Need help with replacing caps on old Multivox amp

[Amplitude]

I need to replace the following cardboard caps (may restuff) on this 1950s Multivox amp. As you can see, it's old enough to have an electro-dynamic field coil speaker set-up, and a selenium rectifier (I'll deal with that later).

Here's what I need to know:
First, the two capacitors on the speaker sub-chassis are 7 mfd/uF @50volt each. I'm unsure about the type of caps I need, whether they need to be electrolytics or not (I'll probably increase the values slightly). Are the dotted-line markings on the cardboard shells the "negative" end? What caps would you recommend here?




Second, the following cap is a multi-stage unit that attaches to the selenium rectifier and tube rectifier pin#1. It's a 50mfd/50mfd @250volt. Again, I'm confused about the polarity for the replacement caps needed to replace this unit. The cardboard shell states the "common" (yellow wire) is "+ -". Will a positive+/negative- end of each cap be wired together for the "yellow" wire, (with separate positive+/negative- outs for the black and red-dotted wires)? So, what do you recommend here? Thanks!

[Enzo]

It is a crossover, yes? SO the caps would not be polarized regardless of type. Electrolytics are used because for a given capacitance thay can be much smaller than other types. No cap "needs" to be electrolytic, they just specify them for cost and size. But technology has advanced in the last 60 years. CAps are smaller. Besides, you have TONS of room there. A couple 7uf film caps should be easy to find and will work fine.

Normally a dual cap like that has two positive terminals on one end and a common negative on the other end. However, this looks to me like it could be an end to end pair of caps with the connon coming out the center of the two. hence the +/-. How is it wired in now? ANy chance the two caps are in series across B+, and this was their way to get a higher voltage rating?

What model is the amp, so we might look up the circuit?

[Amplitude]

Thanks Enzo, that's what I was just told on another site as well regarding the 7uF caps. I have a couple of Solen 8.2uF's on the way. As to the large multicap, it turns out I have the caps inside wired correctly, the two negatives are wired together and feed out with the yellow "common" (I have 2 Illinois 47uF 350v caps inside the cardboard tube). The two positive ends go to the selenium rectifier that feeds power to the 12" speaker field-coil. The main speaker in this amp is an "electrodynamic" unit, no magnet used. Very primitive! The amp is a 1954 Premier Model 71, like the one Link Wray used back in the 1950s/60s (Rumble, Rawhide, Jack the Ripper, etc). I haven't located a schematic yet, but have been using one off a Harp amp site for a Model 76, which is somewhat similar to the Model 71. Thanks again.

[Enzo]

I remember an LP in the store for cheap, by Link Wray. The title was "Twangy Guitar." I am still kicking myself for not grabbing it.

[Chuck H]

FWIW changing the value of the caps on the speaker sub chassis (which looks like a crossover circuit) will change the frequency knee of the crossover. The value of the caps in this sort of circuit isn't such that you can fudge much or a "a little bigger is probably better" kind of deal. Why is the crossover there? Are there two speakers in the amp? Is the amp one part of a more elaborate system? This could be pretty important since the crossover circuit and how it is to be used, as well as the proper speaker compliment will affect the reflected impedance on the tubes. With tube amps the wrong impedance can be a bad thing.

[Amplitude]

Chuck, this amp has 3 speakers. There's a Jensen 12", and two smaller Jensen 3-1/2" speakers (some call these tweeters, but they're actually full range drivers). As mentioned above, the 12" speaker has an electro-dynamic coil system, there is no magnet on the speaker. Voltage going through the speaker field-coil creates a magnetic field. The 3-1/2" speakers have regular, permanent magnets. I don't believe that's a crossover on the sub-chassis, but I'm certainly no authority on this kind of set-up. I wouldn't think an ohm or two difference on those caps would matter greatly, if at all, but this stuff is way over my head, lol.

Here's the cab (chassis and speakers are out at this time):

[Amplitude]

On second thought Chuck, I suppose it could be a crossover. The 4 wires exiting the terminal strip near the two coils/caps are all connected to the various speaker terminals.

[Chuck H]

If it IS a crossover, changing the value of the caps in the circuit will change the crossover point. That could damage a speaker not designed to handle it. Your proposed value difference probably isn't significant but I was just pointing out that in this particular type of circuit using bigger caps doesn't improve filtering or any other positive result. It just changes the pass frequency.

[Amplitude]

Yes, I see what you mean. I might should have gone with some 6.8uF caps. They would be closer to 7uF than the 8.2uF's. I've been trying to keep close values/tolerences with the other caps/resistors that I've replaced. I may call Tube Depot Monday and see if I can get a couple of the 6.8uF caps before my prior order is shipped out. Thanks.

[riz]

Beautiful amp!

Enzo--I actually just DID bring home a copy of an English-pressed compilation LP of Link Wray, from '72 by the cover... It's the good stuff, and it's on the turntable right now--it rocks! And it's long out of print... so PM me if you want, I'll digitize it for you.

[Bruce / Mission Amps]

If the Premier is 71 or "like"... it should have a couple parallel wax capacitors feeding the 3.5" speakers off the + lug of the 12" driver and I think they are in parallel.

[Amplitude]

Hi Bruce. I didn't see any signs of broken or removed wires at the speaker terminals when I took the speakers out, although it's possible I could have missed something. This appears to be the earliest version of the Model 71 (it's a 1954), and I believe by 1955 there were some changes to the 71 series (all 3 speakers permanent magnet by then). On some later versions I've seen a cap (or two?) near the woofer. I wish I could get my hands on a c.'54 schematic.

[Bruce / Mission Amps]

Yeah now that I hear what you're saying, I think you are right. The last one I had here did have the original PM speaker in it and it did have the parallel wax-paper coupling caps to the little 3.5" speakers.
I think Chuck could be right and it is a cross over, it certainly looks like one.
Some of the old Gibson amps with two speakers would have little 2.2uF to 4.7uF coupling cap to the small speaker... I've assumed just to block the 120Hz and below freqs.
To be honest, none of these builders documented their amps very well.

[Amplitude]

It is a crossover, yes? SO the caps would not be polarized regardless of type. Electrolytics are used because for a given capacitance thay can be much smaller than other types. No cap "needs" to be electrolytic, they just specify them for cost and size. But technology has advanced in the last 60 years. CAps are smaller. Besides, you have TONS of room there. A couple 7uf film caps should be easy to find and will work fine.

Normally a dual cap like that has two positive terminals on one end and a common negative on the other end. However, this looks to me like it could be an end to end pair of caps with the connon coming out the center of the two. hence the +/-. How is it wired in now? ANy chance the two caps are in series across B+, and this was their way to get a higher voltage rating?

What model is the amp, so we might look up the circuit?

Enzo, you are correct on the wiring of the 50uF/50uF cap. I've been informed the cap is part of "a half-wave voltage doubler circuit " (now that's way, way over my head, lol), and is wired like you stated. So I have unstuffed it and re-wired the caps inside correctly. It's a good thing I started second guessing myself and got a few more opinions. Thanks.

[Bruce / Mission Amps]

Interesting... I wish you could reverse engineer this and schem it up for us....
Two caps in series has less total capacitance then the smallest capacitance of the two, or half the capacitance if they are the same value... but the voltage dividing capability of the two caps means that the total voltage is shared between them... so, if they are the same capacitance, the reactive effect is to have each carry half the voltage.
BUT... those caps say 7uF@50v... two in series is 3.5uF@100v, similar to my previous statement about 2.2uF to 4.7uF caps found in some other old amps... so I doubt they are 100v worth of B+ supply filtering.
The two coils are still odd (if not part of the speaker system). Open air coils like this are rarely more then a few mHys not full Henrys like a choke in a B+ rail.... and I didn't think they were needed for a simple capacitor charge voltage doubler... what's that all mean?
I still think they might be part of an audio cross over. ha ha ha.
OK I just reread Enzo's post and I see he is talking about the single multi-cap with three leads... sorry.
That could be correct and makes sense.
Also.. when I check those kind of caps, the molded end where the wire leads come out is cracked.
9 times out of 10, being 50-60 years old, they are BAD.
For reliability, replace them and the (Se) selenium rectifier, even if you aren't sure about their true condition.