[Hammarlund] HQ-129x RF Amp. 6SS7 pin 3

[Kenneth G. Gordon]

On 22 Jan 2007 at 20:34, RJ Mattson wrote:

> My HQ-129x schematic shows the suppressor grid (pin 3) of the 6SS7 RF
> amplifier should be tied to ground. On mine, pin 3 is connected to the
> cathode (pin 5). This makes the suppressor always positive.

How do you arrive at that conclusion? Have you measured the voltage?

> Which is
> correct? Pin 3 to ground or to pin 5?

According to my copy of the manual, it should go to ground.

> Was this a mod?

It may be. It may also be a factory change. Think of it this way:

In MOST tubes which have suppressors, at least in most beam-
pentodes, that grid is tied INTERNALLY to the cathode. 

This is what makes those types of tubes unsuitable for use as electron-
coupled oscillators, BTW.

The cathode should be tied to ground, or at least very close to it. 
Sometimes the cathode is raised "above" ground a bit so that the grid 
can biased properly, or so that the stage's gain may be varied.

This is called "cathode bias", in case you may have forgotten.

In the case of the HQ-129X, the cathode of the RF amp stage is first 
bypassed by a big capacitor, C-3 a 0.02 MFD job, then tied to a couple 
of others tubes' cathode resistors, which then all go to the RF gain 
control, labled "Sensitivity", the low end of which is connected to a 100 
ohm resistor, the other end of which goes to ground.

In any case, that external wire from the suppressor pin to the cathode 
pin may not be a mod, but a factory change to make the RF gain control 
more linear since the suppressor grid voltage changes with the cathode 
bias.

Tieing the suppressor to the cathode MAY also make the RF stage a bit 
more sensitive.

If the voltage on the suppressor measures positive, and if the wire and 
solder joints are all OK, that positive voltage will be only a few volts and 
will be identical to the voltage on the cathode.

You might want to test the receiver as wired, then change it to match 
your schematic and test it again to see what the difference might be.

In power pentodes such as the 803, the suppressor voltage can be as 
much as 60 VDC positive.

Ken Gordon W7EKB