resistor on 6jb6 screen?
 

Hello,

I am restoring an "old" drake TR-6, I just noticed a 100 ohm resistor
connected between pin 1 and pin 7 of
the three 6JB6 finals. I understand these two pins are both connected
internally to the screen grid, so
I wonder what's the purpose of those resistors which should just be
shorted by the internal connection
of the tubes. Looking at the TR-4 schematics I saw they have 68 ohm
resistor in the same place, so it
surely has a meaning but I can only guess it's a sort of protection in
case of a failure.
Does anyone know what they are for?
Thanks and best 73

Francesco IS0FKQ

resistor on 6jb6 screen?
 

I am restoring an "old" drake TR-6, I just noticed a 100 ohm resistor
connected between pin 1 and pin 7 of
the three 6JB6 finals. I understand these two pins are both connected
internally to the screen grid, so

Looking to the schematic diagram, that resistors are in series to the screen, so originally they could not be shorted by the tube.

Are you really sure that the resitors are connected between pin 1 and pin 7? They often use the unsed socket pins as standing posts. So, if the resistor would instead be connected between pin 1 and pin 9, then all would be explained (pin 9 is unused).

Does the wire taking voltage to the screen go directly to pin 1 or pin 7?

Another explanation is that the previous owner, wanting to have a little more power, moved the resistors from pin 9 to pin 7, so effectively shorting them. In this way he could easily restore the original configuration by just resoldering the resistors and the screen feed wire.

73

Antonio I0JX

resistor on 6jb6 screen?
 

Antonio Vernucci ha scritto:

I am restoring an "old" drake TR-6, I just noticed a 100 ohm resistor
connected between pin 1 and pin 7 of
the three 6JB6 finals. I understand these two pins are both connected
internally to the screen grid, so

Looking to the schematic diagram, that resistors are in series to the screen, so originally they could not be shorted by the tube.

I have the original schematic and it shows the 100 ohm resistor between
pin 1 and 7, you can find the
TR6 schematic at the following link (look at the last page of part 2):
ftp://bama.sbc.edu/downloads/drake/tr6/
You can see the 100 ohm resistor between pin 1 and 7 of each final tube
and the additional 100 ohm
in series connected at pin 7 of each tube, which isn't actually correct
as in my unit only one tube
is connected this way, the other two are connected to pin 1.

Thank you and best 73

Francesco IS0FKQ

resistor on 6jb6 screen?
 

wrote:
Antonio Vernucci ha scritto:

I am restoring an "old" drake TR-6, I just noticed a 100 ohm resistor
connected between pin 1 and pin 7 of
the three 6JB6 finals. I understand these two pins are both connected
internally to the screen grid, so
Looking to the schematic diagram, that resistors are in series to the screen, so originally they could not be shorted by the tube.

I have the original schematic and it shows the 100 ohm resistor between
pin 1 and 7, you can find the
TR6 schematic at the following link (look at the last page of part 2):
ftp://bama.sbc.edu/downloads/drake/tr6/
You can see the 100 ohm resistor between pin 1 and 7 of each final tube
and the additional 100 ohm
in series connected at pin 7 of each tube, which isn't actually correct
as in my unit only one tube
is connected this way, the other two are connected to pin 1.

Pin 1 and pin 7 probably connect to opposite sides of the screen. Perhaps
the inductance of this connection creates an opportunity for a VHF parasitic
resonance, and the 100 ohm resistor is there to dampen it out?

-Chuck

resistor on 6jb6 screen?
 

Hello,

Chuck Harris wrote:


Pin 1 and pin 7 probably connect to opposite sides of the screen. Perhaps
the inductance of this connection creates an opportunity for a VHF parasitic
resonance, and the 100 ohm resistor is there to dampen it out?

Yes, this can be the reason, thanks for the hint!
Probably is anyway a good thing to add similar resistors in a home made
tube linear.

73

Francesco IS0FKQ