1950s amps, speakers, and output power?

[Rhodesplyr]

The 1953 Multivox Premier 120 guitar amp I worked on recently has a typical output stage with two push-pull 6V6s sharing a cathode resistor, but, unlike many similar output stages, it has no cathode bypass capacitor.

While I had it open, as an experiment, I alligator-clipped in a cathode bypass capacitor and found that it definitely increased the perceived power of the amp (via transient response), but it also worked the stock speaker, a 12" with a fairly small voice coil, fairly hard.

That made me consider why the amp designer left a cathode bypass out of the original design. Was it because they wanted to use smaller, less expensive speakers and, thus, limited the amp's output power on purpose?

Are there other amps from this era that appear to be intentionally limited in output power in some way?

I'm just thinking out loud here, and I could very well be completely off-target.

[Enzo]

Limit output power, also limit bottom end. A small coil speaker can be pretty loud, piercing even at higher volumes. The cone won;t have to move far, but the lower the freq, the farther the cone has to move. Keeping the snorting bass out of it allows more sparkling highs to be produced.

It pays to always look at things as a system, not just as parts.

[Rhodesplyr]

It pays to always look at things as a system, not just as parts.

I agree entirely--and in this case, the speaker grille has heavy flocking on it, which would tend to block highs, and it's effectively a ported speaker enclosure, not open-backed. It's an interesting balancing act, assuming that these were all informed choices...

[pdf64]

Bypass caps on p-p shared cathode resistors only do anything much at high signal levels, above the point where the tubes enter cut-off for part of the cycle. For class A amps they can be omitted (though it would affect the overdriven performance).
Are you supplying the amp with a 50s line voltage? If the voltages are higher than the design spec, it may push it into class AB.
Pete.

[Rhodesplyr]

Bypass caps on p-p shared cathode resistors only do anything much at high signal levels, above the point where the tubes enter cut-off for part of the cycle. For class A amps they can be omitted (though it would affect the overdriven performance).
Are you supplying the amp with a 50s line voltage? If the voltages are higher than the design spec, it may push it into class AB.
Pete.

With all due respect, it's not difficult to push a pair of cathode-biased 6V6s into Class AB1 operating conditions, and the preamp/driver stages of this amp are fully capable of doing so. I understand that an unbypassed, shared cathode resistor becomes a source of negative feedback when the stage goes beyond Class A operation. I take that for granted as the basis for this discussion.

I know it's common on these lists for responders to tell posters that they didn't actually perceive what they claim to have perceived, but I clearly observed that with the addition of a cathode bypass capacitor, this amp was capable of transients that would probably bottom out the speaker's voice coil.

I also recently ran across some early 1950s speaker prices, and I might speculate that speakers were a larger percentage of the construction cost of an amp then than they are now.

[Bruce / Mission Amps]

If you want guys want an interesting education... connect your output transformer secondary to a fixed resistor (no speakers) ... with the scope probe hooked up correctly..
Watch what happens when the push pull driver stage pushes the unbypassed, cathode biased power tubes further and further, right into distortion.... the ugly shape of the "sine wave" will astound you.
Now put a small 22uF-47uF 50v cathode bypass cap in (which will increase the apparent output plus, kill off almost all the effects of negative feedback at the power tube) and watch the TOTALLY different looking sine wave and how it starts to flat top at the peak power but stays much more linear before that.
Compare those two images and you'll see why you need a better speaker.
Sorry, if you don't have an O'scope....

[Rhodesplyr]

Bruce,

I do have a scope, but my ears can pick that up just as easily as my other test equipment :-)

I considered fully bypassing the cathode resistor of this amp, but I'm pretty sure that the stock speaker was reaching the limits of its excursion when I did that. As you indicate, a more robust speaker would probably be required.

[trobbins]

Some glib musings:

At the time of the Williamson, and in to the early 1950's, hi-fi speakers or speakers with any substantial power rating beyond a few watts were definetly not common, and expensive as you indicate. But the situation was certainly changing at a fast rate. Magnet capability and suspension technologies were pretty primitive. Whizzer cones and cone breakup modes gave you treble, not tweeters. Port design didn't know anyone called Small or Thiele. Distortion, if measured was at low output level. Guitars didn't play bass notes, and didn't play to noisy large auditoriums. Electrolytics weren't that cheap either. 6V6's were probably cheaper than any other valve designed to generate more than a few watts - so defacto were used to cover all requirements over a wide power range.

[Rhodesplyr]

I found a 1950 price list that showed the replacement 15" Jensen woofer for a Leslie cabinet costing $63.00. Running that figure through an online inflation calculator produced a figure of $560.00 for 2010.

[trobbins]

It would be great to have a price snapshot comparison of some common parts - such as the PT and OT, some valves of the era 6L6 KT66 6SN7 EF86 12AX7 (9-pins were just becoming mainstream by then), a few electrolytics, a pot - and also some total kits and finished amp costs. The hobby mags in Australia provide a few pricings, I may be able to dig up some comparisons for good measure.