Anyone work on tube amps?

I have a sick HeathKit SB-220. I am 30 miles North of Louisville. Any help would be appreciated.

Tube amps are pretty easy. If all the voltages are present on the tube pins and the tube is good, it should work 95% of the time.

What does "sick" mean?

wa1nic said:

Tube amps are pretty easy. If all the voltages are present on the tube pins and the tube is good, it should work 95% of the time.

Click to expand...

That can be true, but I wouldn't necessarily agree with it. You can have voltages within the proper range on the tube pins and still have problems; dried out capacitors, old resistors, dirty switch contacts and other flaky components could lead to issues like ripple, hum and other unpleasantness.

I speak from experience, right down to the slight electrocution from not keeping one hand in my pocket while I was poking around. Learned that lesson the hard way!!!

Yeah, those capacitors can knock ya for a loop!

Also op, would be helpful to know what you mean by "sick"? Loss of some power, or all power, noise on transmit, or not working at all? Tube type amps are easy to work on, just be careful of those big caps, and don't mess with any coils. Give us a better description of what's wrong.

The relays will click when keyed with the manual switch, tubes glow, no wattage output and it pegs my Icom's SWR..
Plate amps show .1 when keyed.

First, please be very careful with the SB220. The power supply produces about 2000 volts
and sticking your finger in the wrong place can easily kill you. Even with the switch in the off
position and unplugged, the power supply filter capacitors can hold a lethal charge for a long
time. If you don't understand the above statement, please don't remove the case.

You said your Icom pegs the meter. That's a very good sign that the amp input circuit has
a problem. Each band has a tuned input circuit to present a low swr (impedance) to the
icom and the swr meter in the icom should have a low reading. The band switch switches
in a circuit for the proper band. Make sure the amp knob is set to the correct band your
attempting to use. I've seen broken switches, knobs slipped on the shaft and not on the
correct band.
If you have a good meter that can measure impedance, hook it to the input connector
and see what it reads. (Should be around 50 ohms.)
Plate meter reads 1 amp ? that's very excessive and maybe a indication that one or
more of the tunes are bad.
Correct plate current would be 600-700 ma and you should never exceed more than
200 ma of grid current or the tubes are in danger of being damaged. If the tubes are
in good condition, the above reading would produce about 1200-1400 watts output
with 80-100 watts of drive.

I can think of many other ideas but most require some reasonable test gear. Do
you have any history on the unit ? Did you have it working when in your possession ?

Good luck and be very careful......

.1 when keyed and tuned correctly it raises the SWR on my rig off the chart.

0.1 Amps is what he said.

Ya Tom, I threw in the "95%" because there can be other problems.

If the input circuit it damaged, turn the power off, open it up, and use your nose and ears, looking for burned things. An ohm-meter will tell you about open coils. Make sure you are not measuring something in parallel with a coil that will show continuity.


I just dug up an SB220 schematic. Have you tried more than one band, and does it act "sick" on all bands? L1, 2, 3, 4, 5 being open would cause a problem but it would be band specific. You can check the continuity of any of those inductors as long as the band switch is selecting a different band.

Burned RF relay contacts could also cause such a problem. That would show up on all bands and I would think it would put on a bit of a light show and you would hear it too.

Less likely but also possible would be one of the capacitors in the network shorting out. If someone drove it hard on the wrong band, there could have been some pretty abnormal voltages in the input network that could have poped a cap as well as an inductor.

I am sure there are a few less likely problems, but I would guess that damage in the input network might be found if you use your eyes and your nose.

Like W3DMV said, there is a lot of volts with a lot of current capability behind it in the box. Use extreme caution. Dont be afraid to be scared of it, because you should be scared of it. Done assume that if it is off that it cant hurt you. After you turn it off, give it a good minute to bleed voltage off and then still short out the power supply capacitor string with a screwdriver... just to make absolutley sure that it is indeed discharged. An open bleeder resistor could leave the capacitors in a lethal state for a long time.

Rick

I have retubed before. Do not fret I will discharge the unit then test it. Thanks guys.