OT Wiring

[jgb5129]

I made a diy 5f6a bassman from scratch and it sounds fine except for a loud, low pitched hum. It starts as soon as i flip the standby switch and is unaffected by volume controls (even when volume is off). I'm thinking it might be from switching the leads on the OT. I'm using a classic tone 40-18001 OT. Right now i have the brown and blue wires coming from the tubes, and i know they are not backwards (tried that). Can anybody tell me the what the correct wiring is supposed to be for the transformer?

[martin manning]

I'm not sure what you're asking here... Brown and blue are the primary, and you probably found out what happens when you swap them. Here's the spec sheet with a schematic: http://www.classictone.net/40-18001.pdf

Re the hum, low pitched probably means it's 60 Hz, and so the issue is possibly with heater wiring. Does your heater string have a grounded center tap or a pair of 100 ohm resistors (one from each side) going to ground?

[jgb5129]

Attached is my PT. The 2 green wires go to the tubes and the green/yellow is grounded. Do i need to ground the green wires through 100ohm resistors too? Also- would it make a difference if the 2 green wires were reversed?

[niversen]

if you're going to use the two 100R resistors to ground method, don't ground the center tap from the PT's heater winding. two more options to consider are the "humdinger" method, or possibly a heater elevation circuit off of a B+ node. I've never used the "humdinger" but i've read lots of people's experience with it. i have used the two 100R resistors to ground and the heater elevation method though. both accomplished my goal.

[Phil_S]

Use the green/yellow center tap or the two 100z resistors, not both. Reversing them won't change anything. Can you post a picture of your build? A good overview picture and two or three detail closeups are a good idea.

[jgb5129]

A wood chasis was a terrible idea for grounding. I basically used star grounding and ran the outlet ground to the 2 star grounds. let me know if you want any specific close ups.

[Phil_S]

Wood holds moisture. It's conductive. This makes it a poor choice of material.

[martin manning]

Your PT and OT are located close together and the laminations are oriented parallel to each other. That's about as bad as it can be for transmitting 60-cycle hum from the PT to the OT. They are also on the preamp side of the chassis, and so close to the sensitive input wiring. You might try rotating the OT 90 degrees and see it that helps any. The wood chassis also means no shielding from stray 60 Hz EMF too.

Looks like you have the plate leads carrying high voltage hanging down and extended with crimped connectors? Scary! I think this is a do-over.

[mmmoser]

I would second the change in the orientation of the power and output transformers. Also, Did you say you have the AC cord ground going to the rest of your circuit grounds? This would be a no no. It needs to have its own separate ground away from the other circuit grounds.

[Phil_S]

How do you provide an AC ground on a wood plank?