Impedence for Matchless at half power?

[Leonc]

My Matchless amp was purchased used earlier this year; it didn't come with a manual and in fact, I've never seen a manual for a Matchless amp. It has a half power switch on the back and 4-8-16 rotary click switch. It's a combo amp in a 1x12 cab, loaded with an 8ohm Matchless speaker which plugs into an internal speaker jack. There's also an external speaker jack.

What I don't know is if you're supposed to switch the output down to 4ohm when playing at half power. Anyone know?

[LSchefman]

This is just my common sense guess, but the impedance switch is designed to match the amp to whatever speaker you're using.

I can't imagine that chaning the output power level would require a change of impedance, unless you're using a different cab with a different impedance when you switch to lower power.

But double check before you take my advice!

[Leonc]

Actually I know of at least two amps with this requirement: the Bogner Ecstasy Classic and the ÷ 13 FTR 37 (and I think the ERT 33 too).

[Mark Kane]

Leon, your right, set it to 4ohms when switching to half power.

[Leonc]

Mark, I was going to write you because I wasn't 100% clear on your original reply. Thanks a bunch for clearling it up!

[Mark Kane]

Leon, I've just been talking to a friend about the impedance mismatch in the bluesbreaker reissue amps and to correct that you set the switch higher, I wasn't thinking when I put up the first post and then it hit me what I'd said. It's the opposite situation. Anyway, those are tough trannies, but you should hear a noticable difference with the switch moved.

[John Phillips]

You switch to half the impedance (ie 4-ohm setting when running an 8-ohm speaker). This is because you need to effectively double the speaker impedance to compensate for the doubled tube impedance (since the pairs are in parallel, all four have half the impedance of two).

It's not absolutely essential - all that happens if you don't is that the remaining tubes are running into half their matching impedance, which wears them faster. The transformer is less efficient due to the mismatch, but the total power being passed is halved, so the overall heat dissipation is still lower.

This applies to any 4-tube amp that you run with only 2 tubes BTW.

[Leonc]

John thanks--I knew I could rely on your for an excellent explanation!

[Leonc]

Yeah, I'll tell Scott you're worth at least twice of whatever he's paying you to do this job .

[LSchefman]

I'm glad you double checked. I was taking a guess. And it proved to be a VERY uninformed one!

Thanks for setting me straight, guys!

All EL84 (and EL34) based amps by Matchless has a Outputtransformer primary impedance of 4K Ohms. The DC30, Jorgensen, Lightning, Spitfire, Hurricane and Tornado.

So switching it to half power you should go with the same speaker impedance.

How ever, setting it to 4 Ohm when using a 8 Ohm speaker, makes the tubes work under the same load as they would at 4 Ohm setting to a 4 Ohm speaker, in full power mode.

2xEL84 likes to see a 8K Ohm load.
4xEL84 ......................4K Ohm load.

Try it out and listen what suits you best, it won´t ruin your amp!

Rock on !

http://www.mp3.com/DIRTYLKARLSSON

[Darby Crash]

I'm reviving an old topic here, but I'm still confused. Someone told me that you should halve the impedance if you pull a pair of tubes to lower the power, but not if you use the half power switch. My ESD cab is wired in series for 16 Ohms; should I decrease the setting on the HC-30 to 8 Ohms when the half-power switch is on? Thanks.

[John Phillips]

Darby, that 'someone' is wrong, because pulling two tubes and using the half-power switch do exactly the same thing - switch the amp from four tubes to two. The advantage of the switch is that you don't have to fiddle around pulling tubes and putting them back again.

I would set it to 8 ohms when using a 16-ohm cab at half power.

[Darby Crash]

Okay--thanks for clearing that up!