Sylvania SCO3 Phono Amp Conversion.

[Last]

Hello everyone!

I recently bought a Sylvania SCO3 Mono Amp (still waiting to receive it).

Anyone have any experience turning a Phono Amp into a Guitar Amp?

It has a pair of 6V6 Power Tubes a 5Y3GT Rectifier & a 12AX7 pre amp tubes.

It currently has an RCA Phono input jack on it. Would it be as simple as just installing a 1/4" input jack in place of the RCA?

I'm thinking yes but nothing is ever that easy.... is it?

Some pics:

Sylvania SCO3 Mono Amp

[img:800:600]http://i145.photobucket.com/albums/r203 ... ure836.jpg[/img]
[img:800:600]http://i145.photobucket.com/albums/r203 ... ure841.jpg[/img]

[img:800:600]http://i145.photobucket.com/albums/r203 ... ure844.jpg[/img][img:800:600]http://i145.photobucket.com/albums/r203 ... ure842.jpg[/img]

[Structo]

Be nice to find a schematic but I think you may have to change the input a bit.
The old phonographs had a lower impedance I believe.
Might be as easy as putting a 1 Meg grid leak on the 1/4" jack.
I wonder if it has a grid stopper on the 12ax7.

Cool looking old amp but, a guitar may blow those speakers depending on their condition.

Also, it will probably need new electrolytics and a three prong cord to be safe.

Oh, and you see that cap on the big speaker?
That is a crossover for the smaller speaker (tweeter).
I would disconnect that small speaker before running a guitar through it.

[Last]

Thank you very much for the very quick reply & advice Structo.

I was able to find a data/schematic sheet on it (gotta love google & ebay) lol.

But it's not here yet either. You made some suggestions that I'm not familiar with such as 1 Meg grid leak on the 1/4" jack & a grid stopper on the 12ax7.

As you can see I'm fairly new to all of this even though I've sourced out quit a few parts & even have had a 5E3 Deluxe Tweed built from a Kit. My wife is a whiz with a soldering iron & ever since I bought her a nice soldering station she's been going to town on my projects! Gotta love it!

This looks like a very informative & friendly forum.

I'm very much looking forward to contributing in any way I can as I progress. I see I've got a lot of reading to do here 1st though.

Thanks again Structo really appreciate the insight.

Oh, here's the PhotoFact Manual I'm getting.

[img:500:375]http://i.ebayimg.com/16/!BZLIE4!CGk~$(KGrHgoOKj4EjlLmY+TiBKlYP+084w~~_12.JPG[/img]

It has the same Model Number but doesn't look exactly the same cabinet wise. I'm fairly certain that it will be accurate.... I hope!

[David Root]

That is an old one! Late '40s, maybe 1950 I would say. I have a '47 Magna Electronics (original name of Magnatone) that is fairly similar except it is an SE design with one 6L6, also a metal tube!

I hope you didn't pay too much for it. It looks like it has a field coil speaker. Maybe both speakers. You can't push them too much they don't like being overdriven. If you can find a schematic I would seriously consider replacing the speaker(s). Also the metal tubes have a tendency to overheat and burn out if you push them, for guitar use you would be better off with a pair of old RCA grayplate 6V6GTs. You might want to increase the plate voltage a bit too, if that's possible, change the 5Y3 to a 5AR4 maybe.
Definitely look at gridstoppers too, good advice.

[M Fowler]

I used to put in a 1/4 jack with 68k resistor and play away. Most of my old amps had a turret strip on top with output transformer taps.

[Sonny ReVerb]

...also a metal tube! ...Also the metal tubes have a tendency to overheat and burn out if you push them, for guitar use you would be better off with a pair of old RCA grayplate 6V6GTs.

I believe those are smoked glass 6V6s. 6V6'GT' means "glass tube" - no? I would bet they're some fine sounding tubes.

[Structo]

Hmmm, I could be wrong but I don't think that is a field coil speaker.

It has a permanent magnet, it's just very small.
The small transformer I believe is the OT.

The large condenser or capacitor is a high pass filter for the tweeter.
I would disconnect that.

Last,
A grid leak resistor is a resistor that goes from the grid (input) to ground.

A grid stopper is a resistor in series with the input grid.

Most guitar amps use a 1meg resistor from the input to ground.

And a common grid stopper can anywhere from 33K to 68K.

A grid stopper is mainly used on the input to filter out RFI or other high frequency interference.

The grid leak resistor helps to keep the impedance high.

Here is some explanations written is pretty simple terms.

http://www.diystompboxes.com/pedals/tubedummy.html

[Last]

Excellent!

Thank you very much for the information. I'll post up some pics when I get my hands & screwdriver on it!

I realize the speakers will have to go but it's somewhat of a relief it can be turned into a guitar amp with the right knowledge & parts.

I paid $65.00USD shipped for it & will probably put another $75 to $100 @ most & will hopefully have a nice vintage sounding amp. (fingers crossed).

Awesome forum!

[Phil_S]

I don't think there is a problem with posting the Sam's schematic. They are not the creator of the product represented by the schematic. I think it would be deemed to be an unoriginal copy under the law. Or posting it might be permissible under the fair use exception (but distributing it would not be covered ). But I am not a lawyer, and don't want to be responsible for making trouble.

That said, you've probably gotten 95% of the basic conversion info already.

If the tubes and speaker are good, you practically stole it at $65 delivered. If this were mine, I'd put a 1/4" jack with a 33K resistor on the tip to the grid input of the 12AX7. This is quick and simple. Clipping that one high pass filter cap that Tom pointed to is probably worth doing, as well. Before you go investing much more than that, see how it sounds. I suspect you can get some decent guitar tone out of it for $10-$15.

Nice score. Let us know how it works out.

Phil

[Firestorm]

The speakers look like alnicos so they're not quite as small as they look. Still not likely to be good for guitar. The PT looks unhealthy. That's an awful lot of rust. Be prepared to replace that too.

[Structo]

Yes, I thought about that power transformer as well.

Some say a rusty transformer can screw up the eddy currents in a transformer.
I was always told that a crusty, rusty transformer should be replaced.

Interestingly enough, I was reading at the Mercury Magnetics site and they have a article about it.
They state that the rust can actually make the transformer run cooler.

The iron oxide is an insulator and most transformer lamination's are intentionally oxidized for this purpose.

Here is the article.
Comments?

http://www.mercurymagnetics.com/pages/n ... mernik.pdf

[David Root]

The 6V6s are metal cases, look at the bases. They are Sylvanias, see the logo on the left, and also marked with a Mullard logo too. Never seen that before.

[Structo]

Hi David,

I don't know much about the old tubes but I believe you are correct in that those are metal tubes.

I see the Sylvania but where is the Mullard logo?

[David Root]

I think you are right, the yellow hexagon looks like the Mullard hexagon, but I can't see the word Mullard inside it.

[David Root]

I think you are right, the yellow hexagon looks like the Mullard hexagon, but I can't see the word Mullard inside it.