Early '60's Alamo Paragon #2567, combo with low output and crossover distortion

[jskeen]

A very good customer brought this thing in to me last week when he picked up a hot rod deville I had recapped for him, and I said I would see what I could do, but no promises :-) I know schematics seem to be nonexistant, But the amp actually works, and he wants to keep it as original as possible, so I'm not contemplating a shotgun recap or anything, but on either channel, the output has to be dimed to get anything out, and it's very low and distorted. Initial assumption is that it's only running on one output tube. swapped in a new set of 6l6's, same thing. So before I get any further, I need to get some basic info. The tubes that came in it were 12ax7, 12ax7, 6sl7gt, a pair of newish TAD 6l6wgc's, 12ax7 and an illegible sylvania that I assume to be a 5y3. All 3 12ax7's are vintage looking tungsol's.

So first question. All the tube sockets are rubber isolated from the chassis, but painstakingly grounded via wires soldered to the screws and shields. So it's not a hot chassis. The original 2 wire cord is rotted, so I assume it is safe to add a modern 3 wire and tack the green onto an existing chassis ground, AND REMOVE THE DEATH CAP? Any objections? So why go to the trouble to rubber isolate the tube sockets then even more trouble to ground them by hand? Mechanical isolation?

Ok, so once I get safe 120vac back into the amp, I need to start chasing down what's up with the output. From what I read, the output tubes are setup with the cathode grounded, a grid leak resistor providing bias control. What should I expect to see as far as voltages on the output tubes to verify that both are biased on and roughly equal, and where would I look for outputs on the inverter? This thing is so compact and the wiring is so overlapped that I haven't made heads or tails of it yet. So, rather than just randomly poking about and drawing bad conclusions, I figured I would ask the experts. Any assistance is most appreciated.

And of course, I took a few pics :-)

[Enzo]

Mechanical isolation?

Exactly that.

The fact the socket frames are mechanically isolated but still connected to chassis does not tell us the chassis is not hot. We can infer teh chassis is not hot from the tube complement. Hot chassis amps have series heater strings and tubes like 35W4 and 50C5. You have a 5v rectifier tube and 6v heaters on the others. That pretty much guarantees a power transformer.

I would start suspecting coupling caps between stages gone leaky, putting DC where it don't belong.

I would go down the row of tubes checking for reasonable B+ on each plate.

[jskeen]

Well, This is one of those times when I almost hate having fixed something. I checked for HV on all the tubes, and everything looked good, so I piped in a 1kh tone and chased it through the stages and it all seemed good. Hmmm, so I unsoldered the mess on the speaker lugs and ohmed the speaker at 6.0 ohms. hmmmm Little lower than expected, so I jumped the output over to a known good 8 ohm speaker, and it sounded fantastic. Darnit, garbage, spit, dirty diapers and other assorted foul language. Went ahead and pulled the speaker, and sure enough, the voicecoil is rubbing with only about 1/8 inch movement. I hate seeing a beautiful vintage speaker with a perfect cone jammed like that. Back to the owner to be sent to the recone shop of his choice. Pop in an old Peavey driver I had sitting around for now and finish checking it out.

Low and behold, the tremolo don't work. Yes, i plugged in the footswitch, and made sure it actually switched. tried a new 12ax7, no dice. if I switch the tremolo footswitch on, crank the trem volume all the way, and run the speed pot back and forth, I can get a little bit of scratch from the pot through the speaker, which goes away if I hit the footswitch, turn the trem volume down, or pull the tube. So Im guessing the trem circuit is conducting, just not doing what it's supposed to. I understand that the alamo trem circuit is unique and patented, but anybody have an idea what it encompasses and why it might do everything but actually change the output?

Thanks again for the Hint Enzo, it got me past the hangups and on to the main problem :-)

[Enzo]

Have we replaced any caps yet? In my experience, when a trem doesn't trem, it is usually the caps.


6 ohms DC resistance - what your meter measures - is typical for an 8 ohm speaker. 8 ohms is the nominal IMPEDANCE, not resistance.

[jskeen]

The only cap I have touched is the one from the input of the fuse to ground. Gotta get that one outta there. But I'll look around and see if I can find where the RC network that drives the oscillation of the trem driver tube is and look at the caps in there. Of course it's hidden behind that goofy push button bass boost/eq board from what I could see wiggling wires.

Surprisingly, all those original 50 something year old sangamo filter caps seem to be hanging in there, it don't hum, buzz, crackle or pop.

As for the speaker, I usually see readings between 6.4 to 7.2 DC resistance on a speaker rated for 8 ohms impedance. 6.0 dc ohms may not be impossible, but it's unusual, at least. but the killer is the mechanical resistance of the cone to movement and the audible scraping and grinding of the voice coil and tube inside the magnet gap. poor thing either got too hot, or mechanically damaged, or something, and I suspect some of the coil is shorted, reducing both the resistance and impedance.

Which begs the question. Somebody tacked in an external speaker output jack in the back and soldered it in parallel to the existing speaker. SO, if they were using a 8 ohm extension cab, the OT was seeing a 4 ohm load. That being the case, would we expect to see more or less current in the voicecoil of the internal 8 ohm speaker than it would usually see by itself?

Thanks
James

[mozz]

That's not a 5y3, that's a 5u4gb. Alamos are known to be big time Fender copies. I have seen Fenders with rubber grommets under all tube sockets and ground wires from the sockets soldered right to the chassis also. Change out any of those cardboard caps no matter how they test, you are asking for trouble if you don't.

[Enzo]

The footswitch jack probably needs to be shorted across, if the shunt contacts are dirty or bent, it won't trem. Check.

The green Sangamos are not filter caps, they are paper or film caps. (The cap fans can say which) Check for DC on one end and no DC on the other. (exceptions are possible.) Those yellow caps are electrolytics and are real suspect.

All the Alamo drawings I have show a similar trem circuit, three 0.02 caps for the feedback loop.

[jskeen]

Well, I got that tone matrix? bass boost module? whatever that mechanical monstrosity is, out of the way and spent some time wiggling green and black wires to see where they go, and it looks like the RC network that drives the Tremolo is actually those 3 purple .05 at 600 paper caps. They are the only .05 caps in the amp, while most of the rest are .02, which begs the question. Why would they go to the trouble to source and use larger caps for the tremolo, when they already use a bunch of .02's in the rest of the amp, and Per Enzo, they use .02 in their other amp trem's? The Paragon was Alamo's top of the line amp, and I think it had the highest output power rating of the line at the time as well. Does output wattage effect the design parameters for a tremolo circuit, if it is the type that works directly on the output tubes? Is that why later, higher powered amps like fender twins and such went to the optical type design, to decouple that somehow? Inquiring minds want to know. :-)

Oh, and as a side note, that whole little push button tone thingie actually takes an input off of the volume pots, does whatever it does to it, and then feeds it all the way down to the positive speaker terminal! Not sure what's up with that either, anybody know of any other amp that has that sort of arrangement?

But on the positive, side, 3 new orange drops fixed the tremolo circuit right up, and it's now ready to go back home and wait on the reconed speaker to return it to original (or as close as possible for now).

[g1]

Oh, and as a side note, that whole little push button tone thingie actually takes an input off of the volume pots, does whatever it does to it, and then feeds it all the way down to the positive speaker terminal! Not sure what's up with that either, anybody know of any other amp that has that sort of arrangement?

I think that is going in the opposite direction. A sample of the output signal is being routed back to the volume pot, some kind of selectable NFB circuit (or presence or resonance etc.)

[jskeen]

You are obviously correct, no way an unamplified signal off the volume pot is actually gonna effect the final high power output signal at the speaker. I was typing faster than i was thinking, at that point :-)