Selecting an OT for 150W and a 2 Ohm secondary

[kleptocracy]

Hi all,

I'm currently (very slowly) building a bass amp based on a modified Sunn Model T schematic (The PI and power amp is basically identical), and I'd like to drive my current cab, which is a 2 Ohm 4x12. I'm having trouble finding an output transformer that will work with 4x 6550s pushing ~150W into 2 Ohms. The best I can find so far (without breaking the bank) is a Hammond 1650R, which is a 5K:4-8-16 OT rated for 100W, and I'm just considering running it as a 2.5K:2-4-8 OT at 150W.

I've seen people say that because those Hammond OTs are meant for HiFi, that you could push a bunch more power through them with minimal sacrifice to guitar, but would it be noticeable in a bass amp? Especially given the fact that I'm running with a load on the secondary which it wasn't designed for? Is there a much better option for only a bit more money?

Keep in mind that this thing will probably be run at or close to full power 90% of the time, if the way I abuse my 160W Peavey AlphaBass is any indication, so if it's really truly gonna be borderline, I should probably find something else.

Thanks in advance!

[Chuck H]

Well, part of the reason the Hammonds can be half rated for guitar amp use is the narrow frequency band of the instrument and the limitations of the circuit. A bass amp is much more a reference amplifier AND bass frequencies require more power than any other. That's the reason you need a 400W bass amp even though the guitar player is using a 50W amp. Still...

I think the Hammond 1650R will probably be fine though. Not that it's a direct correlation to power handling, but the thing weighs 12 pounds. I use a Hammond 10W off the shelf unit in my personal amp. I run it flat out all the time and have measured 25W output. What's more I have, on a couple of occasions, run it for a moment with an open load at that output. Fully overdriven 25 watts complete with voltage spikes, etc. and no load. The OT is fine. Those off the shelf Hammonds are nearly bullet proof.

[Enzo]

How about calling PV parts to see if they still have any 70500044 OT for Alphabass? WHat the heck.


And if you are cranking the bejesus out of a 150 watt amp, perhaps consider a larger amp. If you like tube sound, get a tube preamp with a nice strong solid state power amp. Guitar amps are about loud, but a 800 watt bass rig is not about blow the roof off, it is about how smooth and effortless the sound is that comes out. Bass is very peaky, and having hundreds of watts of headroom means those peaks come through clear and smooth, not lopped off and scrunched.


Think of a Honda CIvic cranked up for all it is worth pulling a house trailer down the highway, compared to a Chevy SUburban doing the same job. That Chevy is a good strong bass amp. COMpare to a guitar amp. A guitar amp is like a 100 horsepower motorcycle versus mom's sedan.

[Steve Conner]

Or rewire your 4x12. If it's 2 ohms, it must have 4 8 ohm drivers in parallel, so you can rewire it for 8.

Not that it's a direct correlation to power handling, but the thing weighs 12 pounds.

Yes it is!

[kleptocracy]

Thanks guys!

I probably should have been a little more specific...I actually play guitar through the AlphaBass. My band is kind of an experimental sludge/noise thing, and has a droptuned guitar (me) and a droptuned bari (the other guy) both through bass amps with no actual bassist. The 2 Ohm 4x12 was intentionally wired as such with two bass-voiced drivers and two guitar-voiced drivers. And in our experience, our amps only give us the right timbre when they're cranked to within an inch of their lives...which occasionally causes sound guys (and our drummer) severe headaches. So...to some extend this is going to be a full range amp, not strictly a bass amp in the modern sense.

At any rate, it sounds like the 1650R is just the iron for the job, although I am now very curious as to the AlphaBass OT. I'm going to check into that....

Thanks!

[J M Fahey]

Hi. Steve beat me to this, by suggesting you rewire for 8 ohms and make everybody´s life (including yours) much easier.
Then I read your answer, and I understand what you want to achieve, the sound of 2 different types of speakers mixed "acoustically", "in the air" and not "electrically", inside the box.
Well, since you have 2 different types (if you had 4 different it would be something else) you can keep the sound you like if you wire 2 speakers of one type in series (because their impedance, resonance, etc. will track each other perfectly by definition); doing the same with the other 2 (same considerations) and then the two pairs in parallel.
Guaranteed you will not lose the sound you have now.
And will have many transformers to choose from, plus making your box easier to drive with other heads, if anything bad happens during a gig.