1962 Magnatone Custom 410 low output power, thumpy vibrato

[glebert]

Working on a project amp I picked up a while ago. Generally works, but not getting the magic Magnatone sound I expect. This is supposed to be a 5 watt amp, most I can get out of it before serious distortion is 2 watts into speaker or resistive load. Replaced the tubes with new (or at least newer ones) but no change in output power. I don't have a 12DW7 at this moment for the vibrato and have a 12AU7 there (this is what was in amp when I got it). I don't know that that tube should factor into output power if it is just for the vibrato. Vibrato seems thumpy, without noticeable pitch shift, but that may just be because it has the wrong tube (?).

I haven't worked on amps this old before, or tube rectified amps in general. I haven't opened it up yet to check anything. What would be normal suspects? Low voltage due to either bad rectifier or leaking caps? Anything in particular to look for?

https://www.magnatoneamps.com/schema...natone_410.pdf

Thanks in advance,
Greg

[Rhodesplyr]

This amp is 58 years old, and with amps this age, you really need to take what I call a "global" approach, which means that you can't take anything for granted. To be honest, I wouldn't even turn on an amp this age without opening it up first to check its operating condition.

You may have leaky interstage coupling caps, worn-out B+ filter caps, severely drifted resistors, etc . . . The death cap needs to go.

The schematic shows 10uF filter sections, and those are small filter values. If they have lost capacitance or increased significantly in ESR, they will not be doing their jobs.

The high-mu section of the 12DW7 is used as the vibrato oscillator, so I wouldn't necessarily expect the vibrato to sound right with the wrong tube section as the oscillator. Sometimes you have to rebuild the whole oscillator section in vintage amps to get the oscillator to work correctly. And like everything else, an oscillator circuit depends on a healthy power supply. Then there are the varistors, which we hope are not damaged since they are not easy to replace.

It could also be a bad output transformer, but we always hope it isn't and that other repairs will make the problems go away.

I would first make sure there are no leaky interstage coupling caps and that the power supply is in good shape.

[glebert]

Took some gut shots and power supply voltage measurements. I have 375 VDC at first power supply node, 353 VDC at second, and 254 VDC at third. So the first two are above what is called out on the schematic and the third is about right. I have 122 VAC at the switch.

As you can see in the pictures the 20uF section of the multi-cap is not being used and has been replaced by the silver 35uF silver Sprague cap. It looks like a handful of resistors have been updated at some point as well. I will plan on replacing the paper and formed paper caps, the disk caps I believe are ceramic and may be later replacements as well. The story I got with the amp is that it came from the estate of a guy who collected antique radios. I knew that it had been powered up not long before I bought it and I had taken a cursory look inside before I powered it up.

Oh, and death cap removal and 3 prong cord are also happening. Oh, and a fuse.

[glebert]

Thinking out loud, but was wondering if I should expect to see 5w coming out in the configuration I have it in. The amp has an jack for an external speaker which just in parallel, not a switchjack. The amp had a back panel on it which would have made it inconvenient to disconnect the internal speaker. Is is possible that full power only comes on when this has an even lower output impedance? I also don't have a tweeter in mine (some had them, some didn't, the ones that did all blew supposedly). I have a 16 ohm extension cabinet, I could try hooking that up and see if my output power goes up some.

[g1]

I had thought about the same earlier. Whenever I get an oddball low power amp that is delivering even less than I expect, I try different impedance loads to see if that is the issue. Sometimes they are not optimized for max. power output.

[The Dude]

A less than ideal impedance match is certainly a possibility. However, whether the tweeter is there or not shouldn't make much difference as far as output impedance. It has a crossover cap inline, and has little effect on anything except high frequency.

[g1]

I have a 16 ohm extension cabinet, I could try hooking that up and see if my output power goes up some.

Can you cobble together a 2 ohm 5W resistor?

[glebert]

Can you cobble together a 2 ohm 5W resistor?

Yes, or I can put my 4 ohm resistor in parallel with the internal speaker. I figured I'd sneak up on it a bit by doing 4||16 and 4|||8 first to see if it looked like it was trending right.