Speaker impedance puzzler

[Rhodesplyr]

I was asked to take a look at an early 1950s Premier Multivox 120 that had had the basic repairs done by another tech who told the owner that it would never sound good with its original speaker, so the owner removed the speaker. I took the original speaker back to my shop and tried it out with another amp, and it sounded fine to me and to two vintage guitar amp aficionados who had happened to drop by at the right time. It's a twelve-inch speaker made by Best Speaker Company, and it has a Premier label on the back, so it appears in every way to be the original speaker. My brother, who used to recone speakers professionally says there's no sign it was ever reconed. It has a 4 Ohm voice coil

Since no schematic is known to exist of this particular amp, I had no way of knowing what impedance speaker it was intended for, so I hooked up an 8 Ohm resistor and measured the primary of the output transformer with an impedance bridge. I got ~7k Ohms plate to plate for two push-pull 6V6s at 1kHz, which would suggest to me that the output transformer was intended for a speaker of at least 8 Ohms (based on datasheet ratings for 6V6s that show 8k-10k). It has no taps for speakers of different impedances.

The speaker does sound better driven from the 4 Ohm tap of a small Grommes amp we had handy with push-pull EL84 outputs.

So, I'm puzzled. We've got a speaker that appears to be original to the amp, but it looks like it's not exactly the ideal load for the output transformer. I don't have enough experience with these amps to know if, for some reason, the OPT was intended only to present 3.5k to two 6V6s. On the other hand, I've been doing this long enough to know that both amp and speaker builders sometimes make mistakes or cut corners. (I had a speaker reconed with an 8 Ohm voice coil that was supposed to be 16 because the reconer grabbed the wrong voice coil.)

Any ideas?

[pdf64]

From your findings, I'd guess that a likely explanation was that the manufacturer fitted the wrong speaker. Does the amp put out more clean power into 8 ohms, compared to 4 ohms?

[Chuck H]

Premier was not exactly a, uh, premier brand. I'm sure it's entirely possible they just stuck the speakers they had into a bunch of amps to fill an order.

[Rhodesplyr]

I haven't measured it with test equipment, but, subjectively, the amp sounds a bit "thin" driving the 4 Ohm speaker, compared to hooking it to a similar 1950s 8 Ohm Rola speaker I have. On the other hand, the Rola and the Best speaker sound fairly similar when driven by the Grommes amp (4/8/16 OPT) from the appropriate OPT tap.

I suppose one solution to retain a certain amount of the vintage value of the amp would be to have the speaker reconed with an 8 Ohm voice coil by Weber or another reconer that has a good stock of vintage-appropriate parts. The original cone is very similar to the one used on the Weber 12A100T. Weber even offers a 12 Ohm voice coil option, which would bump the primary to 10.5k.

[Rhodesplyr]

Premier was not exactly a, uh, premier brand. I'm sure it's entirely possible they just stuck the speakers they had into a bunch of amps to fill an order.

Having looked under the hood, I'd agree with that assessment. I hear they are popular as harp amps. For guitar, it's not in the same league as a '53 Valco Oahu I worked on a few months ago with its original Rola field coil speaker.

[trobbins]

A lower output impedance level may be an appropriate loading for a 6V6 PP if the B+ is low, and the screen is high, relative to 'typical' design conditions for Vg=0. Such conditions are probably more likely to be seen in old amps with no screen resistors and small filter caps.

It's also worth considering that the frequency range of most concern for the application may be where the speaker is presenting a relatively high 'impedance', due perhaps to little damping, or high inductance (relative to a more modern speaker).

Ciao, Tim

[J M Fahey]

the Rola and the Best speaker sound fairly similar when driven by the Grommes amp (4/8/16 OPT) from the appropriate OPT tap.

Then the Best is quite a good speaker .
In the 50's, there weren't many ways to make speakers: they had thin light paper cones, round_copper_on_paper voice coils, made out of thin wire and glued with (nitrocellulose) "speaker cement", all other parts glued with that same cement, , Alnico magnets, etc.
What am I aiming at?: you already have a perfectly working, aged, original speaker.
Is it 4 ohms? Use it as such where approppriate !!!
Why recone it?
Even if somebody used all original parts, adhesives and techniques (which I find difficult), you would still lack the 60 year age.
Of course, if the cone were crumbling .... but it's not.

[jrfrond]

A 2- 6V6 amp du jour suggests a power output of approx. 12-14WRMS or so. If I had the amp in the shop here, I'd just run it into 4 and 8ohm loads, drive the input with 1kHz, and see which one delivered the rated output. That would be the correct speaker load.

For the record, most of the Premier amps I've repaired have had 4ohm speakers installed.