Speakers for a JTM45 in a bluesbreaker cabinet

[bristol]

Hi,
I am planning to build a JTM 45 and mount it in a BB cabinet. I am thinking about using a Celestion Gold + a 30 watt G12H (one of the options on the GPM 45 Combo). I want to get the early Clapton Blues breaker tone (like everyone else who wants a JTM 45) as well as Cream, Hendrix, AC/DC, etc. - all the classic Marshall tones. Fido recommended the speaker combination. Do others agree with that combination?

How should it be wired? Two 8 ohm in parallel, two 8 ohm in series or something else? I have also seen comments that recommend setting the output impedance of the amp higher than the load - 16 feeding 8 ohms or 8 feeding 4. Is this safe? To be able to do this, it seems like a pair of 8 ohm speakers would be the best way to go - 16 ohms when wired in series or 4 in parallel. That way I can run 8 ohms driving a 4 ohm load. Assuming it is safe.

Thanks

[Big Mike]

i'd get 2 8 ohm speakers and wired it for a 16 ohm load, then you're using the whole transformer, and can mismatch the 16 at 8 if you like.

Gold/G12H would probably sound very good, but also pretty bright.
I'm not exactly going for Beano tones, but I use a scumback M75/H75 mix for a 'greenback and H-30' pre rola tone, and they sound great. Might also consider a scumnico/H75 for a better version of that celestion Gold/H30 sound.

I'm a big fan of mixing speakers for extending the frequency range. Seems that if chosen right, they make up for each others weaknesses.

[bristol]

Thanks. So many choices. I am glad I have been pointed in the Suumback direction. The seem to blow Celestion away.

[shakti]

If you want the Beano tone exactly, the Scumnico is what you want. I use a pair in a style I Bluesbreaker build, and now that the speakers have been properly broken in, I'm just blown away at how they positively *nail* that tone dead on to a T.

As far as the other tones you mention, I feel that a closed cab is a must to properly emulate those sounds. So I'd rather go for one at a time, maybe start with an open-back 2x12 with Scumnicos, then later add a closed-back 2x12 or 4x12 with ceramic Scumbacks. That should give you the most variety.

[shakti]

BTW, as far as mismatching speakers, I'm not so sure about that...I'm using two 16 ohm Scumnicos in parallell for 8 ohms, and the amp at 8 ohms. Running mismatched with amp at 16 ohm into the 8 ohm load yields a slightly looser, softer but more overdriven sound. I think the matched sound is closer to the Beano tone.

Original Bluesbreakers may have run mismatched in the earliest years when they were using the RS OT which doesn't have selectable impedance (you essentially hardwire it for one impedance at a time), but as soon as they moved to the Drake OT in late '64, there was selectable impedance, and no real need to mismatch it. It's perfectly safe to do, it just means the tubes are running into a lower primary impedance (normally 6.6k for two KT66s, mismatch it like that and they run into 3.3k).

I'd go for 16 ohm speakers and wire them in parallell, for two reasons;
1) the WWMD rule ("What Would Marshall Do?")
2) 16 ohm is the standard Marshall impedance for 4x12s, so if you end up swapping the speakers into a 4x12, you can still use them

[Big Mike]

Edit: Nevermind...reread your post and it makes sense about the 8 ohm load.

I usually like 16 ohm load because it's using the whole tap on the transformer..seems to respond a bit better IMO..but the diff. between 8 and 16 (matched) is negligible.

[bristol]

Thanks, lots to think about.

[bristol]

I am wondering about the output impedance when two speaker cabinets are connected. It appears from the schematic that the two output jacks are in parallel. Is that correct? If so, if I plug in two cabinets that are 8 ohms each, wouldn't the load be 4 ohms?

What I am wondering is if I put two different speakers in the combo cabinet, could I plug in one or the other or both together? (I am thinking about using a scumnico and a G12 scumback together.) If they are both 8 ohms and I plug in both together they would be in parallel would present a 4 ohm load and one at a time, 8 ohms. Or I could get two 16 ohm speakers and present an 8 ohm load together or 16 each individually.

Does this make sense or am I very confused??

Thanks

[Big Mike]

There's a way to wire that up, I think it would take a switching jack in the cab.
Not a bad idea really.

yes if you plug into 2 individual 1x12 cabs that are both 8 ohms, the amp will see 4 ohms.