TWIN 4 or 8ohm

[Kevin Mincke]

I recently acquired one of my Twins back I had sold years ago. At the time I bought it (1977)the origional owner had put a 15"
D130-F in it (which it still has). I was wondering if Twins & Vibrasonics were all 4-ohm and should I have the speaker reconed to 4-ohm also. How does this affect the amp, sound etc. What model speaker did the 70's Twins/Vibra's come with?

[Brandin]

Twins are 4 ohm, and vibrosonics from the 70's are 8 ohm. The vibro's came with
JBL's or EV's. If that's a original JBL D-130, I would leave it alone. Get one of those Black Widow speakers in 4 ohm's.

[C Dixon]

Assuming one never goes below a tube amp's output impedance, the only thing that happens if the speaker's impedance does not match, is a loss of power. The further one gets from a perfect match between and amp's output impedance and a speaker's impedance, the less power the amp delivers to the speaker.

No loss of fidelity for all practical purposes.

Note: never go below the output impedance when connecting speaker(s) to a tube amp. Reason: I can burn up the output stages when played at sustained maximum Power.

God bless you in whatever you do,

carl

[Larry Bell]

That's interesting, Carl
Any idea why MosValve amps are rated at 4ohm but recommend using 8ohm speakers?

Jack, I know we've discussed this -- seem to recall you went for the 8ohm when you built your cabinets.

What's the story? My Peavey 115e cabinets are 4ohm and sound ok to my ears. Would 8ohm speakers sound better?

LTB

[Jack Stoner]

Larry, in ref to the MosValve 500, the "best performance" or optimum is 8 ohm load, which would lead me to believe the max power transfer occurs with an 8 ohm load. The factory tech told me not go below 4 ohms so I would assume that is the minimum safe load.

I've never measured the power output at different loads (and don't have the equipment to do it) so I don't know how much power is really lost with a 4 ohm load.

If someone has a manual on the MosValve 500 maybe it lists the rated max output for the various resistance loads.

[Donny Hinson]

The 8-ohm tranformer (used in the single-15 Showman and Vibrasonic) is Fender Part No. 022897. If you check the transformer in your amp (it's the one near the center of the chassis) it should have a part number on side. If it says 022889, you have a 4-ohm transformer for two 8-ohm speakers wired in parallel. I would recommend changing to the proper transformer for the speaker combination you are using. The stock Twin Reverbs will only put out 60-80 "clean" watts, and you need all of that for a good sound.

Most of the Twin Reverbs from that era came with Jensens, or "stamped steel" JBL's. These are both good quality speakers, but there are much better speakers available. The D-130F (cast frame) JBL sounds quite good, but is only "average" in efficiency.

[Bill Crook]

It has been my feeling that to use an 8 ohm transducer on a 4 ohm output device results in :

1) lower output power sound-wise, the amp isn't decreased in it's ability to swing Pos/Neg tho

2) a more bassy sound
3) less chance of clipping
4) decreases the chances of blowing the output Q's under sustained max volumne levels

Personaly, I dont think most of us would realize that an 8 ohm transducer had been used due to the fact that we most allways put a due amount of EQ in to our settings.

Never-the-less, using a 8 ohm aginst a 4 ohm output stage, will not hurt your amp.

I modified a "NASHVILLE 400" once using 2 power MOSFETS from a motor control unit by displaceing the 6-8 output Q's with 2 MOSFETS, resulting in a output stage that would TOTALLY accept a direct short at the output without distroy-ing the amp. Beings that the power rails were +/- 50 to 55 volts at approx 6 amps, MOSFET C/E max = 600 volts,150 amps, you can see that it's hard to distroy the output stage in case of a speaker shorting out or under a no-load condition.

The drawback to this schemne:

1) Power mosfets are expensive: $150.00 to $800.00 a piece. (You need at least 2 of them.)

2)It's an vast over-kill to a output stage.

3) Do you really want that much power to sound ratio ?


I was able to do this mod as I had access to a defective VFD (Veriable Frequency Drive)unit that was going to be discarded.

All top-end Power-amps use this technology now, but as some of you know by now, it's very expensive.

[Michael Brebes]

The Fender Twin is rated 4 ohms minimum and will lose some of the available power by using an 8 ohm speaker. But the JBL D130 is more efficient than the stock 12s so there might not be that much of a hearable power loss. As for the Mosvalve amp impedance question. Transistor/mosfet amps are different from tubes in that they don't have an output transformer and they have a very low inherent output impedance. The reason that you can't go below a 4 ohm load is because you will start drawing more current than the output devices are rated for and some or all will start to fry. Otherwise the amp doesn't care whether it sees 4 ohms, 1000 ohms or completely open. The amp outputs voltage so 10 volts squared equals 100. Divide that by 8 ohms and you get 12.5 watts. Divide that by 4 ohms and you now get 25 watts. Or in other words, a 4 ohm load will give you twice the power versus an 8 ohm load, on a Mosvalve or any other transistor/mosfet power amp. Hope that clears up some questions that were brought out.

[Kevin Mincke]

I checked the transformer number as Donny mentioned above, #022889 is what I have denoting a 4-ohm. Upon closer inspection, the speaker will most likely need reconing. I guess I have the option of keeping the speaker an 8-ohm and changing the transformer to a 8-ohm (like the Vibro's) or have the speaker reconed 4-ohm.

[Ricky Davis]

For the interest of this situation I did a test and here is the results from my ears.
I've been using a E-130 4ohm in my twin for a couple of months and sounds great. Redd from Merle Haggards band loaned me a couple of D-130s to use and try out maybe to buy. One of them is 8ohm from a '60s vibrosonic and one is a 4ohm re-cone original D-130.
Well the D-130F 8ohm speaker has just blown all the others away. So much better sound than the 4ohm recone(JBL kit)and the E-130; it ain't even funny. The only difference was I had to turn my volume up a half a notch(Hello!! a half). Redd told me the 8ohm speaker will take more low end before it craps out than the 4ohm; well he is right. I have found the speaker of my dreams and he is kind enough to let me have it> I'm on cloud 9 and thank you Redd. Oh btw; don't put the 8ohm in a solid state that requires a 4ohm load; then you will have troubles; as Redd tells me; but no prob with the tube amp; because that is all the difference in power I experienced(half a click) and it is a superior sounding speaker than the 4ohm cause the higher the ohm the more it will take.
So I've learned.
Ricky

------------------
Ricky Davis
http://hometown.aol.com/sshawaiian/RickyHomepage.html
http://users.interlinks.net/rebel/steel/steel.html
sshawaiian@earthlink.com

[Kevin Mincke]

This is pretty much the results I got when using the D130-F in the Twin years ago........never questioned it as it "just sounded good" and never thought about any potential damage to the amp. Think I'll have it reconed an 8-ohm and throw it back in the Twin! As stated above, the only difference between the Twin & Vibro is the 4 vs. 8-ohm transformer (2) 12"s vs. (1) 15".Hmmmm......?