Alamo Capri Model 2560

[Goldhedge]

Hi all. I just acquired this little 3 tube amp. It's from the 1960's. All original and fairly clean. The circuit is really simple compared to what goes on here.

It has a 60Hz buzz and I'm not sure what could be causing it.

After perusing here for the past year I've actually learned a few things - twisting wires to reduce hum, shielding inputs etc. but this is a unique amp. Note the wiring.

I just tested the tubes and they all came out in the green zone on an RCA tube tester. (There's a recycled electronics place in town with every component one could imagine and they have the RCA unit there.)

Tubes:
It uses a 12AU6 pentode sharp cutoff for the input
A 50C5 beam power amplifier to power the speaker
and a 35W4 half wave rectifier to heat it up.

The power rail has no resistors to alter the voltage so they all get 90v right from T1 (according to the schematic).

The unusual component is the "UNE CAP". It's 3 caps in one!

If I were to guess, I'd have to say the UNE CAP is the noise inducing culprit. Either that or the lack of proper wire routing? Though I assume they built it with no noise and the wires haven't moved in 60 years. Most of the caps are ceramics.

There is only one manufactures schematic which is nearly impossible to read, so I used DigiKey's "SchemeIt" online program to create the schematic as best I could. It wouldn't allow values to be inserted so I had to use an editing program to effect that. The "35W4" icon on my drawing isn't schematically correct, couldn't find a halfwave. The on/off switch is in the Tone pot.

I also took photos of the inside wiring etc.

I haven't measured voltages yet, but will get to that eventually.

Any suggestions as to what to check? Correct? I'm not sure if I can even find that UNE CAP anywhere, so I may have to use 3 caps if removed??

[Goldhedge]

some more pics

I did search the site, but didn't see this particular Alamo model. Thought I'd add it to the archives as there's nothing on the net about it.

I'm thinking that the 'UNECAP' is acting like a filter for the half wave rectifier.

[xtian]

The hum is likely because of the aging electrolytic caps (the "UNI" cap). Replacing the elecrolytics is standard maintenance for vintage amps.

[Structo]

I'd probably go with something like this.

http://www.tubesandmore.com/products/C-EC60401025

Or if you want to keep the firecracker style, this one.

http://www.tubesandmore.com/products/C-ER33-33-47

[Goldhedge]

I'd probably go with something like this.

http://www.tubesandmore.com/products/C-EC60401025

Or if you want to keep the firecracker style, this one.

http://www.tubesandmore.com/products/C-ER33-33-47

Thanks!

It has a holder for the firecracker style and would be an easy replacement. I think I'll give that a try. The cap you suggest isn't the same uF's. Would that be a concern? Might be forced into the 'can' version...

Just curious about the ceramics, would they ever go 'bad' like the electrolytics?

There's really not much to this design. Pretty simple.

[xtian]

The cap you suggest isn't the same uF's. Would that be a concern? Might be forced into the 'can' version...

No problem. In very broad strokes: if you use too little filtering, you may get hum. Too much and you may get a "sterile" or less responsive amp. If you have a tube rectifier, there is an upper limit to the filtering at the first (reservoir) cap.

Just curious about the ceramics, would they ever go 'bad' like the electrolytics?

No. Electrolytics go bad eventually because they're made with a goopy paste which dries out over time. But ceramic and film caps do not dry out, and should last much longer.

[Structo]

Also tube rectifiers have a maximum filter size that can be used on the first filter.
This is due to the voltage at startup.

The 35W4 tube data sheet says 40uF max.

This is usually not etched in stone but should serve as a guide for filtering.

Also most E caps have a 20% tolerance at least.

You could also buy individual caps and mount them point to point.

Data sheet for 35W4
http://www.mif.pg.gda.pl/homepages/fran ... 3/35W4.pdf

[Goldhedge]

Also tube rectifiers have a maximum filter size that can be used on the first filter.
This is due to the voltage at startup.

The 35W4 tube data sheet says 40uF max.

This is usually not etched in stone but should serve as a guide for filtering.

Also most E caps have a 20% tolerance at least.

You could also buy individual caps and mount them point to point.

Data sheet for 35W4
http://www.mif.pg.gda.pl/homepages/fran ... 3/35W4.pdf

Interesting.

I just rechecked the wiring of the cap and they put the 60uF lead on to pin 7 of the 35W4 halfwave! I wonder if that would cause a problem...?

[Firestorm]

No. That's correct. See their schematic.

[Goldhedge]

Thanks!

I installed the cap and it works! Had some hum, but then I plugged in a guitar and it went away. Has a nice warm overdrive on the neck pickup. Not too loud. I'm thinking this is about a 2 to 5 watt amp.

It comes and goes depending on the angle of the guitar, but that's typical for these old amps...

The 60uF is on the half wave though the data sheet states 40uF. Maybe that's how it buffers the DC? The cap has 80uF so that should help it even more? I'm thinking back to filter caps on a rectifier and the larger the reservoir the better the filtering.

[Firestorm]

Cool. There will always be a little hum in an SE amp. Rectifier tubes don't actually have maximum capacitance ratings for filters; they have maximum current ratings. 40uF for this tube is only a "typical operation" number. 60uF is fine at these voltages, especially since the tube has an indirectly heated cathode and no standby switch. Maximum Class A output for the 50C5 is 3.5W, so you probably have about 3W.

[surfsup]

Did you ever measure voltages? It looks like from the schematic from the company there is supposed to be ~150V going to the transformer and the PT plate has ~140V on it. Are your voltages similar? Maybe that first cap is leaky...C6?

Oh I see you have 140V on your schematic as well. Was that measured or copied from the original schemo?

Looks like C6, C7 and C8 are the three only electros. As xtian said if you're in there and you don't care about "vintage" and rather want a working amp, just swap 'em for new ones and I'd at least separate the cathode cap as a separate one.

Do you still have the 5n on the leg of the powersupply?

[Goldhedge]

Did you ever measure voltages? It looks like from the schematic from the company there is supposed to be ~150V going to the transformer and the PT plate has ~140V on it. Are your voltages similar? Maybe that first cap is leaky...C6?

Oh I see you have 140V on your schematic as well. Was that measured or copied from the original schemo?

Looks like C6, C7 and C8 are the three only electros. As xtian said if you're in there and you don't care about "vintage" and rather want a working amp, just swap 'em for new ones and I'd at least separate the cathode cap as a separate one.

Do you still have the 5n on the leg of the powersupply?

New cap is installed and measurements taken. The 140v and 90v were from original scheme.


I updated the schematic I drew off the original. I think it is correct this time. I included the pin 2 leg to pin 6 on the 50C5.

I also measured DC and AC voltages. They are in blue as indicated.

Pin 2 on the 12AU6 looks to be going to ground? or at least it appears to, because there is a wire soldered across to pin 7 (see pic). The original scheme shows a C1 0.005uF cap off of the input resistors, but I don't see one on the amp. I do see C2 which ties into pin 1 and the 2.2M resistor.

I did measure a 12vac off of pin 2 which means it isn't going to ground.

Doing my best to decipher this schematic vs the actual amp. I'd like to get it as correct as possible.

[Goldhedge]

Just curious about putting a 3 prong plug on this amp.

The only benefit I see (other than not killing myself) is to keep the correct polarity on the transformer input. Hot on the hot side and ground on the ground side of the chassis.

With the 2-prong plug one could be electrocuted if touching ground while fiddling with the amp.


Is there anything else technical I need to consider before attempting this?


And thanks for all the advice and help.

[Goldhedge]

Looks to be the same circuit - same tubes, two transformers, however on this one the tranny isn't right off the AC.

Must have been a popular inexpensive circuit in its day.

Same 60Hz hum.

I ordered the firecracker cap and installed it.
33uf, 33uf, 47uf at 160VDC

The old was a Temple 18-57-2
Red 100Mfd
Yel 60Mfd
Grn 30Mfd
Black common

I also put a 3 prong grounded plug in and grounded the chassis.

Turned it on and nothing!

Not sure why I get no sound.

Tubes heat up. The 12AU6 takes longer to heat, but eventually it does.

Any suggestions? Does the cap need to have closer values? Too small?