Gibson GA-17 scout PT replacement

[smokey]

Hi there,

My Gibson GA-17 needs a PT replacement. The original PT has a 6.3vac-0 winding for the 6ca4 heater. I have a spare transformer at home with 3.15-0-3.15 heaters winding. Is there anyway I can wire the heaters in order to have the correct voltage for the rectifier ?

thanks......S

[Tom Phillips]

Smokey,

3.15-0-3.15 is a 6.3V center tapped winding. You don’t need to use the center tap. Just hook up the two “3.15” leads and leave the “0” lead unused (taped or heat shrinked) and the hookup will perform the same as the existing 6.3 – 0V winding in your GA-17.
Of course you will also need to verify that the high voltage winding in your replacement transformer is OK as well as the current capabilities of both windings.

Note: Someone may suggest an upgrade to change the heater wiring in the amp and ground the center tap to possibly improve the hum level. I’d try it with the stock wiring first and then decide if the hum level is acceptable to you. The upgrade would involve a complete rewiring of the heater circuits.

Cheers,
Tom

[smokey]

Hi Tom, thanks for your answer.

Note: Someone may suggest an upgrade to change the heater wiring in the amp and ground the center tap to possibly improve the hum level. I’d try it with the stock wiring first and then decide if the hum level is acceptable to you. The upgrade would involve a complete rewiring of the heater circuits.

As an upgrade, how would you rewire the heater circuit ?

thanks......S

[Tom Phillips]

...As an upgrade, how would you rewire the heater circuit ?...

Basically you need to inspect your heater wiring and remove the ground to the chassis that is currently installed on one of the legs. The schematic suggests that you may have only one ground point but often there is a grounded heater pin at each tube socket to simplify the wiring. If so, you need to replace the heater wiring with a twisted pair. Then you ground the 6.3V center tap to the chassis. A good spot is where the high voltage winding center tap is grounded. A better spot is at the cathode of one of the power tubes. This elevates the heater potential an gives better hum reduction.

I see that the GA-17 uses a rectifier tube that runs off the same 6.3 V heater line. I'm now wondering it that has anything to do with Gibson's choice to ground one side of the heater circuit. Anyone know?

Remember - Your amp may sound just find without any changes. If you don't have excessive hum then it may be best not to mess with the wiring.

Regards,
Tom

[Enzo]

While grounding a center tap might be incrementally better than grounding one side of the 6V, the majority of the hum reduction occurs just by grounding, whichever means you chose.

Lots of old amps just grounded one side. A CT winding costs more for not all that much less hum. Plus it adds to the wiring, which ups the cost of making the amp.

I am sure Gibson grounded the heaters for hum abatement, and it is coincidental the recto is a 6v heater.

[smokey]

Thanks to both of you for sharing your knowledge

cheers....Smokey