Tube parallel and matching speaker impedance

[Guus]

Hi Guy's,

A few day's ago I posted a message about 'stand by' switching.
The 'problem' is solved, but it sets me to thinking again.
I'm lost, so please help.

Situation:
I build a 2 stroke guitar amp, single ended, where I can switch from 1x EL 84 to 2x EL84 in parallel. Works realy fine.
What I do is when switching to low power:
- I switch 1x EL84 in mute (lifting cathode from that tube from ground;
- in the same time I switch the OT output to the right impedance (from 2x EL84 to 1x EL84)so to keep a matching of 8 Ohm.

I wonder:
When I put 1x EL tube (from the two in parallel) in 'stand by', does this change the impedance to the speaker?

I thought yes, but now I'm hesitating because:
- when I pull the tube, obious yes the impedance changes;
- but when the non active tube stays in the socket with leaving on a connection on the plate and screen?

regards
Guss

[tubeswell]

Yes the 'inert' tube is not conducting, so there is only 1/2 the amount of tube current, so all other things being equal it wants to see 2 x the load at the same plate voltage

[Guus]

Hi Tubeswell,

Thank for the respons.
I begin to see some glow in the dark.

If I understand your message:
- when there is floating no current in de 2e parallel tube then the OT see's only 1x EL84.
This is the case when de cathode of the tube is completely lifted from ground. Plate/screen voltage can be connected and have no impact because there is no circuit.

But the situation that:
- I connect the cathode of the tube, that I don't use, via a high value resister (say 47kOhm) to ground. In that case there flows only a few milliamps but there is a connection/circuit.

So I think: the amount of mA doesn't play's a rule but the OT see's the (intern) resistance of both tubes? One tube a little more then the other.
So I guess, when a resister (even a high value) is connected from cathode to ground the OT see's 2x EL84?

Right?

Guss

[Firestorm]

Only AC passes from one side of OT to the other so if the tube is not passing signal it essentially isn't there from the OT's perspective.

[martin manning]

Only AC passes from one side of OT to the other so if the tube is not passing signal it essentially isn't there from the OT's perspective.

In this case the second "OT" stands for "other tube" ;^)

[Guus]

Hi Firestorm,

once again you helped me out.
Thank you very much.

Guss