Heathkit SB-220

[555]

it can deadkey 500 and modulate to 1500 to 1800 all day long and it deadkey more if i have to

its all in 1

my next step will be the transformer upgrade
i have a guy that wants to buy my upgraded transformer and then i will buy my next transfomer but it will have its own cabinet. the transformer im looking for and was quoted by Harbach would cost little over 1000.00
then i will change out the filament transformer and upgrade the zs to zg

 

[Shockwave]

it can deadkey 500 and modulate to 1500 to 1800 all day long and it deadkey more if i have to

its all in 1

my next step will be the transformer upgrade
i have a guy that wants to buy my upgraded transformer and then i will buy my next transfomer but it will have its own cabinet. the transformer im looking for and was quoted by Harbach would cost little over 1000.00
then i will change out the filament transformer and upgrade the zs to zg

The 1000 pf red doorknob capacitor mounted on the aluminum plate on the back of the tune variable is likely to fail. It already looks overheated. If you notice the box starts to lose it's tune the longer you're keyed it's this red cap that's breaking down. Replace it with a ceramic style. The bigger transformer is also likely to make the plate tune variable cap arc under full power. You may be able to take it apart and remove every other plate on both halves to increase the gapping. Otherwise consider a vacuum variable for the job.

The original 5.1 volt 10 watt zener diode could be upgraded too. If you're using it on AM you might want to step the voltage up to at least 10 volts and let the tubes run cooler by removing a little bias. Make sure to use a 50 watt zener if you replace it. I like to directly ground the 3 grid pins on each socket too. Each pin tied right to ground with low inductance, not all 3 pins tied together and then grounded. It may increase IMD the slightest bit but it increases stability by miles and makes it worth the tradeoff.

 

[555]

yah im a bit worried about this box it was built back in 89
guy been running it all these years then decided to get a bigger z box
last year and since i had a 5 tuber he bought that then i bought the 3 tuber
had it for almost a year

 

[Bionic Chicken]

I'm sure the box works great but the tubes are a mounted a little close together for good air flow to them. But that is what happens when you have it "all in one". Outboarding the HV supply (plate transformer, rectifiers, and caps) will let you "line" them up and provide cooling enough for all three. You can also trim the chassis on the underside to provide more airflow to the tube sockets, a weak point on the SB 220. When you have been hammering on it they get hot due to very liitle cooling on them and that's when the solder melts out of the tube pins. Upping the bias voltage is something needed if you operate on AM all the time. 12 volts will cool the tubes down alot especially if you are driving it hard. Also add a 1 amp fuse in series with the zener. If you have an arc on a tuner or tube this will protect the rectifiers and other components. Here is a good link for more info on the SB 220. Check out the replacement for the zener diode. I have used this mod for years.

Circuit Improvements for the Heath SB-220 Amplifier