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What Happens When...(multiple OT taps)


 
11/29/2004 8:46 AM
Ferdinand The Bull What Happens When...(multiple OT taps)
I don't know why I never considered this...  
 
I have several project amp heads that use OT's with multiple secondaries. Instead of a selector switch (like Marshall), I installed three jacks and labelled them 4, 8, and 16 ohm.  
 
What happens (theoretically) when more than one secondary is used at the same time? For example: connect a 4ohm cabinet to the 4ohm jack, and an 8ohm cabinet to the 8ohm jack simultaneously.
 
11/29/2004 9:55 AM
MatthiasM
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Hi Ferdinand,  
 
It's just some simple math with parallel impedances and the (constant) impedance ratio of an OT:  
 
The 4ohm cab on the 4ohm jack gets transformed to maybe 8k (for a PP EL84 amp in this example), the 8ohm cab on the 8ohm tap gets transformed to 8k as well, both 8k loads (= the cabs' impedances transformed up) are parallel, so the power tubes see 4k, which is too low.  
 
If you use a 16ohm cab on the 8ohm tap, the powertubes see it as a 16k load. Add a 8ohm cab on the 4ohm tap, the powertubes see it as 16k, too. Both 16k loads are parallel resulting in 8k -> Everything's fine.  
 
Does this help?  
 
;Matthias
 
11/29/2004 10:09 AM
Bruce /Mission Amps
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Are you sure about that?  
Magnetics has never been one of my stronger suits and sometimes obvious conclusions are wrong.  
 
When having some custom transformers made, I asked this exact question to an engineer at Hammond Manufacturing a few years ago.  
After a few phone calls, with a couple in house conferences and a lot of uhh... uhmm... uhh... it became apparent that the two or three guys there didn't know either.  
 
Save some Hammond OTs, there are not seperate secondary windings in most all OTs so why wouldn't the zed of the lower ohm taps be swamped by the lumped LCR 16 ohm load across the whole secondary and change the overall zed ratio of any winding below the 16 ohm load?  
 
Bruce
 
11/30/2004 6:22 AM
rooster
I'm not at all sure that it is an obvious question, Bruce. I had the same thought about different jacks, and whether you could run off the 4 and 16 ohm tap, f'rinstance, at the same time. It really sounds like a more complex hookup than merely plugging in the taps.  
 
rooster.
 
11/30/2004 8:49 AM
Dutch
email

quote:
"why wouldn't the zed of the lower ohm taps be swamped by the lumped LCR 16 ohm load across the whole secondary and change the overall zed ratio of any winding below the 16 ohm load? "
 
 
Bruce--  
Short answer: because the tapped secondary would act like an autotransformer and the 16 ohm load's stuff would be transformed down to the lower taps' levels. Matthias explained it correctly. Take the example of a 6.8k p-p transformer with multiple secondary taps:  
 
When you hook a 16 ohm load to an 8 ohm tap, the transformer reflects this mismatch to the primary side, which makes an 6.8k p-p load act like a 13.6k p-p load.  
 
Similarly, if you hook an 8 ohm load to a 4 ohm tap, you do the same thing.  
 
Hook both loads at the same time, you get your 6.8k p-p back.  
 
Looking at it another way, if you had a seesaw that was mounted off-center, and you put a skinny kid on the long end, you could have a realllllly fat kid sit on the short end and make it balance. But, if you didn't have a realllllly fat kid handy, you could sit a chunky kid and a skinny kid on the short end, with the skinny kid (assuming they're sitting on the seesaw in the normal manner, facing the other side) behind the chunky one (since you can't sit them in the same spot) and it would balance....  
 
HTH!  
 
C ya,  
Dutch
 
11/30/2004 10:13 AM
Dave H
email

quote:
"Save some Hammond OTs, there are not seperate secondary windings in most all OTs so why wouldn't the zed of the lower ohm taps be swamped by the lumped LCR 16 ohm load across the whole secondary and change the overall zed ratio of any winding below the 16 ohm load?"
 
 
If you assume the transformer is ‘perfect’ i.e. its winding L is high enough to look like an open circuit to all audio frequencies and its stray LC is negligible then it’s only the turns (=voltage) ratio that matters. If two separate windings have the same ratios as a single tapped winding they will work in the same way. (I think) :)  
 
Dave
 
11/29/2004 10:06 AM
Dave H
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The OT primary impedance would be halved. To avoid that you could connect an 8ohm cabinet to the 4ohm jack, and an 16ohm cabinet to the 8ohm jack simultaneously.  
 
Dave
 
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