PT primary 125 v 120 leads

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PT primary 125 v 120 leads

Postby centervolume on Tue Dec 09, 2014 4:24 pm

Hi all
enjoying my annual retro creation, a 6G3 with quickswitch 6L6-to-6V6 option.

I picked up one of weber's 125P23B (https://taweber.powweb.com/store/025130sch.jpg).

It has 3 primary leads (black - 120, brown - neutral, and blue - 125). I love the 125 option given the wall voltages in my area.

My question has to do with the polarity of the primaries: does it matter which color goes to the switch versus the fuse? or is it moot given the rectification of AC to DC later in the circuit?

Probably a pretty basic question, but have not seen it asked before ... a lot of the original PTs from the 50s, 60s have both primaries as black iirc, so I'm thinking it's perhaps not an issue.

Any thoughts on this ?

hope all are well during the holiday season!
a fish in the dish is worth ten in the stream
centervolume
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Re: PT primary 125 v 120 leads

Postby pdf64 on Tue Dec 09, 2014 7:56 pm

Vintage electrical equipment / practices fall way short of what is acceptable today.
Any switch / fuse should act on the high potential line conductor, never just the neutral (this should be grounded somewhere in the supply system).
Double pole switches are the modern standard, these act on both line and neutral; this may be safer, as line and neutral might have been flipped over somewhere in the chain.
http://www.thegearpage.net/board/showpo ... stcount=38

My question has to do with the polarity of the primaries: does it matter which color goes to the switch versus the fuse? or is it moot given the rectification of AC to DC later in the circuit?

Why do you think it's moot?
The PT acts to isolate secondary circuits from the line.
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Re: PT primary 125 v 120 leads

Postby centervolume on Tue Dec 09, 2014 9:29 pm

just because the originals have 2 black leads serving the primary.

for this application, I want to use the blue (rather than black) and then the brown.

so to follow your points, the blue is hot/high voltage and the brown is neutral. As such, the blue should go to the fuse and the brown to the switch (correct?)

-------------

another thing that has cropped up: the center tap (green w/ yellow) for the heaters is dead shorted to each of the heater leads, reading .1 ohms relative to each. So after blowing a fuse, I desoldered the center tap from ground and now am going to isolate it and use the 200 ohm pot to reference each of the green heater leads to ground as a hum control.
a fish in the dish is worth ten in the stream
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Re: PT primary 125 v 120 leads

Postby pdf64 on Tue Dec 09, 2014 10:19 pm

The fuse and switch should act on the live conductor.
The standard is that brown is live, blue neutral.

the center tap (green w/ yellow) for the heaters is dead shorted to each of the heater leads, reading .1 ohms relative to each


What resistance do you think is acceptable?
Please consider that there's a thick piece of wire (ie the heater winding) connecting then all together.
My band:-http://www.youtube.com/user/RedwingBand
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Re: PT primary 125 v 120 leads

Postby centervolume on Wed Dec 10, 2014 1:31 pm

I had no idea it would be so close to a dead short, based on resistances of other center-tapped windings on the PT>but measured centertap to heaters on my 62 princeton and it's basically that. So the heater may be ok, will have to back up and look at blowing fuses from wider perspective
a fish in the dish is worth ten in the stream
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Re: PT primary 125 v 120 leads

Postby centervolume on Wed Dec 10, 2014 5:56 pm

blown fuses issue solved; she is passing signal but now about that horrible 60 cycle hum...

double checked my heater wiring to ensure that one side was only pin 2s on power tubes and 4/5 on preamps and that the other side was only pin 7s on power tubes and pin 9 on preamps.

I have the twisted pairs lifted straight up off the tube sockets by about an inch or so.

The pilot light assembly must be insulated from the chassis? or is it just the connection points where the heater windings from the PT solder to the assembly that must not go to ground? I did not receive any extra insulation board for that assembly, may have to fashion one.

wondering where else to check for this nasty hum?
a fish in the dish is worth ten in the stream
centervolume
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Re: PT primary 125 v 120 leads

Postby pdf64 on Wed Dec 10, 2014 6:23 pm

What was blowing fuses then?
If you're dealing with new builds, PTs etc, then a light bulb limiter is a vital bit of kit, which you may consider prioritising.
The pilot lamp fitting shouldn't need isolating from the chassis 0V.

See Geofex debug pages http://www.geofex.com/ampdbug/ampdebug.htm

Get a feel for the hum, note what controls affect it.
Remove V1, investigate what affect that has on the hum, replace.
Remove V2, repeat above, then same for V3.
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Re: PT primary 125 v 120 leads

Postby centervolume on Wed Dec 10, 2014 9:13 pm

thanks for the geofex page, have that one in the memory banks and visited it shortly after posting my last message, then gravitated to the Tom Mitchell text - he has a couple great flow charts for troubleshooting in his repair your own tube amp book.

regarding the fuses - was unaccustomed to having 3 choices of primary leads, and the color codes were not familiar. I had originally had it set up for brown and black for 120vac. Then started thinking 125 would be better changed to blue and brown (mistake) this morning went to black and blue.

I'm wondering now if my problem with the hum is that the power supply grounding comes off two points under the doghouse, one at each end of the board. They both pass through the chassis into the main circuit board area where one grounds to the brass plate and the other to the chassis close to the point where the PT center taps go to ground. So I tied that 2nd power supply ground to the same point as the center taps. I have to wonder if the AC current grounding at the same point as the power supply is presenting a problem.
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