Potential impedance mismatch problem

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mnemonic
Bottle Rocket
Posts: 2
Joined: Tue Feb 06, 2018 8:41 am

Potential impedance mismatch problem

Post by mnemonic » Tue Feb 06, 2018 9:01 am

Hey guys, new poster here but I’m have a peculiar issue with my recently purchased 3 channel Dual Rectifier (non-multiwatt).

Im using a 16 ohm cab off the 16 ohm tap, and I am very happy with the sound. Yesterday I decided to try out a ‘safe mismatch’ as described in the manual, by hooking my 16 ohm cab to the 8 ohm tap. I really liked what I heard, it was a bit louder (kinda like flicking from tube to diode rectification is a bit louder), it was puncher, tighter, a bit stiffer, basically sounded better in every way.

This is at odds with what the manual (and various forum posts I’ve read) said would happen though. The manual says:
MIS-MATCHING: When running a higher resistance (for example: 8 ohm output into a 16 ohm cabinet), a slightly different feel and response will be eminent. A slight mismatch can provide a darker smoother tone with a little less output and attack. This response is a result of the amplifier running a bit cooler.
So far my experience is the exact opposite of this. The darker, smoother, less output sound is running 16 ohm cab on the 16 ohm tap.

I know my cab is definitely 16 ohms, I wired it myself (two 8 ohm speakers in series), I even measured it with my multimeter today and it measures 14.2 ohms resistance which is about right for a 16 ohm cab.

Is this experience typical? Or could something be wrong with the 16 ohm tap on the transformer?


For reference I’m using orange and red channels on Modern, at loud bedroom volumes (loud enough to rattle the walls but not enough to deafen myself), FX loop is on, using tube rectifier, powertubes are new as of last week (Mesa 6L6GC STR440) but rectifier tubes are probably original (I’d guess 15 years old). Cab is a 2x12 with Celestion V30 (out of a Mesa cab) and Celestion G12K100, both 8 ohms and wired in series.

Thanks for any help guys

IronSean
Mark III
Posts: 237
Joined: Mon Sep 29, 2014 12:17 pm
Location: Canada

Re: Potential impedance mismatch problem

Post by IronSean » Tue Feb 06, 2018 10:46 am

Maybe try it in vintage mode and compare? Modern mode removes the negative feedback between the speakers and the power amp, so the way a mismatch behaves in that mode might differ possibly.
Mark V
Studio Preamp
formerly: Quad Preamp, Rocket 44 112

mnemonic
Bottle Rocket
Posts: 2
Joined: Tue Feb 06, 2018 8:41 am

Re: Potential impedance mismatch problem

Post by mnemonic » Tue Feb 06, 2018 1:50 pm

IronSean wrote:Maybe try it in vintage mode and compare? Modern mode removes the negative feedback between the speakers and the power amp, so the way a mismatch behaves in that mode might differ possibly.
Good call, I just tried it on channel 3 in vintage mode and 16 ohm into 16 ohm cab sounds louder and puncher than 8 ohm into 16 ohm cab. The mismatch is slightly quieter, less punchy and a bit darker.

I guess you’ve got it right, must be something about the lack of negative feedback in modern mode that makes it react different to a mismatch, at least at the volume I’m playing at.

Crisis averted! Thanks for your help!

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