1.5W top boost-style amp

[martin manning]

Hooked up half of the heater xformer again to get 3.2Vac, got the following voltages on the primary taps:
brown-blue 36.7Vac
brown-red 24.5
red-blue 12.5

You didn't say which secondary taps you connected the 3.2VAC to, but the brown-red and red blue voltages (and resistances) should be much closer than that. The impedance ratio is much too low, even for the larger brown-blue voltage, assuming you were on the 8 ohm secondary.

[oxbow_lake]

Secondary was connected black to white, which with an 8ohm load should yield 3k on the primary according to the datasheet. With the measured voltage ratio of ~11, I calculate a primary impedance of only 1k.

The guy at Hammond said the primary DCR on the 125Bs he had were all much closer to 300R. He's sending a new one to try, so I guess we'll get a definitive answer one way or another!

[bepone]

With nothing hooked up to the OT, I get the following resistances (in Ohms):
Primary:
brown-blue 238
brown-red 174
red-blue 63

from other things but also now on from here, is obvious that this primary is not ok, and transformer need to be sent back for reclamation, this looks like half of one side of primary with UL tap, and all other half is missing, anyway mistake in production

[oxbow_lake]

Update: received the new 125B output transformer from Hammond. The amp now works as expected. There is mild hum when the tremolo is engaged, but I am fairly sure this is due to excessively long flying leads in my build. Otherwise, it is nice and quiet.

Attached is the final schematic, with measured voltages.

Thanks everyone for your help. This forum is truly a valuable resource for those of us wishing to learn!

[bepone]

no problem!

[johnnyreece]

Glad it's sorted out! I had a hummy trem a while back, and it's because I had a ground loop (the footswitch jack was grounded to the chassis and to the ground bus). I disconnected the wire from the ground bus, and the hum went away. Good luck with yours, and congrats on sorting it out!

[ViperDoc]

@oxbow-lake: I assume you're using KiCAD/Escheema for your schematic drawing? If so, what version are you running, and how did you get the junctions to print on your pdf files? Since I upgraded to the latest version 7.0.5-0, my prints no longer print the junctions. It's more than annoying. Thanks!

EDIT: I see on your print that it's version 6.....DO NOT UPGRADE. Beware of gremlins.