>
>
>-----Original Message-----
>From: 2@vc.net [mailto:2@vc.net]
>Sent: Wednesday, July 31, 2002 06:08
>To: Jim Thomson; W6frmarv@dslextreme.com
>Cc: w8ji@contesting.com; stevek@jmr.com; R. L. Measures; AMPS
>Subject: Re: FW: diode strings...bias
>
>
>
>
>>
>>
>>>
>>> They also tried the grounding just one grid pin of each socket
>>> and all reported the same 20w reducrion in drive power.
>
>** Frequency didn't matter?
>
>#### (VE7RF) No, freq didn't matter. There was a reduction in
>drive power on all bands... including 10M.
>
** If 600pF of grid bypass to gnd was causing the drive requirement to
go up, there should have been a difference between 3.5 and 29MHz. At
3.5MHz, 600pF has 75-ohms. At 29MHz, 9.5-ohms.
>
>>> The fellows with the SB-221's also reported that their output
>power
>>> on 10M shot up from 700-800 watts to 1350-1400 !!
>>>
>** 10m is the band where 600pF of grid bypass [XC=9.4-ohms] is
>seemingly enough. Since a typical "Zero-inductance" 3-500Z
>grid-grounding strap has c. 15nH of XL at 10m, a 600pF grid-bypass will
>put the grid closer to ground than a strap.
>
>#### (VE7RF) That was my thought as well. But like the guys at
>Nasa say, "one test is worth a 1000 opinions."
>
>>> The fellows with the TL-922's reported that they could now
>>> remove the AG6K nichrome suppressors, and the TL-922's were
>now
>>> rock stable.
>>>
>** 922 / rock stable . . . I own one and that's laughable. They are
>[factory-fresh] squirrelier than a SB-220 or an AL-1200 with
>"neutralization".
>
>#### (VE7RF) That's cuz the stock 922 uses the 600pf bypass caps.
>600pf isn't much of a bypass, esp on the lower bands.
** the 922 does not oscillate at HF.
>You can try it urself, and even leave in the stock 3 x 200pf
bypass
>caps + rf chokes. In any event, it works, the 922's are stable,
>no nichrome needed, and 20w less drive required.
>
** No nichrome is needed to reduce vhf-Q if one happens to have a <15nH
resistor that can dissipate 25w. If such a resistor is used, increase
L-supp by c.60% and vhf-Q should be low enough. 25w dissipation is no
problem, however, 15nH is. Cesivid Co. said they could build such a
resistor for a $10,000 minimum order for 1k units.
>#### (VE7RF) Perhaps in a GG amp, one actually needs to ground
>the grids !
** The grids will be closer to ground at 29MHz with 600pF than with an
L=15nH strap.
>We then insert a fast grid fuse in series with the
>grid shunt.
** Good
>A 100k R is placed in parallel with the fuse holder.
>
** paralleling a dime-size 120v MOV might be good.
>
>### (VE7RF) Ur idea of replacing the stock chokes with 30 ohm grid
>resistor "fuses", just creates unneeded, and varying bias.
>
The V-drop across the 30-ohm resistors amounts to c, 5% of the
cathode-grid voltage. What effect did you observe?
>...
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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