Supro Help

[52 Bill]

It's the 0.05 uF cap that is connected between the power switch and chassis ground. And yes, just remove it when you replace the AC cord with a 3-wire one.

The cap connected the chassis to one side of the ac line as a shielding/grounding method. This was before the standard ground pin was added to wiring.

[52 Bill]

I couldn't find the right schematic in my files, maybe you can find one on the interweb.

I am attaching something that is close, it will help you understand the circuit, even though the component values may be different.

[Appaloosa]

Hey Thanks a lot 52 Bill!

I just pulled out the death cap and installed a 3 prong cord. I moved the hot to the tip of the fuse. It seemed that before the supro had a grounding post on one of the power transformer bolts, so i used this. Is this an OK spot for my chassis ground? maybe that spot isn't stock? I have a picture of it, see what you think.

I was thinking before I order some parts for this amp (caps and resistors) maybe I would get doubles of everything a build a clone of this amp. But is this a good design to rebuild in terms of tone? is there maybe a better design to base my first home built amp on? what do other builders of these type amps do for the speaker transformer, as it seems most of these old amps have them mounted to the basket? thanks again

[52 Bill]

I would have put a knot in the power cable to keep it from being pulled out through the rubber grommet. The ground at the transformer mounting bolt is a very common solution, but officially it should go to its' own chassis point.

As for first builds, I wouldn't recommend this circuit. Probably a better choice would be a Fender Tweed Deluxe, as there are entire sites devoted to the cloning of these, so you'd be able to get a lot of advice and help and an easy source of parts.

As for the transformer question, it was just a common practice for the output transformer to be mounted to the speaker basket. Mounting it on the chassis instead will work just as well. Your amp has a field coil magnet, which also acts as a power supply choke.

I did find a closer schematic at the Valco Pages web site. You should go there and see all of the great stuff there about Valco amps.

[Appaloosa]

Yah, I need to put a knot or one of those slip-on AC cord stoppers on the power cord- the rubber gromit is toast from age as well. I just put it in there quick to see if It would work. thanks for the valco page tip, I'll check that out.

[trobbins]

As 52 Bill indicated, you really should take the mains earth PE to a separate chassis bolt - its your (and others) safety at stake here. Your photo shows a rather suspect connection as it is not direct on the chassis - transformers sometimes use fibre washer insulation on bolts to alleviate partial shorted turns - and some use rubber insulators for vibration control. It is also better to use a single star connection to chassis at another point than the PE connection. Does the tag strip have a bolted connection, or soldered - it may be more appropriate for the PE - or drill a new hole, and ensure bare metal contact.

[olddawg]

Just a thought. If you are really into aesthetics, many times you can "gut" the can cap and mount the new caps inside with a little hot glue. The modern caps ar much smaller than the originals ans can usually be common grounded and shoved up inside the can. If you take your time you can make in indistinguishable from the original, but you have to remove the can, use a file or a dremel to get the bottom of, and mcgyver it all together.

[Appaloosa]

thanks for the can cap tip, I might try that when I get the new caps.

I will also move the AC ground to its own contact point, thanks trobbins. should I also move those other grounding wires? I take it that the trxformer mount bolt is not a stock grounding post.

[trobbins]

Identify all the 0V connections in the amp, and check out some google background on distributed star grounds for amps - yours is simple, so you may be able to bring all 0V to the one tag point, and then take to a chassis point - which may be an input or output jack - it's easier if you use an isolated input or output jack.

[Appaloosa]

I cut out the "preamp out" jack some one else installed and used the jack as my main chasis ground. I put all the new caps in cleaned the pot. and the amp sounds really good/ unique, it seems to breakup really quick but it seems to be a good distortion sound, I assume this to sound "right"? Any tips to make this amp sound better?

Thank you to everyone who helped me out along the way!!

[trobbins]

If your amp is the same as the 510-1C schematic posted earlier then some options could be:

- increasing one of the input 47k resistors to say 1Meg and then if you want a high impedance input you can plug the guitar in to a normal 47k input, and plug an 'open' jack in to the other 47k input socket (then you're just left with 1Meg grid leak).

- add grid stoppers to the 6V6's - search for suggested values (eg. 4k7)

- add more than 10uF on the 6SC7 supply filter

- lift the heater CT from 0V, and add a 200R (or similar) pot across the heater and take the wiper to either star 0V, or a filtered divider off the 6SC7 supply (to get about 50VDC).

- check the 270k, 270k and 6k8 values with a meter to see if the paraphase is correctly set up.

- add screen stoppers to the 6V6's (eg. 270 up to say 1k), or a series RC from B+ to supply the stoppers (to lower the screen voltage a bit and to filter it and to add some sag control).

- add a 1R resistor in series with each 6V6 cathode - to allow cathode idle bias to be checked.

Googling each of the options can give you an idea of what others have experienced.

Ciao, Tim

[tubeswell]

A few more supro schematics on this page:

Amplifier Schematics

Maybe something here too:

http://www.el34world.com/charts/Schematics/files//supro

[52 Bill]

I don't think that these amps break up all that early. How old are the tubes?