Impedance-matching transformer – any cons?

[jimmywaleseswhale]

Hi,

I managed to get a new Marshall Origin 5C for a very decent price. The most controversial part of the amp seems to be the tiny 8' speaker that it's equipped with. I'm lucky to have a 1x12' cab equipped with a Celestion v30 8 ohm speaker which I was hoping to use with the amp. Unfortunately, the Marshall amp only has a 16 ohm out.

I read up on impedance conversion boxes, mostly from Weber. I went on Ebay and managed to snap up a 'Box of Matches' impedance matching device (like this https://web.archive.org/web/20180303233 ... s-30W.html). Unfortunately, the device doesn't seem to be in production and has no manual. My understanding it is built around a T1 transformer. Am I right in my thinking that I just need to plug the amp into the 16 ohm socket on the top row and the cab into the 8 ohm socket on the bottom row?

I guess the best way is to find out is to test it with a multimeter. As far as I know, you shouldn't turn on the amp with no speaker attached, so I probably should test it through the speaker? Would it work if I first measure the impedance of the cab (unplugged), then plug the conversion box and measure the impedance again?

More generally, I'm having some second thoughts about the whole impedance-matching thing after discovering that a used v30 16 ohm isn't much more... However, it's already being shipped. Any experience in using these boxes and thoughts whether they affect the sound much would be appreciated.

Apologies for a load of novice question and thanks for your help!

[xtian]

I think you're worried about nothing. Your 16-ohm tap on the Marshall should work fine with an 8-ohm speaker load (a bit less output power).

[jimmywaleseswhale]

Good to hear! I don't really have a good sense of how resilient are amp transformers and other forums made me somewhat scared (though I'm sure everyone is just passing on dogmatic statements they read on other forums themselves)

[R.G.]

xtian's right. Don't sweat exact matching.

Impedance matching is what the designers in the Golden Age of tube amps did to try to get a useful amount of power out of high-voltage/low-current tubes into low-voltage/high-current speakers. I have seen 1950s charts of tube output power versus varying load impedances on the tubes, and there is a broad, flattish maximum in power out at the exact perfect match. But about an output stage that is otherwise stable is damaged with mismatches of 2:1 or 4:1.

The legend of damaging your tube amp by mismatching speakers has a tiny grain of truth. If your amp oscillates, the high frequencies can overheat the output tubes and/or power supply and cause failures. The design of the whole power amplifier including any feedback affects how much abuse on the outputs it takes to make the thing oscillate. No-feedback tube amps rarely can be made to oscillate, and are just fine with anything between an open circuit and a shorted output in general. Amps with feedback can sometimes be made to oscillate if the output load impedance is too high, hence the advice that you will kill a tube amp by running it with no load. Yep, sometimes you can, and it's a good idea not to do that in case yours is one of the ones that self-oscillates and dies.

But in the case of mismatching of 2 or 4 to one, you can consider mismatching to be something of a tone control with some change in max power.

[xtian]

The only time I ran into trouble, I was demo-ing my 50w plexi-style with Bassman (2ohm) OT, and plugged it into a friends 16-ohm cab, and cranked it to max. Flyback voltage zapped from plates thru tubes, thru heaters, thru 100R faux-heater center tap. Smoke! But no major damage, just had to replace tubes and those 100R resistors.

[ChopSauce]

Not to add anything at all to the above answers to your questions, but just a grain of salt, with respect to the kind of comments that a professional tester may do about this sort of amp:

I really do not know what you mean by "most controversial" but a small speaker is sometimes what's needed for a small amp to sound right at home volume, if this is the intended usage.

Even that way, the amp might be too loud and I've rather been told about people selling their small (5W) Marshall because they are too loud for home use, rather than because of the speaker.

[jimmywaleseswhale]

My impression so far is that it is pretty loud to get to a high-gain 'Marshall' sound but... I don't really play that much high-gain stuff. It also has a low (0.5w?) / high output modes but I must admit that it's pretty loud at high-gain low-out mode.

I was considering getting a cheap attenuator like this one https://www.yerasov.com/catalog/other-p ... muffler-15 https://www.yerasov.ru/sites/default/fi ... fler15.pdf but no idea if it's good. Seems like attenuators range in price between a price of a cheap pedal and 2-3x what I paid for the whole amp!

[jimmywaleseswhale]

Also think it's a fair comment re 8' speaker. Hoping to try out both and see if I still need a cab

[xtian]

Jimmy, you can buy at 50w L-Pad, and it makes a great attenuator for low wattage amps:

https://www.parts-express.com/L-Pad-50W ... hm-260-252

[jimmywaleseswhale]

Thanks, might be a fun project to do! In practical terms, are most passive attenuators basically the same?

[pompeiisneaks]

Not necessarily, I can think of at least two types of passive attenuators.

Some are pure passive, resistive loads and they tend to work well but have definite tone sucking problems especially the more you send signal to the resistors.

Then there are those that use things like speaker coils, and inductors and other passive components that simulate how a speaker works more realistically and reducing the tone suck quite a bit.

One of the more commonly suggested ones here on the forums that's purely resistive is the Ken Fischer designed airbrake:

https://ampgarage.com/forum/viewtopic.php?t=8733

Here's a thread with at least one schematic that shows a more reactive load with inductors and caps etc. (I've seen some with speaker coils but no cone so it can still 'do' what the speaker does in the coil, without making noise.

https://ampgarage.com/forum/viewtopic.php?t=32878

Up to you the path you follow.

~Phil