HELP needed - installing an output transformer

[bad daddy deville]

I found some posts here at The Amp Garage about replacing the "very puny output transformer" on the Epiphone Galaxie 10 amp. So, I bought an OT8SE transformer from Musical Power Supplies and want to install it. Sadly, I'm functionally ignorant about electronics (though I know enough to know that there are lethal voltages inside an amplifier and I need to be careful. I wear latex gloves, and don't touch anything I don't need to touch.)

[img:491:426]http://www.hallamgallery.com/silvertone ... former.jpg[/img]

The OT in the Galaxie 10 has four wires coming off it: on one side, a black wire leads to one lug of the power tube...

[img:551:338]http://www.hallamgallery.com/silvertone/black-wire.jpg[/img]

...and a yellow wire leads to an area on the circuit board marked 4:

[img:452:373]http://www.hallamgallery.com/silvertone/closeup.jpg[/img]

The other side of the transformer has a yellow and a green wire which appear to go to 1 and 2 of that same area.

Here's the section of the Galaxie 10's schematic which shows the output transformer:

[img:583:475]http://www.hallamgallery.com/silvertone/schematic.jpg[/img]

The replacement OT has three wires coming off one side of it:

Red = B+
Brown = Plate 5KΩ
Blue = Plate 7KΩ

On the other side, there are four wires:

Black = 0Ω
Green = 3.2Ω
Yellow = 8Ω
Orange = 16Ω

My guess is that either the Brown or the Blue wires are the ones that go to the power tube lug (since they refer to a "plate") and therefore the Red wire on that side of the OT should go to 4 on the circuit board. The schematic lists that wire as 5K, so I assume that means that the Brown 5KΩ wire is the one to use, and that the Blue one should just be ignored. (I assume that the Red wire is power because of the + sign, but I don't know what the B stands for.)

My other guess is that since the Galaxie 10 has an 8Ω speaker, I need to use the Yellow 8Ω wire and ignore the Green and Orange wires from that side of the transformer. I assume since two wires are needed, the Black 0Ω wire must be the other one. Both those wires would go to 1 and 2 on the circuit board, but I don't know which one would go to which point (or if that even matters.)

Rather than desoldering the old OT's wires and soldering the new one's wires to the circuit board and the power tube lug, my plan would be to cut the wires and splice the ones from the replacement OT, covering each joint with shrink-wrap. That way, if there's a nasty hum, or if the new OT doesn't work for some reason, I can easily put the old one back in just by reconnecting the same-colored wires.

So, that's my situation. I live on remote Nantucket Island, so there aren't any amp techs here I could take this to. Does anyone who has experience with this operation have any ideas? I don't want to do anything stupid...

[martin manning]

Welcome.

...I wear latex gloves, and don't touch anything I don't need to touch.)

The gloves are probably false security if they are the thin "surgical" kind. Do you know how to drain the filter capacitors? Do you have a multimeter to check for voltage on the capacitors? I assume you have a soldering iron and rosin core solder, and you will need de-soldering braid to remove the solder from the existing connections. Any of this can be obtained at the Radio Shack store on Sparks Ave in Nantucket.

On this amp the easiest way to drain the caps would be to ground pin 1 or pin 6 of the 12AX7 with a clip-lead (pins are numbered clockwise from the gap between pins 1 and 9, looking from the bottom). That will create a path to ground that goes through some substantial resistance (>100k ohms). It will take a minute or so, and measuring voltage to ground at the the standby switch will confirm that the remaining voltage is at a safe level. A lead with insulated alligator clips (which you could get at Radio Shack) is convenient for this operation, but you could connect a piece of wire from chassis ground to the blade of a small insulated screwdriver and touch that to pin 1 or 6.

The replacement OT has three wires coming off one side of it:

Red = B+
Brown = Plate 5KΩ
Blue = Plate 7KΩ

On the other side, there are four wires:

Black = 0Ω
Green = 3.2Ω
Yellow = 8Ω
Orange = 16Ω

My guess is that either the Brown or the Blue wires are the ones that go to the power tube lug (since they refer to a "plate") and therefore the Red wire on that side of the OT should go to 4 on the circuit board. The schematic lists that wire as 5K, so I assume that means that the Brown 5KΩ wire is the one to use, and that the Blue one should just be ignored. (I assume that the Red wire is power because of the + sign, but I don't know what the B stands for.)

Yes, all good. The "B" refers to the B battery supply, a holdover term from when tube equipment was powered by batteries: A for the filament, B for plate, and C for the bias voltage.

My other guess is that since the Galaxie 10 has an 8Ω speaker, I need to use the Yellow 8Ω wire and ignore the Green and Orange wires from that side of the transformer. I assume since two wires are needed, the Black 0Ω wire must be the other one. Both those wires would go to 1 and 2 on the circuit board, but I don't know which one would go to which point (or if that even matters.)

Yes on using the 8 ohm lead and the black lead. It does matter which one goes where since the circuit has a feedback loop. The phasing of the feedback signal must be correct or you will get positive feedback and a (loud) howling oscillation. The orange wire should replace the existing wire that connects to the speaker/speaker jack and the black wire replaces the one going to ground, shown on the schematic connecting at the bottom of C12.

Rather than desoldering the old OT's wires and soldering the new one's wires to the circuit board and the power tube lug, my plan would be to cut the wires and splice the ones from the replacement OT, covering each joint with shrink-wrap. That way, if there's a nasty hum, or if the new OT doesn't work for some reason, I can easily put the old one back in just by reconnecting the same-colored wires.

That will work, but I would rather not splice wires. You could make temporary connections that way to test, and then lift the PC board and make permanent connections. On the other hand, the odds are this thing has lead-free solder, and dealing with that (higher melting temperature) and a PC board may get you into trouble. I guess if you can make a good splice, that might be the safer way for you to go. Got a tiny marlin spike? ;^)

[Leo_Gnardo]

Sounds like a good plan. Tape off the unused ends of your OT's wires.

And discharging hi voltage caps - see C11? You can wait a couple minutes after powering down, be double safe by unplugging the amp from AC power, short across C11 with a alligator-clip lead & leave it there for a minute. That will also discharge the other hi voltage caps. Then you can do without the rubber gloves. (Thin ones won't do much to stop a shock anyway, and I can't imagine working on circuitry with thick "linemans" gloves.)

If you're impatient and can't take a couple minutes, make a discharging cable and use that. What I did for mine, solder a 1000 ohm 5W resistor into the middle of a stranded insulated wire @ 1 foot long, solder an insulated 'gator clip at each end. Heat shrink over the resistor/wire connections. Clip one end to chassis (usually where the - end of filter caps are attached) and press or clip the other end to the cap you want discharged. Leave it on for a minute or 2. The R value of the resistor you use isn't critical, anything you have on hand, say 470 ohms to 10K should be good. The idea is to prevent a sudden cap discharge (spark) which isn't the best thing for the life of the cap. Although I've yet to kill one that way.

If you decide your new OT is the bees knees then clean up your mod by doing away with those splices. Might mark the inside of the chassis with the original color code in the event you decide to re install the original part.

One of my old friends spent a youthful summer on Nantucket, brought over his guitar, amp & VW bug. Instantly got gigs, had a great time, still talks about it. So long ago he spotted Felix Pappalardi hanging out "downtown" with his purple sunglasses on. That was the good old days, late 60's.

[Dr-Joned]

If that is the correct schematic, then the output is 4 ohms. You would use the Black and Green. This is your common and 3.2 ohm output.

[Leo_Gnardo]

Oops, thanks Dr-Joned for pointing out you want to connect the 4 ohm lead.

Also, since C11 is connected back-of-the-board, you can attach your cap discharge lead to either side of the 470-ohm "ant coffin" resistor right next to it, and that'll do the job.

Some good ideas from Martin too. We were up early this morning! Radio Shack on Sparks Ave. how appropriate.

[martin manning]

Since the OP said it has an 8 ohm, it may have. Wouldn't be the first time the article didn't match the schematic, eh?

[bad daddy deville]

You guys are amazing! I posted my query very late last night. I woke up this morning and it was like the elves had come in and answered all my questions, solved all my problems! For one thing, I now know how to discharge the voltage inside the amp, and how to verify that it's been done successfully. Tomorrow I'm going to assemble a discharging wire with a resistor and alligator clips.

I'd read at music-electronics-forum that "the amp is a single 6l6 into 8 ohms" so I bought an 8 ohm Weber speaker for it. Now that it's been pointed out that the schematic says the speaker is 4 ohms... maybe I'd better put the stock speaker back in. Or, can I ~make~ it 8 ohms by using the other wire from the transformer? Maybe the rest of the circuit isn't designed to handle that.

Tomorrow's going to rain here, so I can play with the soldering iron all day. I'll let you all know how my tranny swap goes.

And thank you all, so much! It's so great to reach out for help----and then actually get helped.

By the way, I first came to Nantucket Island back in the early '70's and it truly was a paradise here then. I arrived with my chrome dobro and a van I lived in and had a great time here. Man, those were the days! I moved here full time in 1980, and it's been a great place to have a modest little life. I raised my son here and he had a childhood like 1950's America--- nobody locks doors, he could take off with his buddies and we didn't have to worry that some weirdo or some gang-banger was going to cause him harm. Hey, it's still a great place to vacation, so why don't all you guys come here next summer!

[martin manning]

Or, can I ~make~ it 8 ohms by using the other wire from the transformer?

By all means do that! It's the output transformer that takes care of the matching, so you are good to go. You have some other options too- use the 7k primary tap and run a 6V6 in there, for example.

[bad daddy deville]

And there's this I just found:

"I bought this Epiphone Galaxie 10 - $40 CL. ... Tone has the output tranny with 4 and 8 ohm taps so about any 8 ohm 10" speaker would go in it."

[Leo_Gnardo]

I'd read at music-electronics-forum that "the amp is a single 6l6 into 8 ohms" so I bought an 8 ohm Weber speaker for it. Now that it's been pointed out that the schematic says the speaker is 4 ohms... maybe I'd better put the stock speaker back in. Or, can I ~make~ it 8 ohms by using the other wire from the transformer? Maybe the rest of the circuit isn't designed to handle that.

By the way, I first came to Nantucket Island back in the early '70's and it truly was a paradise here then. . . . .snip. . . . Hey, it's still a great place to vacation, so why don't all you guys come here next summer!

You could put both 4 & 8 ohm outputs on separate jacks then plug in appropriate speakers. You'd be amazed how good a little amp can sound running big speakers. Full grind into a 2x12 or 4x12, and not so loud you can't talk over it. Juicy tones.

I'll have to drum up some cashburgers to visit Nantucket. 1995 I got to work for @ 2 weeks on the next island over, MV. Rehearsals at Carly's pool house. It was OK but overrun with tourists, also cops looking to slap fines on tourists speeding. I just barely didn't get caught. Nantucket sounds more my speed.

[bad daddy deville]

Well, I couldn't wait until Sunday. We had some friends over for dinner, and after they left, I busted out the soldering iron. First, I made up one of those dischargers, with alligator clips and a resistor:

[img:816:612]http://www.hallamgallery.com/silvertone/discharger.jpg[/img]

I discharged any current that might have been lurking, then removed the old, puny Output Transformer and mounted in the new one. The old OT was technically 10W and the new one only 8W, but this new one is much more massive in size. (And it has bell ends.) I joined the wires by splicing them---without the benefit of a tiny marlinspike. I covered the joins with shrink tubing. I covered the ends of the transformer wires that I didn't use. I'm pretty certain I got the wiring right but if I get a feedback loop howl, I'll have to disjoin two of the wires and reverse them.

[img:816:612]http://www.hallamgallery.com/silvertone ... former.jpg[/img]

I used the 8 ohm line from the transformer. I dug out the original Celestion speaker that came mounted in the Galaxie 10 stock, and it's stamped 8 ohm. So, I guess a mistake was made on the schematic drawing.

My wife was asleep when I finished, so I'll still have to wait until tomorrow to put the amp back together and see if it works. *fingers crossed!*

--Jack

[bad daddy deville]

Nantucket's crawling with tourists in the summer, just like the Vineyard. Both islands were busier this year because some people who used to go the the Jersey Shore didn't go back because of destruction left by Hurricane Sandy.

Martha's Vineyard gets more celebrities while Nantucket gets more Elite 1%-ers. We riff raft never see them, though, since they hang out in their compounds and don't mingle with us.

[martin manning]

I would rather have insulated clips on the discharging lead. You could make the ground connection with one clip and then carefully, holding the insulated sleeve, touch the other clip to a point on the power supply string until the voltage drops to a safe level, but it's easier if you can attach the clip to the point you're draining from (while it's still "hot") and leave it there.

[bad daddy deville]

Got it! I'll pick some up from the Radio Shack store. You know, the one on Sparks Avenue.

[martin manning]

Incidentally and in case you haven't run across this already, if the amp is "working," that is to say the power tube is installed and conducting current, just turning the main power switch off (leaving the standby "on") will drain nearly all of the energy from the filter caps within a few seconds. This is because the cathodes will remain hot enough to maintain the current flow for a short time. You should always measure the voltage to be sure, and clip the discharging lead on as well since there will be some energy remaining. Some amp circuits have a permanently installed "bleeder" resistor or other circuit features which allow the stored energy to dissipate completely, but this one does not.