>Hi Pete,
>
>> I have a Round Emblem model. I was told that the RF cable
>used to connect
>> the Xmiter (KWM2 etc) to the input of the 30L1 was of critical length. It
>> was explained that the Amp could go into runaway on 21 Mhz and above if
>> this cable was not used.
>
>Your problem illustrates my point about the grid system in the 30L1,
>and the silly super cathode driven system applied to triodes.
'Super cathode driven' applies to a tetrode where the screen is grounded
and the grid is operated at 0 RF-V with respect to the cathode. Thus,
there is never any grid current in a super cathode driven amplifier
circuit.
>The 30L1 lacks two things important for stability, it has the grids
>improperly grounded and it has NO neutralization circuit. Both are
>serious mistakes with four 811A tubes in parallel.
...snip...
A push-pull grounded-grid amplifier can be neutralized.
A single-ended grounded-grid amplifier can not be neutralized. [see
*Care and Feeding of Power Grid Tubes*]
To check out whether or not Mr. Rauch knows what he is talking about in
the area of grid-grounding, use a dipmeter to measure the grid's resonant
freq. in a 30l-1 with the Collins-stock 220pF grid byass C in place.
Remove the 220pF capacitor. Replace it with a short length of wide
copper ribbon. Re-measure the grid's resonant frequency. If the grid
resonance increases in frequency with the grid directly-grounded, then
the amplifier would be more stable with that modification, and Mr. Rauch
is 'absolutely, positively, 100%' correct.
Rich---
R. L. Measures, 805-386-3734, AG6K
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