Questions on speakers

[darkmonohue]

I've got some randomly assorted questions about speakers. I recently acquired a 1968 Ampeg Reverberocket, whose speaker has been replaced. The new speaker's only identifying mark is a sticker that says "Weber Blue 12F-16-50w," with the number "12790713" below. Am I correct in thinking that the 16 in this designation refers to the impedance (16 ohms)? The Ampeg is rated for an 8 ohm speaker--according to my extremely limited understanding it's safe for an 8 ohm amp to drive a 16 ohm speaker (but not a 4 ohm speaker)--is that right?

Does anyone know where I can find sensitivity ratings for current production speakers? I like the tone of the Weber in the Reverberocket, but find it's awfully loud for bedroom playing--I'm thinking of substituting a less efficient speaker (though I have no idea how efficient the current speaker is). Am I right in thinking that this would reduce volume at an equivalent level of power output, and does anyone have any recommendations of low sensitivity speakers that would work well with the Reverberocket? Thanks!

[Robert M. Martinelli]

What about a dummy load?

Hi,

well, your assumption is not completely true, every time you connect a speaker with an impedance which differs from the one "expected" by the amp you get an impedance mismatch, and, even if the speaker has a bigger impedance it doesn' t necessarily mean that' s a good practice - talking ' bout tube amps, you must remember you have a transformer between the tubes and the speaker, and the transformer' s turns ratio is calculated exactly with this formula :

turns ratio = Sqrroot( Zin/Zout ) where Zin is obviously the tubes' output impedance and Zout is the speakers' impedance.

With the above equation satisfied you have a perfect coupling between the amp' s output stage and the speakers.


As to the speaker you' re using, I' m pretty sure the "12" refers to the diameter in inches and the "16" to the impedance, anyway, to state the real speaker' s impedance, simply measure its coil' s DC resistance with a multimeter, the reading should be slightly lower than the impedance, so expect a, say, 6 Ohm reading for a 8 Ohm speaker and a, say, 12-13 Ohm for a 16 Ohm speaker.

If you connect a speaker with a higher impedance you' ll probably experiment a sound quality decay, the efficiency will be lower and you' ll also have less current flowing through the transformer, so the DC plate voltage at the primary winding ( and at the plates ) will be higher.

I read that you' re also having "volume" problems, so I suggest you a simple but effective solution that would fix both of your problems.... it' s worth trying a 16 Ohm ( well, the closest commercial value is 15, or you could parallel two 33 Ohmmers to get 16,5 Ohms which is close enough ) power resistor paralleled to the speaker as a dummy load.

This would bring the total impedance to 8 Ohm, and half of the output power would be dissipated on the resistor, which would act both as an impedance adapter and a "power brake" rendering the amp quieter....Of course you' ll need a power resistor capable of dissipating half of the power without overheating, so you' ll probably need a 20-25 Watter.

I definitely think you should try this ( a power resistor is very cheap compared to a new speaker you don' t probably need ).

Hope this helps

Best regards

Bob

[stokes]

I dont think a simple impedance mismatch is going to get your volume down to "bedroom" levels.Neither will a speaker of less efficiency.The difference in volume level is going to be negligable in either case,you will likely notice a change in tone,which may or may not be all that extreme.As for the impedance mismatch you describe being safe,it is probably more precise to say that such an impedance mismatch is "safer" than a mismatch in the other direction.It should also be said that some transformers are more tolerant of mismatches than others.Your best bet to achieve bedroom volume levels would be a power attenuator.

[Robert M. Martinelli]

I dont think a simple impedance mismatch is going to get your volume down to "bedroom" levels.Neither will a speaker of less efficiency.The difference in volume level is going to be negligable in either case,you will likely notice a change in tone,which may or may not be all that extreme.As for the impedance mismatch you describe being safe,it is probably more precise to say that such an impedance mismatch is "safer" than a mismatch in the other direction.It should also be said that some transformers are more tolerant of mismatches than others.Your best bet to achieve bedroom volume levels would be a power attenuator.

Hi folks,
Stokes is probably right saying that having an impedance mismatch on the "high" side ( speaker impedance > output impedance ) is "less unsafe" than having one in the other direction, of course it' s always better to have a good match, and this was what my previous post was all about; I really think a parallel dummy load is a good solution for both problems because it acts like an impedance adapter AND a power attenuator achieving both the goals of matching the impedance and lowering the volume.

Best regards

Bob

[Chris - CMW amps]

I've got some randomly assorted questions about speakers. I recently acquired a 1968 Ampeg Reverberocket, whose speaker has been replaced.

Not to spam over here but I got an august '63 Jensen C12-R (8 ohms) speaker in my shop from a '63 Ampeg Reverberocket in good condition.

[mac1amps]

A 16 Ohm "dummy" in parallel is definately NOT going to bring the amp's operation to "bedroom" levels (unless you have about a 1600 sq. foot bedroom. Unfortunately [IMO], an attentuator is probably your best bet with a Reverberocket (I personally don't care for the use of attenuators, primarily since it takes a "toll" on your amp, + wears out your power tubes prematurely. They also tend to reduce speaker dynamics significantly). They DO tend to sound much better than master volumes though (Then again, ANYTHING sounds better than master volumes, unless they're set on "10")

[Robert M. Martinelli]

A 16 Ohm "dummy" in parallel is definately NOT going to bring the amp's operation to "bedroom" levels (unless you have about a 1600 sq. foot bedroom. Unfortunately [IMO], an attentuator is probably your best bet with a Reverberocket (I personally don't care for the use of attenuators, primarily since it takes a "toll" on your amp, + wears out your power tubes prematurely. They also tend to reduce speaker dynamics significantly). They DO tend to sound much better than master volumes though (Then again, ANYTHING sounds better than master volumes, unless they're set on "10")

Er...telling the truth I' m currently sleeping in a 5000+ sq.ft. bedroom ( my wife just threw me out of the window together with all my guitars and amps so I' m sleeping and playing in my garden )

Ok...let' s get serious... I don' t like "real" power attenuators for the same reasons Mac highlighted, the solution I proposed was the simplest and most cost-effective IMHO, and though it is true that 20+ Watts is still too much for bedroom playing, these 20 Watts require the volume pot to be cranked all the way up, so there still is plenty of room for adjustments on the low side.....

Talking 'bout master volumes, I must agree with Mac, in fact I' m playing most of the time through my beloved 1964 Vox AC30, and when I try amps with master volumes they sound crappy unless the master volume is set to 10 or even 11 ( ever heard about Nigel Tufnel ? )

Best regards

Bob

[darkmonohue]

Thanks for all the information and ideas. Chris, I suspect that sending a speaker all the way from the Netherlands is cost-prohibitive--I'm located in the US. I wrote to Weber, and they confirmed that the speaker is rated at 16 ohms; I'll replace it with a speaker with the proper rating. I think I'll try an attenuator to deal with the volume issue. The resistor would be less expensive, but the attenuator isn't so permanent, and to tell the truth, I'm a little scared by the idea of a resistor dissipating that much power--wouldn't it get hot enough to fry eggs? At any rate, the amp has a beautiful clean tone at bedroom levels--maybe I should quit worrying about what it can't do and enjoy what it can.

[Regis]

This home built attenuator will give you what you want.

I've built several like this and they sound fine.

You should probably put a speaker with the correct impedance in the amp too.

Good Luck,

Regis

[Chris - CMW amps]

Regarding MVs/soaks: depends on the used (type of) circuit, values and components quality if it will work/play/sound greately or not imho .

@darkmonohue
Shipping wouldn't be (very) cheap but if really wanted/needed I'd rather send it to someone really needing an original replacement than about anything else. Also got the cabinet but no chassis/parts/etc.

[Robert M. Martinelli]

to tell the truth, I'm a little scared by the idea of a resistor dissipating that much power--wouldn't it get hot enough to fry eggs?

You should worry about the resistor heating up to dangerous levels only if it wasn't properly rated, but with a 20/25 W resistor this is not the case, it can dissipate - continuously - half of the amp' s power played at full volume, so I don' t think it' s going to heat up with the amp played at low volumes; just for safety' s sake / peace of mind and if you plan to gig with the amp at high volumes you could build a little metal grid around the resistor to keep anyone' s fingers from touching it ( very unlikely though, as the resistor is supposed to be positioned inside the amp ), and you could make the circuit "less permanent" adding an "impedance switch"....

Anyway, it' s up to you, if you like it you might as well leave the amp the way it is...

Best regards

Bob

[Enzo]

A resistor is not going to help.

Cutting power in half will reduce the loudness by only 3 decibels. And 3db is not very much. To reduce volume by half requires a 1/10 power level. SO 2 watts is half as loud as 20 watts.

That should demonstrate that simply adding some load or a less efficient speaker is not going to get to bedroom levels.

[Robert M. Martinelli]

Enzo, you' re right, as usual, ( P=10*log(P1/P0) [dB] ), I was just trying to propose something simple and cost-effective to match impedance and reduce volume, and thought about a resistor, but I must admit that, while quite effective in matching impedance, mine is not the "best" solution to tame the amp' s power; just thought it was worth a try.

Regards

Bob