Power Transformer education

[dkevin]

Hello All,
I asked for some help on the Vintage Amp side but no one responded. Perhaps I can find some help over here.
I have acquired a Gibson GA-7 amp with a dead power transformer. It repeatedly blew fuses despite removing tubes, disconnecting electrolytics and finally, disconnecting all the wires. When I tested it for continuity the wires that attached to the 5Y3 pin 4-6 showed none. I am a novice in this area so let me lay out my plan and let you shoot holes in it.
I am planning to have the original re-wound if financially feasible. In the meantime, I am shopping for a suitable replacement. The amp is a GA-5 Les Paul Jr. amp with a larger (10 inch) speaker. It is single-ended with a 6SJ7/6V6 and a 5Y3 rectifier. I have found several possibilities but have a few questions. Edcor sells an XPWR041 that provides 680V (340-0-340) @ 120 mA. This is exactly what the schematic says is needed at pins 4&6 of the 5Y3. The 6.3V is broken down as "3.15-0-3.15". Is this what the old transformer provided? There were two heater wires at pins 2 & 7 of the 6SJ7 and 6V6. I'm a bit confused at this point!
Lastly, the 6.3V is at 4A and the 5V is at 3A. Is this too high for the tubes? What is the net effect of running too high an amperage to the heaters? Can I do anything in the heater circuits to reduce the amperage to the tubes?
I really like the transformers I have purchased from Edcor. They are an American company and I want to give them my business. However, if the application won't fit, I'll have to look elsewhere.
Thanks for your help!
dkevin

[roughcut studios]

power transformer ed.

The 3.15/0/3.15 is just 6.3v center tapped. Was the original center tapped? The amperage rating of the filament windings needs to be equal to or greater than the actual current being drawn, (greater than will keep the temp. down). The 5y3 draws 2 amps from the 5 volt winding, so you are fine with 3 amps available. The tube will only draw what it needs. 4 amps available on the 6.3 volt winding is plenty for your tube lineup. Rewinding will be more expensive than a replacement. ....Jon

[dkevin]

Always learning...

Hi Jon- the GA-5 Les Paul Jr. amp schematic (same as GA-7) does not show a 6.3V center-tap wire. What would I do with that wire if I did get the Edcor? The original PT had a couple of extra wires that went to chassis ground. Maybe it was center-tapped but not noted on the schematic. That would certainly not be a first for Gibson.
I am concerned about any detrimental effects caused by running the tube heaters with a higher-than-needed amperage. One person suggested that it might shorten the life of the tube. Have you ever encountered such a condition?
I realize that re-winding the original power transformer would be more expensive than buying a new one. The amp is a very rare variant of the Gibson amp line and although it is in rough-looking condition, I would like to restore it with the original parts as much as possible.

[Wilder Amplification]

Hi Jon- the GA-5 Les Paul Jr. amp schematic (same as GA-7) does not show a 6.3V center-tap wire. What would I do with that wire if I did get the Edcor? The original PT had a couple of extra wires that went to chassis ground. Maybe it was center-tapped but not noted on the schematic. That would certainly not be a first for Gibson.
I am concerned about any detrimental effects caused by running the tube heaters with a higher-than-needed amperage. One person suggested that it might shorten the life of the tube. Have you ever encountered such a condition?
I realize that re-winding the original power transformer would be more expensive than buying a new one. The amp is a very rare variant of the Gibson amp line and although it is in rough-looking condition, I would like to restore it with the original parts as much as possible.

Most amps with center tapped heater windings ground the center tap, then the heaters "float" across the full winding (i.e. neither side of the heater string should be referenced to ground).

As Jon stated above, the current rating is just the rating that states what the winding is capable of supplying. Current isn't FORCED into the load. The load draws what its load impedance allows it to at a given voltage. Ohm's Law applies here as it does everywhere. If you have a 6.3VAC @ 4 amp heater winding and your heaters only pull 2 amps, they're only gonna see 2 amps. As long as the heater voltage is at 6.3VAC, your heaters ain't gonna know what the current sourcing capabilities of the winding are and will only draw what their resistance/impedance allows them to at that voltage.

[d95err]

I am concerned about any detrimental effects caused by running the tube heaters with a higher-than-needed amperage. One person suggested that it might shorten the life of the tube. Have you ever encountered such a condition?

Running with a higher (or lower) heater Voltage than 6.3V, could shorten tube life. The tubes draw as much heater current as they need, no more.

[MWJB]

Dkevin "Hi Jon- the GA-5 Les Paul Jr. amp schematic (same as GA-7) does not show a 6.3V center-tap wire. What would I do with that wire if I did get the Edcor?"

You would either:

1) tape it off & wire up as per the original PT, just using the secondaries, or..

2) Rewire your heaters as a "twisted pair", eliminating the grounded secondary and use the centre tap as a ground reference. I very much suspect that your Gibson currently has one leg of the heater winding grounded & the other wire feeding half the tubes' filament connections (the othe tube filament connections will be grounded)?

Nearly every amp you will find ruuns the heaters over 6.3VAC, this is to ensure at least 6.3VAC with a full tube compliment & current draw. As long as your heater voltage is <7VAC you don't have a problem.