Id start by moving the coil tap a turn or two in both directions and chart
what happens.
With that low efficiency I would also look for something else amiss such as
the plate choke, wiring error, etc. That LDG tuner has been
discontinued......possible design problem?
Carl
KM1H
----- Original Message -----
From: "Chuck Curran" <ccurran@wi.rr.com>
To: "'AMPS List'" <amps@contesting.com>
Sent: Sunday, January 02, 2011 5:00 PM
Subject: [Amps] 3-400Z Linear Problem
> Hello to All:
>
>
>
> Here is the problem I am dealing with. I built a home brew amp using a
> pair
> of Eimac 3-400Z's, which I obtained brand new in 1968. I hauled them
> around
> for years, became active again in Amateur Radio and built this amp.
>
>
>
> I used available information in 2008 to calculate the values needed for
> the
> PI-NET output circuit, and then found a NOS B&W 852 tank coil at a Ham
> Fest
> and bought it. I found that this tank coil had proper inductance,
> relative
> to my needs, on all bands except 80 meters. I modified this tank coil to
> have exactly what I believed I needed. If you look at a 2007 ARRL
> Handbook
> in the PI-NET area, page 18.7, you can see my values are very close to
> what
> is listed for a plate load impedance of 2100 ohms, which was about what I
> expected during construction.
>
>
>
> Upon completing the amp, I fired it up on 75 meter SSB and it ran
> beautifully. I was getting about 62% plate efficiency, so I thought all
> was
> well. I tried it on 20 meters, into a dummy load and as I recall, it ran
> fine. I do not use it too often, mostly running just my exciter.
>
>
>
> Recently I tried to load it up on 40 meters, but I immediately discovered
> a
> horrible efficiency, something in the 25% range. Nice cherry red glow to
> the plates! So that is the issue and reason for this posting, I am trying
> to understand why my 40 meter efficiency is so bad. (80 meter was still
> great at this time.)
>
>
>
> Now for a bit of past history. I used the formulas for PI-NET component
> calculations available to me at the time. They were based on the updated
> formulas offered by Elmer Wingfield, so I felt I was in a safe zone. I
> also
> have a General Radio 1656 Impedance bridge, which allowed me to measure
> the
> tank coil values on each band, to what I perceived to be a very accurate
> level. As a point of information, I obtained an original B&W sales
> brochure
> and it gave the inductance values of their 852 coil on each band. My
> measured values were very close "except" on 40 meters, so I convinced
> myself
> that my impedance bridge was running OK. The PI-NET uses a Comet 5-500 pf
> tuning cap and a Russian 15-1200 pf load cap.
>
>
>
> For my calculations, I used an on-line calculator and obtained the
> following
> for a Q=12, plate load of 2100+ohms:
>
>
>
>
> Band
>
> C1, in pf
>
> L, in uh
>
> C2, in pf
>
> As Advertised B&W 852
>
> MY Modified B&W 852
>
>
> 80 Meters
>
> 246
>
> 9.44
>
> 1352
>
> 7
>
> 9.3
>
>
> 40 Meters
>
> 123
>
> 4.72
>
> 676
>
> 3.72
>
> 4.5
>
>
> 20 Meters
>
> 61
>
> 2.36
>
> 338
>
> 2.34
>
> 2.34
>
>
> 15 Meters
>
> 41
>
> 1.57
>
> 225
>
> 1.34
>
> 1.34
>
>
> 10 Meters
>
> 31
>
> 1.18
>
> 169
>
> 0.95
>
> 1
>
>
>
>
>
> The first columns above were the targeted area, the far right column is
> the
> measured inductance as installed. The amp is actually running a bit above
> 2800 VDC with a plate current of 650 ma during tune up. I can vary the
> plate voltage up to 3300 VDC, just as a FYI. I originally did the
> calculations assuming a slightly lower plate voltage, but the 2800 VDC is
> what I got in the end.
>
>
>
> OH, another interesting point. When loaded on 80, I am actually using a
> measure 280 pf on tune cap and 1630 pf on load, not real close to expected
> values----
>
>
>
> Since trying the amp and finding the issue on 40 meters, I tried using the
> currently available on-line calculators, and my numbers are not matching
> as
> closely as I would have expected. However, whether it ends up with a Q of
> 10 or 12, I would not expect that to result in an efficiency of 25% on 40?
>
>
>
> I have checked the components using both digital and the trusty General
> Radio 1656 Impedance bridge, where possible. The caps are all measuring
> in
> a consistent manner between all three meters, I can not check the
> inductance
> on multiple meters, the digital types do not have the required
> capabilities
> in uh. They can not go that low, at least the ones I own.
>
>
>
> The input circuit uses an LDG AT-100AMP auto-tuner, which does a real nice
> job, my SWR is always down around 1.3-1.4 or better. I can drive these
> tubes to my targeted value of 250 ma on grid current, on both 80 and 40,
> and
> beyond - no shortage of drive is present. I have typically set up the
> linear on 80 meters to 1200 watts output, the tubes have no issue on
> getting
> there.
>
>
>
> I have a significant number of pictures, and feel at this point I know the
> circuit quite well. If anyone might have any comments or questions to aid
> me, it would be appreciated. I have pulled the tank coil and again
> measured
> the inductance with it sitting on the bench. Unfortunately, all seems
> well.
> No issue was detected. I am thinking of putting it back in and hanging a
> load resistor of 2100 ohms from plate to ground and sticking my MFJ-259B
> meter on the output, and seeing if I can resonate on 7.175 MHz - that is
> my
> next idea, anything you might have as a better approach?
>
>
>
> Oh I do find a peak in output while tuning on 40 meters, just can't think
> of
> anything else to try so that is why I through that out---
>
>
>
> Thanks,
>
>
>
> Chuck W9POU
>
>
>
>
>
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