Ohm mismatch

swlabr

Hi everyone

Just got hold of a lovely Vox AC4C1 BL. The celestion vx10 is a 10 inch 16 ohm speaker. According to the instructions the speaker out should only be used with 16 ohm cabs, but people at TGP and TPRI have been trying them out with 8 ohm speakers.

According to a post at offsetguitars the ac4tv is capable of giving a 8watt output with the mismatch.

I've come across an ibanez tsa112 cab with an 8ohm speaker in it for half the RRP at amazon.es

I was wondering if experts here (as I don't trust those posters) would consider it safe to use it with the mismatch. I would eventually consider upgrading the celestion seventy 80. Otherwise I will be returning it.

Cheers and Happy New Year to all!

Edit: changed the link which was wrong

lustycourtier

I gig week in week out with a cranked ac15 with a 15 ohm blue, and the amp is rated to 8.

ICBM

It would be interesting if it was mismatched as stock since the type of sound the AC4TV produces - which personally I did not like - is actually quite characteristic of a high mismatch, come to think of it! I didn't consider that at the time I've tried them, but it is a possibility… it wouldn't be the first amp to be made with a mismatch as standard. Impedance matching is not as critical for valve amps as often believed, but it tends to affect the tone and the power output a bit.

It will be fine with an 8-ohm load, and may even be a better match for the amp, but you won't get as much as 8W - even if the output is matched accurately for 8 ohms (the correct match gives maximum power, not the other way round, so if it produces more power at 8 then it's a better match), mismatching to 16 would only cut the power by about 25%, so the most you'll get at 8 ohms is about 5W - which is the most you can get from a single-EL84 output stage anyway usually.

ecc83

There is no way they are getting 8watts from a cathode biased EL84 IMHO!

The 8R load will be fine. If you were talking about a rip snorting 50-100W fixed biased amp then there is a case for saying you would overun the op valves and shorten their lives (but even then you would have to drive the arse off it almost all the time) but a cathode biased amp will simple shut down the valve progressively as you demand more anode current.

In any case the "mismatch" is going the right way. The biggest danger to valves is where the load is higher than the rated output tapping but even here the error has to large, e.g. 4R tap 16R speaker. Amp makers give dire warnings in their manuals because generally...

1) Amps sound best when properly loaded (NB.."loaded" not "matched". Nothing audio is ever matched anymore.)

2) The company lawyers insist on an ass saving clause!

Note (as is always said in such threads) Transistor amps must NEVER be loaded below the book Z. A really badly loaded valve amp will probably protest a bit, even pop a fuse and be ok. Transistors die in microseconds.

Dave.

swlabr

Cheers guys, helpful as always.

Really wanted to keep the cab (really beautiful). Anyway ICBM, I'm not sure the ac4tv and the ac4c1 are the same circuits . The latter has master volume to preamp valves and is supposed to be a low watt version of the top boost channel.

I'll see how it gets on with the seventy80.

ICBM

swlabr said:

I'm not sure the ac4tv and the ac4c1 are the same circuits . The latter has master volume to preamp valves and is supposed to be a low watt version of the top boost channel.

I'll see how it gets on with the seventy80.

No, they aren't the same circuit (I hope not! I hated the AC4TV ) but it's quite possible they use the same transformers.

I also dislike the Seventy80, so don't immediately blame the mismatch if it doesn't sound great . Definitely needs a Blue, Greenback, G12H-30 or some other classic Celestion.

swlabr

Absolutely right ICBM! Does need a proper speaker (will have to start saving), but the seventy80 is an upgrade to the 10inch vx10.

Was thinking of a celestion gold or the blue, as you said, but I might have to manage with a cheaper greenback.

BTW tried it with the English v30 in another of my amps and sounded quite good too.