Question about a recap

[torea]

Hi everyone, I'm recapping the electrolytics in an old tube amp. I'm replacing the old can caps with radial caps. One of the can caps had the positive connect straight to ground. How do I wire the radial for this? I thought I wasn't supposed to short the positive and negative leads.


Thanks!

 

[(*steve*)]

You would connect the +ve to ground.

Why do you think this would short the positive and negative leads? Is the -ve connected to ground also?

Are you sure you've got things right?

 

[torea]

You would connect the +ve to ground.

Why do you think this would short the positive and negative leads? Is the -ve connected to ground also?

Are you sure you've got things right?

Well, in the can capacitors, the can exterior is the negative, correct? The original can is connected to the chassis by twist bits that are part of the exterior, so the negative is connected to the chassis ground, right? And the +ve terminal connects to a ground rivet inside the amp. So both connect to ground, right?

I guess I could solder the -ve of the new radial to the old cans twist bit, and solder the positive to the ground rivet. But it seems like it's all the same thing to my inexperienced mind.

Thanks!

 

[(*steve*)]

I would expect that one of your assumptions or observations is wrong.

I can't tell you which one, but it is almost certain that one is.

 

[shrtrnd]

In old TV's and some radios there are two 'grounds'.
The chassis ground, and B+ (which is at an elevated voltage from chassis ground).
Stick to the original polarity of the caps when you're replacing them in the old circuit
and you'll be ok.
You're thinking strictly in terms of chassis ground. That's not what the designers of
the old gear were thinking when they designed their circuits. They thought of the best
way to obtain different voltage levels without adding components. The 'ground' of your
amp for the caps you're confused about might be 30v higher than the chassis ground.
The cap doesn't know the difference. It's 'ground' is just elevated from the normal
'ground' you're thinking of.

 

[torea]

Thanks for the replies. I think the closest way to make it the same as before is to attach the +ve to the ground rivet (like the old can was) and the -ve to the twist bit from the old can.

 

[torea]

Ugh, dumb mistake

I studied the schematic a little more and discovered my problem. I was looking at a newer version schematic, but my notes were for the correct version. The newer edition has C12 (the cap in question) as connected to a 5U4, the output transfer, and another can cap at ground. The correct version for my amp, has C12 with the positive to ground and the negative to a diode with "-33V" labelled. The diode in this version connects to the 5U4 through a resistor. Makes sense since that diode connects to the twist bit of the old can cap. I just didn't make the mental connection.

Sorry for the confusion. Don't know how or when I mixed up my schematics, but luckily everything else is the same.

 

[(*steve*)]

Don't know how or when I mixed up my schematics

But best that you found out

And thanks to Shrtrnd for the plausible explanation.

If you're really confused by something, chances are it is because your assumptions or understanding is wrong.

That's not a criticism, it's a recognition that you've spotted something you don't understand and you're smart enough to realise it.

 

[torea]

But best that you found out

And thanks to Shrtrnd for the plausible explanation.

If you're really confused by something, chances are it is because your assumptions or understanding is wrong.

That's not a criticism, it's a recognition that you've spotted something you don't understand and you're smart enough to realise it.

No offense taken I'd much rather triple check everything with a careful eye than rush it and risk destroying my vintage amp. Especially this one, I probably won't come across another. Finished the recap, tested it and it works so much better!

 

[davenn]

Hi Torea

its great to see you making the effort to restore an old tube amp
I have spent many years prior to moving to Australia restoring old valve radios
there are a few tricks to keep a unit looking as original as possible and the one in your case was to empty out the guts of the old electro caps and mount the much smaller/modern replacements inside the old electro. aluminium case (with appropriate insulation on the leads so it doesnt short out. So looking at the unit... radio, amp etc it looked original. just that nice touch

cheers
Dave

 

[torea]

Hi Torea

its great to see you making the effort to restore an old tube amp
I have spent many years prior to moving to Australia restoring old valve radios
there are a few tricks to keep a unit looking as original as possible and the one in your case was to empty out the guts of the old electro caps and mount the much smaller/modern replacements inside the old electro. aluminium case (with appropriate insulation on the leads so it doesnt short out. So looking at the unit... radio, amp etc it looked original. just that nice touch

cheers
Dave

I actually left the old cans in for the most authentic look =D I covered the old leads with liquid tape. The original design used the twist bits (tabs I guess?) of the cans as major ground points, so I didn't want to have to relocate all of those wires.

It's so nice to bring back an old, rare organ speaker rig like this. And to think, if I hadn't bought this amp (and it's perfect condition speaker cabinet!) the guitarist who owned it was going to gut it to make it 1/4" compatible for his guitar. Which I guess is OK if done right, but this dude did not know what he was doing. He pointed to the insulation on a piece of speaker wire running in the cabinet and told me "yeah we gotta get all that stuff off there, it messes with the harmonics and stuff."