I prefer T-80 or T-94 size cores and use low value Arco 460 series trimmers
to fine tune. The silver micas are 500 or 1000V DM-19 size. The package is
larger than what most used but I like stability and only doing it once. No
heat & No drift up to 200W.
Carl
KM1H
>
> On 02-Apr-10 12:53 AM, Jim Thomson wrote:
>>
>> ## Eimac sez a 3x3 is supposed to be 24.0 pf from anode to grid
>> on a wooden bench. I measure exactly 24.0 pf on the 875.
>> Eimac sez the YU-148 is supposed to be 24.5 pf, from anode to grid.
>> I measure exactly 24.5 pf on the 875. That's good enough for me.
>
> *** Agreed, no reason not to use the 875 or equivalent for this
> measurement. I have used my old homebrew C meter that tests at audio
> frequencies with success. Also used my MFJ successfully.
>
>
>
> throw away those knobs and use fixed capacitors!
>>
>> ## which is why I don't use fixed caps, nor 9 x bandswitched
>> tuned inputs anymore. Those days of fixed SM caps and a
>> tunable Millen 60946 slug or XR-50 are over. Plan B was
>> to use variable arco's and fixed T-50 torroids. Plan C is to use
>> 1 x tapped coil, and 2 x broadcast variables.
>
> *** The comment I made about throwing away the knobs was made tongue in
> cheek. I actually have done that. Interesting experiment but impractical
> unless you ONLY run the amp into a 50 ohm dummy load.
>
> *** I disagree, but only on a philosophical level. I absolutely refuse
> to consider any solution which requires more knobs. Even with logging
> scales and charts, it increases the band change effort well past my
> comfort zone. My eyes are not what they used to be. I can't see the damn
> charts or numbers on the knobs easily any more. I would (and do) spend
> as much time as necessary to implement fixed tuned input networks. Those
> stupid slug tuned coils are about the worst thing I can think of to do
> for an input network. T50-2 and T50-6 toroid inductor are the only thing
> that should be considered (for a fixed tuned network). Arco adjustable
> caps are good but still not as good as fixed silver mica caps. All my
> input networks are constructed using toroid inductors and fixed silver
> mica caps. They are tuned on the bench (usually one time only but
> unfortunately I don't get it right every time) and then forgotten until
> I rebuild the amp for some reason. (I'm never satisfied!) I usually get
> them close with the MFJ and then "fine tweak" for at least 30 dB return
> loss in the middle of each band using the VNA. This will give better
> than 1.1:1 input SWR over the band, including 80/75. If it is an 8 band
> amp, then there are 8 tuned networks.
>>
>>
>> ## I only have the trial version of KM5KG's RF network designer.
>> The full blown version is next weeks project, which should have been
>> done a long time ago.
>
> *** The new trial version is crippled excessively. I have an older trial
> version that has the solenoid coil design feature enabled. I keep
> meaning to get the paid version because it is such a great tool but
> never seem to get around to it.
>
>
>>
>> ## when the tank coil cooks, and BW is narrow, that's usually
>> a subtle hint, that loaded Q is too high. A loaded Q of 8-10
>> using GM3SEK's sheet works good. If drive power is then reduced,
>> plate load Z will rise, and so will loaded Q. A 8-10 loaded
>> Q will rise to 10-12. Eff is still good, no coils get cooked...
>> [ and no bandswitch's getting cooked either]
>
> *** I design for a Q=12 but I also have a separate band switch position
> for each and every band, no doubling up. I have trouble imagining a tank
> so marginal that it melts down when used at a lower power level.
>
> *** Checking SWR bandwidth is a great "feel good" test that can easily
> be done with instruments everybody *should* own. I prefer to actually
> measure the loaded Q (since I have the instruments). For instance the
> last tank I did had design goals of Q=12 for 80-12 and Q=14 for 10. I
> tuned up the finished amp on each band with full drive and recorded the
> logging scale numbers. Returned the amp to the bench, set and measured
> the loaded Q on each band for the actual tuning used for full power. No
> resistors, no guessing. Measured Q's came out to be in the range of 12.1
> to 12.6 on 80-12 and 14.6 on 10.
>
>
>>In all
>> three cases, the MFJ produced measurement results that were at least an
>> order of magnitude better than what VE7RF reported.
>>
>> ## Maybe mine is blown up ?
>
> *** "Blown up" is probably too harsh. The few detector diode failures
> I've seen resulted in reading that were incorrect but close enough that
> you could talk yourself into believing them. The only way to tell is to
> check cal with resistors and capacitors (and inductors if possible) of
> known accuracy at a number of frequencies. Check for both R and X. 50
> ohm microwave terminations work good for R. X should be checked with
> good caps that have a reactance approximately equal to 50 ohms at the
> test frequency.
>>
>>
>> ## u could hook interconnecting leads to mfj.. and measure it... then
>> subtract those readings from the completed coil + interconnects.
>
> *** Sometimes that works OK, sometimes not. The actual inductance of the
> "test leads" varies quite a bit as you move them around. On large coils
> such as used for 80 meters, it's usually insignificant. But on smaller
> coils such as used for 10/12 or 6, it can result in unusable results. A
> guy needs to take each measurement on a case by case basis and PAY
> ATTENTION! Take nothing for granted.
>
>>
>>
>>
>> ## The b+k has spring loaded 3/4" wide slots on front, so bannana
>> jacks/test leads don't have to be used at all. Any strap up to 3/4"
>> wide, can be used as a test lead. Also, things like .01 uf caps can
>> have their leads stuffed deep into the same spring loaded slots,
>> eliminating the stray L from their 20 ga long leads.
>
> *** Seems like no matter how many "fixtures" I dream up for the various
> instruments I have, none are 100% effective. Also seems like simpler is
> usually better. For leaded parts using instruments with UHF connectors,
> I mostly just bend the leads around so they jam into the UHF connector.
> This works well for caps and toroids, don't work at all for air core
> solenoid coils. (Proximity to instrument and fingers change the
> inductance and Q) Those BNC to binding post adapters work pretty good
> for most leaded parts when you need to "screw" them down.
>
>
> 73, Larry
>
>
> Larry - W7IUV
> DN07dg - central WA
> http://w7iuv.com
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