An old transformer

[wmprivett]

I have an old transformer that ran a 5Y3 rectifier and 1.5 amps worth of tubes on the 6.3 v string. I think I can build a Gibson ga-6 with it (1 5Y3, 2 6V6s and 2 12AX7s). My question is, it has a bare wire coming out of it that was soldered to the chassis. Does this wire just ground the transformer, or is there some other factor I need to take into account? Just asking for a friend.
Thank you.

 

[tubegeek]

I have an old transformer that ran a 5Y3 rectifier and 1.5 amps worth of tubes on the 6.3 v string. I think I can build a Gibson ga-6 with it (1 5Y3, 2 6V6s and 2 12AX7s). My question is, it has a bare wire coming out of it that was soldered to the chassis. Does this wire just ground the transformer, or is there some other factor I need to take into account? Just asking for a friend.
Thank you.

It grounds the transformer core, and it may or may not also connect to an internal shield between windings - a sort of "deluxe" feature that reduces the magnetic field escaping from the transformer.

Any numbers found on the outside?

 

[wmprivett]

Thank you.
It's an old Motorola transformer:

25B21248-G

5C41

Its from an old radio.

I appreciate your help.

 

[Lowerleftcoast]

Quick internet search came up with this
www.antiqueradios.com/forums/viewtopic.php?f=19&t=337305

550 volts ct 50ma, 6.2 volt at 2.1 amps and 4.7 volts at 1.8 amps.

Two 6V6 would likely take it over the 50mA. It would be good for a SE though.
Perhaps a skylark or les paul jr.

 

[wmprivett]

Now you really got me... wondering.

The radio had in it:

6K6
6SJ7
6SQ7
6SA7
6SK7

I looked up the tubes, and the current requirement for each is .3 amps for 1.5 total. I'd be grateful if you could check and see if I read the data sheets wrong. One of them anyway. If I'm screwing that up, I need to put down the solder iron, step away from the bench and so some more studying.
Thank you.

 

[tubegeek]

We can draw a few conclusions from the original tube complement. 6.3V heater current is 1.5A or so, 5V heater current is 2A or so. No reason to doubt that, just because the good folks at Howard Sams tested a related unit and got a (very slightly) different answer.

The real question that remains open is, how much current can we rely on the high voltage winding to supply? Probably not terribly much - the single 6K6 is our clue there. 25 mA average, 28 mA peak current, 4 to 8 mA screen current, so let's call it 30 to 40mA required for the 6K6. The rest of the preamp tubes would have added perhaps 10mA current draw on top of that.

So Howard Sams is probably not too far off calling it a 50mA high voltage winding.

Should be ok for a Champ, or even a single 6V6 plus reverb or tremolo, but probably not ideal for a push-pull amp.

Here's a circuit that I've had a lot of fun with, it's a very slightly modified Supro 1616T (a/k/a Gretsch 6151.) Mine started life as a single 6V6 radio, very similar to yours, with a modest-sized power transformer. Buildable on terminal strips, for sure.

Mods were just a few tweaks to the tremolo circuit to slow it down a little and extend the range of speed, the only change to the audio circuit was the switchable cathode bypass cap. I settled on this circuit because I wanted something Champ-ish but with tremolo and this delivers the goods - the trem is quite good if I do say so myself. I used a Hammond 125CSE single ended output transformer, which has plenty of balls for a single 6V6 - maybe slightly overkill - I run it through a single 12" speaker and it's much beefier than you'd expect. You might have the 6K6's output transformer left over from your salvage operation though?

This is what the chassis looked like with the original output transformer still in place, and the big tuning capacitor for the radio removed:

Then I did a little origami on the sheet metal, and swapped output transformers.

Since it's a little head now, it gets to do all the thinking.

For convenience, I left the 2-prong convenience outlet in place, good for pedal power. That must've been there for an optional phonograph originally.

 

[tubegeek]

This schematic seems to be the right one for another Motorola model that used that transformer. (There's a parts list on a subsequent page.) Notice the connections to ground coming from the transformer. It does indeed have an internal shield (represented by the dotted line between primary and core, connected to ground.)

https://www.tubesandmore.com/sites/default/files/schematics/motorola_galvin_55f11_pg17-2.png

 

[wmprivett]

Ah! I see, said the blind man...

The HV current was my missing variable. I do need to go back to the drawing board.
That said, I was originally thinking of the Supro 6606, which has one less tube than your schematic. But I'd love to complicate things a little more than that (hence the push pull). I with try your schematic. Thank you again for the lesson. Always a good idea to ask first.
Thanks,
Bill

 

[tubegeek]

Ah! I see, said the blind man...

The HV current was my missing variable. I do need to go back to the drawing board.
That said, I was originally thinking of the Supro 6606, which has one less tube than your schematic. But I'd love to complicate things a little more than that (hence the push pull). I with try your schematic. Thank you again for the lesson. Always a good idea to ask first.
Thanks,
Bill

Another approach would be push-pull with two lower-current output tubes, like a 6AU6 for example, unfortunately that'd require a socket change, it's a nine-pin, not an octal.

Two 6SJ7's in push-pull would do OK as a very low-wattage amp, great bedroom amp I would think.

 

[tubegeek]

Supro 6606 is much like a Champ, but with grid-leak bias. May not be happy fed with pedals, and may be a little on the early-breakup side. Easy to convert to a typical cathode bias circuit, just wire V1A like a Champ.

 

[tubegeek]

@wmprivett

What's that gorgeous hunk of rusty junk in your avatar may I ask? Guessing a regulated power supply?

 

[wmprivett]

You're good.
That is a Sorensen Nobatron 325 B. It came with a free rodent condo. But, it also came with a couple of large transformers, one of which has 5v, 380-0-380 , and 6.3v windings. I converted the chassis to a 5E3 head, but still have the transformers. Between the two transformers, they ran a 5y3, a 5U4, at least 3 6L6 tubes a couple of VR150s and a OD3 tube. Unfortunately, I installed a plate to convert 3 octal tube sockets to 3 12AX7 sockets ,so I'd have to take the plate off to see what was there. The afore mentioned transformer is the approximate size and shape of a 5E3 transformer, only having some more laminations. Im thinking, that transformer might be able to be used in a ga-6?
Thank you again.

 

[tubegeek]

You're good.

I can count to two (panel meters.)

Not a stereo power amp, so... Current and Voltage!

 

[wmprivett]

Exactly. Here's a pic of what it became:

 

[tubegeek]

View attachment 763894 Exactly. Here's a pic of what it became:

Are those KEY switches?!

Legend has it that the first call studio bass players in NYC equipped a bunch of the Ampeg bass amps with keyed power switches, left one in each studio, and shared the keys with each other (threre was a buy-in cost.)

So, no schlepping.

 

[wmprivett]

Ha! Thats a great idea!
Well, the left key is to turn on the standby, the right key turns it on.
My brother who got me into building amps, said when you look at the amp, you should wonder what it does. I figured if it looks like it should launch miss-i-les, but amplifies your guitar, that fits the bill.

 

[wmprivett]

We can draw a few conclusions from the original tube complement. 6.3V heater current is 1.5A or so, 5V heater current is 2A or so. No reason to doubt that, just because the good folks at Howard Sams tested a related unit and got a (very slightly) different answer.

The real question that remains open is, how much current can we rely on the high voltage winding to supply? Probably not terribly much - the single 6K6 is our clue there. 25 mA average, 28 mA peak current, 4 to 8 mA screen current, so let's call it 30 to 40mA required for the 6K6. The rest of the preamp tubes would have added perhaps 10mA current draw on top of that.

So Howard Sams is probably not too far off calling it a 50mA high voltage winding.

Should be ok for a Champ, or even a single 6V6 plus reverb or tremolo, but probably not ideal for a push-pull amp.

Here's a circuit that I've had a lot of fun with, it's a very slightly modified Supro 1616T (a/k/a Gretsch 6151.) Mine started life as a single 6V6 radio, very similar to yours, with a modest-sized power transformer. Buildable on terminal strips, for sure.

Mods were just a few tweaks to the tremolo circuit to slow it down a little and extend the range of speed, the only change to the audio circuit was the switchable cathode bypass cap. I settled on this circuit because I wanted something Champ-ish but with tremolo and this delivers the goods - the trem is quite good if I do say so myself. I used a Hammond 125CSE single ended output transformer, which has plenty of balls for a single 6V6 - maybe slightly overkill - I run it through a single 12" speaker and it's much beefier than you'd expect. You might have the 6K6's output transformer left over from your salvage operation though?

View attachment 763815

This is what the chassis looked like with the original output transformer still in place, and the big tuning capacitor for the radio removed:
View attachment 763819

Then I did a little origami on the sheet metal, and swapped output transformers.

Since it's a little head now, it gets to do all the thinking.
View attachment 763820

For convenience, I left the 2-prong convenience outlet in place, good for pedal power. That must've been there for an optional phonograph originally.
View attachment 763821

View attachment 763822

I was looking at your schematic. 3 40 uf's. That's different. Is it very quiet?

 

[tubegeek]

I was looking at your schematic. 3 40 uf's. That's different. Is it very quiet?

Hmm. That may or may not be accurate - it's just the original can cap that was in the AM/FM radio that this started out as. I'll take a look and see what's actually in that can, I might not have corrected the original schematic in that area. The rest of it is as built though.

It's quiet enough, yeah.

 

[wmprivett]

That works then. These things are made for editing on the fly anyway. I just wanted to make sure there wasn't some special reason because of some other edit you might need them.
Thanks.

 

[tubegeek]

That works then. These things are made for editing on the fly anyway. I just wanted to make sure there wasn't some special reason because of some other edit you might need them.
Thanks.

I feel like anything above 16uF is a pretty subtle change really.