Parallel output tubes

[jcalvarez]

Hello!

I have noticed some amplifiers using parallel output tubes include one coupling capacitor per control grid (C1,C2 below), while others just connect the grid stoppers together then just one capacitor (C3) to the driver plate.
Is there any advantage of using separate caps?

Regards,
Jose

 

[Kay Pirinha]

In the 1st design you can bias every tube individually, in the 2nd one not.

Best regards!

 

[euro21]

Is there any advantage of using separate caps?

No.
Only one advantage: if one tube has grid-cathode shorting (circa 0.00001% probability) -possible anode current runaway- , the other will survive.

 

[euro21]

In the 1st design you can bias every tube individually, in the 2nd one not.

The second one also, if you use cathode biasing.

 

[jcalvarez]

Thank you all for your advice. I did not think about the biasing options. For fixed bias, top is preferable, self bias, bottom one, less components.

 

[Kay Pirinha]

Component count isn't the important point here, me thinks 😉. For self bias I'd chose individual cathode resistors and capacitors for each tube. Anyway you're right, individual coupling caps aren't necessary then.

Best regards!

 

[jcalvarez]

Component count isn't the important point here, me thinks 😉. For self bias I'd chose individual cathode resistors and capacitors for each tube. Anyway you're right, individual coupling caps aren't necessary then.

Best regards!

Agree. Individual cathode resistors and caps is a must, in my opinion, when paralleling output tubes and using self-bias. Easier to match the tubes.

 

[kodabmx]

When using cathode bias, only one coupling cap from the driver is needed. When using fixed bias, each needs it's own coupling cap.
Also when using cathode bias, each tube should have it's own cathode resistor (or CCS) and bypass cap though.

 

[6A3sUMMER]

The schematics have something missing:

There needs to be a grid leak resistor, one end to the grid stopper, and one end to ground;
Or in the case of fixed bias, one end to the grid stopper, and the other end to the fixed bias voltage.

Since grids have maximum grid leak resistance ratings, the schematic with only one coupling cap - requires a resistor that is 1/2 of the maximum rated value (it has to return 2 grids).

Boats are meant to be floated.
Grids are not meant to be floated.

 

[jcalvarez]

The schematics have something missing:

There needs to be a grid leak resistor, one end to the grid stopper, and one end to ground;
Or in the case of fixed bias, one end to the grid stopper, and the other end to the fixed bias voltage.

Since grids have maximum grid leak resistance ratings, the schematic with only one coupling cap - requires a resistor that is 1/2 of the maximum rated value (it has to return 2 grids).

Boats are meant to be floated.
Grids are not meant to be floated.

I apologise for my laziness, I just put the diagram quickly, did not include leak or bias resistors.

 

[6A3sUMMER]

jcalvarez,

It is OK.
You know, and most of us know.

And thank you for attaching a schematic in your original post.
So many posts should start with a schematic, but do not.

I only mentioned the grid leak resistors for the benefit of Newbies.

 

[jcalvarez]

jcalvarez,

It is OK.
You know, and most of us know.

And thank you for attaching a schematic in your original post.
So many posts should start with a schematic, but do not.

I only mentioned the grid leak resistors for the benefit of Newbies.

And I appreciate your comments, I follow your posts here with interest, they are very informative.