Collins 30L1 Heathkit SB200 Arc protect and Bias Kit (requires user experience)
$ 19.01
Description
Note: We will have a neutralization kit soon. The Collins 30L1 and Heathkit SB-200 share nearly identical grid and bias circuitry. Keep in mind the Collins system Heathkit copied was designed in the 1950s before we had inexpensive readily available Zener bias diodes. As such, Collins just used what was technically available. Heath copied or patterned their relay and grid bias system after the 30L1. The primary difference is Collins runs four 811A tubes at nearly 2000 volts while Heathkit ran a pair of 572B tubes at 2400 volts nominal. Due to similarities of a negative 120V nominal voltage relay and a poor unstable resistor divider bias system, one kit can fit both amplifiers. We also have an updated relay system and soft key or relay buffer for the SB200 and 30L1, although the 30L1 is a squeeze. Control grid voltage has a large effect on anode current. A good design never has ripple or anomalies in bias voltage, especially when using low-bias high-mu tubes. A good design also will not allow operating bias to move around with modulation, with very few exceptions. Bias can track modulation or envelope levels if in phase and at or near radio frequency rates, since this adds negative feedback or linear gain compression. However, if that bias shift is delayed in phase or pushes the tube into cutoff, IMD (splatter) will significantly increase. Taking tube bias into cutoff places the amplifier into shorter conduction angles, causing the tube to have distorted transfer and become a mixer of multiple tone drive signals modulated by the out-of-phase or time delayed bias changes. This is why we should not use a simple resistor divider in amplifiers that have control grid current. Grid bias is very “loose” in both the Heath and Collins amplifiers. While neither amplifier has stable predictable well-filtered operating bias, Heath added C19 to the poor Collins bias system. C19 reduces bias hum and noise at the expense of increasing unwanted time lag in bias voltage RF envelope tracking. Latency in bias tracking decreases IMD performance increasingly at higher modulation rates (audio frequencies) on SSB, which is exactly the worst frequency range for SSB adjacent channel IMD products (transmitter channel bandwidth). Not only is the resistor divider bias horribly unstable with grid current, the entire filament-cathode current, which is the sum of plate and grid currents, flows out the relay control path. Any voltage drop on the relay grounding path adds to tube bias. This is why the Harbach soft key, already at the edge of control voltages, sometimes develops high levels of 60Hz pulses in bias voltage in these amplifiers. IMD in both amplifiers varies considerably with operating band, amplifier tuning adjustments, the particular tubes, and even exciter characteristics. The only certain thing, in either the Collins 30L1 or Heath SB200 amplifier, is this “loose” bias scheme does not follow good engineering practices today. These parts correct and update these units.
Item arrived in excellent condition and appearance appears to be of high quality. Blue Ray read/\write CD drive at this price is an exceptional value. Thank You for a great deal.
Very many thanks for the spares to get my 30L1 up and running again. Excellent packaging and prompt shipping and I would certainly purchase from the seller again.
The item arrived quickly, well packaged, and as described in the listing. 👍
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