Alamo Capri 2560 Problem: No Audio

[MrG]

I have a Capri 2560 that powers up but has no audio i.e. no hiss, no hum etc...

Opened it up and found the 35w4 rectifier tube plugged into the 12au6 socket and vice versa. The power tube(50C5) is correctly plugged into the center socket.

What would happen if the pre-amp 12AU6 and diode 35W4 tubes were accidentally plugged into each others socket and then powered up?

One of the first things I checked was the speaker. I disconnected the leads and checked with an ohm meter and found it to be open.

What would be the best way to approach this problem? Are any of the tubes toast? Could this blow the speaker too?

The speaker looks to be a 8 inch. What impedance (4, 8)? The original speaker only shows a part number from Alamo Electronics.

I sure would like to rehab this old amp.

[Chuck H]

Rare to see an amp with that tube compliment that has a power transformer!

Start by removing the tubes and discharging the power supply filters. Then inspect for any obvious problems like burned stuff, bulging or leaky caps, tubes that show white and powdery where they should be reflective, funny smells like a shorted toaster, etc. When anything obviously bad has been corrected and a working speaker has been hooked up you can plug it in and take some voltage readings.

Here's the schematic:

[MrG]

Dropped off the original speaker at a shop that does rebuilds. $30 IF he can repair it. Otherwise I'll need to replace it. Nine inch diameter if I remember. Anyone know the impedance? I'd guess 4 ohms but... Until I replace the speaker I want to have the correct load on the drive (speaker or 10w resister) before I swap the tubes and power it up.

[Chuck H]

You can actually determine the likely impedance of the speaker by testing the output transformer. You'd need a way to generate low voltage AC, preferably between 400Hz and 1200Hz and capable of a little current into a load, and a volt meter.

[Enzo]

8 ohm, 4 ohm, won;t matter, it won;t hurt the amp to be "wrong."

[MrG]

Update:
Relocated the pre-amp and power tubes to their proper sockets, hooked up a 8 ohm test speaker and powered up the amp. Happily, I heard some static and 60Hz hum. At least there is audio now, and no smoke!

Injected a 400Hz tone and checked stages with the oscilloscope. Clean form and stage gain with the exception of the final output. Waveform was a little sketchy probably due to hum getting in there.

I hooked up a 4 ohm speaker from another amp cab, plugged in a guitar and tried it out. Fairly clean sound, 60Hz hum notwithstanding. The volume level output seems low though, even for this small amp.

Any thoughts on the output level? I have voltage readings from the circuits if needed.

[The Dude]

It's likely your filter caps are dried up given the age of the amp. Check the AC content at the filter caps and/or tack in good one(s) to see if the hum goes away. Or, you could just replace them since they are likely past it anyway. As far as output level, having only the schematic for reference, I notice that the B+ caps are only rated for 150V. This tells me that the B+ is something less than 150VDC. One could then infer that the amp is not going to be horribly loud. How loud is hard to tell without seeing actual voltages on the schematic and a few other specs/readings.

[Justin Thomas]

For fun I looked up the data sheet for a 50C5... class A1amplifier - 120v on plates, 2.4W signal power output. Max plate volts is 150, plate dissipation is 7W. The original schemo seems to say B+ is 140, to my young eyes... Yeah, probably not going to be very loud at all. Might be a great little recording amp, though! Good luck!

Justin

[The Dude]

Yes, agreed on the recording amp thing. I have a little amp like this at home. For the life of me, I can't recall the brand or model right now, but it's a simple little thing with a couple of tubes built probably in the late 60's. A friend of mine found it cleaning his barn and gave it to me. The pots were all completely frozen and it was beat to hell. It has a little 8" speaker. I cleaned it all up, got the pots unfrozen and cleaned, checked all of the tubes, etc. and it works fine. It sounds amazing dimed and recorded. Real nice breakup at low volume.

[Enzo]

The tubes are right out of an All American Five, so this amp is basically a table radio,and will have similar performance.

[MrG]

The tubes are right out of an All American Five, so this amp is basically a table radio,and will have similar performance.

That's a good point. I figured a guitar amp should be "loud" but maybe not in this case.

Amp is working now but has a considerable amount of 60 cycle hum that probably should be looked into. Changing all the electrolytic caps would be a good start. I do not currently have an easy way to test these size capacitors.

Here are a few observations and readings: (BTW, this my first tube repair project)
Waveform on pin 7 of the rectifier tube is saw-toothed so the cap (currently a 85uf) is doing something but I would expect a smoother wave at that point.

Power tube (50C5) plate voltage is 160V. A check of the mfg. data sheet shows a max of 135V.

Pre-amp (12AU6) plate voltage (pin 5) is 80V. Grid #2 (pin 6) is 150V.

Thanks for indulging my on going quest for knowledge.

[MrG]

Thanks to those of you who offered advice. After checking voltages and wave forms, I ended up changing out the big three electrolytics but because I couldn't find exact values I had to use a 22, 47, and 66 mfd in lieu of the 20, 40, 60 mfd as on the schematic. I had to build up the 66mfd B+ filter using three 22mfd caps.

Because the rectifier tube showed signs of overheating I decided to replace all of the tubes with NOS replacements from a seller on ebay for about $15.

Though there is still some hum in the audio, it's not nearly as bad as it was. Scope shows cleaner waveform and much higher gain than I was seeing before. The final audio level is much louder now, very warm tone and nice breakup at full volume.

Still waiting for word on repairing the original speaker. I may have to get a replacement.

All in all, a fun project.