Vibroverb PT - no centre tap on heater circuit

[Howard Smith]

I have a loud hum in my new build AB763 vibroverb copy. I found by accident that this can be eliminated by earthing one of the heater circuit wires - this seems not to affect the voltages around the circuit and the amp works OK. The PT is a MOJO762 with export taps and there is no centre tap on the 6.3V circuit. The schematic and the Super Reverb that I have copied both have an earthed centre-tapped heater circuit.

Any advice gratefully received. Thanks.

[txstrat]

None of the PTs I usually use has a center tap for the heaters. I just ground each heater wire via a 100 ohms 2 watts resistor. That should be enough to eliminate heater wire hum. Some tend to use a trim pot in between te heater wires instead. But in my builts so far I found out it made no difference.

[Howard Smith]

Heater wire resistors

Thanks for the quick reply TXStrat - my Super Reverb with a 5U4GB has those resistors but the AB763 schematic I was following did not use them - now I understand what they are for! I should get a chance to try them out tomorrow.

Thanks again.

[withnail]

Just reading old posts... I wouldn't use anything over 1/2 watt for those 100ohm resistors... they are there to act as a fuse ,and fail/burn up instead of your PT. A higher wattage may not protect your PT.

[Rob Mercure]

While some builders may use resistors as a fuse I truly doubt that Fender did. When they needed a fuse they installed a fuse. And in my mind the idea of fusing a circuit with resistors is hokey as a fuse doesn't have the designed in circuit opening time a fuse does. In any case the resistors would only serve as a fuse when you've got a heater to cathode short in the output tubes or, more commonly, when a tube with a broken locator key is inserted into a socket. These are the only times I've seen the resistors open up. The more I think about it if they were designed as fuses you'd think that they would be located where they are easily replaced but Fender frequently stuffed them under the pilot light where they're hard to get to. Also carbon composition resistors are known to catch fire when they fry and newer Underwriters code requires flame proof resistors in certain circuit locations. Again I don't think that a manufacturer would place a part that could create a fire in a tight location if they were going to use them as fuse. Ya gotta be careful about "lore" - especially rumors that sweep the web.

Rob

[Howard Smith]

Heater Circuit Resistors

Do you think I should stick with the 2W carbon film resistors that I've installed? They have certainly sorted my original problem and the amp is all working now. Many thanks for all the replies.

[Enzo]

They are perfectly fine. Enjoy your working amp.

[withnail]

Although Fender didn't intend for the 1/2watt 100 ohm resistors to act as a fuse, they are a cheap, easy way to protect the PT.