[Central-Electronics] 10B 6ag7 100 ohm resistor question

[Jim Candela]

Steve,

  My first guess is that you are losing your blocking bias, and the 6AG7 pulls heavy current when in 'standby' mode. 

I would scrutinize the bias circuitry first. If C32 is the original cap, it is surely leaky. All those large bypass caps in the 10a,b & 20a transmitters are leaky, and need to go. All the electrolytics need to go too. Your problem could be a flaky contact with the 6AL5 tube socket, or a bad 6AL5. I have had this issue before.

Looking at Bama for the 10b pdf download, CE had several changes to the 6AG7 support components such as a cathode coil added, a series grid 1 resistor added, and they moved the parasitic choke from the grid to the plate. Which configuration does your rig have? 

During 'standby' nothing bad should happen so long as the 6AG7 is biased into 'cutoff' and C37 or C35 are not conducting DC current. If the bias isn't right, the tube might go into a parasitic oscillation, pull heavy current, and 'warm-up' that 100 ohm resistor.

 Does the failure occur when in 'Calibrate' mode where the transmitter is used to VFO SPOT (the T/R relay is not actuated)? If so, the 6AG7 has no load on it, and the darn tube is likely going off on a VHF parasitic. I have run into this issue with some of my QRO 20a projects. I bet the CE mod's on the 10b tried to address this. Another possible fix for this is to add a VHF High Pass filter with 50 ohm termination so that VHF energy has a 50 ohm load regardless of whether the transmitter is keyed or not.

Yes those resistors are fuses. They also use 560 ohm fuses in other areas. Don't replace with WW. Use carbon film 1/2 watt, and your still protected. 

BTW, you can measure across that 100 ohm resistor to measure 6Ag7 plate current (I=E/R). Pull the tube out to see if either C35 or C37 are conducting current. It helps if you know the DC resistance of R34 before you start.

Hope this helps,
Jim
WD5JKO



----- Original Message ----
From: Steve Hobensack <stevehobensack at hotmail.com>
To: central-electronics at mailman.qth.net
Sent: Sunday, November 19, 2006 7:30:44 AM
Subject: [Central-Electronics] 10B 6ag7 100 ohm resistor question

I am on my third r34 100 ohm resistor in the past ten years on my CE 10B. 
This resistor is just before and in series with the plate rf choke that 
feeds the 6AG7 output tube. They seem to eventually go up in smoke when the 
6AG7 output tube goes South. The rig was not in transmit mode when the 
resistor & tube go bad. Funny thing, the tube still checks good on the tube 
tester. Those carbon composition power resistors are getting hard to find. 
Are they used as a fuse in this rig? I am leary of putting in a 100 ohm 
wirewound resistor. In each case of failure, the 6AG7s were RCA. Is there a 
source for these resistors? Is there a more trusty brand of 6AG7 for this 
rig?
Thanks
..Steve...N8YE

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