Feedback resistor

[Apprentice]

Any thought on this? I have built the blues junior conversion from scratch and checked and checked but I cannot seem to connect the 75k feedback resistor if the meat pot is anything less than half way clockwise without a very high pitched whine coming from speaker, oscillating I think.
Using a Hammond opt with blue and brown correctly fitted to o/p valves. Bias set to -12.8v. Using 8ohm tap on secondary.
If I remove feedback link amp works fine?
Any ideas?

[pompeiisneaks]

This usually means your output transformer leads are out of phase and you're creating PFB instead of NFB, desolder them, swap the output tubes and resolder and you're golden.

~Phil

[Apprentice]

Thanks Phil you were exactly correct. I was using a Hammond opt and the colours must be reversed to a Mojo which I think is the one used by Hoffman. I managed to work it out with the help of M Blencowes book.
Great to know there are helpful people out there. Thanks again

[pompeiisneaks]

No problem, also sometimes they just accidentally reverse the colors at manufacture time.  Either way it's a 50/50 tryout, you'll get it right half the time ;)
It's always best to leave those leads long and wait until first power up, then if you're right, cut and resolder the same ones, if wrong swap, test, if good, cut and resolder that way :P  (in some cases that doesn't do it and it's some kind of oscillation, but it's better to confirm that first and remove it as a possibility.  If it get's worse, you'd need to reverse them again and look for some other issues/oscillation etc).

~Phil

[dude]

Hey Phil, do you usually cut your wires to the exact length, no slack, when you're sure everything is running well. I usually always left an inch or more than needed, for repairs, etc. I find usually after a few solder, de-solder, the wire sometimes needs to be be cut a quarter inch or so.


al

[pompeiisneaks]

For the transformers, I like to leave a bit of slack so that if they ever need tweaking or get re-homed, there's some spare room.  On the other connections I always cut them pretty close, either direct or with a few bends but as close as possible.


~Phil