> On Dec 23, 2025, at 10:09 PM, Brian L. Stuart <[email protected]> wrote:
>
> On Mon, Dec 22, 2025 at 01:42:54PM -0500, Paul Koning wrote:
>> I glanced at a few of them. Was surprised to see an inverter using a 6L6.
>> Then looked a little further and came across an element that uses an 807.
>>
>> The 6L6 and even more the 807 are rather substantial power tubes. 807s are
>> commonly seen in amateur radio transmitters in the 1940s and 1950s, good for
>> 50 watts or so power output. I have a couple, they are very nice tubes. I
>> don't remember the 6L6 ratings; 20 watts, perhaps? Both are also rather
>> large, the 807 especially, when compared to typical "receiver" tubes.
>
> Good catch. The majority of the 6L6s are used in parallel pairs in
> the circuits identified as "transmitters." They are drivers for
> transmission lines that can be up to about 80 feet long with taps
> about every foot. The pulses they're transitting are about 2
> micro-seconds long. So they really are like small RF transmitters
> pushing long transmission lines pretty hard.
>
> When I first saw those designs, I thought in terms of over-design
> from the context of TTL signals on busses measured in inches. Then
> again, we're talking about a 50 volt swing charging up a big long
> coax capacitor. So the more I've dug into it, the more respect I
> have for the design. In fact, there are also pulse amplifiers
> that can be inserted to bump the signals on long lines.
Sure, that makes sense. And there's a fair amount of power involved to produce
fast pulses of significant amplitude.
It reminds me of the CDC 6000 series mainframe console display. That uses what
amounts to oversized oscilloscope tubes -- electrostatic deflection tubes, but
14 inches diameter vs. the usual 5 inches or so you would find in a typical
oscilloscope. We sometimes referred to them as "radar tubes" but classic radar
displays use electromagnetic deflection (with a rotating yoke).
Since the display rate is pretty fast -- a vector-drawn character takes about
2.5 microseconds, with 100 ns per stroke step -- and the deflection waveforms
are at least several hundred volts, the deflection final stage is a push-pull
pair of 3CX100A microwave transmitter tubes, rated at 100 watts at several
hundred MHz. In the DD60 display they are DC amplifiers, with 2 kV anode
voltage. Not the usual environment for a computer technician to work in.
paul
