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Half Power switches and speakers

lhrocker

Member
Messages
116
If your amps have a half power switch, do you have to change the output impedance of the speakers, like when you pull a pair of power tubes?
 

BCJek

Member
Messages
902
It depends on the circuitry used to drop the power to "half". Some amps do remove two of the 4 power tubes from the circuit [often by lifting the cathodes from ground] and that will require an impedence change. Others use lower B+ voltages, which does not require a change. Others again use triode/pentode switching on the output tubes, whcih does not require a change.

Which amp are you thinking of?
 

rockon1

Platinum Supporting Member
Messages
13,181
My Peavey Ultra Plus cuts out two of thye four power tubes. Technically speaking the impedance should be changed to match but evidently Peavey isnt worried about the mismatch as they do not mention it in the owners manual. Its got big beefy iron. Bob
 

lhrocker

Member
Messages
116
It depends on the circuitry used to drop the power to "half". Some amps do remove two of the 4 power tubes from the circuit [often by lifting the cathodes from ground] and that will require an impedence change. Others use lower B+ voltages, which does not require a change. Others again use triode/pentode switching on the output tubes, whcih does not require a change.

Which amp are you thinking of?
My Carvin Legacy has a half power switch. Can I just throw the switch without worrying about the speaker impedance?
 

Austinrocks

Member
Messages
7,020
mesa mark iv is not a power switch, you can lower the power several ways on the amp, simulink is 4 tubes 85 watts, class A is 2 tubes 50 watts, tweed will reduce the power by about 80% compared to full power, the speaker does not change,

when I changed the 6L6s to 6v6 which have half the power I also changed the 8 ohm speaker to the 4 ohm output on my mark iv, not a big deal the mesa techs were not sure when I asked them, they added it to the manual I noticed after I asked them.


I also do this on my fender Hot rod deluxe, a dummy plug in the extension speaker jack will change the transformer from the 8 ohm tap to the 4 ohm tap, like on the mark iv, the JJ6v6s will replace a 6L6 or EL-34 if you rebias it, the plate voltage is similar, the power is half, so you can use them to reduce the power of the amp, and still keep the power tube sockets filled.
 

rockon1

Platinum Supporting Member
Messages
13,181
My Carvin Legacy has a half power switch. Can I just throw the switch without worrying about the speaker impedance?

Like my Peavey Ultra Plus they are evidently not worried about it. They make no mention of it in the manual. Not surprizing as most tube amps will handle a 100% mismatch no problem. Bob
 

rick13

Silver Supporting Member
Messages
769
Most of the boutique amps are overbuilt....so some mismatch isn't an issue. When I spoke to the guys at Two Rock, they said to change the impedance when I went to half power. They also said that since the transformers are oversized....it's not really as much of an issue as we tend to make of it in the forums.
Rick
 

bish0p34

Member
Messages
327
Anyone know about a Triamp... I have a 16ohm cab, but I run it on half power... What ohm setting should I use?
 

rockon1

Platinum Supporting Member
Messages
13,181
Most of the boutique amps are overbuilt....so some mismatch isn't an issue. Rick
Almost any amp will tolerate a 100% mismatch-not just boutique amps.Certainly any amp that has a 1/2 power switch that cuts out 2 of the tubes will. They wouldnt put a 1/2 power switch on an amp expecting guys to change the impedance every time they used it IMO. Heck,the 1/2 power switch on my Ultra is even on the front panel. Bob
 




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