Alamo Capri Model 2560

[Goldhedge]

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?? I'm thinking this could be the culprit as its 60uF, 40uF, 20uF seem to be acting as some sort of filter.

[soundguruman]

If the amp is ON, and the volume is turned UP, no input plugged in, it's normal to get 60Hz buzz.
Turn down the volume, is the buzz still there?
The input jacks don't short to ground when the guitar is unplugged, therefore, buzz is normal with the volume turned up( no guitar plugged in).
Now, with the volume turned down, is the buzz still there?
In this 60s era, not a lot of attention is paid to 60 Hz noise in guitar amps. Hum at some level, is normal. It's built that way.
However, caps go bad with age. 20 years or older can be an indication that the caps are going bad...but not necessarily for certain.
There are triple caps, but they are metal cans, and you can buy them from CED / Antique electronics supply.
BUT in this case, separate caps is probably where it's going to...
AND your last problem, you might not be able to buy those tubes anymore, so check first, before you put a lot of time and effort into it.

[Goldhedge]

Thanks!

If the amp is ON, and the volume is turned UP, no input plugged in, it's normal to get 60Hz buzz.
Turn down the volume, is the buzz still there?

Yes.

The input jacks don't short to ground when the guitar is unplugged, therefore, buzz is normal with the volume turned up( no guitar plugged in).
Now, with the volume turned down, is the buzz still there?

Yes.

In this 60s era, not a lot of attention is paid to 60 Hz noise in guitar amps. Hum at some level, is normal. It's built that way.
However, caps go bad with age. 20 years or older can be an indication that the caps are going bad...but not necessarily for certain.
There are triple caps, but they are metal cans, and you can buy them from CED / Antique electronics supply.
BUT in this case, separate caps is probably where it's going to...
AND your last problem, you might not be able to buy those tubes anymore, so check first, before you put a lot of time and effort into it.

The tubes are still good, however I did check at the (AES) Antique shop and they do have them! $25 for all three in fact.

I'm thinking it's the UNE CAP as well because it's an electrolytic - if my memory serves me.

The hum is there at low volume and at max. It's not a high wattage amp. I'm guessing 5-10w maybe?

[Jazz P Bass]

If you want to see what is going on, get out your volt meter.
Set it to read Volts ac.
Black to chassis ground & red to the preamp plates.
What reading do you get?
That reading is the 'ripple' voltage.
If it is high enough it makes the plate voltage ripple & that comes through as hum.
You will need a high voltage cap that you can parallel with the other ones, one at a time.
If the measured ripple drops, then that cap is bad.
Another item that you can check is the coupling caps on the preamp.
They will be hooked up to the preamp plates.
There job is to pass the ac signal voltage yet block the Volts dc from the plate.
One side will measure the plate voltage (dc) while the other side should read close to zero Vdc.(the actual value will depend on the next circuit)
Those tubes are widely available in NOS by the way.

[olddawg]

You are lucky. It's a series filament amp with an isolation transformer. Usually amps with those tubes are widow makers.

[Goldhedge]

You are lucky. It's a series filament amp with an isolation transformer. Usually amps with those tubes are widow makers.

Would a 3 prong plug 'fix' that?

[Goldhedge]

If you want to see what is going on, get out your volt meter.
Set it to read Volts ac.
Black to chassis ground & red to the preamp plates.
What reading do you get?
That reading is the 'ripple' voltage.

12vac pins 3 & 4

If it is high enough it makes the plate voltage ripple & that comes through as hum.
You will need a high voltage cap that you can parallel with the other ones, one at a time.
If the measured ripple drops, then that cap is bad.

I tried a spare ceramic 0.05uF and the hum did not dissipate.

Another item that you can check is the coupling caps on the preamp.
They will be hooked up to the preamp plates.
There job is to pass the ac signal voltage yet block the Volts dc from the plate.

16.24vdc which is across the 0.05uF cap on pins 6 and 7


How are these measurements?

[52 Bill]

Adding a 0.05uF cap across the filter caps will not really do much of anything as far as your hum is concerned. What JPB was suggesting was to parallel an equal rated cap across each of the filter caps to see if the hum would go away. If you worked on amps for a living or if you were an advanced hobbyist with a well stocked parts drawer, you would have a spare cap of similar ratings to do this with.

As the filter caps age they often dry out and basically become open circuits and can no longer remove the ripple from the dc power supply. Temporarily adding a cap in parallel with the existing one is a quick test to see if the original one is doing its job or not.

There are three caps in one tube. Two of them are used to filter the main power supply and one is used to bypass the cathode of the power output tube. You can order a new cap that is similar in construction with three units in one case or you can purchase three separate caps and replace them individually. Either way, you will need to replace all three caps.

[Goldhedge]

Adding a 0.05uF cap across the filter caps will not really do much of anything as far as your hum is concerned. What JPB was suggesting was to parallel an equal rated cap across each of the filter caps to see if the hum would go away. If you worked on amps for a living or if you were an advanced hobbyist with a well stocked parts drawer, you would have a spare cap of similar ratings to do this with.

As the filter caps age they often dry out and basically become open circuits and can no longer remove the ripple from the dc power supply. Temporarily adding a cap in parallel with the existing one is a quick test to see if the original one is doing its job or not.

There are three caps in one tube. Two of them are used to filter the main power supply and one is used to bypass the cathode of the power output tube. You can order a new cap that is similar in construction with three units in one case or you can purchase three separate caps and replace them individually. Either way, you will need to replace all three caps.[/QUOTE]

Ok thanks! I do have a similar 3-caps-in-one tube, but as an 80/40/20uF. I looked hard for the precise measure, but alas it was not to be.

I re-examined the wiring of the cap. The spec sheet for the 35W4 half wave says 40uF for the filter input cap, but they attached the 60uF lead to pin 7...!? I wonder if they did this in error? The 40uF lead is on C7, with the 20uF being on C8.

EDIT: yes the original is correct. 60uF on the half wave.

If I make C7 80uF how will that will effect the 50C5 screen grid? If it absolutely must be 60uF I think I can find one at the local electronics junkyard.

Thanks for everyone's patience. I'm more of a digital-head. What I've been learning over the past year is that tubes are 'living and breathing' - to use an analogy - Digital is hard and cold. I wish they still taught tubes! They actually make sense!

Don't use this updated scheme - it's incorrect.

[52 Bill]

If I make C7 80uF how will that will effect the 50C5 screen grid? If it absolutely must be 60uF I think I can find one at the local electronics junkyard.

Don't worry too much about the exact values. Enzo recently reminded us of the fact that at the time that this amp was manufactured, most electrolytic caps were rated as being +80 % to -20% of the marked value. Modern caps are much closer in tolerance that the old days.

[Goldhedge]

Ok, so I swapped out the old can for the newer can. It's a Sprague and looks fairly new.


It still has that 60Hz hum. The only components left are the ceramics, resistors, and the tranny's.


Any tips on how to find a hum? ;-)


Or is this as suggested 'built to hum'?

It's a learning experience I don't mind having. I wish I had a scope to view the wave forms!


EDIT to reveal a bit of ignorance...!

I plugged in a guitar and guess what! No more hum! Well, there is a little bit depending on the angle of the guitar to the amp - typical, but it sounds great. Has a nice warm overdrive to it.

Thanks everyone who helped guide me. I hope the schematics will help someone else in the future!

This is the correct schematic

[soundguruman]

Ok, so I swapped out the old can for the newer can. It's a Sprague and looks fairly new.


It still has that 60Hz hum. The only components left are the ceramics, resistors, and the tranny's.


Any tips on how to find a hum? ;-)


Or is this as suggested 'built to hum'?

It's a learning experience I don't mind having. I wish I had a scope to view the wave forms!

Did somebody put a 3 wire power cord on it? The heaters need to be lifted off ground if the power cord is changed to a 3 wire grounded one.
Other than that, you could send it here, but you have to pack it carefully...

[Goldhedge]

Did somebody put a 3 wire power cord on it? The heaters need to be lifted off ground if the power cord is changed to a 3 wire grounded one.
Other than that, you could send it here, but you have to pack it carefully...

No, but that is what I should do to it.


How do I lift the heaters off ground?

[Enzo]

There are many sources of hum, and each has its own cure. I have discussed this before. If you have a grounding problem, all the extra filter caps in the world will do nothing to help it. And likewise, if your filter caps are weak, all the extra grounding and shielding in the world will not help. You have to apply the cure for the kind of hum you have.

SOme reactions in no particular order.

The 40uf and 60uf are your B+ filters. note the 20uf is only 25v, it is not a filter, it is the cathode bypass cap for the power tube. Some older products, many in fact, used the aluminum can mounted to the chassis for multisection caps. This format you have is not unusual, other than in guitar amps we see fewer of them. A cardboard cylinder with wires out one end. You can find caps like that, usually dummied up from smaller caps. But usually i just replace those with individual caps. A modern cap is lots smaller anyway. The 22uf 25v caps are common in Fender amps and elsewhere, and are not much larger than a resistor. That would make finding a 40/60 easier.

Those main filters are only rated for 150v, so this whole amp is a lower voltage affair.

Your drawing has a mistake. The real drawing looks right. You have C7 wired to the screen grid of the power tube, right. But then you have R7,R8 from the preamp tube wired to the suppressor grid on the power tube. They should be wired to the screen grid too, or at least C7 somewhere. The suppressor grid of the power tube is connected to its cathode. Since the amp works, I will assume the error is only on the drawing.

Jazz asked you to take some readings of AC voltage. We expected AC on pins 3 and 4 of the rectifier, since they are AC heater voltage from the transformer. I think what he wanted, and I agree, was reading the DC voltage at pin 7 of the rectifier, and expecting close to 150vDC, but then switch the meter to AC volts. Ideally we would get zero volts AC, meaning zero ripple. That won;t happen, but the less we get the better. I'd expect a few volts of ripple there. But on the screen grid supply, I'd expect basically no ripple - no AC.

Cap values are not critical. And the factory made no error in wiring. The book may say 20uf or 40uf for the rectifier, but guitar amps break those rules all the time. Leo fender made amps with the voltage on 6V6s 100v higher than the book said. Did it for decades. The screen doesn;t care what size the cap is, all it wants is smooth DC there.

Ripple is just one source of hum. This amp is basic, with only twp high voltage nodes. Measure the ripple on each of those nodes. If ther isn't any, then more filters can't reduce it further. We need to know that, so we either DO have ripple and can work on it, or we do not, and we should move on and waste no more time.

Where did you get the pin numbers for the 50C5? My book shows the plate as pin 7, the cathode as pin 1, the control grid as pins 2 and 5, and screen grid as pin 6. The suppressor is internally wired to the cathode.

Ripple: there is only so clean we can get the first filter stage to be. To get the first node clean, we'd probably need to add another filter stage, not just larger caps. But wired this way, a little hum is inevitable. This is a single ended amp, not push pull. Push pull amps by their nature cancel out hum in the power tube stage. A single ended amp cannot do that. There wil always be a small amount of hum in most single ended amps, at least the ones that draw plate voltage from the first filter.

But there are other hums. Turn the volume to zero, any hum reduction? I see a two wire power cord, it should be made a three wire. But until then, does turning the plug around in the wall socket make a difference?

[Enzo]

Please explain why the heaters would have to be lifted off ground. They are on the secondary side, they are not connected to the mains at all.