PT lead help

[gary sanders]

This thing is huge and I dont think I will plug it in till I know what is what.The blue and red to the diodes confused me,not what Im used to as far as what the books say color should be.I guess this is a half wave rect.?The giant greens are heaters,the browns are heaters to a 6DN7

[David Root]

What is it, maybe an old '50s oscilloscope amp? 6DN7 is an old TV tube. I have seen odd PT lead colors on those before.

[tubeswell]

Hi Gary,

What are the VAC to ground on each secondary (esp for the blue and red)? The tube types give a reasonable clue as to the current draw parameters.

Pete

[Cantplay]

No grommet or strain relief on incoming power line, be careful not to cut the wire on sharp edge.

John

[passfan]

This looks like an older Conn stereo tube amp. 7868 outputs with 12ax7 drivers. The 6dn7 forms a voltage regulator in conjunction with an OA3 regulator tube. Did I guess right ?

[gary sanders]

This looks like an older Conn stereo tube amp. 7868 outputs with 12ax7 drivers. The 6dn7 forms a voltage regulator in conjunction with an OA3 regulator tube. Did I guess right ?

Yep thats it.

I removed the strain relief for the plan of eventually putting a 3 prong on it.The original power cord was dry rotted and crumbling so it got cut off.All I need is to leave what works the power amp.I have yet to turn it on but will get the reading on the VAC and post.I just dont get a blue and red going to the diodes,(big diodes with hieroglyphics on them!....) unless it was like an old bias tap off the B+ but I dont think that is the case here because its cathode biased.

[gary sanders]

Im not in any danger of damaging the transformer by turning it on long enough for a V reading am I? This thing has seen a few hours of hard work.(Point to point....wheres my alligator clips?)

[passfan]

I removed the OA3 and the 6DN7 from their sockets as they regulate voltage elsewhere in the organ and not in the amp. Mine worked fine without them. As with all older circuits bring it up on a variac or a bulb limiter or both, you just never know. Here is a gut shot of the one I had before I sold it.

[gary sanders]

I will try it on the bulb limiter,back with results.......

[gary sanders]

OK heres the voltages with the 6DN7 and the Regulator OA3A.It all seemed very low but the heater readings were nuts! would this have anything to do with the 38 12AU7s it run? Anyway,I think the Red and Red/Green are my B+ and I dont know what the 35 volt would be for in this circuit but I would like to drop a tube rectifier in but with this low of B+ unloaded that may be a bad idea.What do you guys recommend to get this thing into a good guitar power amp?

I was going to salvage all the components on the OA3A/6DN7 circuit and maybe use those sockets for a 6S*7 since I have the space.

My biggest concern is the heaters,those monster leads look to be for some big current

[gary sanders]

I removed the OA3 and the 6DN7 from their sockets as they regulate voltage elsewhere in the organ and not in the amp. Mine worked fine without them. As with all older circuits bring it up on a variac or a bulb limiter or both, you just never know. Here is a gut shot of the one I had before I sold it.

Same as mine.The sockets for those tubes may become the preamp in mine

[Phil_S]

Back up a minute and try to establish which leads are paired and/or have center taps. Your meter set to Ohms will tell you. No winding is perfectly symmetrical because the outer winding has more wire required to make the same number of turns as the inner winding, but they will have approximately the same reading. For example, a center tapped winding might read 49 from the outer leg to the CT, and the other might read 51 from the outer to the CT, and it will meter 100 across the two outer legs. You'll need to lift any ground connections, pull tubes, disconnect anything else that might interfere.

From there, you will know what pairs with what and you should be able to get reliable readings. Use clips to get ohm and voltage readings. This is much more reliable. Of course, you don't use the light bulb limiter when taking voltage readings -- it will make the low and they will not stabilize.

Nothing is certain in wire colors, but I'm guessing the following, some of it based on the info you gave:
High voltage, Red-Red/Yellow-Red/Green 244-0-244 without a load, probably ~ 220-0-220
Filament, Green, Green/Yellow, Green might be 25V. Each side of the CT is 12.6 and that is good to power 38 12AU7's. It seems a little odd to do it that way. Maybe there was some weird series parallel thing that allowed the use of a 25V supply? You can probably get enough Amps out of the green winding to run it through a voltage multiplier until you get a decent B+.
Brown-Brown could also be 25V, but seems wrong. What is 25V?
Yellow-Blue...maybe not even a pair, but at 35V, a bias winding?

[gary sanders]

edit

[gary sanders]

red to yellow/red 300v
red/green to yellow/red 300v

both greens to green/yellow 6.5v

Its starting to make sense