Supreme (Garnet) Amp

[Enzo]

Got crazy readings on the can cap, must replace

I must say, that really doesn't help us help you. What does "crazy readings" mean? Be specific.

[g1]

DigiKey website drives me crazy - neverless I still don't know which of the 3 iso trans I need. All presuming 'customer' will pay... Sheesh

Use the 68, it has the highest VA rating, so most capable. The $ difference is negligible so may as well use the best one.

[The Dude]

FYI: You don't need a 3 cap can. You could save money by using separate caps and even more money if you use radials. I would use radial caps and terminal strips. There's usually plenty of room in those older amps.

[glebert]

Don't know the fuse rating - 2A in there now. But you are right, bigger one is only $2 more..

When I put an iso into my Melody amp I found there really wasn't any place to mount it up on the chassis, and would have actually made the amp quite top-heavy since it is such a minimal amp. I ended up mounting the iso transformer down on the bottom of the cabinet and then used a set of 5 pin molex connectors to take make a quick disconnect so that it wasn't like wrestling a squid when I tried to work on it.

[mtlbasslad]

I must say, that really doesn't help us help you. What does "crazy readings" mean? Be specific.

My apologies Enzo (& everyone else) for that foolish post, must not post anything after happy hour.
So to do it right I removed the cap can & got these readings - 750uF section reads 865uF, the 2 100uF sections read 112 uF.
As has been pointed out, that is too much for this little amp & rough on the rec tube... but if the can cap wasn't dead, where is the hum coming from?
I will take the good advice & source radial caps & terminal strips... I wonder if I can find those at DigiKey. I'm pretty sure the nearest parts place would have them, but it's 50km away & the price of gas makes it better to order online.

@g1 can you email me the schematic please? I could not print it out from your post, sigh.

[glebert]

Does the hum go up and down with the volume? Also, is it 60 or 120 cycle hum? If necessary download an audio spectrum app for your phone.

[Enzo]

Oh, I thought your readings were going to be voltages. 865uf is only about 15% high for 750, not a problem. 750 may be a problem for the rectifier, but the cap itself can easily be that far off. In those days 20% was close tolerance.

[mtlbasslad]

Does the hum go up and down with the volume? Also, is it 60 or 120 cycle hum? If necessary download an audio spectrum app for your phone.

No, hum is a constant - 60hz & 120hz. No apps on my phone, only thing around here dumber than my phone is me...

[mtlbasslad]

Use the 68, it has the highest VA rating, so most capable. The $ difference is negligible so may as well use the best one.

I managed to survive the DigiKey website with your help about the 68. Parts on the way.

[mtlbasslad]

FYI: You don't need a 3 cap can. You could save money by using separate caps and even more money if you use radials. I would use radial caps and terminal strips. There's usually plenty of room in those older amps.

I took your advice & actually managed to find terminal strips, rubber grommets, iso trans at D-K. Couldn't find radial caps though? Went with axial. Last time when I really needed axial, I had to take radial, go figure.

[mtlbasslad]

Got the parts & installed the ISO trans. The secondary has a 3rd wire called 'static shield' - what do I do with that?
Also the ISO trans secondary is 140V with a 125V input - with no load is that normal?
Now trying to figure out if the connections to the old can cap were in the right place before I install the 3 new caps...

[Helmholtz]

The secondary has a 3rd wire called 'static shield' - what do I do with that?
Also the ISO trans secondary is 140V with a 125V input - with no load is that normal?

Wire the shield to safety ground/earth, typically amp chassis.

All transformer secondary voltages sag with load. So no-load voltage being higher by around 10% than with rated load is normal.

[mtlbasslad]

Wire the shield to safety ground/earth, typically amp chassis.

All transformer secondary voltages sag with load. So no-load voltage being higher by around 10% than with rated load is normal.

Thanks for that Helmholtz - I'll wire that shield to ground now, last step before I plug it in. Hoping for no 'magic smoke'. And that hum is gone - crossing my fingers.

[mtlbasslad]

Put everything together & fired it up - no magic smoke, but still a bit of 60hz buzz. Would adding another 50uF cap (in stock) help that? Or is this little amp prone to some hum?

[Enzo]

Power supply ripple is 120Hz, so filter caps won't help 60Hz hum.