Q: Ampeg Gemini speaker impedance mismatch?

MWaldorf

Member
Messages
32
Searched around and couldn't find an answer - I've got a '65 Ampeg Gemini I. I've been running it with a JBL D130, which sounds amazing but is killing my back. I've been looking at neodymium speakers, but I don't see any 8 ohm neo guitar speakers in current production. Can the Ampeg OT handle an impedance mismatch like Fenders can?
 

MWaldorf

Member
Messages
32
Sorry, I should have made myself clear - I've been using the Gemini with a 15" speaker. Ted Weber used to make an 8 ohm 15" neo guitar speaker, but at this point the only 15" neo guitar speaker I'm aware of is the Eminence EPS-15, which is 4 ohms.
 

Leonc

Wild Gear Hearder
Platinum Supporting Member
Messages
18,165
Searched around and couldn't find an answer - I've got a '65 Ampeg Gemini I. I've been running it with a JBL D130, which sounds amazing but is killing my back. I've been looking at neodymium speakers, but I don't see any 8 ohm neo guitar speakers in current production. Can the Ampeg OT handle an impedance mismatch like Fenders can?
Ah you said, Gemini 1, and those use a 12" speaker. Sorry I misunderstood you.
 

sharpshooter

Senior Member
Messages
4,012
Maybe I'am not understanding you, the Gem I uses a 12in speaker, and in stock form, does not have an external speaker jack.
Do you use the JBL instead of the internal speaker, or by somehow, in addition to the internal speaker.
With the exception of some Altecs/EVs, your JBL is already at the top of the heap, any other speaker is only going downhill from what you have.
For the price of a neo you could get wheels/hand truck/hand cart/something,, maybe pay-off someone to cart the load with a few beers,,, just sayin.
 

MWaldorf

Member
Messages
32
Yes, the Gemini I comes with a 12" speaker, but with a new baffle a 15" fits quite nicely. The JBL is a great speaker, but I'm my own roadie, especially shleppin' amps into and out of my car. I've had good experience with the Weber Neomag 15s but they're no longer in production.

All of those neo 15s are described as bass speakers. My experience using speakers voiced for bass as guitar speakers hasn't been good. Not to say that these wouldn't work, but in the absence of confirmation otherwise, I don't want to drop a bunch of dough to find out.

Anyway, it seems worth asking, if only to see if Ampegs can handle an impedance mismatch. Like I said, vintage Fenders will, but I don't know about others.

Thanks, Mel
 

sharpshooter

Senior Member
Messages
4,012
Ah, now I get it, you know the stock output x-former in the Gems, (I have a '66 Gem I,) is really a little pint-sized unit, which looks to be about half of the size it should be,, but that aside, I would give the guys at Fliptops a call/email, etc., and see what they think. They are really the experts for all things pertaining to vintage Ampegs.
http://www.fliptops.net/

Also, you got my intrest up,, I have a beautifull Altec 418H, (15in), which just might find a home in my Gem, to replace the stocker,.
 

jcs

Member
Messages
8,065
I own a 67 Gemini II....I love my original 1964 Chicago Jensen C12N in this amp but the Eminence Legend 151 is great too but both are fairly heavy.

I have these 70's Eminence alnico 15's with fairly big alnico magnets but they aren't that heavy....I am really tempted to try reconing one of them as they are a bit too dark for this amp.

I would look for a used Weber Neo and have it reconed by Weber.
 

zenas

Member
Messages
8,756
Even a normal ceramic magnet 15" will be lighter than that JBL130. I've got a couple of old 15" alnicos out of church organ cabs that are also real light.

How about a cart to save your back?

Been thinking about one of those since I have two Twin Reverbs. But my heaviest amp is a 1987 Marshall in a tweed Twin cab with two JBL D120Fs.
 

enorbet

Member
Messages
46
Just FYI back in the 60s and 70s (and later) quite a few guitar players preferred Bass speakers especially guys who always played through the Bright Channel and especially those playing Tweeds, Marshalls and Top Boost AC30s. What this means to you, MWaldorf, is that it may be possible to tweak what you have not only to save the expense of a new amp but to great effect. Whether you try a treble booster pedal, reducing your Bass control (and possibly increasing your Volume control to compensate), using a Bright switch or channel, or adding a Bright cap to your Volume control (assuming you're not playing dimed where the cap has zero effect) you might possibly be in for a sweet surprise. Also dropping some Bass content early in your amp an getting it back further down the chain (and speaker is just about as far as it gets) improves headroom. Just a suggestion. You might find a new dimension in your existing amp.

Oh yeah and regarding impedance generally if your speaker is lower than what the amp wants to see that's more often than not, not a good thing tonewise and longevity of tubes. Higher than spec'd drives he amp further towards Class A which tends to sound smoother but with a loss of headroom affecting clean sounds ie: less punchy. You won't damage the amp by going higher though since signal voltage goes up but current goes down under that condition.
 
Last edited:

drbob1

Gold Supporting Member
Messages
27,836
enorbet, unfortunately what you've written doesn't go far enough to explain the problems with unmatched impedance.

Since impedance of a speaker varies with frequency, and since tube amps are tolerant of this it's easy to say that an exact match isn't necessary. Lower speaker impedances do tend to run the tubes hot, and may wear them sooner, and a bad tube failure MAY damage something else, but it's generally safe within a 2:1 mismatch (so running a 4 ohm speaker in your 8 ohm amp). A higher speaker impedance will reduce current and start out safe, but it seems likely that there is a point at which the "flyback" voltage is multiplied by the OT enough to cause arcing of the tube sockets or even in the transformer primary (voltages just get too high). This may not be true with a straight resistive load like the Ultimate Attenuator, but with a reactive load, as the speaker is returning to rest, voltage equal and opposite to what was fed into the speaker comes back thru the OT. If you've doubled the impedance the voltage will be higher, quadruple the impedance and it'll be a LOT higher, beyond the thermal capability of the wire.

To the OP, this doesn't matter since you're not going in that direction, but it's worth keeping in mind. And the smaller and less robust the OT the greater concern there is with these things (there's a lot of stories out there about taking out Champ transformers with an 8 ohm speaker, for example, but very few stories about hurting Fender Twins with ANY speaker load).
 




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