>Tom,
>In the mid 70's I went to work at Heathkit as a Ham Tech. One of the first
>rigs I had to work on was a SB220. I replaced burned chokes/caps on the
>tube
>sockets and sent the rig back to the customer. I then contacted Benton
>Harbor on my "technical assistance" line and asked why these parts were
there. A
>week or so later I was told they "thought" they had something to do with
>metering.
>Not much of an answer and I never did get one that made sense.
** that's because it doesn't, Bob. The only possible reason is that if
the 3 grid caps are fairly small, they increase the grid resonance by a
couple of MHz. However, this seemingly makes little or no improvement in
amplifier stability.
>Bottom line
>is there is no valid technical reason to have them in the circuit.
** An early 2, 3-500Z circuit from Eimac's Bill Orr used chokes/caps on
the grids. Heath, Drake and Kenwood copied the idea. However, I have
yet to read an account of Bill Orr's reasoning behind the idea. As I see
it, lifting the grid off of ground allows one the opportunity to use a
grid fusing element. However, in the directly grounded grid config, it
is possible to install a grid fusing element in series between the grid
current meter shunt R and ground.
>
>Personal experience with the 25 to 30 SB220s Ive restored/repaired/modified
over
>the years indicates directly grounding the grids makes for a more stable
>amplifier.
>Sounds to me like the resistor and cap suggestions came from someone with
>parts to sell.
** Good point, Bob. Rich and company charge $0.05 each for pn 88, the
30-ohm, 1/4W carbon film grid fusing resistor. For pn 91, the 470pF,
2000v grid bypass caps, we charge $0.25 each. After >6000 parasitic
suppressor retrofit kits. I have yet to hear of anyone who is using a
grid fusing resistor reporting a grid-filament short. We are currently
making so much money, there could be a new BMW 750 on the horizon.
However, after my trip to Sees Candy yesterday to buy St Valentines day
chocolates, the new Beemer project is, alas, switched from Operate to
Standby.
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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