Bell 2122 mono amp

HomerJSimpson

HomerJSimpson

Addicted Member
I have this Bell mono tube amp that I am tring to get going for a friend. There are 2 wires coming out by the transformer. One goes to the chassis. the yellow one goes to the transformer. The yellow one measures -400vdc.

This amp has no sound. All the tubes light up.

My question is: is the yellow wire supposed to be connected to the brown wire?

This amp has been recapped, and looks like this is a new transformer too.

I don't want to blow anything up including myself!
 

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Short answer: there's no way to tell from your post. It's like asking "my car won't start. Should I connect the brown and yellow wires to fix it?"

400V is a lot of volts to have on a bare wire outside the chassis, don'tcha think?

If the brown wire goes to chassis, and the yellow wire is 400V, and you connect them together... I'd have to say no, it doesn't seem like a good idea to apply 400V to a metal chassis with an ungrounded plug. But maybe I interpreted what you are saying incorrectly.

I don't know what's gotten into people today 'cuz this is the 2nd time I've given this warning, but THESE ARE LETHAL VOLTAGES we're talking about here. If you don't understand what you're doing, a really really good rule of thumb is: DON'T DO IT.

These amps contain resistors, capacitors, the odd choke, transformers, and a rectifier of some sort in addition to the tubes. They're all simple passive components which can, with the exception of the tubes themselves, be checked with rudimentary equipment such as a DMM. Diagnose it like you'd diagnose any piece of any type of electronic equipment. Is it getting power to the transformer? Do you have power at the secondaries? Is the rectifier working? Are there inline fuses and/or diodes and/or fusible resistors which have done their job and gone open circuit? Have any signal path coupling caps failed? Do the tubes glow when it's turned on indicating that heater voltage is present? Have you measured the output transformer's primary and secondary for conductivity to ground?

Good luck.
 
Tubes light up = heater voltage present
400V on yellow wire = HV present

Look up the pinouts for the various tubes, check that voltage is getting to the plate of each tube. If it is, check out all of the interstage coupling capacitors. Check all of the plate resistors for tolerance. If there's any way to easily bypass the tone control network, do it, it's one less thing to worry about. You know to NEVER power up a tube amp with no load (speakers or BIG power resistor) attached, right? Tube amps handle a dead short across the speaker terminals better than no load. Check the output transformer as I suggested, you'll likely need to de-solder the secondary side where it goes to ground (chassis).

Post results.

And please cap that yellow wire.

Oh, and where does the yellow wire come FROM? I mean what does it connect to underneath the chassis? Same with the brown? Take a picture of the underside of the amp and post it, it'll answer some questions.
 
dspear99ca said:
Oh, and where does the yellow wire come FROM? I mean what does it connect to underneath the chassis? Same with the brown? Take a picture of the underside of the amp and post it, it'll answer some questions.
Click to expand...

for the 3rd time now:

Brown wire to chassis

Yellow wire from transformer.
 
By the usual transformer color code, a yellow wire would be rectifier filament, and would NOT ever be connected to the chassis. But this might be a non-standard transformer, or a random color wire spliced on...

The center tap of the high voltage winding does connect to chassis ground (ideally the minus terminal of the first filter cap). And it's POSSIBLE that somebody had wired an eternal STANDBY switch between those two points... but I'd do a bit more investigation before connecting anything.
 
I don't know how he could have DC volts coming from a power transformer winding... maybe it's an output transformer that it's coming from.

Sorry about asking the same question, I assumed that one end of the brown wire connected to chassis, I wanted to know where the OTHER end connected to. I am guessing both wires come from the transformer. It very well may be an output transformer if there's DC on it. If the B+ was not connected, it might also explain the lack of sound.
 
The yellow wire is actually spliced on to a red wire coming out of the tranny.

I tested the DC voltage across the yellow (red) wire and the chassis.

The wire is coming out of the POWER transformer.

it is a replacement transformer.
 
Ahhh, now we make progress. Commonly you'll have two red wires, or a red and a red/other going from your transformer's high-voltage secondary winding to the rectifier plates. If you're running a 5U4GB or 5AS4 or similar this would be pins 4 and 6 on the rectifier tube. A third red or red/other running to ground from the transformer would be the HV center tap. You'd then have two yellows or a yellow and yellow/other running from a 5VAC transformer secondary to rectifier pins 2 and 8, the heater/cathode of the rectifier. The high voltage DC, or B+, is taken off of this filament loop. That is what your yellow wire is.

You'll need to look underneath and see if this yellow wire is an EXTRA B+ (purpose unknown) or if it is the main B+ feed which should go to the (+) of your first filter capacitor, i.e. look to see if any other wires go from the filament loop (the red wire the yellow is spliced into) or it's complement on the far side of the rectifier filament to one of the capacitors in the silver can in the middle of the chassis. If there is no other connection to the filament loop, you've found your problem. You'll need to post some close-up pictures of the underside of the chassis and we can figure out where it needs to be reconnected.

I am concerned about the brown wire as well... it must be there for a reason. I am thinking that maybe it would ground the negative side of your filter caps. Is that big silver can capacitor a replacement, or does it look original? If it were a recent replacement, perhaps it was never connected up.

What model # is the rectifier tube, and what are the values of capacitors in the big silver can? The values will be marked on the outside, there's likely 2 or 3, or even 4, inside the single can.

Dave
 
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My eyes! My eyes! Holy sh#t that has got to be the ugliest rework I've ever seen.

Get rid of ALL the electrical tape, the heat will destroy it in short order anyways. If any leads are touching any others, you'll have to bend them to separate as best you can, or desolder them and put on some sleeving, or put the tape back afterwards. The can cap is not in use, just the ground lugs. The OPT is a replacement, the original was mounted under the chassis just behind the power switch. An additional can capacitor was removed to make room for the larger output tranny on top. I can't follow the transformer leads from the pictures, it's just too crowded in there and the tape, well, let's just say the tape doesn't help. Can you determine that there are four leads from the power transformer to the rectifier tube? They will connect to pins, 2,4,6 and 8. Can you confirm that the red wire that the yellow (400VDC) wire is connected to goes to either pin 2 or 8 on the rectifier? Can you determine if there are any leads running from either pin 2 or 8 to the positive side of one of the large, blue electrolytic caps? what are the value of these two caps? the largest cap attached to the rectifier directly should be 20uF, but it's not uncommon to use a resistor between the rectifier and cap and up this value.
 
It's quite possible, then, that it may be the center tap of the high voltage secondary winding and that it SHOULD be connected to the brown. I think Tom's guess about a standby switch may be correct.

First, MAKE SURE there are four wires from the transformer to the rectifier like I told you. If one is missing and the yellow wire is it, connecting it to the brown will elevate the whole chassis to 400V. We do not want to do this.

To determine if the yellow wire is in fact the centre tap, measure the resistance between:

1. yellow wire and pin 4 of rectifier
2. yellow wire and pin 6 of rectifier
3. pin 4 and pin 6 of rectifier.

All 3 should have conductivity. #1 and #2 should add up to #3. Ideally you'd discharge all your filter caps first to avoid smoking your meter or zapping yourself.

If this checks out, hook the yellow and brown together. Put the whole thing onto a nonconductive surface and connect your voltmeter between the chassis and earth ground. HOOK UP A SPEAKER LOAD. Power up by plugging it in (or better yet using a switched outlet), NOT by touching anything on the chassis including the power switch. Watch the meter to determine if the chassis is live. If it is obviously something ain't right.
 
There are 4 wires going to the rectifier tube. 2 solid red, and 2 solid yellow.
(the wire in question is red with a yellow stripe)

1. yellow wire and pin 4 of rectifier
2. yellow wire and pin 6 of rectifier
3. pin 4 and pin 6 of rectifier.

No connectivety on all 3 accounts
 
dspear99ca said:
Can you determine if there are any leads running from either pin 2 or 8 to the positive side of one of the large, blue electrolytic caps?
Click to expand...

no
dspear99ca said:
what are the value of these two caps?
Click to expand...

10uf

dspear99ca said:
the largest cap attached to the rectifier directly should be 20uF, but it's not uncommon to use a resistor between the rectifier and cap and up this value.
Click to expand...

The 2 grey ones are 22uf. And connect to pin 2 of the rectifier.
 
HomerJSimpson said:
There are 4 wires going to the rectifier tube. 2 solid red, and 2 solid yellow.
(the wire in question is red with a yellow stripe)

1. yellow wire and pin 4 of rectifier
2. yellow wire and pin 6 of rectifier
3. pin 4 and pin 6 of rectifier.

No connectivety on all 3 accounts
Click to expand...

My bad. Try

1. yellow wire and pin 2 of rectifier
2. yellow wire and pin 8 of rectifier
3. pin 2 and pin 8 of rectifier.

Same deal, if the yellow wire is the center tap, it's resistance to pin 2 and pin 8 should be about the same, and about half of the pin 2 to pin 8 resistance.

Sorry, 4 and 6 are the plates, not the filament and wouldn't have conductivity with anything.

You are close.

The 2 x 22uf is a lot for the 5Y3GT if wired in parallel just downstream of the rectifier. If they have a resistor in between them they're good. When you switch on the power, the caps try to charge up as fast as they can. They pull REALLY hard on the rectifier, and using oversize caps as the first cap in a pi filter generally beats on the rectifier and shortens its' life. Better off to use a 20uf (22uf is okay), then a resistor, then a larger (as large as you want, I mean you could cram a 10,000uF cap in there without harming anything) cap as the 2nd cap in the filter.

Dave
 
pin #8 from the 5Y3 should be connected to a 20-40mfd @ 450volt capacitor, and then from there runs most of the B+
 
dspear99ca said:
My bad. Try

1. yellow wire and pin 2 of rectifier
2. yellow wire and pin 8 of rectifier
3. pin 2 and pin 8 of rectifier.

Same deal, if the yellow wire is the center tap, it's resistance to pin 2 and pin 8 should be about the same, and about half of the pin 2 to pin 8 resistance.

Sorry, 4 and 6 are the plates, not the filament and wouldn't have conductivity with anything.

Dave
Click to expand...

1: nothing
2: nothing
3: .5 ohms
 
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