Help me out with an ohm question, please and thanks.

War_in_D

War_in_D

Well-known member
Apologies for the long read but I'm kinda dumb when it comes to all this "Ohms law" stuff and I've usually just matched whatever Ohm setting the cab was, and matched that ohm setting on the head I was using. Easy enough. However, I'm getting ready to start plugging in all my heads/cabs into the head switcher (Delisle 8x8 Deluxe), and was planning on running the Fryette PS100 in the attenuator loop of the switcher to both provide reactive load/attenuation as well as being a common effects loop for all the heads. Problem is, it just dawned on me that one of my cabs is 8 ohm (Mesa Oversixed Recto) and the other two cabs are 16 ohm. Not to mention that one of my heads (Mesa Single Rec) is 4/8 ohm only with no option for running at 16 ohm. I'm looking for the best/safest way to set all this up. There are switches on the back of the Fryette for amp input impedance selection, and speaker out cab impedance selection. The instructions just say to match the impedance of the amp on the Amp In, and same with the cab on the Speaker Out but what happens when you're using the Fryette in the context of a cab switching setup where not all the impedances on the cabs are the same?

Would I need to set all my heads at the lowest common denominator of 8 Ohm (since that is the max Ohm load of the Single Rec), and set the amp input on the Fryette at 8 Ohm (which would then match all the heads) and then set the speaker output at 16 Ohm, since two of the three cabs are 16 Ohm. Would setting the speaker out impedance on the Fryette at 16 Ohm, and running the 8 Ohm cab be a safe mismatch?
 
With reactive loads you can’t assume the same tricks with impedance mismatching of output transformers because of the RLC (resistance, inductance, capacitance) designs within the reactive loads to have resonant peaks at certain frequencies. If you mismatch, you run the risk of damaging your amplifiers or the PS100 due to an unintended frequency response from the impedance mismatch.

With that said, the output of the PS100 itself is just another tube amplifier and not just a reactive load or speaker emulator. I’m not sure however how the output is given - off the secondary from another transformer internal to the PS100, or some other manner.

It’s not the easiest, but to minimize the risk of damage to the PS100, You’ll have to order speakers for the 8ohm cab to re-wire it to 16ohm so that all speaker cabinets are what the PS100 expects.
 
glpg80 said:
With reactive loads you can’t assume the same tricks with impedance mismatching of output transformers because of the RLC (resistance, inductance, capacitance) designs within the reactive loads to have resonant peaks at certain frequencies. If you mismatch, you run the risk of damaging your amplifiers or the PS100 due to an unintended frequency response from the impedance mismatch.

With that said, the output of the PS100 itself is just another tube amplifier and not just a reactive load or speaker emulator. I’m not sure however how the output is given - off the secondary from another transformer internal to the PS100, or some other manner.

It’s not the easiest, but to minimize the risk of damage to the PS100, You’ll have to order speakers for the 8ohm cab to re-wire it to 16ohm so that all speaker cabinets are what the PS100 expects.
Click to expand...

Yeah, the thought had crossed my mind to switch out the speakers but I don't know if that's what I want to do at this point. Good point on the output of the PS100, I may try reaching out to Fryette and see what they say.
 
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