Silvertone 1451: death cap and more

[otto pärt]

I have an old Silvertone 1451, which is essentially the same as the 1430 whose schematic is attached here.

As you'll see, there's no power transformer, and the ground side of the power supply is connected directly to one side of the power cord. Then theres a 68k resistor and a .05 cap from there to the chassis.

I have installed a grounded power cord. Any suggestions as to how to make this atrocity as safe as possible?

[JHow]

I think people have made these circuits safer by re-doing the power supply using 1:1 isolation transformers. Do a search on the forum for triad n68x. Ithink there is discussion of this somewhere.

Edit: Here is a guy doing something similar:

http://www.clarkhuckaby.com/AmpMods/Silvertn.html

[olddawg]

Personally, I have never found anything redeeming in the sound of these types of amplifiers and they are definitely a dangerous shock hazard. IMHO, if you must use it your best bet is to use an isolation transformer as mentioned previously. You can wire a couple of 12v transformers (or whatever same ratio pair you have laying around) back to back if they have a sufficient current rating. People usually mount the isolation transformer on the bottom of the cab and hard wire it. Otherwise, leave the circuit as is.

[Phostenix]

You can replace the death cap with a new Y rated cap that is UL approved to open if it fails, like this:

PME271Y547MR30 Kemet Film Suppression Capacitors

[otto pärt]

Success

I put an isolation transformer in this thing. It ended up giving me almost 140VAC to work with. (Should that happen?) I put in a huge cap to bring the hum down to a reasonable level, and I added a couple of resistors to balance the heater in the preamp tube. You'll notice a couple of other tweaks if you compare to the original schematic. Also, the 6" speaker was crap, so I put in an 8" Jensen C8R.

Now she's louder and better sounding than she has any right to be!



(Note to search engine: Silvertone 1451 and Silvertone 1430 are essentially the same.)

[JHow]

Cool. Where did you locate the 1:1 xformer? Do you have pix to share?

[otto pärt]

Cool. Where did you locate the 1:1 xformer? Do you have pix to share?

The transformer came from Mouser. Click here.

I'll try to post a couple of pictures ...

[otto pärt]

A few pix:

[JHow]

Oh, I see, the 1:1 is at lower right, n'est-ce pas?

[otto pärt]

Oh, I see, the 1:1 is at lower right, n'est-ce pas?

Yep.

The output transformer is mounted on the outside of the chassis; it was previously mounted on the original speaker. The transformer inside the chassis is the 12v 1:1 transformer for the preamp tube heater.

[Prairie Dawg]

Personally, I have never found anything redeeming in the sound of these types of amplifiers and they are definitely a dangerous shock hazard. IMHO, if you must use it your best bet is to use an isolation transformer as mentioned previously. You can wire a couple of 12v transformers (or whatever same ratio pair you have laying around) back to back if they have a sufficient current rating. People usually mount the isolation transformer on the bottom of the cab and hard wire it. Otherwise, leave the circuit as is.

Depends. I had a series string Danelectro Leader for a while that had 2x6SJ7, 2x25L6 in push pull, and a 25Z6 rectifier. Once I figured out what was wrong with it it was a nice sounding 5-7w amp. The things people did to avoid buying power transformers.

[pbarnrob]

Depends. I had a series string Danelectro Leader for a while that had 2x6SJ7, 2x25L6 in push pull, and a 25Z6 rectifier. Once I figured out what was wrong with it it was a nice sounding 5-7w amp. The things people did to avoid buying power transformers.

Keep in mind, not all power mains in the US were AC until sometimes in the '70s, NYC still had Mr. Edison's DC (AND his original nickel-iron batteries!) in some neighborhoods, so AC/DC radios and other gear were (and still can be) quite useful. No sound? Reverse the plug. Now with all-AC, polarized plugs are practical, and even better/safer, three-wire with bonding ground.

The back-to-back filament transformers to make an isolation depend on the wattage rating of the (best if identical) units, and you can tap the 12V for heaters. Figure Watts = Volts x Amps, and allow about 50% more for safety margin, cool running.

Oh, and if you're adding or rewiring a fuseholder, make sure the 'hot' line-in goes to the back-end of the fuseholder, so with the fuse out, you can't touch the hot circuit. Obviously, open on the bench, you could, but remember, these voltages (line/mains AND B+) are *LETHAL*! One hand in the pocket if you HAVE to work live, dry floor, rubber-sole shoes, no metal stools etc. etc.

[R.G.]

Keep in mind, not all power mains in the US were AC until sometimes in the '70s, NYC still had Mr. Edison's DC (AND his original nickel-iron batteries!) in some neighborhoods,

This fascinated me, so I went and looked it up.

There is STILL a DC distribution network in San Franscisco! PG&E maintained a DC distribution grid up through ?2009? when a short in a faulty splice in DC cables started a fire in nearby oil-filled AC distribution stuff and shot flames out of a manhole cover, and interrupted power to a neighborhood. They then split the DC network into a large number of DC "islands", each serving several customers and not interconnected to limit collateral damage.

They do this because of a large number of DC powered elevators in hotels; this was the only way to do good elevators until the 1940s/50s? and they're still there. Replacing them with modern motors is hugely expensive, apparently.

I was wowed. But then I'm easily amused.

[Helmholtz]

I put an isolation transformer in this thing. It ended up giving me almost 140VAC to work with. (Should that happen?) I put in a huge cap to bring the hum down to a reasonable level, and I added a couple of resistors to balance the heater in the preamp tube. You'll notice a couple of other tweaks if you compare to the original schematic. Also, the 6" speaker was crap, so I put in an 8" Jensen C8R.

Now she's louder and better sounding than she has any right to be!



(Note to search engine: Silvertone 1451 and Silvertone 1430 are essentially the same.)

Tube rectifier have limits on the C value of the first filter cap to limit peak currents. Have you checked with the tube's datasheet? 350µ seems extremely high.

[Enzo]

HI Helm, Otto has not logged onto this forum in about five years. He may not see your post.