Question of Ohms

Grumpyrocker

If I had an amp head that only had a 16ohm output, but my speaker cab was a 2x12 rated at 8Ohm, does that mean I couldn't use that amp head with that speaker cab?

tone1

https://i.imgur.com/jSMMlM8.jpg

Grumpyrocker

Anyone with an actual answer?

ICBM

What amp is it? Why does no-one seem to include this information in the question?

Bridgehouse

Are the 2x12 speakers rated as 8 ohms combined? 

Grumpyrocker

ICBM said:

What amp is it? Why does no-one seem to include this information in the question?

It's a little Marshall DSL1HR Head, 1Watt amp. Only has one speaker out. Says it's 16Ohm.

Bridgehouse

For clarity my question was what’s the cab input rated at? If they are 8 ohm speakers then it depends on how they are wired up

Grumpyrocker

Bridgehouse said:

Are the 2x12 speakers rated as 8 ohms combined? 

Yes it's a Harley Benton 2x12 V30 cab. 8Ohm mono, or 16Ohm Stereo.

ICBM

Generally with a valve amp it will be safe, just result in a small loss of power and a bit more compressed sound. But it will also stress the output valves more heavily and might not be a good idea to crank it up continuously like that.

I have no idea how that amp will tolerate it though, it's quite different from most proper amps since the output valve is a ECC82 preamp valve and the transformer is likely to be tiny - I haven't seen inside one of the new ones.

You can use it through just one of the speakers if you set the cab to stereo, that will be fine - I don't think you need another plug in the second jack to turn off the speaker, but you'll be able to tell just by trying it.

Grumpyrocker

ICBM said:

Generally with a valve amp it will be safe, just result in a small loss of power and a bit more compressed sound. But it will also stress the output valves more heavily and might not be a good idea to crank it up continuously like that.

I have no idea how that amp will tolerate it though, it's quite different from most proper amps since the output valve is a ECC82 preamp valve and the transformer is likely to be tiny - I haven't seen inside one of the new ones.

You can use it through just one of the speakers if you set the cab to stereo, that will be fine - I don't think you need another plug in the second jack to turn off the speaker, but you'll be able to tell just by trying it.

Thanks for the reply. I don't think I'll risk it. I had been looking at the new baby Blackstar (which would have been fine with my cab) but it has no FX loop. Whereas the Marshall does.

I think I'll keep looking for a different amp.

BTW The cab has two inputs (one labelled mono) and a switch to go from mono to stereo. 

ICBM

Grumpyrocker said:

Thanks for the reply. I don't think I'll risk it. I had been looking at the new baby Blackstar (which would have been fine with my cab) but it has no FX loop. Whereas the Marshall does.

I think I'll keep looking for a different amp.

BTW The cab has two inputs (one labelled mono) and a switch to go from mono to stereo. 

It will probably be fine. If you're not cranking it right up there's no risk from running it at the wrong impedance anyway.

ecc83

Without casting any aspersions!! The HT-1 was the first 1W amp AFAIK to use an ECC82 in push pull as an OP valve. Now the HT-1 can have any speaker impedance fro 4 to 16 Ohms  and since there not many ways you can wire that circuit I bet the Marshall is very similar, (not saying they copied it of COURSE!) .

Dave.

ICBM

ecc83 said:

Without casting any aspersions!! The HT-1 was the first 1W amp AFAIK to use an ECC82 in push pull as an OP valve. Now the HT-1 can have any speaker impedance fro 4 to 16 Ohms  and since there not many ways you can wire that circuit I bet the Marshall is very similar, (not saying they copied it of COURSE!) .

I did think that too, but when you've seen the failure rate of the transformers in some Marshall amps, I'd be inclined to err on the safer side than I normally would...

Grumpyrocker

Yes it was references online to Marshall transformers that got me to start the thread in the first place, as I saw the Blackstar said it could handle any speaker. 

ICBM

Grumpyrocker said:

Yes it was references online to Marshall transformers that got me to start the thread in the first place, as I saw the Blackstar said it could handle any speaker. 

It really depends on whether the voltages that can be developed at only 1W (probably nearer 2W fully overdriven) can be sufficient to cause arcing in a transformer, even if not brilliant quality. If not, then it doesn't matter what the impedance or how hard you drive it, it won't blow. Although it could still shorten the valve life, they're only about a tenner.

What I don't know is whether by marking it for 16 ohms only Marshall know it's a bit fragile, or do they simply intend it to be run with one of the own cabs which are typically 16 ohm.

Fenderish

Grumpyrocker said:

Bridgehouse said:

Are the 2x12 speakers rated as 8 ohms combined? 

Yes it's a Harley Benton 2x12 V30 cab. 8Ohm mono, or 16Ohm Stereo.

Then set the cab in stéréo and use only one input/speaker

ecc83

"I did think that too, but when you've seen the failure rate of the transformers in some Marshall amps, I'd be inclined to err on the safer side than I normally would..."

IC, ^ do they fail from arcing or overdissipation? If the former connecting a lower load than 16R will actually help there. Won't of course be kind to the ECC82 but they are tough lil buggers and in the One at least are cathode biased so anode current is inherently limited anyway.

For the (my) record. I had just one HT-5 OPT fail, punched through pri to sec and blew HT fuse, that's one of thousands by now. Never had an HT-1 OPT fail but I retired pretty soon after its launch but it has been a successful amp so we'd know? !!

While we are on the One? DO NOT  ye gitist fiddlers fit anything BUT an 83 and 82 in the right places! Nothing else will handle the output and the load Rs in the pre amp section are pretty wee and might burn out. Not heard of it happening, jus sayin...Guess that also applies to the Marshall but then, not that fussed if you ***k them!

Dave.

Grumpyrocker

Thanks once again for the replies.

A bit annoying really - I could just use one side of my cab. Or seek an alternative amp. But the Marshall (speaker output aside) is the one that had all the features I wanted. 

ICBM

ecc83 said:

IC, ^ do they fail from arcing or overdissipation?

Not sure/either/both - probably depends on the model too, it goes all the way from the 20W DSL upwards.

Whichever it is, the rate is exponentially higher than any other brand - I know Marshall have sold a lot of amps, but I’ve easily replaced more transformers in them than all other brands put together. In many cases just one model more than most other brands! Even very common brands like Fender and Peavey.

It’s true that some of them are caused by poor design and component choices in the speaker circuit, but it also applies to PTs as much as OTs, so I think it must be a design spec or supplier quality problem.