Silvertone 1457: trani wiring?

[nee]

Further examination of the Silvertone 1457 described in an earlier thread shows what seems to be something weird about the wiring of the power transformer.

There are two yellow leads for the heaters, no probs. There's a green lead.

Now, the schematic calls for the black leads to go to pins 1 and 6 of the 6X4 rectifier. On my amp, pins one and 6 are jumpered. One black lead goes to pin 6. The other black lead is tied to the green lead, which goes to earth, and then is jumpered to pin 2 of the 6X4.

Is this normal? Or safe??

Thanks!

[morcey2]

It sounds like someone tried to convert it to a half-wave rectifier. I'm not sure what pin 2 on the 6X4 would do as it doesn't look like it hooks up to anything. That setup would probably push B+ up into the 450V+ range. And it would hum like nobody's business.

Where is the center tap for the HT winding hooked up?

Matt

[nee]

Matt: If by center tap you mean the green lead (there are only 5 leads: 2 x yellow, 2 x black, 1 x green) it and one of the black leads are soldered to an earth lug. From there another lead runs off to pin 2 of the 6X4.

Thanks!

[Phil_S]

I also replied to year earlier thread. I suggest there are some unknowns or at least, as you observe, some weirdness here. I'd go back to basics on this one and test the PT to sort this out. For very little effort, you can be reasonably certain about what's what.

Desolder the green and black wires. Let's leave the yellow, as they seem to be OK. Clip the wire on the earth lug so that you leave a bit of unshielded wire braid as you'll need this to re-solder whatever turns out to the the center tap. Typically, you need a high wattage iron to solder to the chassis and you should leave yourself a low-wattage soldering solution.

With all three wires disconnected do a simple test by measuring ohms across the various paired combinations of the three wires. You should see about half the ohms when you have the outer leg to CT, compared to when you have the meter on the two outer legs.

Then, with no tubes in the amp (remember, we left the filament supply in place), taking care not to fry yourself or the tranny, power up the amp and use your meter to measure VAC and verify what the previous continuity test suggested. Solder the leads for the outer legs to the rectifier socket and the center tap to the bit of wire you left attached to the chassis.

Provided you got output that is approximately the 310v you expected, you should be confident that you've fixed this. Remember, a/c supply voltage then was 110-115 and today it is often above 120. 120/115 = 104%. Expect things to be about 4% higher.

[nee]

Thanks for your replies to these threads, Phil.

On closer examination, and after doing as you suggested, it's clear that this is a "found" transformer that someone has installed to get the amp working on 240VAC mains power. There is NO centre tap for the HV secondary, only for the filament supply.

I think my options are to try and get a suitable replacement, run with the half-wave rectification as it is now, or install a diode rectifier.

[morcey2]

Thanks for your replies to these threads, Phil.

On closer examination, and after doing as you suggested, it's clear that this is a "found" transformer that someone has installed to get the amp working on 240VAC mains power. There is NO centre tap for the HV secondary, only for the filament supply.

I think my options are to try and get a suitable replacement, run with the half-wave rectification as it is now, or install a diode rectifier.

You could convert it to a hybrid bridge rectifier:
http://www.freewebs.com/valvewizard/bridge.html

It's about half way down that page. It keeps the voltage drop and sag of the 6X4, and you only need to add 2 diodes.

Matt

[nee]

Thanks Matt - I saw that page, which gave me the idea. Does a hybrid rectifier require a centre tap? I see at the top of the page he says a bridge rectifier needs a non-centre-tapped transformer, but it's not so clear with the hybrid.

Forgive my newbiness! I'm still trying to dredge up what little about electricity I learned at school a few decades ago!

[morcey2]

Thanks Matt - I saw that page, which gave me the idea. Does a hybrid rectifier require a centre tap? I see at the top of the page he says a bridge rectifier needs a non-centre-tapped transformer, but it's not so clear with the hybrid.

Forgive my newbiness! I'm still trying to dredge up what little about electricity I learned at school a few decades ago!

It doesn't require a center tap since it's just a variation on a bridge rectifier. We're all newbies at something. I haven't been doing this very long, so if I can learn some of this stuff, it should come pretty easy for everyone else.

Matt

[Phil_S]

...it's clear that this is a "found" transformer that someone has installed to get the amp working on 240VAC mains power. There is NO centre tap for the HV secondary, only for the filament supply.

Stop here. There is one more thing to check. Some older PT's have the CT built into the frame and the ground is established by the fact of bolting the PT to the Chassis. You should take your meter and Ohm the outer leg to the copper band on the PT (if it has one) or to some other bare metal on the exterior of the PT. Then test for voltage. See if you can establish for sure there is no CT.

I think my options are to try and get a suitable replacement, run with the half-wave rectification as it is now, or install a diode rectifier.

If the secondary HT voltage is suitable, the path of least resistance and cheapest thing to do is to use what you have. You can use the tube socket to build the SS rectifier of your choice. You DO NOT use a CT with a SS rectifier. If you ground the CT with a SS rectifier, you will let the smoke out and, trust me, you don't want to do this.

Remember to ground the filament CT. As a precaution, I'd remove the filament supply from the rectifier socket if I built a SS rectifier there. You can just clip flush and heat shrink the leads.

There is nothing inherently wrong with a SS rectifier. They are easy to build and parts will set you back less than $1. You've little to lose and everything to gain. Try it. If you don't like it, you can do something else.

[nee]

Hi Phil,

I did as you suggested and checked for centre tap via the transformer chassis. No luck.

I installed a hybrid diode/tube rectifier which works, though the voltage is pretty low - only 285VDC, whereas the schematic calls for 330V at pin 7 of the rectifier tube. The amp works now, quieter than it did though with much less hum.

After about 5 minutes playing the transformer developed a terrible mechanical rattle and hum - it's pretty had it I think. Wonder what a good (cheap) replacement might be? How about this:

http://store.triodestore.com/pa2powtrandy.html

Many thanks!

[Phil_S]

Ian,
That one won't do the job at all. It does not have the mA capacity on the HT secondary or the filament winding.

Try here: www.musicalpowersupplies.com

Sometimes, what you want is out of stock.

For a few more bucks, try www.edcorusa.com

[nee]

Thanks for the tips Phil.

So what transformer specs should I be looking for to provide rectified 330VDC, and enough juice to power a 12AX7 in the preamp, a 12AX7 tremolo tube, and a 6V6GT power tube?

[morcey2]

Thanks for the tips Phil.

So what transformer specs should I be looking for to provide rectified 330VDC, and enough juice to power a 12AX7 in the preamp, a 12AX7 tremolo tube, and a 6V6GT power tube?

For a center tapped tranny, something close to a 275-0-275 will get you 330VDC with the 6X4 rectifier. An 18-watt PT would get you pretty close, but would be a little high. If you're going solid-state rect, something like a 250/260VAC would get you in the right area. With a SE 6V6 power amp, anything with a HV winding rated above 60ma would work. For the heaters, you would only need 1.5A@6.3v, which should be really easy to find.

The PT275 from musicalpowersupplies.com would be plenty. Matt (at MPS.com) is a great guy to deal with too. A hammond 270DX or 270CX would work too. Or 370CX/DX if you don't have 120V mains.

Matt.

[Phil_S]

I don't mean to deter you from asking questions. Please, do that if you are no clear.

I think, perhaps, you can benefit from struggling with some of the math and drawings, etc. You are painting by numbers here and will do better if you move up the knowledge curve a little bit at a time.

Although you have already received an answer, the attached should allow you to verify for yourself whether the advice you received as good advice. In the end, you are responsible. Good luck.

BTW, the price on the one from MPS is really hard to beat.