2 amps, 1 cab.

bandmaster188

Hi folks, in need of some wisdom please.
my mate has just bought a 4x10 cab with Jensen c10rs (100w @ 8 ohms) to run his harp amp into (5watt champ circuit)
he’s also considering down sizing his guitar combo to a pro jnr or similar type.
i suggested a small head type amp and utilise the 4x10.
is this possible to run 2 lowish watt amplifiers into the cab? And if so, how’s best? Could both amps be used simultaneously?
the harp amp is 4 ohm.
cheers!

ecc83

The cab could be rewired to 2x 2x 10 each 8 Ohms therefore  4 Ohms per side.  That would make it rather odd for other uses, you could not then run both sections in parallel  at 2 Ohms (well,  4R valve amp might be ok)

Anytime you need to run 1 cab, two amps you need the rather expensive Radial switch box.

Dave.

bandmaster188

Cheers dave! Are there any less spendy switching options that would still be up to the job?

ICBM

He should probably have done more research before buying the cab... the 4-ohm Champ-type amp *may* be safe at 8 ohms but it's not a good idea - these are some of the few amps that don't really like a 2:1 high mismatch, the OT is rather fragile and I've seen a couple blown by running at 8 ohms.

You can't use two amps simultaneously into the same speakers.

The best thing to do is what Dave said - divide the cab into either two 4-ohm or one 4 and one 16-ohm (depending on what is most suitable for the other amp) speaker pairs.

If you do 4 and 16 you would need some rather complex switching to run it properly as a single cab, although you could actually get away with the two halves in parallel, giving 3.2 ohms which is safe for a 4-ohm amp - two of the speakers will be louder than the other two, but you'll probably not notice much.

If you do two 4-ohm pairs you will need a series connection to run it as a single cab really.

prowla

Well, you can bridge power amps...

ICBM

prowla said:

Well, you can bridge power amps...

Yes, but then you're effectively using them as one amp.

bandmaster188

ICBM said:

He should probably have done more research before buying the cab... the 4-ohm Champ-type amp *may* be safe at 8 ohms but it's not a good idea - these are some of the few amps that don't really like a 2:1 high mismatch, the OT is rather fragile and I've seen a couple blown by running at 8 ohms.

We did have it on good authority from the builder that it’s ok to run the 8 ohm cab, but will get him to double check this. I believe they are mojotone transformers.
Cheers for the advice as always chaps!

ICBM

If it’s got a heavier spec transformer than the stock 5F1 type it should be OK.

prowla

All-in-all, it might be a better option to simply sell that cab and get another two.

bandmaster188

prowla said:

All-in-all, it might be a better option to simply sell that cab and get another two.

Or keep his combo!

Winny_Pooh

Wht not chop down the cab to a 2x10. Not too hard to do and you can even use the existing vinyl if careful & flog off the remaining speakers.

ICBM

Winny_Pooh said:

Wht not chop down the cab to a 2x10. Not too hard to do and you can even use the existing vinyl if careful & flog off the remaining speakers.

Really don't - it's never as good as building a cab properly from scratch. If you want a smaller cab, sell the big one and buy one the right size.

I've seen probably a couple of dozen cut-down cabs over the years, every one looked like a DIY bodge job. It's really not as easy to do it properly as it sounds.

bandmaster188

No fear of that. The 4x10 was bought purely for the harp amp and sounds great. Nice and light too. Just my interfering thinking that he might have been able to kill to birds with one stone.

ICBM

He can, just not at the same time.

Or has he tried playing guitar through the Champ? That would work! It even has two inputs.

I used to have a ‘59 Champ which I sometimes ran through a 4-ohm-wired Marshall 4x12”... it sounded absolutely immense.

bandmaster188

ICBM said:

He can, just not at the same time.

So a switching unit like dave mentioned could be an option?

ICBM said:

Or has he tried playing guitar through the Champ? 

Yep and it did sound great. But not enough headroom to keep up with the band. Micing up solved this but then different settings were needed for harp and guitar . Far too complicated!

ICBM

bandmaster188 said:

So a switching unit like dave mentioned could be an option?

Yes, but so could simply unplugging one amp and plugging in the other. Unless you actually need to switch on the fly in a song, don't make things more expensive, complicated and potentially unreliable than they need to be.

If both amps have standby switches it's safe to leave the unused amp powered up on standby without the cab connected.

But given the impedance issue as well, I think making the cab into a stereo 2x10" pair is probably the best solution.

bandmaster188

Cheers icbm. Will pass all this on.

robinbowes

If your friend decides to go the switching route, I have a Radial Headbone VT that I'm not currently using.

R.

bandmaster188

robinbowes said:

If your friend decides to go the switching route, I have a Radial Headbone VT that I'm not currently using.

R.

Thanks Robin, will let him know.

robinbowes

bandmaster188 said:

robinbowes said:

If your friend decides to go the switching route, I have a Radial Headbone VT that I'm not currently using.

R.

Thanks Robin, will let him know.

I'd want £170 + postage.

[my general rule-of-thumb] is 2/3 of new price, assuming good condition, perfect working order, etc, and the cheapest I've seen it is £255 at Gear4Music]

R.