Hello Bob:
Just got back from the 4th weekend, and noticed your e-mail and the
commentary on the B&W 852 tank coil. I just finished an amp using this tank
coil in April of this year, so I hope I can share a few items that may
provide some help for your project.
First, I bought the tank coil to use in a linear utilizing a pair of
3-400Z's. I have been hauling around six brand new tubes since 1969, so it
became the time to use them when I started my amp project in 2006. I bought
a beautiful 852 coil on ebay and thought based on the specs it would be
perfect for the expected plate load impedance I would be dealing with on the
pair of 3-400Z's amp. I was wrong.
I agree that you have the correct inductance values, as listed in your
e-mail. I have an original B&W brochure from about 1965, and the B&W
figures and yours match fine. I measured my coil, using a calibrated
General Radio 1656 Impedance bridge, and I measured the same values as
listed in their brochure, within .05 uH. I am puzzled by your measurements,
since mine were quite similar to the advertised values.
The snag I ran into was the total inductance, when operating on 80 meters.
If you select a particular plate load impedance and calculate the required
inductance for whatever plate load and Q, all of a sudden you will see that
10-40 meters fall into place, but 80 is way short on inductance.
Use any of the on-line pi-net calculators, or the one in any ARRL handbook
after 1996. The formula prior to 1996 handbook editions was a bit off, and
a fellow down in Texas offered a correction during the 1980's that was
finally accepted in 1996 in the ARRL Handbook.
I ended up cutting out the 80 meter coil section, and inserting additional
turns. My final inductance was 9.4 uH for 80 meters and all is well with my
amp, on all bands.
Back in mid 2006, several people on this list server commented on the fact
that the 852 was a "screwed up" tank coil. At the time, I did not fully
appreciate their reasons. After working with this 852 coil, I now would
have to agree with those comments from back in 2006 when I started this amp
project.
Bob, if you want pictures of the modified tank coil, let me know and I will
send what I did to your attention.
Chuck, WA9POU
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Bob Maser
Sent: Thursday, July 03, 2008 10:42 AM
To: AMPS
Subject: [Amps] B&W 852 Impedance Measurements
The B&W 852 that I bought arrived today and is in good condition. I took
some inductance measurements for each of the band positions and found that
all of them are low as compared to what I thought they should be.
Original Specs Actual measurement
10 .95 uH .61 uH
15 1.34 .96
20 2.34 1.67
40 3.72 2.56
80 7.00 5.41
My measurements were made on a AADE unit which has always been very
accurate. The 852 was not mounted on the aluminum chassis, just sitting on
a wooden bench. I cannot see any signs of alteration, it all looks stock.
In order to use this 852, the Q would be between 15 and 20, very sharp
tuning. The Original specs correspond to a PLI of 2000 with a Q of ±12.
Any comments would be appreciated.
Bob Maser W6TR
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