Getting a 16 Ohm Speaker down to 8ohm

[greg]

I have purcheased a 1x12 cabinet with a 16 ohm Celestion G12H30 which was accidentally overlooked while shopping on Ebay. Unfortunately, the head that I bought the cabinet to go with only has an 8ohm tap on the OT. While I understand that it should be safe to run this head with the 16 ohm load, I don't think it sounds particularly good. It seems much less punchy.

I'd just sell it and buy a new one, but this speaker has been broken in by Avatar (the Hellatone line) and does sound noticably nicer than a new G12H30 I have A/B'd it with. I'd work on breaking a new one in myself, but, well, you know - I have this one now and want to see if I can make it work for me.

I am assuming that I could put some kind of an 8 ohm load in parallel with the speaker to bring it down to 8ohm, but a) is that going to sound different than going straight to the speaker? and b) if this is a viable option, what is the best thing to use? A resistor? A speaker with no cone?

Thanks in advance.

[Enzo]

Well first you would need a 16 ohm second load to parallel with the other to make 8 ohms total, not an 8 ohm one.

If you do this, of course half the power of the amp goes to the dummy load.

Really the way to do this is to swap speakers in the cab.

[greg]

Yes - 16 ohm. Whoops.

That is what I expected (that I'd be wasting power). Oh well.

Thanks for the reply.

Greg

[Ray Ivers]

Greg,

What kind of head are you using with this cab?

Ray

[greg]

It is the little 6 watt amp I have been talking about on the Mods and Tweeks board (the Solen cap thread, on which, by the way, I just posted a question for you specifically - please take a look when you get a chance). It is a West Labs Pico Fire using their 6 watt PT/OT set. A little monster!

Thanks
Greg

[Ray Ivers]

Greg,

Oh, OK - I was hoping it was a four-power-tube amp that you might be able to pull two tubes from, and thus match up to your 16 ohm load. I couldn't find a schematic to the Pico-Fire at Dave's site, so I don't know what the circuit is.

Ray

[greg]

No he hasn't posted the circuit and would only give it to me upon purchase of the PT\OT set. I think is planning on offering a kit for it. It is a single ended amp utilizing a single EL84. Sounds like a new speaker is my only good option here.

Thanks
Greg

[tboy]

Sounds like a new speaker is my only good option here.
Well... You could get another 16 ohm cab and use the two in parallel. :D

[Sock Puppet]

Sounds like a new speaker is my only good option here.


You might also consider a 2:1 impedance matching transformer.

S.

[Enzo]

A 2:1 SPEAKER transformer?

[R.G.]

They exist, but are possibly not a good choice.

I'm surprised that no one has mentioned the obvious: using an output transformer with 8 and 16 ohm taps as a stepdown autotransformer. You get what is probably a good enough frequency response, and the power/sizing is easy - just pick one rated at enough or more output power. Tape off the primary leads - those will have hundreds of volts on them.

[Tom Phillips]

My 2 Cents:

Enzo: Weber sell's impedance matchng transformers. See https://taweber.powweb.com/store/zmatch.htm
I never heard one and the concept doesn't sound like a tone preserver or enhancer to me so Greg, I'm not recommending that you go that route.

However, if you are considering buying transformers, I'd put my money into a new OT for the amp and replace it with one that has 4, 8 and 16 Ohm taps. Then you can match your current speaker and any future speaker cab.
Tom

[Enzo]

OK, I only think of those things when matching into 70v systems. No reason it couldn't be used otherwise I guess. But when we buy the thing, then arrange it into the amp, it seems like just replacing the OT with a multitap would be cheaper and simpler.

[Albert Kreuzer]

They exist, but are possibly not a good choice.

I'm surprised that no one has mentioned the obvious: using an output transformer with 8 and 16 ohm taps as a stepdown autotransformer. You get what is probably a good enough frequency response, and the power/sizing is easy - just pick one rated at enough or more output power. Tape off the primary leads - those will have hundreds of volts on them.

Hi all,

just a thought: What about using a real autotransformer (117/230V)? It should work impedance-wise (tone is another matter), no?
You could transform your speaker impedance one notch up or down as you need it...
Has anybody tried this?

Cheers,
Albert

[R.G.]

Hi all,

just a thought: What about using a real autotransformer (117/230V)? It should work impedance-wise (tone is another matter), no?
You could transform your speaker impedance one notch up or down as you need it...

If you had the correct tap. The impedance is transformed by the square of the voltage ratio. So to transform 16 ohms to 8 ohms, you need a voltage ratio of 1 to 0.7071. A 4 ohm tap on an output transfomer is half the voltage of a 16 ohm tap.

Yes, an autotransfomer would work, and that's what I rejected mentally before realizing that the secondary of an OT is already the correct set of voltage ratios.

[Steve Conner]

I bet you could make something out of a toroidal transformer meant for powering solid-state gear, with a 120V primary and a 35V secondary (two 0-17v secondaries in series would do) You can wire this as an autotransformer to convert between 16 and 8 ohms. (you hook the 120V winding across the 16 ohm side, and use the voltage induced in the 35V winding to "buck" the 16 ohms down to 8.)

I guess if you have a trannie that has a 120V winding and two 35V windings, you can make a do-hickie that converts roughly between 16, 8, and 4 ohms. None of the ratios is really that critical.

I spent quite a while thinking about how to do this with off-the-shelf transformers, but I got stuck because I only considered trannies with 240V primaries. Then I would need 2x70V secondaries, which you only seem to get in crazy sizes like 1kVA+. But thanks to you guys I remembered I can get 120V units here too :-) I ought to try it sometime, since my favourite amp only has a 16 ohm output.

[greg]

Thanks for the input on this thread. I have to say that the majority of it was over my head, but it is appreciated. I have taken the easy route and have purchased the proper speaker for the amp.

I gotta keep life simple where I can!

Thanks again
Greg

[R ski]

On my first tube amp build, the output tranny impeadance taps were
wired to a 4,8. 16 ohm tap selector plate. Moving the tap lead didn't
seem that significant soundwise on a 10watt amp.