some help with my amp

walter · Guest

i'm under the impression that my marshall's not performing to its total capability, i later realized that the power level output mode is switched to low, with the impedance at 8.
can i solve my problem by switching it to high mode, and does that mean i have to alter my impedance along with it or do i leave it at 8?

TimC · Posted: Tue May 13, 2003 10:53 am Post subject:

Hi Walter. I've never owned an amp that had this type power selection. The owners manual should explain about switching impedances when switching power output. I imagine it simply cuts two of the power tubes out of the circuit. If it does this, then the switch should be set to half of what it is with all four tubes. If the amp has two 16 ohm speakers, the normal switch position would be 8 ohms. Cut out two power tubes, and now the switch would be set to 4 ohms. But now the speaker load is wrong for the output transformer. Adding an 8 ohm extension cabinet would then match the speaker load to the output transformer.
With an amp that can cut off two power tubes, it would seem wise to regularly switch the inside and outside power tubes. This way they would all wear out together. Ken Fischer explained that Marshalls sound best turned way up. In the old days, players would yank two tubes to play small venues where the glass would be blown out with full power.

walter · Guest

i appreciate your help, unfortunately...i don't have a manual (bought it used). the impedance switch is at 8, the opposite position indicates 4 & (16). i don't wanna risk anything by switching the output level to high without being sure, understand. i've got a feeling the 8 is where it should be, should just switch it to high......i don't know. think i should?
thanks again.

TimC · Posted: Wed May 14, 2003 6:12 am Post subject:

Walter, is this a combo or just a head? If you post the model type, maybe someone with an amp like yours can help out. If it's a combo with two speakers, set the switch for 1/2 the impedance of what's written on one speaker. Set the power switch to high, and rock on. If it's a single speaker amp, match the impedance switch to what's on the speaker. One way to check the impedance of unmarked cabinets is to set your multimeter to ohms and test at the amp end of the speaker cable. The reading will be lower than the actual impedance since you're just measuring resistance, not impedance. ( about 3/4) This works for single speakers too.

paganicon · Posted: Wed May 14, 2003 7:07 am Post subject:

I had a JCM 900 with half and full power switch.. I'm pretty sure the impedance doesn't matter. It depends on the cab.
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walter · Guest

done. thanx a lot!

TimC · Posted: Mon May 19, 2003 4:53 pm Post subject:

In an earlier reply about impedance matching after removing two power tubes, the information was taken from an old amp book I had. After further research, it seems the impedance of the speaker load should be doubled if nothing else is done. Players used to disconnect one speaker of a two speaker amp when removing two power tubes. Maybe the amps with power switching have separate output transformers or dual primary
windings and the match between secondary and speaker would be unaffected. If someone can explain this, I'd sure appreciate it.