Wiring up a nice, new power transformer....

[dehughes]

Hello all,

I'm about to get to work installing a nice, new Mercury Magnetics Woden clone power transformer in my AC/DC30ish amp. This is the first amp I've built (bear with me..... ) so I'm slightly unclear on a couple of things.

1) On the primary, there are many wires, which as I understand, and per the tranny's schematic, are for different voltage setups. The Red wire is 0v, and the White is 117v. I assume that those would be the two "main" lines on the primary, for my purposes, and the White would connect to "hot" wire from my power plug (IEC receptical, actually) via the "power" switch on the amp, and the Red would be connected to the "negative" wire from the power plug via my 2 amp mains fuse? Please correct me if I'm wrong....

2) Also on the primary, there is a Pink wire that is listed as "shield" on the schematic. Is this an equivalent term for "ground"? Where then is the best place to ground the primary?

3) On the secondary of the PT, the B+ lines (about 303v....) and the tube filament lines (6.3v of course) have center taps. Can I ground the center tap of the B+ line and the center tap of the filament lines in the same place? (I may or may not put the standby switch on the center tap...still researching...). If not, then what do you all recommend?

Thanks in advance for putting up with my semi-newb questions. I've done some repairs and some tinkering, but never anything from scratch and with my own synthesized schematics. Your help is appreciated greatly.

[skyboltone]

Hello all,

I'm about to get to work installing a nice, new Mercury Magnetics Woden clone power transformer in my AC/DC30ish amp. This is the first amp I've built (bear with me..... ) so I'm slightly unclear on a couple of things.

1) On the primary, there are many wires, which as I understand, and per the tranny's schematic, are for different voltage setups. The Red wire is 0v, and the White is 117v. I assume that those would be the two "main" lines on the primary, for my purposes, and the White would connect to "hot" wire from my power plug (IEC receptical, actually) via the "power" switch on the amp, and the Red would be connected to the "negative" wire from the power plug via my 2 amp mains fuse? Please correct me if I'm wrong....

2) Also on the primary, there is a Pink wire that is listed as "shield" on the schematic. Is this an equivalent term for "ground"? Where then is the best place to ground the primary?

3) On the secondary of the PT, the B+ lines (about 303v....) and the tube filament lines (6.3v of course) have center taps. Can I ground the center tap of the B+ line and the center tap of the filament lines in the same place? (I may or may not put the standby switch on the center tap...still researching...). If not, then what do you all recommend?

Thanks in advance for putting up with my semi-newb questions. I've done some repairs and some tinkering, but never anything from scratch and with my own synthesized schematics. Your help is appreciated greatly.

1) TE: Let's start with some differet terms. The red wire is "common" to several taps on the primary side. The white wire in your household system is the neutral not the negative. We use "negative only in DC circuits. In a normal 117V outlet, the taller of the two blades that slide into the wall is the neutral. Hook up the red wire to that one. Then, with your transformer, the white wire gets hooked up to the shorter blade of the plug via the power switch. That is the "hot".

2) Ground the "shield" or pink wire to any convenient chassis ground spot. The closer the better.

3) I am not wild about referencing the filaments directly to ground. All of the schemes on this board show the filaments above ground by at least 100 ohms. This is to keep the cathode, which may be a couple of volts above ground from feeding electrons back wards through the plate to cathode circuit. There is an excellent discussion of this phenominum here:

http://195.178.239.50/ax84/media/ax84_m35.pdf

[dehughes]

1) TE: Let's start with some differet terms. The red wire is "common" to several taps on the primary side. The white wire in your household system is the neutral not the negative. We use "negative only in DC circuits. In a normal 117V outlet, the taller of the two blades that slide into the wall is the neutral. Hook up the red wire to that one. Then, with your transformer, the white wire gets hooked up to the shorter blade of the plug via the power switch. That is the "hot".

2) Ground the "shield" or pink wire to any convenient chassis ground spot. The closer the better.

3) I am not wild about referencing the filaments directly to ground. All of the schemes on this board show the filaments above ground by at least 100 ohms. This is to keep the cathode, which may be a couple of volts above ground from feeding electrons back wards through the plate to cathode circuit. There is an excellent discussion of this phenominum here:

http://195.178.239.50/ax84/media/ax84_m35.pdf

THANK YOU. Much appreciated...

1) Sweet. So the white wire on my tranny is called the "hot" wire (at 117v), and the red wire is called the "common" wire (at 0v). The pink "shield" wire is then the ground. So then, to think of it in terms of a household wiring system, the tranny's common wire connects to the "neutral" wire of the house, the hot wire of the tranny connects to the "hot" wire of the house, and the shield or ground wire of my tranny will be connected to the ground wire in my house. GOT IT! Of course, I'm oversimpifying for the sake of clarity (for the sake of my newbness....).

2) The closer the better. GOT IT!

3) I'll read up on that for sure. Many thanks.

Any other words of wisdom for a 1st timer?

[skyboltone]

Remember though:
The white wire in your house is the "neutral" and the black or red is the hot>

[dehughes]

Right on. Thanks.

[skyboltone]

Any other words of wisdom for a 1st timer?

Yes, during construction there are times when you will want to have your amp energizd while you work on it; checking voltages, using a scope to look at the signal, etc. When ever looking into a live circuit it is REALLY important, actually essential, that you remember that you are taking your life in your own hands. There are voltages in there that WILL KILL YOU! The old saw "keep one hand in your pocket" is a good one but not always possible. Please be careful.

By the way, get yourself an old Simpson Model 60 Volt Ohm meter. They are vastly better for working on tube amps because they don't load the circuit like a Fluke will. Don't get me wrong, I have a fluke on the bench, but for looking at a plate or screen voltage on a loaded tube, you'll get a better picture with the Simpson.

[dehughes]

Very cool. Thanks man.

By the way, what do you all use to measure/test caps? I'd like something that can test them in a circuit... Right now, I only have a so-so DMM that has little inputs for cap testing. It's okay for all else, though...

[dehughes]

Well, I have it all wired up, and turned it on.....and I DIDN'T DIE! That right there is a success. On top of that, the voltages all measured out correctly, the recto tube heated up nicely, and all the swtiches seem to be functioning normally. However, I did notice that when I have the amp on standby (wired it up so that it lifts the B+ center tap from ground), pins 4 and 6 of the recto tube have different readings, with one pin reading MUCH higher than the other...one pin reads close to 0 volts, whereas the other actually rises from about 310v to 389 once the standby switch is on. Is this a normal thing to see on a recto socket if the standby switch is wired to lift the B+ center tap from ground?


THANKS!

[skyboltone]

Well, I have it all wired up, and turned it on.....and I DIDN'T DIE! That right there is a success. On top of that, the voltages all measured out correctly, the recto tube heated up nicely, and all the swtiches seem to be functioning normally. However, I did notice that when I have the amp on standby (wired it up so that it lifts the B+ center tap from ground), pins 4 and 6 of the recto tube have different readings, with one pin reading MUCH higher than the other...one pin reads close to 0 volts, whereas the other actually rises from about 310v to 389 once the standby switch is on. Is this a normal thing to see on a recto socket if the standby switch is wired to lift the B+ center tap from ground?


THANKS!

And these voltages are measured relative to what? Remember now, the only connection that the windings have to ground is the center tap and the neg side of the filter caps downstream. A more meaningful measurement would be between the two anode pins, where you should see 700 VAC more or less. This is quite normal. When the center tap is restored, one side of the tube will make B+ on the positive side of the cycle and the other anode will make B+ on the negative side of the cycle. Thus, DC is born. Lumpy for sure, but that's what the filter network is for. More than likely you are seening the positive side of the cycle between the tubes anode and the ground on one of the caps. That's why I say, NEVER CONFUSE A STANDBY SWITCH WITH A SAFETY SWITCH. I think everything is normal. Another standby configuration might include turning off the rectifier filaments.

Dan

[dehughes]

Oh Dan....you're great. Thanks for nailing it. I went with this standby wiring as it seemed to be that the general consensus was that it is the "correct" way in terms of saftey, etc., based on replies that you and others made on my post about standby switch wiring. Thanks for clearing up my thoughts.

While you're at it, I'd like your suggestion on this: What should/shouldn't I do when powering it up for the first time with the filter network installed? I've now wired up all the filter caps, choke, and OT...the only thing not wired up is the eq section and the input jacks, and the heaters. Is there something I should do when hitting the filter caps for the first time, or any other component for the first time?

THANKS!

[skyboltone]

Well, that's a good question. Even if you've been doing this stuff for a long time you never feel absolutely certain about throwing a switch the first time. Especially on high power tube circuits. I use a Variac and bring it up slow the first time. A variac is a variable transformer that you can dial a volt with. You start out with a few volts and start moving the dial clockwise and while you look and smell for smoke. If you don't want to buy one (variable prices on Ebay, I've seen the 3.5 amp jobs go for $10. Mine is a 25 amp. If you have no load to speak of, and are just testing a disconnected power supply a 3.5 amp will do. Otherwise, wire a big light bulb in series with the hot lead from the wall plug. Again watch and smell for smoke. Test the voltage you are getting at the power supply with the bulb in the circuit. If everything seems hunky dory take the bulb out and turn that sucker on. Let it cook for awhile. It's good for the caps to be doing their job for a few hours the first time out.

[Elcabong]

Otherwise, wire a big light bulb in series with the hot lead from the wall plug. Again watch and smell for smoke.

Something like this do well http://www.geofex.com/Article_Folders/SPO_Test.htm

[dehughes]

Very nice. So should I have the tubes in when I turn it on for the first time? How about speakers plugged in? All I'd really be doing is just testing the power section and letting the filter caps break in a bit, right?


david

[skyboltone]

Take it slow David. No tubes, then add the rectifier tube, then add the pre tubes, then add the power tubes and Phase Inverter. All of this with no signal. Check voltages, plate, cathodes, heaters, each stage of the power supply. It's likely that you will have variations in the expected voltage at B+2, B+3, and B+4. Check for DC at grids of tubes. Shouldn't be any, otherwise you have a leaky blocking cap. Take your time, noodle it out bit by bit.

Good Luck
Dan

[doctord02]

This is one of the best step by step guides around:

http://www.paulrubyamps.com/info.html#FirstPowerUp