OT centertaps

[5v333]

i think i have found the problem.

first of, i tried hooking up the output with two 10K tied to ground and the pic got steady on the scope.
however when sweeping the lows there was obvious imbalance between the two phases. it needed a centertaped load of about 270R to get it right...

today i removed the local feedback around the 6sn7, which i was very happy about.
now we dont need any load at all on the output to get a steady pic on scope and there is balance again between the phases.
i guess the feedback net around the 6sn7 put some strange loading on the OT...

 

[5v333]

stryke that..!

reinserted the local feedback net and balance is still good. mustof been a loose joint or something.

now atleast i need an external balanced load to get a clear pic on the scope.

 

[5v333]

it worked for  a while then the perfect symetry was gone again...

what the frak is going on....

 

[PRR]

If you had one loose joint, you probably need to go over the build and make ALL joints GOOD.

 

[CJ]

if the amp is right on the verge of becoming unstable, sometimes the external cables can push it over the edge, ie ground loops from input and output devices,

but you want it to be able to handle that stuff

my guess is the transformer is not wound for the correct coupling between the primary and the secondary wind you are using as feedback

you can see the structure needed for a feedback tap right here>

https://img1.wsimg.com/blobby/go/0e07cc59-2f5a-481f-967d-12c7363b86fa/downloads/1ctutvur7_110772.jpg

I believe Lundahl uses a simple pri-sec structure for their dual coil transformers, nothing wrong with that, many other classic transformers are wound that way, Neve, UTC A-10, HA-100 X,

i had the same osc problems with the exact same circuit while trying out DIY versions of the S-217-D without the print,

 

[5v333]

interesting CJ!

still no luck. but i have some more things to try out.

still need a ground reference on the output whether it is a ground referenced load or grounding the center tap.