Half power switch, impedance mismatch question

[markmalin]

Hi all,


I'm building an AC30 and plan to do a 1/2 power switch to remove two of the EL34's.  My question is, I was talking to Patrick at Mercury Magnetics and he said I'd need to use a multi-tap primary so there wouldn't be an impedance mismatch when I switched the 2 tubes in/out.  Is it that much of a consideration? I understand the reflected impedance is going to change, but is it common when switching to 1/2 power that you would also switch primaries on the OT?  Which makes me wonder about these powerscaling devices...they don't take that into consideration if I understand that correctly.  I'd appreciate anyone's input on the topic.

Thanks everyone!
Humbly,
Mark.

[Willabe]

My question is, I was talking to Patrick at Mercury Magnetics and he said I'd need to use a multi-tap primary so there wouldn't be an impedance mismatch when I switched the 2 tubes in/out. 

I think you heard him wrong or he misspoke. Not a multi-tap primary, but a multi-tap secondary.


                  Brad   

 

[markmalin]

My question is, I was talking to Patrick at Mercury Magnetics and he said I'd need to use a multi-tap primary so there wouldn't be an impedance mismatch when I switched the 2 tubes in/out. 

I think you heard him wrong or he misspoke. Not a multi-tap primary, but a multi-tap secondary.

                  Brad   
 

No, it really is a multi-tap primary.  I think it has a 6.6k and a 5.7k primary, and then 4, 8 an d 16 ohm secondaries.  I just don't see wiring up TWO primaries somehow -- seems to me that when people do a 1/2 power switch they would use a standard (appropriate) OT and let there be an impedance mismatch when running only two tubes.  But maybe I'm mistaken??


Mark.

[sluckey]

When you switch out two of the tubes the primary impedance will double. Ideally you would double the secondary speaker load as well. You could do this by using a different speaker cab, or you could connect an 8Ω speaker to the 4Ω OT secondary tap. There are several ways to double the load reflected back to the tubes.

But your whole purpose is to reduce power. Having a mismatch of impedance is inefficient but will reduce power as well.

[VMS]

Little bit offtopic, but I was reading the fender tone-master schematic and owners manual and they encourage you to use the impedance switch together with the ppimv or to reduce the power. However they don't tell or warn you what happens when for example you use 4ohm speaker with impedance switch at HI position.

http://support.fender.com/manuals/guitar_amplifiers/Tone-Master_manual.pdf

http://support.fender.com/schematics/guitar_amplifiers/Tone-Master_schematic.pdf

[Glennjeff]

You could wire that dual Z primary to a DPDT switch (BIGISH ONE 240V 10 A)) to have selectable primary impedances. I have a 5.2 k / 6.6 k primary wired that way in a D'Mars 4 X EL84. With 2 X EL84's 6.6k is still a bit low but OK. With 4 X EL84's 5.2 k is about right or maybe a tiny bit high. Those switches should never be used while the amp is switched on however - big sparks and arcs, possibly blowing transformer or valves. That is why you don't see them - monkey proofing.

The lower impedance tap is typically used in an ultra linear design for the screen connections.

[markmalin]

You could wire that dual Z primary to a DPDT switch (BIGISH ONE 240V 10 A)) to have selectable primary impedances. I have a 5.2 k / 6.6 k primary wired that way in a D'Mars 4 X EL84. With 2 X EL84's 6.6k is still a bit low but OK. With 4 X EL84's 5.2 k is about right or maybe a tiny bit high. Those switches should never be used while the amp is switched on however - big sparks and arcs, possibly blowing transformer or valves. That is why you don't see them - monkey proofing.

The lower impedance tap is typically used in an ultra linear design for the screen connections.

That's an extremely helpful comment!  This is going out to a customer, so he just wants to toggle it to half power - I doubt he'd take the time to power the amp off.


Given that, seems like for this situation the best route is:


- a single switch for removing 2 EL84's from the circuit
- using a single primary (the 5.2k, yes?)
- suggesting he use a double speaker load (I know he has multiple cabinets).


FWIW - I had a Bad Cat 30 on the bench this week and it has a 1/2 power switch.  I didn't look real close (those amps are not easy to trace visually for me...all done with flying components).


Mark.