There is a possibility that the 12BY7 is faulty (excessive grid emission), despite the fact that it seems to function ok, in the FT-101. Perhaps the dc grid circuit resistance is lower in the FT-101 vs the TX-1??
A problem that can occur in "high milage" tubes, involves migration of the cathodes oxide coating, to the control grid. The rate of this migration problem is greatly accelerated if the heater voltage is excessive, a common situation with todays higher line voltage levels.
The unwanted oxide coating, on the control grid, causes it to begin emitting electrons--not a desired function of the control grid!
This type of failure can manifest itself as a run away type condition. The tube seems to function ok, when "cold", but when the grid temperature rises sufficiently (due to heat radiated by the cathode), grid emission commences. Perhaps you checked the control grid voltage, when the tube was cold?? If grid emission is the cause of the problem, I would expect the grid voltage to rise in a positive direction, and the plate current to rise substantially, as the tube heats up.
GL & 73 de Ron
From: "tvman1954@..." <tvman1954@...>
Sent: Friday, October 18, 2013 8:05 PM
Subject: [Heathkit:Yahoo] TX-1 Apache Problem Brand New Member
I have mostly restored this TX-1. Big Iron and Lots of Plate Modulation. But Modulation is the problem. I have changed the Cathode Resistor and Cap to ground. Isolated the Input Grid with another New .01 Capacitor. Pin 9 of the 12BY7 is Grounded as it should be. B+ voltage is present on the Screen and the Plate through the Driver Transformer. I have purchased and replaced the Audio Driver Transformer Twice.
On my Scope, the Input Audio Signal is Nice and Clean on Pin 2 of this 12BY7 tube. In my mind and now out of my mind, I think that everything has been done. When placed in my FT-101 the 12BY7 Tube works great so we know the tube is good.
The problem is really Distorted Audio from the Secondary of the Audio Driver Transformer and also the Plate of the Tube will turn Cherry Red quite quickly. Pin 1 the Cathode of the tube is going to over 180 volts instead of 10 volts as the Schematic references. Gentlemen, this thing has me Stumped. It is apparent that the tube is turned on hard and drawing lots of current. With 0 Volts on the Control Grid pin 2 I can't see why it wouldn't be doing that. Please Help. I have never seen a Pentode wired as a Triode much less Zero Bias on the Grid..... Help ... Help... What am I missing? Someone has surely had this problem..... Also a Brand New Member here just waiting to help others....
73 from Wade/KJ4WS