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Stancor A-3852 wiring in '55 Gibson GA-6

Rockinrob86

Gold Supporting Member
Messages
3,937
This gibson came with an old Stancor OT. It has 6 secondary taps, so I pulled up a chart online with the pinout:



If you zoom, in this is readable. According to my understanding, for the 6V6 primary load I am looking for 8K, and that the closest way to wire this for 8 ohms is to use the 5 and 6 taps on the secondary? The overhauled amp worked with a different speaker, but regrettably I didn't note what taps the transformer utilized.

I had the original speaker reconed by one of the best, and received it in the mail yesterday

The amp sounded bad - with the volume turned up most of the way it would crap out almost totally. I realize it may not be the OT, but I just wanted someone else to confirm my choice of taps seems reasonable.
 

HotBluePlates

Member
Messages
12,236
This gibson came with an old Stancor OT. ... According to my understanding, for the 6V6 primary load I am looking for 8K, and that the closest way to wire this for 8 ohms is to use the 5 and 6 taps on the secondary? The overhauled amp worked with a different speaker, but regrettably I didn't note what taps the transformer utilized. ...
You do know your penance is to repeat 500 times, "I will always photograph/diagram the existing wiring connections before teardown/overhaul."

Push-pull 6V6 could be anything from 6kΩ to 10kΩ (maybe higher), depending on supply voltage & class of operation.

I notice when looking at any of those (6-10kΩ) columns, secondary pins 5-6 most often turn up something near-ish to 8Ω. Where it doesn't (for low primary impedance like 6kΩ), 4-6 is used.

I think you're fine using pins 5-6.
 

Rockinrob86

Gold Supporting Member
Messages
3,937
You got me to thinking, "I always take pictures of stuff, why didn't you think to check dropbox??"

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2 and 5! It looks like the chart is showing per tube, and it needs to be doubled?! The sound crapping out was either the tubes or the OT when I hit it hard. I'll at least try it the other way and see. May end up just swapping this with one of the classictone tweed deluxe ones - those are great and a better match according to the chart.
 
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HotBluePlates

Member
Messages
12,236
You got me to thinking, "I always take pictures of stuff, why didn't you think to check dropbox??"

2 and 5! It looks like the chart is showing per tube, and it needs to be doubled?! ...
The chart implies you'd need 1/2 or less of the speaker load to get remotely-close to a good primary impedance for 6V6's.

Remember, it's the speaker load attached to the secondary that determines the primary impedance, after it is levered-up by the turns ratio. The Stancor chart might make you assume the opposite, but they're really just trying to help you determine which sets of taps gives a good turns ratio to land at the desired primary impedance.

Sadly, the best test of an OT that "don't sound right" is to tack in a known-good OT.
 

dkevin

Member
Messages
96
Judging by the picture, the electrolytic caps appear to be aged and the interstage caps look like they are originals too. The "crapping out" may be due to E-caps, bad tubes or leaky interstage caps. Oh, and maybe drifted or dead resistors as well. This is not to say the OT is hooked up correctly...only to point out these other factors affect performance too.
 

Rockinrob86

Gold Supporting Member
Messages
3,937
That was a before picture, but thanks for the toughts.


This amp is so simple and has so few parts in it that it actually now only has a few parts left!
 




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