El Basso

[dkevin]

A friend of mine was recently gifted a Massie Lancer model 363 also referred to as an El Basso amplifier. I was tasked with getting it up and running. It came with 5881 power tubes and 12AY7 preamp tubes. According to the schematic (ahem) it was supposed to have 6V6's and 7025's. I got the amp up and running but there were a few bugs. I replaced the filter caps and cathode bypass caps but a steady, low frequency hum persisted. It did not become louder with volume but seemed to disappear with the volume wide open. I replaced a few leaky interstage caps but the hum endured. Finally, I rewired the tube heaters and the heater windings gave up the ghost. I wonder if the hum was a warning that the windings were about to poop out. Have any of you had similar experiences? As a footnote, I decided to rewire the heaters because one leg of the heaters was grounded and the heater wires were connected without regard to proper phasing...ie 6v6 pin 2 to 6v6 pin 7...etc. I will post a schematic and a few pictures for the curious.

 

[robrob]

Out of phase push-pull power tube heaters can cause hum that isn't affected by a volume or master volume control. I'd triple check my new heater wiring before trying a new heater transformer. Most likely the new wiring has a problem (probably a short).

 

[jhundt]

I'd love to see a picture of that amp. It sounds like the first amp I ever had (actually it was borrowed) back in the 60's. It seemed so big and loud back then. The neighbors hated it! Does this one have a closed back?

 

[dkevin]

https://imgur.com/ff2jKkO
https://imgur.com/gZs4Ljl
https://imgur.com/erm3j5n
https://imgur.com/ay7seDl
https://imgur.com/NRAsc7n
https://imgur.com/9PLJhQ0
https://imgur.com/EzfgNbT
https://imgur.com/NbemXn1
https://imgur.com/NoYyOlr
https://imgur.com/uheKMIu
https://imgur.com/vJ9GisL

The amp cabinet is closed-back with a 15" speaker..it weighs a ton!! During the time it was working it was very loud. Could running a pair of 5881's ( in place of the 6V6's it was designed for) have shortened the lifespan or weakened the heater windings?

 

[jhundt]

when you rewired the heaters, did you add a virtual center-tap? Like Fender does, with 100 ohm resistors from each side of the heater winding to ground. You need those.

 

[jhundt]

https://imgur.com/ff2jKkO
https://imgur.com/gZs4Ljl
https://imgur.com/erm3j5n
https://imgur.com/ay7seDl
https://imgur.com/NRAsc7n
https://imgur.com/9PLJhQ0
https://imgur.com/EzfgNbT
https://imgur.com/NbemXn1
https://imgur.com/NoYyOlr
https://imgur.com/uheKMIu
https://imgur.com/vJ9GisL

The amp cabinet is closed-back with a 15" speaker..it weighs a ton!! During the time it was working it was very loud. Could running a pair of 5881's ( in place of the 6V6's it was designed for) have shortened the lifespan or weakened the heater windings?

did you ever figure out the heater problem?

 

[dkevin]

I ordered a new 6.3vac transformer (costs as much to ship as to buy) and rechecked my filament wiring. It is up and running and sounds good. I found some real-estate in the center of the chassis and bolted it up there. The amp voltages seem within reason and I am getting ready to send it back to Tim. It is a lot quieter than it was before...the low frequency hum is greatly diminished. I had revoiced the amp reducing the PI interstage caps from .22uf to .1uf. I hope he is happy with the amp but if he wants more help, he knows where to find me. I asked him to keep the cabinet while I worked on the amp chassis. The cabinet must have weighed 50lbs without the chassis installed. https://imgur.com/a/JCPQR

 

[jhundt]

did you replace those cap cans? In your first post you said you replaced the filter caps, but the photos only show the old cans, I think.

 

[dkevin]

I spliced in 3 new e-caps at first when I suspected the original cans of causing the low frequency hum..but when I realized they did not, I reconnected the original cap cans. I will let them roll as is. I don't usually do that (reuse original e-caps ) but space is at a premium because the filament transformer takes up a lot of room.

 

[jhundt]

do you trust those old electrolytics? They must be 55 years old by now. Normally I would replace something like that just as a matter of routine maintenance and safety.

 

[dkevin]

Not my first choice...when I went to order my individual caps, the F&T 33uf/500 caps were out of stock and back ordered. I used some lesser caps to troubleshoot (33uf/450) but the originals worked fine. This amp will be a living room amp...ie. not gigged with, so I feel OK about going with the ones it already has. I am also juggling the "lack of space" issue due to the addition of the filament transformer. If he needs them, he knows where to find me.

 

[dkevin]

Tim came and picked up his amp. We listened to it on the bench (disassembled) and put the chassis back into the cabinet (did I mention that cab weighs a ton??) and tried it again. Channel 1 input sputtered and stopped working. Tim says he won't use that channel anyway...but it bugs me. I'll look at it another time. Channel 2 sounds rich and full. The speaker in this amp absolutely roars. This amp is one of the loudest 2 ea. 6V6 amps I have ever heard. T tell me he loves it! He hasn't been able to play it above 3 or so in his house. He's gonna play it for awhile and bring it back for a checkup. I'll take a close look at the input jacks for channel 1. As loud as it is now, I can't believe someone put 5881's in it. All-in-all it was an interesting amp to check out.

 

[nasdak]

Change the filtering caps !
You don't want them to short and fry your pt or tubes...

 

[charlie chitlin]

Change the filtering caps !
You don't want them to short and fry your pt or tubes...

Yup. Those old cans gotta go.