Jtm 45 attenuation

midiglitch

Hi folks, Loving the new forum - looks great. great job, well done and thank you to all those who made it happen. Quick question regarding attenuation on my jtm45ri. I could do with just taking it down a notch in certain circumstances. I'm not talking gig levels down to bedroom levels -far from it, just loud gig down to quieter gig for smaller venues and when playing with our very light handed female drummer. I'm playing into a 16ohm cab. If I was to make a 16ohm load just from, say, a 100w resistor and run that in parallel with the cab (setting the amp output to 8ohms) would it: A) work and be safe for the amp? have a noticeable impact on the Volume of the amp at a given setting? Hope that makes sense? Cheers

midiglitch

I don't know why my post has formatted itself without any line breaks. That makes it hard to read, sorry.

matone

Have a look at the Powerplug attenuator ,it`s a simple resistor based unit,cuts either 6 or 12 db and would possibly do what you want.
They have a good website with clips.
I bought one to try and tame an 18watt head for home use but it didn`t cut enough volume for
that scenario.
hth.

goldtop

Your plan will work fine, although half power is only -3dB and that's subjectively not very much reduction. You must also build the 100r power resistor into a decent chunky metal box and heatsink it to the chassis properly.

Maybe borrow a HotPlate for your next gig and try that at the -4dB and -8dB settings. This forum software doesn't show location during browsing, so I don't know where you are, but if you're near Cambridge I can lend you mine.

ICBM

Yes, a parallel dummy load will work and is safe for the amp, but it won't have very much effect on the total volume. A parallel load of the same resistance as the speaker impedance will only cut 3dB - although it might sound a bit more than that because a plain resistive load will have a proportionately lower impedance compared to the actual speaker at high frequencies, so you'll lose a little more treble than bass. To get a rough impedance match you want a 22-ohm resistor, not 16 - the average impedance of a speaker over the audio range is a bit higher than the nominal value.

You can also go further and use a parallel load of about 6 ohms (the theoretical figure would be 5.33) with the amp set to 4 ohms; that will cut about 7dB.

midiglitch

Great replies, thanks chaps. Gold rocker - I'm in the new forest but thank you for the offer. Are there standalone tonal differences when the amp runs at 4ohm vs 8ohm vs 16ohm mode? (If it is even possible to identify those independently of speaker changes / attenuators effects etc.)

ICBM

midiglitch said:

Great replies, thanks chaps. Gold rocker - I'm in the new forest but thank you for the offer. Are there standalone tonal differences when the amp runs at 4ohm vs 8ohm vs 16ohm mode? (If it is even possible to identify those independently of speaker changes / attenuators effects etc.)

Yes, there will be differences, but as you say, identifying the individual change - transformer tap, speaker impedance, even the wiring configuration (eg running the same pair of 8-ohm speakers in series to give 16 ohms or in parallel to give 4 - which changes the effective speaker damping as well as the impedance) makes the question a bit moot since you can't change one without at least one of the others. It will sound different to some extent no matter what you change.