The input SWR is bad on my amp, what can I do?"

A Many "older" HF amps had no tuned input network, or had tuned input networks that were badly designed. If your amp has no input network, it might be a complex project to build and install one. In this case, it would be best to use a rig's built-in ATU (Automatic Tuning Unit) if available, or use and outboard tuner of some type. If the amp has a "tuned" input network and the SWR is bad on only one or two bands, and good on the rest, the network might need to be retuned, or repaired. Sometimes components in these circuits change value and "de-tune" the network. The parts should be replaced and the network re-tuned for minimum SWR. If the input "Z" of the tube(s) are known, a resistor equal to that Z value can be placed on the output end of the network, and an SWR analyzer like the MFJ-259 can be placed at the input of the network to aid in tuning of the unit without "powering-up" the amp. A slight "touch-up" may be needed when the amp is placed into service. Some amps use a passive resistor input network, and these networks provide a constant resistive 50 ohm load to the exciter, while absorbing some of the RF drive signal and converting it to heat. The rest of the RF drive is applied to the tube. This type of network is very broad-banded, however harmonic suppression is not very good. Harmonic distortions are not attenuated. In many cases the ATU in the radio can be used to "tame" the input SWR of most amps. Here is more about tuned inputs.

Q "I'm not sure if I'm tuning my tube-type amp properly, how should I tune-up?"

A My tune-up article should explain this in a very in-depth manner. Read this.

Q "My amp acts strangely on certain bands, arcing and causing TVI, what is the problem?"

A It is very possible that the unit has what is known as a "series resonance" on or near that band. Many commercially built amps have this problem when operated near 25 MHz. How to fix it can be complex, but the first area to try is to change the plate choke for a choke that has a higher inductance. Seek the advice of an experienced amp tech to help you with it.

Q "I have heard that capacitors can go "bad", when and how can I tell if mine are "going" bad?"

A "Computer-Grade" electrolytic capacitors have an average service life of from 15-25 years dependent on the level of ambient heat they operate in. These caps fail in only two mode, "open" meaning loss of capacitance, or "short" meaning a dead short, usually resulting in an explosive destruction of the cap. A shorted cap would be pretty obvious, but if your caps are "open" how can you tell this? If the caps are open, there will be a marked drop in the plate voltage under a "no-load" condition. Also, when under load, the plate voltage will drop severely, and it is possible that an AC hum or severe distortion can be heard on the transmitted signal from this unit. If your amp is over 20 years old, it might be a good idea to buy a replacement set of capacitors for your amp. Always replace the full set of caps.

Q "Can "any" amplifier operate QSK/Full break-in mode?"

A If the unit has an open-frame mechanical relay for T/R, it is not a good idea to operate QSK CW with it. Open-frame relays are much to slow and prone to arcing to operate high speed QSK. If the unit has vacuum relays such as the Jennings RJ-1A which switch at about 6ms, should be capable of up to 40wpm QSK operation. The fastest QSK system uses PIN diodes for T/R switching, but care must be taken not to "hot" switch them, or they might fail. An Enable/Disable keying system must be used to prevent "hot" switching. If your amp makes a loud "thump" or click, it will not be a good idea to use it on QSK CW. PIN diode T/R systems are also very sensitive to SWR, so any time the antenna SWR goes over 1.5:1, an antenna tuner should be put in-line between the amp and antenna.

Q "My amp does not have 17 meters on it, can I still use it on that band?"

A If your amp has 15 meters on it, you can use it on the 17 meter band. Simply use the 15 meter band switch position and tune it up normally, on some amps the 20 meter band position must be used. However, there is not a "tuned-input" network for 17 meters so the input SWR might be fairly high (1.7:1 or more), just use your rigs ATU or an outboard tuner to match the input. On 12 meters, the 10 meter band position is used, but this can be a problem band if there is a series resonance at or near 25Mhz like that mentioned in the preceding paragraphs.

Q "I have tried to use my amp on 17 meters, by using the 15 meter position, but it doesn't work, why?"

A Check the input SWR of the amp first, it might be that the SWR is high enough to cause the exciter to reduce RF output to the point that the amp is not driven enough for output to be useful. If it is found that the input SWR is high enough to cause SWR "cut-back" in the exciter, engage the ATU (if available), or use an outboard unit. If input SWR is NOT a problem, try the unit on the 20 meter position, if it still does not work, it is likely that the tank coil is to far out of resonance to generate any useful amount of RF output. To fix this would require the tank to be "re-tapped", but this might cause the loss of 15 meters. The user would need to decide if that is worth getting operation on 17 meters. It might be better to get an amp that operates on 17 meters without any troubles.

Q "Which is better, a "Pi" or "Pi-L" output networks?"\

A Pi-L by far, is better, my article can explain in greater detail.

Q "Can I use a "CB" amp on the ham bands?"

A Most "CB" amplifiers are very poorly designed and constructed. Most are designed to operate on AM and use a class-C circuit which is NOT linear and will be distorted and full of harmonics. Most CB transistor amps use a method of biasing that makes it impossible to transmit a clean SSB signal, no matter the drive level. CB amps will operate from 10-40 meters typically, but may or may not cover that many bands due to design and construction variations. CB amps are NOT type accepted by the FCC, so it is illegal to use them on the hams bands or any other band for that matter. In other words, use one at your own risk.

Q "My amp runs very hot, even when idling for extended periods, what is the deal?"

A If your unit is an "older" amp, it's likely that most of the heat (aside from the heat of the tubes), is coming from the HV bleeder resistors, which are of a lower resistance than they should be. For example the Heathkit SB-220 uses 30K ohm 7 watts resistors as HV bleeders, which dissipate a great deal of heat needlessly. Change these resistors to 220K ohm 2 watt "flame-proof" 2% type, which will greatly reduce the heat output, extend the life of the capacitors they are in close proximity to, and stiffen the plate voltage. The only draw back is that the bleed-down time will be much longer, but that is a better thing than the heat. If this modification does not reduce the heat level enough, the air-flow may need to be increased.

Q "There is a "ticking" sound from inside my amp, other than that it works fine, what is going on in there?"

A A small "ticking" or random "crack" sound from inside your amp might be a high voltage arc, where it has "ionized" a path to ground somewhere in there. These are hard to find, so here is how it's done: First you need to take the cover off the unit (be careful with HV exposed!), next defeat the interlocks and shorting mechanisms (be very careful!!), now power-up the amp, and darken the room. Now your looking for the light from the tiny arc that is the source of the sound. If you see where it is, note it's position, and repair it, or clean the dust or carbon track it is following, it might require you to actually move a part to break the path of the HV. Restore the amp to operational condition. A sharp point in the HV circuit will readily arc, so file or trim down any of these you find. Wires that are left with a sharp point, or edge will be a likely suspect. Trim them to a blunt edge or point. Rounded and smooth is the answer here.

Q "My amp starts "humming" louder after it has been on for a while, why is it doing this?"

A A growing "hum" in an amplifier, is usually from the power transformer, which as the transformer heats up, the laminations become loose, and begin vibrating in the magnetic flux that flows in the iron core of the transformer. No real easy way to fix this aside of replacing the transformer. There has been some success with re-tightening the bolts that hold the transformer together, this does not always work. Another cause may be a steel cabinet cover that is vibrating in the presence of the magnetic field that extends out from the transformer. A piece of brass or copper attached to the cover in the area of the transformer may reduce the vibrations. Other methods of vibration damping may work but it will be necessary to experiment with them to find the best fix for your situation. Lastly, the hum might be caused by a fan/blower that is "out-of-balance", many times this is caused by a build-up of dust in the fan blades. Fan/Blowers that use "sleeve-type" bearings can cause vibrations also, especially when old and worn down.

Q "My amp starts drawing plate current as soon as it is turned on, what is wrong?"

A Check the T/R relay, insure that the contacts are not fused in the TX position. Next, check the operation zener diode (if used), insure that it is in operational condition. If all these are OK, it may mean that one or more of the tubes have a grid-to-filament short. This does happen from time to time with power tubes. Check the unit to tell if any AC hum is heard on transmit. If AC hum is found, a grid-to-filament short is confirmed, replace the tube or tubes, and the unit should operate as normal again. If no AC hum is noted, and the transmit signal sounds normal, it is likely that the "cut-off" bias is inadequate or not functioning properly. It will have to be repaired.

Q "My amp "blows" primary fuses from time to time, aside from that, it operates normally, what is the cause of the "blown" fuses?"

A This problem often happens to units that have no "step-start" circuit, or a circuit that is not functioning properly. Another possibility for this might be a "flash-over" of HV inside the tube. Insure that your amp "step-start" circuit is operating normally. Also, insure that the plate voltage used, is not in excess of the tube rating. If you have installed new tubes recently, they may be "gassy", it will be necessary to "burn-in" the tube by allowing the unit to idle for at least 24 hours prior to operations. Check the filter capacitor bank for shorted caps. If the fuses used are to close to the maximum operating current level, they can burn out over a period of time. It is best to check the operation manual for the proper value of fuse that should be installed in the unit. Always use 250v fuses, never, ever use 32v fuses.

Q "How long does the typical power tube last?"

A Life spans vary from one type to another. An example is: a 3-500Z should last about 12,000 hours of transmit time. Most metal/ceramic tubes are rated at 24,000 hours. This means that (barring abuse or mechanical failure) the typical tube should last, (based on the following formula: 2 hours per day X 6 days a week) = 19 years for a 12K/hour tube, or 38 years for a 24K/hour tube. Excessive tune-up time, and filament voltages will drastically reduce these numbers. EIMAC tube data states that for each tenth of a volt over the filament voltage rating, the tube life will be reduced by 10%. For example: a tube with a filament rated at 5V, and an amp that uses 5.5V, the tube may have a 50% reduction in life span. Excessive "tune-up" times & excessive RF drive levels are the primary cause of premature failures in most of the tubes used in ham radio.

Q "My amplifier has been in storage for several years, how should I power it up?"

A The old practice of using a variable transformer to slowly increase the primary voltage is not really necessary, however, a few precautions should taken in powering up the amp for the first time in many years. The main problem in the first power up is not the current "in-rush", but rather air contamination of the tube vacuum. The first power up of an amp with tubes that have become heavily contaminated by atmosphere can result in an HV flash-over and resultant plasma damage inside the tube(s). To prevent the possibility of a flash-over, disconnect the plate/anode connectors to the tubes. Next place the amp in the low voltage of CW plate voltage mode, then turn on the main power. If no arcing, smoke, or explosions take place after power up allow the unit to idle with no RF applied for no less than 10 hours. This "burn-in" time burns off, or absorbs a large amount of the contamination in the tube, which will reduce the possibility of an HV flash-over and resultant damage to the tubes, and or the HV power supply. As for the possibility of the HV filter capacitors exploding, or "smoking", even with a variac, there is no way to prevent this from happening. The caps will either work, or they will not. If the caps fail, replace the entire bank, it was likely long past time to do that anyway. If the unit was stored in a damp or high humidity area, the power transformer may have high levels of moisture in the windings, which can be removed by placing the amp in a kitchen oven, and adjust the oven temp for about 200 degrees F. Leave it in the oven for about 3 hours, then remove it, and allow the unit to cool to room temp before powering it up. It would be best to do this procedure with the tubes removed from the unit while in the oven. Another thing to look for, insects or rodents that might have taken up residence in the unit during storage. Clean out any dust that may have built up too. Check for any component damage and make repairs prior to first power-up.

After the unit is "powered-up", look and listen, your looking for smoke, arcing, or other signs of abnormal operation. Your listening for the sound of arcing, unusually loud humming, unusual vibrations, or the sound of the fan/blower either working abnormally, or not working at all. Look at the meters, the HV meter should read normal, and the plate and grid current meters should not read anything during "burn-in" period. If an odor is noticed, and seems to get stronger as the unit continues to run, it would be a good idea to shut it down and investigate the source. Another warning sign, is unusually high levels of heat that seem to be increasing even though the unit is idling, again shut the unit down and investigate the source of the over-heating.

If everything checks out good, and no problems happen during the "burn-in" period, shut the unit down (allow the unit to sit and cool for a few minutes), re-install the plate/anode connections, and again power-up the unit. Again, use the "look & listen" procedure. If no problems happen during this final step, your ready to operate with RF drive applied. If all appears normal, you are, "good-to-go". Be sure to use the "tune-up" procedure.

Remember, the use of a variac to power-up and amplifier is no guarantee that a problem will not happen. Just turn on the main power, whatever happens, is going to happen regardless of whether you did it with a variac or not.

Q "I just installed new tube(s) in my amp, and now it acts strangely and trips breaker or burns the fuse."

A When installing new tubes, there is a possibility that the tube(s) will have a "flash-over" problems, this is due to residual gases inside the tube(s). "Burn-in" the tube(s) for several hours before applying RF drive to them. Read this article for more details. Another possible problem could be shorts within the tube, such as a grid-to-filament short, this will only happen in directly heated cathode tubes. With indirectly heated-oxide coated tubes the short could be from the heater to cathode. Once residual gas is removed as a cause of the flash-over, if the short still exists it could be one of these other causes.