OHMS Questions

[jdorrier]

Can someone please explain to me the OHMS equation? I am looking at this combination but need to make sure it will work. I am looking at buying this setup used, from 3 different people so I can't test it out:

Cab - AMPEG CLASSIC SERIES SVT- 1x15 - 8 OHMS
Cab - AMPEG BSE 4x10H - 4 Ohms
HEAD - 70's Ampeg V4B 147 watts tube amp

Also, any idea what a fair value of this is in pretty good condition?

JD

 

[bongomania]

Check the FAQ for the equation and much more. Short version in this case: 4 ohms with 8 ohms gets you 2.67 ohms, which a majority of amp heads can't do.

 

[René_Julien]

... 2.67 ohms, which a majority of amp heads can't do.

And with the V4B it's not sure if it can handle a minimum of 2ohms.
I think the earliest models can handle it, but from a certain production year V4B heads can handle only minimum of 4ohms.
I played a V4B once, one of the lasts I believe, and it had 4ohm minimum. (Don't know what year it was).

jdorrier, ask the person that is selling it if he got the specs, or if he can see on the back of the head what the minimum load is.

Or if someon here knows from the top of his head what the minimum impedance is for a V4B 147w head.

 

[neaderhiser]

if an amp is rated 200 watts @ 4 ohms, could anyone tell me what it would be rated @ 8 ohms? thanks

 

[mikezimmerman]

if an amp is rated 200 watts @ 4 ohms, could anyone tell me what it would be rated @ 8 ohms? thanks

Not without knowing what the amp is. My understanding (I've never owned one) is that tube amps have the same output into any of the cabinet impedences they are rated for. Solid state amps typically put out less into a higher cabinet impedence, but exactly how much less depends on the design of the amp.

Mike

 

[René_Julien]

if an amp is rated 200 watts @ 4 ohms, could anyone tell me what it would be rated @ 8 ohms? thanks

A simple (and inaccurate) answer would half the wattage: 100 watts @ 8ohms.
That is if impedance can be treated the same as resistance, but it can not.

In most cases it's more than half when doubling the impedance.
Realisticly: anywhere between 100 watts and 150 watts. It depends on the manufacturer really.
I don't exactly what factors in poweramp desings influence the wattage according to load.
(Also general difference between power section of a typical bass head and a dedicated poweramp.)

 

[jdorrier]

So if am I better off to buy a 4x10 cab with 8 ohms, a 1x15 cab with 8 ohms and with the 147 watt tube amp head?

 

[bongomania]

That will get you a 4 ohm total, which is great; but can the 1x15 handle (or sound good with) the exact same amount of wattage as the 4x10? The wattage will be split equally between them.

 

[JTE]

The V4b is a tube head, so it's got an output transformer that will provide the same power into whatever loads the transformer is made for. I think that's 150 WRMS at either 8 ohms or 4 ohms (depending on where the switch on the back is set- if I'm recalling old Ampegs correctly- it might have a 16 ohm setting too, but I don't recall ever seeing one with a 2 ohm setting). It might work OK into the 2.66 ohms you'd get with the mis-matched cabs you're looking at, but it might not like that and cause the transformer to fry.

I'd get two 8 ohm cabinets so I have plenty of options. One 8 ohm box for smaller gigs at 150 WRMS, or two cabinets to move more air for bigger gigs again at 150 WRMS (or there-abouts).

jte

 

[fjbass78]

My early 70's v4 head has a ohm selector switch in the back. It can do 8-4-2 ohms. Take a look at yours. If you see a 2 ohm option, you are good to go.

 

[anderbass]

As far as I know, all the various early to late 70's V4/V4B/VT-22 models had a 3-way switch to select between 2, 4 or 8 ohm operation. I could be wrong but I believe its only the newer type reissue V4BH models that only have 4 or 8 ohm selections. The 2 cabs you mentioned would be a combined load of 2.67 ohm and should be safe for a 70's V4B.

I dug up an old thread for ya about running Ampeg Tube amps with a 2.67 ohm cab load that recommends using the amps 4 ohm settings.

Here's a quote from one of this forums leading Ampeg authorities on this subject:
(Quoted from this thread)
http://www.talkbass.com/forum/showthread.php?t=439371

Either should be OK. Setting it for 4 ohms with a 2.67 ohm load (one 8 ohm and one 4 ohm cab) is safer, but a 2 ohm setting isn't instant death.

As far as the going price for these amps, I always like to do a nationwide search for all craiglist ads to get an idea of the current asking prices. You'll find some V4B/V4 prices in these 2 links:
V4B: http://www.google.com/search?source...z=1T4DMUS_enUS221US222&q=craigslist+ampeg+V4B
V4: (basically the same amp + reverb) http://www.google.com/search?source...lz=1T4DMUS_enUS221US222&q=craigslist+ampeg+V4
(you could search to compare your cabs prices the same way...)

I'd expect the V4B should work fairly well with the 1x15 you mentioned, but I have no idea how that 4x10 would end up sounding with this combination.

If your V4B is a mid to late 70's model, (plastic rocker switches for power/standby)
you'll find quite a bit of info at this site:
http://www.ampegv4.com/

If its a early 70's model, (metal toggle switches) You'll find a wealth of info in this link:
http://www.google.com/custom?hl=en&...eg+V4&btnG=Search&sitesearch=www.talkbass.com