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How to tune a tube amp.

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Captain Kilowatt

Professional Amateur
Staff member
Apr 6, 2005
Nova Scotia,Canada
Typically, the procedure is this::

1. Hook everything up (radio to amp, amp to wattmeter, wattmeter to dummy load which is capable of handling the full output of the amp for as long as it take you to perform the tune-up).

2. Set your radio's output to as low of a setting as it will go to, typically one watt or so.

3. Set the amp's " LOAD " control to mid-point and the " TUNE " control to about the same point.

4. Key your radio and " QUICKLY " turn the " TUNE " control back and forth to get a feel for the proper direction to increase power, then " UNKEY " and wait a few seconds for things to cool down.

5. Key up the radio again and watch your " OUTPUT " wattmeter while rotating the " TUNE " control for maximum power out. The " PLATE " meter should indicate proper neutralization of the tubes " IF ", you see a pronounced dip which co-incides with maximum output. If it doesn't, you need to neutralize/balance the load power between the tubes. There is a control " INSIDE " the amp for this purpose if your amp is worth a plug nickle. " BUT " *** BEWARE *** of the " EXTREMELY HIGH VOLTAGES " present inside the amp! " THEY CAN BE DEADLY!!! "

6. After tuning the " TUNE " control for max smoke (highest wattage as seen on an external wattmeter) then unkey and let things cool a moment.

7. Next key the radio again and adjust the " LOAD " control for maximum smoke.

8. Unkey and wait a moment, then go back and re-peak the " TUNE " control again.

9. Finish up by adjusting the " LOAD " again for max smoke while keeping plate current to a safe level.

10. Now you have preliminary settings, and can begin increasing your radio's drive power. Increase to the wattage you expect to be driving the amp with, and repeat the tune procedure.

11. Once tuned into a " DUMMY LOAD ", then switch to your antenna, and re-peak. If your antenna is adjusted properly, you should not need to re-adjust the amp much, if at all. If your antenna is mis-tuned, then you'll have to re-adjust the amp.

12. For frequency excursions within the same band, you should be able to leave the load control alone (pretty much) and simply tweak the " TUNE " control a bit to re-gain power lost by changing frequencies. If you seem to lose a LOT of power, you may have to re-adjust the " LOAD " control accordingly.

13. Always make sure that your " PLATE CURRENT " dips at practically the same tuning point as " MAX SMOKE ". Slightly off a bit is " OK ". Some people advance the " LOAD " control slightly to lessen power a few watts because it supposedly makes the amp sound better. Use signal reports from others to see if this adjustment benefits you or not.

14. If you don't dip the plate, you're going to wind up burning up a set of tubes.I know some of you will do this even if you do dip the plate current.Remember the filaments are supposed to glow bright orange not the plates! :LOL:

Blanket said:
QRN said:
It's the same for ALL tube amps that have both a TUNE and LOAD control.

some amps don't need to have thier tubes nuetralized, so dipping the plate current doesnt do anything but effect the output. Nor will the adjustment of plate and tune have any effect on modulation, the only effect it may have is if it lowers your dk and your pep power remains constant, you will have an apperent increase in power.

TUNE and LOAD still work the same way whether a tube is neutralized or not. Max O/P may occur slightly off of dip in a non-neutralized amp. As for advancing the LOAD control after tuning for a dip,like it said,"Some people advance the " LOAD " control slightly to lessen power a few watts because it supposedly makes the amp sound better. Use signal reports from others to see if this adjustment benefits you or not." In some RF amplifiers this increases the stability of the amp and improves associated IMD products and thus distortion products. The info presented is good for 99.9999% of the time. ;)
Per the manual for the Pride DX-300:

1) Check to see that the TUNE/STANDBY/OPERATE switch is in the center or STANDBY position. Apply power to the unit by switching the POWER switch to the ON position. You will hear the cooling blower come up to speed. Wait 60 seconds for the tube to come up to operating temperature.

2) During the time that the amplifier is warming up or at any time that the TUNE/STANDBY/OPERATE switch is in the STANDBY position, the exciter may be operated normally "straight through" the amplifier.

3) Set the band switch to the same band as that the exciter is tuned to.

4) After warm up, place the TUNE/STANDBY/OPERATE switch into the TUNE position. The OPERATE indicator will light up, indicating that the unit is ready for operation. If the RECEIVE switch is on, the RECEIVE indicator will light also, indicating operation of the receive preamplifier.

5) Rotate the PLATE LOADING control to the nine o'clock (counter clockwise) position.

6) Apply several watts of drive from the exciter and quickly peak the PLATE TUNE control for maximum output as indicated on the front panel wattmeter. The SCREEN overload indicator may glow at this time, and will peak in brilliance at approximately the same place of tuning that maximum output occurs.

7) The PLATE LOADING control is then turned clockwise 1/2 division or so, and the PLATE TUNING control is peaked again for maximum output. The brilliance of the SCREEN indicator will peak again, but not quite as brightly as before. This process is continued until a further increase in the PLATE LOADING control will give no further increase in power output. This will normally result in approximately 100watts output for three to four watts
of driving power.

8) Un-key the exciter, and place the TUNE/STANDBY/OPERATE switch in the OPERATE position. When the exciter is keyed again, the power output will be somewhat higher, and the SCREEN indicator may glow brightly again. Repeat the procedure as before, tuning the PLATE LOADING and PLATE TUNING controls alternately for maximum output. The PLATE TUNING control should always be the last control that is adjusted. Power output should be about 250watts for five watts drive at this point in the tune up procedure. When proper loading is accomplished, the SCREEN indicator should glow only dimly. The plate current, as read on an external meter, should be in the neighborhood of 200-250ma.

9) For SSB operation, drive power should now be gradually increased to 10 or 12 watts, and the unit peaked for maximum output. This can be done using the carrier insertion control on some exciters, or if this is not available, a steady tone may be applied to the microphone input. When maximum power has been achieved, operation consists of simply keying the microphone and speaking. The SSB delay switch should be in the on position,
which will give the R.F. actuated relays a time delay, so that they won't chatter during transmission. During the SSB transmission, power output peaks will occur so rapidly that the output meter will not be able to follow the signal. If a monitor scope is available, it can be seen that the peak power is considerably better than the 400 watts or so that was obtained during tune up. The SCREEN indicator will flicker with the transmission level, and will reach full brilliance at high peak power levels. This is a faster indicator of peak drive conditions than is the power output meter.

10) For CW operation, tune up as in step nine above. Power output will be nominal 250 watts for 5 watts of driving power. The SSB delay switch may be left in the on position to prevent relay chatter during keying.

11) The 4CX-250B transmitting tube has been proven is commercial and military uses where long life and reliability are major factors. Although it is designed to work a little harder in amateur service, it will still be extremely reliable if a few common sense rules are applied.

a) Make sure that the tube always receives a good airflow. Remember that a lot of watts are being packed into that rather small package, and that the only way to keep it cool is to get air through the tube fins. Do not block the top of the cabinet, or stack other equipment on the amplifier. Make sure that the rear of the chassis (where the blower gets its air) is open to cool room air. If the bottom cover of the unit has to be removed for service, see that it is replaced properly prior to plugging in the unit (for safety also). The bottom chassis must be pressurized
in order to force air through the tube fins.

b) Wait 60 seconds before applying drive to the tube. Allowing the tube to properly warm up will help you get the longest service from it.

c) After a long period on the air, let the amplifier run in the STANDBY mode for a minute or so before turning off the power switch. This allows the blower to bring the tube down to a cooler temperature fairly rapidly.

d) Avoid excessive R. F. drive to the amplifier.

e) Use care in tuning most tube failures are the result of operator error, not equipment failure. These transmitting
tubes are used for thousands of hours in commercial service, and similar results should be yours with reasonable precautions.

Copyright © 2004 CBTricks.com
ok guys, i have a dumb question...lol..my Varmint xl-250 has LOAD, PLATE and DRIVER PLATE knobs. I am confused on how to tune it in case I wanna use it as I dont wanna blow any tubes or hurt radio :) SO if someone could tell me how to tune it, I would GREATLY appreciate it :)
Right or wrong... this is the way I tune up my amp. I run a Superstar 360FM radio into a Golden Falcon 150 transistor linear which drives a Heathkit SB221 tube amp (2kw pep). The radio and transistor amp are setup for 100 watts output (dead key) which is what the tube amp requires. First, I turn on the filament voltage and let the tube amp warm up for at least 30 minutes so the filaments are at proper operating temp, and the cathodes are properly heated. Then I key up the radio (no modulation) and adjust the TUNE and PLATE controls for highest output. Next, I enable the roger beep circuit in the 360FM to generate a continuous 1000 hz tone (easy mod...lift D92, install a switch). Then I keyup the radio, and adjust tune and plate for maximum output (modulated). After all, I want max output when I'm talking, not when I'm deadkeying. AND... I do this into the antenna...NOT into a dummy load. I want max match (and wattage) into my antenna. Sorry, ham guys, tuning up on the air is a bad thing . This procedure works good for me. Like I said, " Right or wrong this is the way I tune up my amp."

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