[Amps] GS35B Amps Common Problem / few more comments
Jim Thomson
jim.thom at telus.net
Thu Nov 7 12:00:50 EST 2019
Date: Thu, 7 Nov 2019 09:01:52 +0900
From: Jeff Blaine <KeepWalking188 at ac0c.com>
To: amps at contesting.com
Subject: Re: [Amps] GS35B Amps Common Problem / few more comments
<The grid on the 3000 is indestructible where the 8877 needs good
<protection.? On the other hand, the output C of the '3000 is quite a bit
<higher than the 8877 (about 25 pf vs 10 pf) and for a guy cooking a
<mulitband amp, it's a factor when it comes to building the output net.?
<An L-PI will fix it, but I mention that because it's a complicating
<factor.? Definately not the right choice for a first build.
<73/jeff/ac0c
## output C on the 3000 is exactly 24.0 pf per eimac.
I measured 24.0 pf on both my svetlana 3000s.....on a wooden
bench, with my LCR meter.
## However, that increases to 33 pf.... with tube in socket.
The extra 9 pf comes from proximity of lower anode fins ...to chassis.
## a simple L-PI solves that issue. Easily designed on the GM3SEK
PI /PI-L spread sheet.
## My 3000 covers 160-15m..... no 10+12M. I used a .66 uh coil,
made from 7 turns of .375 inch OD cu tubing...with a 1.5 inch ID. Wired
between plate block caps....and C1 tune cap.
## The 33 pf C between anode + grid.... plus the .66 uh coil, forms a step
down L network on 15M. That reduces the plate load Z...to a much lower value.
Low enough, that I can use a Q of 8 on 15M for the main PI net. PI net is
designed around the lower....transformed plate load Z.
## The 15M tap on the main tank coil is re-used on 17M. Q rises to 12....on 17M band.
## That L-PI...... and also L-PI-L trick has been used on 10m, 12m, 6m, 15M.....and
on tubes with a LOT higher anode to grid C. YC-156, 6000, 10,000, 15,000, 20,000 tubes.
The worse case I designed for was a pair of 15,000s which had 120 pf from anode to grid. 60 pf
per tube.... in their sockets.
## RF parts sells both the socket... and their newly designed superb chimney. Socket is good for 100A....and
also used on the 6000 tube.
### Dayton list a small blower that is rated for 80 cfm at 1.2 inch pressure. For 4 kw CCS max anode diss, 67 cfm at 1.2 inch
required. Thats with an intake air temp of 104 deg F.
## For typ ssb + cw use... and 70-75 F shack temps... WAY less airflow required.
200w drive = 5.5 kw out. Input Z of the 3000 is 50 ohms. PI tuned input is simple.
I used a pair of broadcast variables + tapped 4 uh coil. 6:1 jackson bros vernier dial drives
on both broadcast caps. Input swr is 1:1 from 1.8 to 25 mhz 4 uh coil made from
17 Turns of 6 ga cu wire.... with a 1.5 inch ID....suspended in air between the caps. Only
2 uh used for 80m, etc. Again, PI spreadsheet can be used for PI tuned inputs. Tested with
800w CCS cxr..through the hb tuned input..then bird..then DL. Beware, the spreadsheet will not
handle 2 equal Zs... so use 50 ohms on the input, xcvr side..and 49 ohms on the output, cathode side.
## Only grid protection I used is a fast 3AGC fuse, on rear panel, wired between chassis..and negative
terminal of grid meter. IF grid fuse opens, no path for DC grid current, amp cant be driven, PO drops to zero.
Input SWR rises to infinity, xcvr po drops to zero.
## HV fuse in series with a 50 ohm glitch R.... in B+. Grid diss is actually a lot more when in GG. Grid is
bonded to cold chassis..via finger stock. 6000 also uses the same 225 w grid.
## 3.5 to 6 kv works good on a 3000. Measure the fil V via a pair of small chokes..at the cathode. Fil V set
for 6.5 vac. Tube will last forever.
Jim VE7RF
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