Discussion in 'Amateur Radio Amplifiers' started by N4IDA, Jun 30, 2019.
So Heath released the SB-220 after revision of the original 3-500Z from zero bias. Interesting.
3-500z tubes were originally designed for some operating bias above 2500 plate volts. It was the 3-400 that didn’t need bias up too around 3000 volts. The Drake L4 only used around 2600 volts and originally used 3-400’s. The B version used 3-500’s at zero bias also put the idle current is fairly high but that’s ok for SSB. HealthKit bumped the HV up to just over 2950 volts and saw the idle current was I’ve 200+ mills. To bring it down where they wanted it too around 5.6 volts so from the very beginning all sb220 amps had that operating bias. Many hams even slightly bias L4b amps these days. The W7RY QSK kit for the 220 allows for adjustable bias. But yeah, Heath 220’s all had some bias.
Thats crazy talk!
Sharp, zoomable,small file size...
Crazy talk I tell ya...
Thanks for the bias info.
Somewhere on the WWW was an Eimac spec sheet showing 3-500Z as having a mu of 200 just like the 3-400.
Supposedly, it was reduced to 130.
I was 5 years old at the time so......
Key isn't the average current rating (3A) of the 1N540x rectifier (datasheet). It's the 200A peak surge rating vs the energy stored in the HV filter capacitors. Limiting C to a sane value is a good idea. In the typical 2x 3-500Z amplifier, it's generally agreed that 270 μF (∕8 ≈ 33 μF total) is plenty.
Early versions had a two section TRW Load cap which was pricey, Heath part # 26-97. They then went with a much cost reduced Oren Elliot Products single section Part 26-145 that is still used in Ameritron amps.
The Tune cap was initially a Johnson 154-9, which became the OEP Part 26-131 sometime very early in production as both appear to use the same Part# .
There was also an important, but simple mod to the AC switch wiring to prevent the filament transformer from shorting during a tube short. This applied to 240VAC line only.
That was the Philips graphite version that Eimac labeled as their own while working out their own 3-500Z problems which reduced mu to 130......likely due to instability.
The limiting factor in the SB-220 family is the unobtainium power switch life due to higher surge current. With the Step Start kit Ive used 330 uF/450V CDE 381LR Snap Ins which fit the original cap blocks just fine as well as 100K 3W MOX equalizers since the more modern caps have far better tolerance of +/-20%.
The general agreement is that up to 470uF is OK with a step start with whatever extra benefit the user wants to believe in.
In all cases a surge (glitch) resistor in the HV lead in place of RFC2 can often save a lot of damage during a tube arc. I use 20 Ohm 25W Ohmite vitreous enamel ONLY as cheap ones can shatter themselves plus the tubes.
Ahh ... see. I found new direct replacement switches that can handle even the inrush current at 120VAC wiring, for both the SB-220 and the HL-2200 paddle type. They are out there, you just have to search ... hard. I wish I had my normal computers and was at home (Had a house fire so I don't have the links or remember them here in the hotel on the laptop).