Basic ohmage question.

[RFR]

I'm familiar with wiring up cabinets with multiple speakers to get a certain final ohmage. However, here's my question.

I have a 4 ohm bass head that I'd like to wire up a single 15 inch speaker to for the sake of making it more compact and portable.
However the speakers I have are 16 ohms.

Is there a resistor or resistor network I could hook up to make a 16 ohm speaker compatible with a 4 ohm head?
Thanks!

 

[ecc83]

Not enough information RFR!

Is the head transistor or valve? I am going to guess the former and if that is the case the impedance of the speaker(s) does not matter SO LONG as it is 4 Ohms or higher.

Generally sstate amps deliver max power into 4R and 'in theory' 1/2 that into 8R and 1/4 into 16 but you generally get 2/3ish into 8 and about 1/2 into 16 but much depends upon the design. Do not go below 4 Ohms or you risk blown transistors.

Valve amps need a load within 50% of the tapping so, set to 4R 8 would be ok PROVIDED you are not driving the nuts of it into distortion (not likely for bass?) 16R would be safe for backstage tuning but no louder than conversational.

So, tell me EXACTLY what you have. EXACTLY what you want to do and I will draw something up!

Dave.

 

[rob aylestone]

I have no qualms connecting a low impedance source to a higher impedance destination. If the amp is designed to drive 4 Ohms - which is, after all, very close to a short-circuit, then it can cope with the changes to the damping factor low impedance drivers introduce. If you run a valve/tube amp into a higher than expected impedance, all that happens is poorer power transfer, as the available current is lower - so net result = lower volume, but an easier time for the amp. Solid state designs rarely get upset by drive impedance greater or lower than optimum because they normally have short circuit protection, and open circuit just means no current flows anyway.

I would happily connect a 4 Ohm head to a 16 Ohm speaker and unless you turn the volume up too much because it's quieter, and start to generate distortion, there's no problem. Resistor networks are not really a solution, because they could easily be designed to produce a matched load, as part of that design, they would dissipate power, making the network electrically functional, but practically very quiet - much quieter than just hanging the 16 Ohm load onto the head.

 

[ecc83]

" If you run a valve/tube amp into a higher than expected impedance, all that happens is poorer power transfer, as the available current is lower - so net result = lower volume, but an easier time for the amp."

No Rob, valves don't work that way. Almost all OP valves are pentodes and they are essentially Constant Current devices which means regardless of the load, they will deliver the same current. Since the reflected impedance to the anodes is proportional to the speaker Z squared, a bit of math soon shows that the peak anode voltages can reach terrifying levels, kilo volts. (and yes, I know that is way above any likely HT voltage but trust me, it 'appens) . The output impedance of a common 30W amp, Vox AC30 say, is in the order of 50 Ohms. Even big 100W jobs are likely 10 Ohms or more so there is no "power matching" going on here.

Valves are very rugged devices* and will cope with quite a lot of abuse but ultimately they will arc over and that can not only bugger the valve but also 'carbonize' the valve holder/PCB and in some cases burn out the OP transformer. Result, a big repair bill.

As you say Rob, "Tickling" a valve amp into 4X its set load will do no harm but you can never rule out the idiot factor and ***t'appen, just needs someone to pull a jack plug out of the guitar end and for it to touch metal..BLAAAAAA! Sure you can go years as an individual and not have a problem but talk to a busy amp tech (ICBM thefretboard.com) and he will give you the horror stories. "Our own" valve amps have a 'gizmo' that serves as SOME protection. Cannot give details because it cannot, AFAIK be patented and Behringer rip them enough! I never had a traff burn out but a couple of widdlers DID try! 4 EL34s down the Swaney and some fuses.

The idea of loading a 16R speaker with a resistor is a good one. Yes, it will rob some power (but as explained above, not noticeably) and get bloody hot! Still, a tenner's worth of allyclad R in a tin could save you 10 times that in repairs.

*Modern valves are not nearly as tough as the old originals. The absolute specc for the EL34 e.g. says it can have 800V on its anode? I doubt any of the present incumbents could!

burnt out tube amplifier pictures - Google Search

^ Sort a thing.

Dave.

 

[miroslav]

Is there a resistor or resistor network I could hook up to make a 16 ohm speaker compatible with a 4 ohm head?
Thanks!

Weber Speakers make the Z-Matcher box...so you can select the input/output impedances to connect unmatched amps and speakers....but it's not cheap.
I believe they have 50 and 100 watt versions.

 

[ecc83]

Weber Speakers make the Z-Matcher box...so you can select the input/output impedances to connect unmatched amps and speakers....but it's not cheap.
I believe they have 50 and 100 watt versions.

Do they BC! I shall look into that. Fine for a valve amp but FFS NEVER connect a low resistance transformer to a transistor OP stage.

Dave.

 

[miroslav]

Z-Matcher - Gadgets

 

[RFR]

Not enough information RFR!

Is the head transistor or valve?

So, tell me EXACTLY what you have. EXACTLY what you want to do and I will draw something up!

Dave.

1964 sears 200 BXL hundred watt SOLID STATE bass head.
1964 McGregor 15 inch alnico speaker @16 ohm. Wattage unknown.

Current configuration is that head into a monster cabinet loaded with 2 each of that brand's 15s and 2 each 12s @16 ohms each.
Wired up to be 4 ohms.

On a related matter, I have a 1969 blackface Fender showman TUBE head with a currently empty 2x12 cabinet. 4 Ohm head.

I was initially hoping to take the 2 12s to put into the fender cabinet. But 2 speakers @16ohms each. I dare not use on the showman as I can only get the 2 down to 8 ohms.
Don't want to damage a vintage Fender head.
There are people who own showmans who say that that head going into 8 ohms is no problem but Im not willing to take that chance.

If the speakers were all 8 ohms, I'd use two speakers for each head, but that isn't the case. Currently I'm using the showman with the monster 4 ohm cabinet, but that's not ideal for transport. (I still gig but and tired of moving big boxes)
When I use the bass amp i unplug the Showman and plug the bass head into the cabinet. But i would like two things;
1) A smaller cabinet for the bass amp
2) be able to use the fender 2×12 cabinet wirh the Showman


There! That enough info for you Dave?

 

[rob aylestone]

Don't want to sidetrack - but a question for Dave. I understand the concept, but thought the output transformer provided the current limiting - the primary/secondary ratio stays the same, so increases by a factor of 4 when the output is loaded with 16, rather than 4? With the HT supply fixed - somewhere between 200-400V or so, increasing the primary impedance lowers the current? Have I got this mixed up? I'm re-thinking my understanding of valves now. It's also stranger because with jack plugs still being quite common on speaker cabs, I'm not sure how CE marking would be possible with HT possibly on the easy to touch jack? I too have had a decent belt from a turn shorted transformer.

 

[bachelorb]

We interrupt this thread for a joke that has probably been seen a million times before.....



....we now return you to the intelligent conversation taking place.....

 

[miroslav]

1964 sears 200 BXL hundred watt SOLID STATE bass head.

Wow...if you had just told me it was a 1964 head...I would have assumed it was tubes.
That must have been part of the very early wave of SS guitar/bass amps...though I think it might have come sooner for the bass amps than it did for guitar amps.

When I was looking at amps back in the early '70s...it was still all tubes, but I'm talking music store stuff...where I'm sure the department stores chains, like Sears, were already doing their own mass production amps...and hey, it was less costly going with SS designs.

So tell me...does it sound decent...?...or are you waiting to get the speaker hooked up before you can give it a spin?

 

[RFR]

Actually it sounds great. It's clean, loud and gets a bit of 'tubey' grit when you push it.

It definately has a character and with the cab containing 2 12s and 2 15s, it moves a LOT of air.

 

[TAE]

Man I've been wrapping my head around OHMs amps and speakers for the last few weeks...UGH!

Yeah for sure no problem hooking up a 16 OHM speaker to a whopping 4 ohm amp...The speaker is just not going to perform as well as a 4 OHM speaker for which the amp was designed and powered for...That said it will totally work and BFD if it isn't utilizing every ounce of power that bad little amp has it will still rattle the doors and do it's job quite well..

I am experimenting making a DIY stick system...using a Peavey Triflex as the amp...had a bunch of speakers and did a trial with cardboard boxes....sounded pretty bitchin...now it's time to build the real deal.... it should be pretty awesome and relatively portable ( I pulled the triflex amp from the original huge box and transplanted it in a KB100) Not exactly light but definitely easier than the original set up...now with the stick instead of satellites I'll be ready, willing and able to do the cocktail hour anywhere...or the old folks rehab home around the block from me...give them some epic stairway to heaven and some Dylan ala Tomco...

 

[witzendoz]

A 16 Ohm speaker on a 4 Ohm amp will work but the Amp will have less than half the power it would have at 4 Ohms. You would be driving the amp hard using the 16 Ohm speaker.

When you say Speakers? Have you got more then one speaker in the cab? If there are 2 x 16 ohm speakers you could run them in parallel and get 8 ohms, better then 16 ohms.

Example below,

Parallel

2 off 8 ohm speakers = 8 divide by 2 = 4 ohms

or in your case using 16 ohms

2 off 16 ohm speakers = 16 divide by 2 = 8 Ohms

Series

2 of 8 ohm speakers = 8 plus 8 = 16 ohms

2 of 16 ohm speakers = 16 plus 16 = 32 ohms

Heres a link to a calculator LINK


Alan.

 

[ecc83]

Don't want to sidetrack - but a question for Dave. I understand the concept, but thought the output transformer provided the current limiting - the primary/secondary ratio stays the same, so increases by a factor of 4 when the output is loaded with 16, rather than 4? With the HT supply fixed - somewhere between 200-400V or so, increasing the primary impedance lowers the current? Have I got this mixed up? I'm re-thinking my understanding of valves now. It's also stranger because with jack plugs still being quite common on speaker cabs, I'm not sure how CE marking would be possible with HT possibly on the easy to touch jack? I too have had a decent belt from a turn shorted transformer.

Ok, think not of "transformer" think "inductor". Basic electrical theory, the voltage across and inductor is proportional to the rate of change of current through it. Ever done a 'buzz' test on a big ole mains traff and got a belt? Everytime you see a relay coil in a sstate circuit you will see a 'clamping' diode across it and possibly a CR 'snubber' network.

So, worse case, no speaker at all. Secondary load is infinity therefore primary load is infinity X n (ratio) squared! In theory, in practice there is interwinding capacitance but the anode (plate) voltage can still get very high indeed. A higher than optimum load will still give excess voltage and two other factors conspire to help the destruction.
1) The impedance of a speaker rises with frequency. It is measured at 400Hz but at say 4kHz it could be 2 or 3 times that value.
2) Distortion produces lots of harmonics, HF (remember the "rate of change" bllx?) So, driving a valve amp into distortion especially with a higher than proper load is fraught.

The fact is almost all the failures in guitar amps are due to valve failure due to overdriving. There have been several designs over the years where not enough testing/thought has gone into what happens when the tits are driven off the thing!

I am not familiar with Fender amp history but I seem to remember that the early one were less rugged than other makes? Nothing against Leo! He was making CLEAN amplifiers and they have always had that reputation for pure, bloody loud sound!

Note, that it is not the VALVE that is the villain here? It is the TRANSFORMER! If you forget to plug a speaker into a valve amp, the traff will "sing" and you MIGHT see a flash in a valve but the seconds or so will not be long enough to damage much more than a fuse. Transistor amps with OPTraffs are rare but try that stunt and POOOF! Buggered in microseconds.

Bottom line. I personally would "misload" one of 'our' amps 1 to 2 and even hit it hard. I would NOT do that to a valuable old Fender. I would misload mine 1 to 3 but only to make gentle "is it on" test noises.

Dave.

 

[rob aylestone]

Great, Dave - thank you.

 

[RFR]

When you say Speakers? Have you got more then one speaker in the cab? If there are 2 x 16 ohm speakers you could run them in parallel and get 8


Alan.

Thanks for the info and link. All speakers are 16 ohm. Cab is wired up for 4 ohm

The cab has 2 15s and 2 12s it's huge!

Goal was to make a 1x15 cab for the bass head and use the 2 12s for my showman head ( also 4 ohms)

Right now I'm using the monster cab for both the bass head and the showman head (not at the same time )

It sounds great but is a bitch to move and get in my car for rehearsals.


So both of these wonderful sounding amps just stay in my studio.

 

[kevin6string]

maybe a matter of symantics and i don't mean to be a smart alec'...however "ohmage"? there is no such word or term as ohmage...the word you are looking for is "impedence" "how much impedence"...just saying

 

[RFR]

maybe a matter of symantics and i don't mean to be a smart alec'...however "ohmage"? there is no such word or term as ohmage...the word you are looking for is "impedence" "how much impedence"...just saying

Thanks for your useful information.......


Not!

Edit: ps 'Ohmage' IS a word. Even if I made it up. And by having it appear in virtual print on this site, it's now publshed. And it's catching on!!!

 

[TAE]

My Amps pay Ohmage to my speakers on a regular basis...

FWIW sorry Kevin you are in error .... Ohmage is the plural of Ohm as per the Merriam Dictionary