Bell Sound 2425 Repair and Restore -- On Hold

[Spenser]

The 4 12AX7's would have been Telefunken (which explains why they are missing!).

Aha!

That multiplex unit on the back is original too but there would have been a shield on that tube.

Look how the 6D10 protrudes beyond the plane. Is this how yours looked? Am I missing a shield? (see attached photo)

P.S. Appreciate your saying something on the challenging, cramped quarters. Fortification for what's ahead.

really depends if the transformer windings can support more load. If the transformer can't give more than the tube can pass, changing the tube for a solid state rectifier won't get you anywhere. If the transformer can give more, than an SS diode with a suitable drop resistor to put the voltages back right might be a solution.

OK. Sounds good. I'm making note of this for later-stage.

Thanks!

 

[gadget73]

doesn't look like a tube that would have had a shield.

 

[Spenser]

Other shields may be missing on this unit. Hard for me to tell because: a) lack of experience; b) my Photofacts is of another slightly different iteration of this model; c) camera angle (chassis top view), plus quality of photographic reproduction of the day.

Shields are intact for the two 12ax7 tubes near the front of the unit. Would there normally also be shields on the other two 12ax7 tubes?

Would there normally also be shields on 6V6GT outputs; and on one or both rectifier tubes?

Repeat sorry everyone; hard for me to tell.

 

[gadget73]

usually you can tell what should have shields because of the base. Pop off the shields that are in place and you'll see what I mean. There will either be fingers or a collar around the bottom of the tube socket that the shield goes into. The shield has to connect to the chassis in some manner or it won't do anything.

No shield on the rectifier or output tubes. Usually you'll only see that on some of the tuner tubes and the phono tubes.

 

[Spenser]

OK. Thank you!

 

[macyjrm]

Exactly what Gadget said. No collar base equals no shield.

 

[steveh1155]

Yes, it had a shield....it was square shaped and attached to the metal case with a clip.

https://www.radiomuseum.org/r/bellsound_bell_trw_2425.html

 

[maxhifi]

can't quite read the later schematic but I do see the two rectifiers. What are they, EZ80 and EZ81? Those sort-of interchange but the EZ81 is rated fora little more current. The heater current is a bigger difference, the 81 wants more.

honestly both of those seem a little lightweight for a stereo amp. An ez81 is only good for 100ma, I'd expect a set of four 6v6 tubes to idle at more current than that. Datasheet says 70ma per pair doing absolutely nothing at 250v, so 140ma for four. Plus you've got the rest of the tubes to power as well.

EZ81 can do significantly more than 100mA at the sort of voltages 6V6 tubes like. Here's the correct page from the datasheet.

I like this tube a lot, I used one in my SE 2A3 amp, and it's been putting out 120mA, at idle for years.

For tubes I would buy Chinese 12AX7, and Russian 6V6. The rest should be NOS American tubes.

 

[steveh1155]

Look how the 6D10 protrudes beyond the plane. Is this how yours looked? Am I missing a shield? (see attached photo)

P.S. Appreciate your saying something on the challenging, cramped quarters. Fortification for what's ahead.

I've had 2 of these units over the years and both had the same multiplex unit mounted like yours and always wondered why they left that tube sticking out the back (it just didn't seem "neat"), but they did cover it with the metal shield that had the same finish as the case.

 

[Spenser]

Yes, it had a shield....it was square shaped and attached to the metal case with a clip.

https://www.radiomuseum.org/r/bellsound_bell_trw_2425.html

Aha. Very good. We've been wondering. So mine's shield got lost somewheres back along the trail on its journey to me. Ah well.

And what a nice resource you've linked here. This Radio Museum site.

 

[Spenser]

For tubes I would buy Chinese 12AX7, and Russian 6V6. The rest should be NOS American tubes.

Thanks. I will test the power transformer over the weekend and if it appears to be good I will start on the caps and tubes picking and ordering.

Happy New Year all!

 

[Spenser]

Hello folks.

Not looking good. High voltage secondary winding is suspect.

According to specs, should be seeing 55 Ohms on one side and 60 Ohms on the other. Red wire to yellow-red wire (55 Ohms), and other red wire to yellow-red wire (60 Ohms).

Am seeing 21 Ohms either side, and 6 Ohms red-to-red.

Red-yellow (tap), wire by the way is disconnected. (It had been connected to one of the can caps.) The two red wires also are free. That is to say, they are only attached to an unoccupied rectifier tube socket.

 

[gadget73]

The reds are usually the HV secondary wires, and go to the plates of the rectifier. red/yellow is the center tap on that and goes to ground.

sounds possibly shorted, are there any signs the transformer got hot? When they short they blow fuses or melt tar out, depending on which is weaker. The stink is also usually a pretty good clue if its gotten hot. A dim bulb test with no tubes installed will tell you pretty quick. With no tubes installed, the light shouldn't do much of anything. With a shorted transformer it will be lit pretty well.

 

[Spenser]

Transformer is toast. DBT testing confirmed. Fiddle sticks.

So now the quesiton, in what kind of condition are the output transformers

Took a quick reading on the primaries of both output transformers. Came up with readings very close to each other. However, total resistance not as high as should be, compated to schematic specs.

Need to isolate output transformers and retest.

Even if the output transistors are good, it might be difficut and more expense than it's worth to try and come up with a replacment power transformer.

I'm inclined to put this project on-hold. Come back to this later. Unless of course if someone out there happened to have a donor receiver ???

I thank all of your for your wisdom and kind assistance on this project.

73
Matt McConnell
W3MAT

 

[gadget73]

output transformers rarely go bad honestly, and when they do its usually going to be an open winding from some sort of over-current condition.

Power transformers are easier than outputs. If you can't find a reasonable sub you can probably get the original re-wound. The Sams may even offer a cross-ref for power transformers. An original may be a bit of a stretch though. Bell gear isn't the most common thing to find.

 

[Spenser]

Good information I did not know. Very helpful to me. Thank you. I will check the Sams and also begin doing some investigating around here. This might get me to a Pittsburgh Antique Radio Society meeting. Been neglecting those guys. I will come back to this in a few weeks.

 

[robert1]

I have this same model that i re-capped a couple of years ago. it did not have a multiplex module installed. in regards to the transformer, it has two separate HV center tapped windings (one goes to the plates of the 6V4 rectifier & the other goes to the plates of the 6CA4 rectifier) both center taps goes to chassis ground. the secondary windings going to the 6V4 are of a lower voltage to supply B+ to the preamp, Phono, & tuner stages. the secondary winding going to the 6CA4 is of a significantly higher voltage for the output stage.
it is possible that the HV winding on the transformer may be "tapped" as to where the lower voltage supply to the 6V4 is derived off of the same winding as the HV supply to the 6CA4. that may be why you are getting low impedance readings, you might be actually measuring between the taps, Not from the ends of the secondary. they both share the same center tap

 

[Spenser]

Thank you Robert1. I will check this. Very much appreciated!

 

[primosounds]

Not to be a party pooper but finding a new PT for this unit might take you awhile since this has 2 B+ lines. I would tend to move on from this one especially because it is your first tube rebuild. With the blown PT there may be other issues as well. Maybe consider working on something easier like a single ended console amp or something with more available parts like a Dynaco?

 

[Blaze++]

Hi Havols,

Great job and thank you for asking safety questions! I apolgize for being so cavalier in communication of status and plans.

Yes, this will be my first tube project. Will move methodically; take my time; follow protocols. Only three '70s semiconductor-type amplifiier repair-restores under my belt. I know we are talking serious difference in voltages plus where can lurk.

Another AK member here in Pittsburgh (responsible for getting me into this sport), has cautioned me, as well, and is standing by to help as needed. I am also a relatively new member of local antique radio club; and a very nice amateur short wave radio club.

Aside detail: I probaby would not really have fired this baby up without checking the resistors during recapping.

Yes I do know the dim bulb test. Thank you. What a wonderful tool.

The schmatic you provided is gorgeous. Thank you so much for sharing. Will compare as proceed. Attached is a picture of the Photofacts schematic I have for the Model 2425. Sorry for poor resolution.

Something I failed to mention. Whoever was working on this before lost a bottom-panel-screw, and, reversed the two rectifier tubes. Small things I know, but also a clue, on competence and/or care. I am hoping what was going on here was a capacitor or resistor problem the person found or did not find and gave up on.


View attachment 1072000

View attachment 1072008

Hi Spenser,

I'm looking for the schematics of the Bell 2445 to fix the tuner section. Maybe I can use the same schematics of the 2425 but the one that you have attached hav a bad resolution.

Could you please send me a pic of that schematics with a better resolution?

I will really apreciate that!

Grazie.

Blaze