Does a #124 style half power switch alter output impedance?

[odourboy]

If you have a 100W amp with a switch arrangement like #124 for lifting the cathodes on two of the output tubes, does the reflected impedance through the output transformer also get changed if you go to the 50W setting?

[llemtt]

yes

you need to move your cab from 4 to 8 (or 8 to 16) to compensate

[talbany]

If you have a 100W amp with a switch arrangement like #124 for lifting the cathodes on two of the output tubes, does the reflected impedance through the output transformer also get changed if you go to the 50W setting? :shock:

Not really.. He puts in a 10k in series to ground on the 50w setting this doesn't completely shut off the tube..Just keeps it idle so the impedance doesn't change pretty cool.. Hope it helps!!

Tony

[Structo]

Here is the schematic for the half power switch.

[odourboy]

Thanks guys!

So I have two differing answers from two respected members of the forum. Do we have a tie-breaker?

[ChrisM]

I'm with Tony on this but my opinion doesn't mean much.
Technically I think the two tubes that are "shut off" aren't really shut off, but barely conducting.

[odourboy]

My gut tells me that if they're not conducting, then they're also appearing as a high impedance in the circuit. ....But my gut has been known to be wrong.

[talbany]

https://tubeamparchive.com/viewtopic.ph ... wer+switch

Tony

[odourboy]

https://tubeamparchive.com/viewtopic.ph ... wer+switch

Tony

Good enough!

[mad_dog]

another question is if your are running it at half power most of the time would you still be wearing out the 2 tubes that are barely used??

Would it be better to just pull the tubes and switch the impedance rather than waste 2 tubes that are never used???

[talbany]

Not really..Since they are hardly conducting they wouldn't see much in the way of wear.. The main issue here would be that the quad would eventually become un-balanced (the functioning set would start to draw more current at idle) you could help this by switching out the set that was turned off or barely conducting with the functioning set every so often so they both have roughly the same amount of hours.. This would help prolong the quads life and keep them better matched..

Tony

[Structo]

I am curious about how this setup sounds?
Those of you using this switch, what is the difference in tone between the full quad and the half power position?

How much does it change the character of the amp?

I know we say there is only a 3db difference in volume between 50 and 100 watts, it seems to me since that difference is small, it would be more of a feel and tone thing.

[talbany]

I am curious about how this setup sounds?
Those of you using this switch, what is the difference in tone between the full quad and the half power position?

How much does it change the character of the amp?

I know we say there is only a 3db difference in volume between 50 and 100 watts, it seems to me since that difference is small, it would be more of a feel and tone thing.

No real easy answer here..If your a livingroom player who doesn't like allot of volume..Don't worry about a 100 watter..

IMO The difference in tone propagated by the extra output tubes changes depending of coarse on the volumes your running (Tubes pulling current) .. For instance your not really going to notice a big change in tone and headroom at wife pleasing volumes..Aside from a slight 3db boost and a slight bump in low end say around 100 Hz that's pretty much all you get with the amp cracked..
Get the amp at gig volumes where the tubes begin to pull some current this is where the impact begins... The extra current generated by the extra tubes X2 (since output tubes amplify current) In this context, we're talking about Magnetic Circuits -- not electrical or voltage circuits. The equivalent of applied voltage in electrical circuits would be Magnetizing Force in magnetic circuits. The equivalent of a current flow is Flux Density.. This is the fundamental force that excites the secondary.. More power on the primary, greater flux density, more induced on the secondary, etc..
This increase in flux allows for greater bandwidth increase touch sensativity...Also at higher volumes you don't get the effects of tube loading as quickly since your doubling the current handling capability,that being of coarse compression saturation and distortion..This is why 100w are punchier tighter and deliver greater headroom.. Some people like the anomolies associated w/50W others prefer it generated at the preamp level..

So yeah it is a feel and tone thing with the impact varying depending on how you set it up..If I could use the analogy .. Think of it like putting a set of high performance tires on your car..At 25mph it's a waste of money.. At 100 your glad you bought em!!
Tony

[BobW]

FWIW, at gig volumes (small or larger venues), I prefer the 100W mode. The difference in 50W vs. 100W seems even more apparent when using the dumbleator.

that's a good analogy Tony.

[martin manning]

If you have a 100W amp with a switch arrangement like #124 for lifting the cathodes on two of the output tubes, does the reflected impedance through the output transformer also get changed if you go to the 50W setting?

The impedance at the OT primary is not going to change, it's still going to be speaker impedance times the turns ratio squared.

But, if half of the output tubes aren't trying to shove current through the OT, I believe the other half will see it as a reduction in load. I'm with llemtt on this one.

MPM