Transformerless Silvertone 1389 amp: a question (and schematic wanted)

[slidincharlie (Carlo P)]

I have just bought a Silvertone 1389 amp. It's a tiny transformerless amp with an oddball tube lineup: 35W4, 12AU6 and 50C5.
My question is: which one is the power tube? Is the speaker driven by the 50C5?
(Attahced is the schematic of a possibly similar amp).

Furthermore, I'm looking for the schematic. Can any of you guys help me getting one?

TIA

[Rob Mercure]

Hey Carlo,

Hope youre doing well. The 50C5 is the output tube, the 35W4 the rectifier and the 12AU6 a signal pentode. This is one of the commonest tube compliments for a line connected set. And speaking about line connected sets: You've been hanging around here long enough to know how dangerous these animals can be without an isolation transformer (and also to know that you can make one easily using two identical AC transformers back-to-back) so please be careful with it. Should be easy to trace out - less than 1/2 hour if the thing's point-to-point. And these little guys can be kinda fun into a good quality speaker.

Rob

[slidincharlie (Carlo P)]

Hi Rob,
nice to talk to you!
yes I know the potential hazard involved in transformerless amps... I was enough stupid not to ask the seller more details!
I'm watching the thread about transformerless amp safety in the main forum and am trying to understand how can I use this little amp safely. I am searching the schematic because a few components have been changed, and want to be sure that they are the correct value.
Cheers,

[Rob Mercure]

I really wouldn't worry too much about changed componenets as long as the amplifier works. But there is a capacitor that isolates the front end from ground that should be replaced with the best quality sub you can get. And make sure that the first electrolytic cap is of a value that doen't stress the 35W4. I'm not sure what the max input capacitance this bottle can stand is but you can find it easily from Duncan's or other sites. Right off 20uf is probably OK and maybe as much as 40uf but not higher (don't take my word - look it up).

I, and others, have offered this up tens of times over the years but are you familiar with using two identical low voltage power transformers to build an isolations transformer? Whatcha do is find a couple of 6 volts trannies - secondary voltage isn't important but being identical is - and wire them secondary to secondary. That is 117:6->6:117 voila you've got an isolation transformer. Now you should put it in a suitable chassis and add fuses on the primary and ultimate secondary as well as using a 3 wire grounding cord with the ground carried all the way through to the isolated 117 source. Again, any secondary voltage will do - I've recycled B&W computer monitor transformers with 17 vac secondaries with success - but there are some gains to using a "filament" transformer pair. Using the filament tranny pair you could rewire your filament string to, oh, 6X4, 6AU6, and 6EH5 (pretty similar to 50C5) and power their heaters from the junction of the secondaries while providing B+ from the isolated 117 (could use SS rectification and eliminate the 6X4 also). Some options.

Rob

[slidincharlie (Carlo P)]

This is too complicated for me I'm afraid...
I'll search for an isolation stepdown transformer.

Another thing:
the amp says "AC or DC" in the back panel. What does it mean? Can it run on DC power? How? With a battery?

[52 Bill]

I'll search for an isolation stepdown transformer.

Carlo- Your country uses 220vac mains voltages correct? If so then the only safe way is to use a step down - isolation transformer. Make sure that it is truly an isolation transformer and not an auto-transformer type that does not isolate the line from input to output.

Another thing:
the amp says "AC or DC" in the back panel. What does it mean? Can it run on DC power? How? With a battery?

Back in the early days of electricity here in the U.S., there were two different systems used to power houses/cities; one was DC and the other was AC. Due to to the problems with long line DC transmission, AC systems eventually won out. Some cities were actually wired up with DC, and during the transition stage from mixed AC/DC to all AC, lots of small appliances were designed to work on either AC or DC. So yes you could wire up a 110 v DC battery pack to power the amp, but I don't think battery life will be very good.

[slidincharlie (Carlo P)]

Thank you Bill.
So this amp could date back to the 40's or early 50's?
I gotta check pot codes.

[Enzo]

As late as the 1960s.

Carlo, any time you are not sure which tube does what, look up the tube. The spec sheet or tube manual page will tell you what it is - power pentode, rectifier, signal pentode.

[slidincharlie (Carlo P)]

??? Puzzled with this amp...

I have still a question.
This amp has been overhauled by some tech. It has new caps and several new resistors, but I don't have a schematic to check if the substitutions are correct.
The schematics of other transformerless amps (see attachment in the starting post for instance) show a sort of transformer tied to the heaters... but I can't locate it in my amp! What is it?
Further my amp has a thingy between the 100 uF cap and one grid of the 12AU6. It looks like a trimmer with a screw on top, and it's labelled ARCO 462.
I've read the resistance between its lugs, and it reads 2.7 kohm. Turning the screw does not affects the resistance.
What is this? Is it a resistor? (maybe R4 in the attached schematic?)

[Rob Mercure]

Carlo,

The "thingy" is a trimmer capacitor but why there would be one in a line Sears phono amp I don't know. Any chance the tech who overhauled the amp put it in there. These parts are usually used at radio frequencies to "dial in" a specific frequency to either enhance it or neutralize it - the capacitance is likely less than 100 pf.

As to the tranny in the schemo in your first post - these are uncommon but not unknown and I expect that it was included so that the heater to cathode voltage of the 12AU6 wouldn't be exceeded if the plug was reversed. I've worked on, and have in the scrap pile, several line connected amps with 12AU6 and non of these have the tranny (and if I remember correctly I have two identical ones that I picked up at a thrift store in Richmond, VA that were made in Italy and marketed under the "Casino" brand!).

While it is possible that the amp was made in the '40s it's much more likely that it dates from the mid 50s - 1960s. In the '40s and early '50s your output tube and rectifier would have probably been a 50L6 and a 35Z4 or 35Z5.

And you can still use two trannies to make an isolation transformer - you just have to use one with a primary made for your line voltage and the second with a 117 VAC primary (such as 240:12 ->12:120 -> to the amp).

Rob

[slidincharlie (Carlo P)]

The "thingy" is a trimmer capacitor but why there would be one in a line Sears phono amp I don't know. Any chance the tech who overhauled the amp put it in there.

This is why I'd like to have the schematic... The amp is workig now, but I'd feel much better if I knew that all the parts in there (many of which are replacements) are on specs.

As to the tranny in the schemo in your first post - these are uncommon but not unknown and I expect that it was included so that the heater to cathode voltage of the 12AU6 wouldn't be exceeded if the plug was reversed. I've worked on, and have in the scrap pile, several line connected amps with 12AU6 and non of these have the tranny

Several other schematics that I've checked have that tranny. So I wonder why this tranny is missing in my amp, and if this affects the sound or operation of the amp

While it is possible that the amp was made in the '40s it's much more likely that it dates from the mid 50s - 1960s.

The pot is coded 137841: CTS, 41st week of 1958?

And you can still use two trannies to make an isolation transformer - you just have to use one with a primary made for your line voltage and the second with a 117 VAC primary (such as 240:12 ->12:120 -> to the amp).

Is the "12V" part compulsory? Or can it be any ac voltage as long as it is the same in the two trannies? Or am I better off with a step-down isolation transformer, if I can find it?
Thanks,

[Tom Phillips]

Several other schematics that I've checked have that tranny. So I wonder why this tranny is missing in my amp, and if this affects the sound or operation of the amp

I suspect that your amp never had the tranny. The clue is that you said that the amp is rated for AC or DC operation. Amps with that tranny (which powers the pre-amp tube heater) would not work on DC power

Quote:
Originally Posted by Rob Mercure
The "thingy" is a trimmer capacitor but why there would be one in a line Sears phono amp I don't know. Any chance the tech who overhauled the amp put it in there.

This is why I'd like to have the schematic... The amp is workig now, but I'd feel much better if I knew that all the parts in there (many of which are replacements) are on specs.

Here is a web page link to a site that describes a Silvertone Model 1430 Amp Modification
http://www.clarkhuckaby.com/AmpMods/Silvertn.html
You should find some useful information there. I also noticed that the trimmer capacitor is shown in his hand drawn schematic of the original circuit before modification so it likely was stock in the Model 1430 that he started with. However, the trimmer cap is not shown in the Model 1430 schematic that you posted at the start of this thread. It’s my guess that the trimmer cap was an undocumented circuit revision by the original manufacturer. There are many cases of circuits that don’t match the published schematics in these old amps. Note that the trimmer cap is in the feedback circuit. It was probably a patch to fix an oscillation problem. Anyway, this indicates that the trimmer cap in your amp could be stock. But...I'd say not needed if the amp had good layout and lead dress.
Cheers,
Tom

[slidincharlie (Carlo P)]

I suspect that your amp never had the tranny. The clue is that you said that the amp is rated for AC or DC operation. Amps with that tranny (which powers the pre-amp tube heater) would not work on DC power

Yeah, you must be right. In fact there is no "DC operation" label behind the 1430 modified in that link. (great link, btw!)
Thanks for the info.
I'll draw a schematic of my amp starting from the 1430 schematics that I have now.

[slidincharlie (Carlo P)]

I wonder if this can be a solution:
wall mains > 1:1 isolation transformer (if I ever find one...) > 220/110 VAC stepdown transformer > amp.
Whadd'ya think?
I am concerned about the sequence of parts in that chain.

[Steve Conner]

Well, how are you running the thing off your 220V mains just now? a step-down autotransformer? Or does it have a dropping resistor for 220 already installed?

FWIW, I think isolating transformers exist that can also convert between 220 and 110. I have two, one was given to me by a friend, and it's badged "Universal Isolating Transformer". I wired it with a switch so it could output either 240 or 120.

The other is a medical unit that has 240/120 switches on both the primary and secondary sides, so it can convert either way.

Last of all, you might not have any luck finding a small enough isolating transformer. But maybe you can find two identical small transformers, each with a 220V primary and two 30V secondaries. Connect all four secondaries in series.