Add Dummy Resistor to Get Proper Speaker Load?

[juniormarbles]

I have a Fender Blues Junior which wants to see an 8Ω load. The Celestion Greenback I want to install is a 16Ω version.
Two questions:

1. Can I wire a 16Ω/20W resistor in parallel with the 16Ω speaker, to get the proper load (and sound!) of an 8Ω speaker?

2. If that is acceptable, should I rather use a 12Ω resistor, to duplicate the passive resistance of a 16Ω speaker, to properly load the amp?

Thanks,
JM

[sluckey]

1. Can I wire a 16Ω/20W resistor in parallel with the 16Ω speaker, to get the proper load (and sound!) of an 8Ω speaker?

That will give a proper load match. Can't say about proper sound. Kinda wasteful to do this.

2. If that is acceptable, should I rather use a 12Ω resistor, to duplicate the passive resistance of a 16Ω speaker, to properly load the amp?

no

[juniormarbles]

Thanks, but... wasteful? I own the vintage Greenback (from 1969) whose sound I very much like, so I want to use it, rather than buying another speaker.

[labb]

Me, I would just plug her in and let it bump...Don't think the amp will care very much, wanting 8 and getting 16. Doubt it will effect the out put power enough to notice.

[sluckey]

Not monetary waste. Amplifier waste. Half the power developed by the amp will be used just to heat that resistor. The sound is not likely to change much. So, what is the "proper" sound of an 8Ω greenback plugged into a Fender BJ? I'd just do as Labb suggests. No harm.

[tubenit]

Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and ridiculous that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

From London Power FAQ     

With respect, Tubenit

[92Volts]

If you look at impedance curves for guitar speakers they're higher than advertised, except at specific frequencies.

Eminence datasheets include this info, but I assume other speakers behave similar (though you might not have data about your particular speaker): https://www.eminence.com/pdf/Legend_1275.pdf

Now, at high frequencies where your "8 ohm" speaker is actually 20 ohms (or your "16 ohm" is actually 40) a parallel resistor will interact differently with the amp than a speaker alone would, and it will absorb more power than the speaker does under those conditions.

So this will affect frequency response and possibly the behavior of your amp, which is why "good" attenuators are more than just resistors.

I wouldn't say it's "wasted" power, plenty of people do use attenuators because they want the amp to overdrive without getting maximum volume out of it.

But it may or may not sound like you want/expect.

[2deaf]

A coil in parallel with a resistor is not the same as a resistor in parallel with a resistor.  A coil with an impedance of 16 Ohms in parallel with a 16 Ohm resistor has a total impedance of 11.3 Ohms.  When the coil has an impedance of 32 Ohms, the total impedance is 14.3 Ohms.  A 9.3 Ohm resistor in parallel with a coil that has an impedance of 16 Ohms will have a total impedance of 8 Ohms.  Still has the frequency effect that 92Volts pointed out.

[tubeswell]

The Fender 'EC' series of amps employ 8R and 16R dummy resistors in their 1/4 power attenuator switching system. (See schematic I did of the EC Tremolux and pictures of the 25W resistors).


As Steve said earlier, they work by soaking up power from the OT secondary (so there is not as much current going into the speaker coil).

[jim]

Mercury Magnetics make a multiple tap OT just for the Jr.  Its a slightly larger sound--maybe by itself not worth it--but with your Greenback could be the ticket if you really love the amp.  I've used it in a couple mods I've done and you will need to drill some new holes which means removing the board which is very fragile.    Jim

[DummyLoad]

do it right... 1760F is a multi-tap replacement for the blues jr.

http://www.hammondmfg.com/pdf/EDB1760F.pdf

http://www.hammondmfg.com/guitarLineOT.htm

won't kill the wallet like MM.

--pete

[juniormarbles]

Thank you all for your most enlightening responses.

What I conclude:

1. Try the 16Ω speaker without any other modification and listen

2. Add a ca.9Ω resistor in parallel, and listen

3. If none of the above results in a decent sound, consider more invasive alternatives (multi-tap transformer, different speaker...)

JM