single ended silvertone clone attempt has bad background hum

[georage]

im trying to build a simplified silvertone 1448 amp clone and it plays well and the tone is ok, but it has a bad hum (i think 60hz). If anyone has any insight, please help!
the hum is loud and constant. it is independent of the volume, but dependant on the tone(a .022uf cap to ground).
so far i have tried: moving wires with a chopstick, redressing heater wires and adding a "virtual ground", eliminating any obvious(to me) ground loops, moving the output tranformer far away from the power transformer and rotating it. adding a grid stopper 10k resistor to the second stage of the 12ax7, replacing all tubes one by one(6v6 and 12ax7) and removing each tube individually accept the 6x4 rectifier.
removing any tube stops the hum and all sound...and the tone control eliminates the hum along with the tone. and shunting input to ground has no effect on the hum.

[georage]

the orange wires are the signal wires and the big wire nut is the star ground and it is well connected. the pcb boards, pt-ct, and chassis all ground to that single point.

[Helmholtz]

Start with posting the schematic.

[The Dude]

Here's a schematic link to the original amp.

https://schematicheaven.net/bargainb...rtone_1448.pdf

If that's what you built, it's dangerous. It's a widowmaker amp. It will need an isolation transformer and a grounded AC cord to be made safe. These amps did have quite a bit of hum as they are single ended.

[georage]

i think it is like this

[georage]

thanks for the assist dude, but thats not the schematic i have. let me look up the exact one. maybe its a different model.

[georage]

sorry guys im using the 1449 schematic oops

[georage]

i guess the amp a i was trying to clone is a 1457...but i used the 1449 schematic and cut out the tremelo.

[The Dude]

Here's an easier to zoom/read pdf of the schematic.

https://schematicheaven.net/bargainb...rtone_1449.pdf

[mozz]

Put a cap across the 4.7k. 22uf/10uf/1uf, something like that.

[georage]

a 5uf cap across 4.7k cathode resistor makes the hum louder. that means the hum is being amplified by the tube. and shorting the input has no effect and shorting the stage 2 grid stops the hum, so....idk

[Helmholtz]

a 5uf cap across 4.7k cathode resistor makes the hum louder. that means the hum is being amplified by the tube. and shorting the input has no effect and shorting the stage 2 grid stops the hum, so....idk

While a (large enough) cathode cap at the input tube often lowers 60Hz heater hum, it als increases gain.
So any hum entering the tube will get louder.

It would really help to know it's 60Hz or 120Hz.

If it's 120Hz it would mean power supply ripple.
As you don't have the tremolo circuit you should add another filter cap (say 10µ) to ground at the node between R20 and R5.
Generally single ended output requires more filtering than push-pull, so consider adding filter capacitance.

I wouldn't trust a wire nut relying on twisted strands/wires for low enough ground resistance.

[mozz]

It's the first tube or before it.

[georage]

well it went from bad to worse. it making a bad tremelo effect and finally died when r7 failed, probably from soldering too hot. i think i am going to rip it apart and start over.

i never checked the power supply ripple. because i dont knownhownto use my oscilloscope to test over 300 volts...im not proficient with it yet....

the problem could be a bad filter cap on the power board, right? and it sounds like 60hz to me, or rather it sound just like the ac unit and the fridge and everything else that make electric humming in the house....

if just the final 5uf preamp cap went bad, would i be able to eliminate that hum with the tone control which grounds the signal between the two stages of the preamp????

[The Dude]

Odd that the cathode resistor should fail. Are you sure you had the right value resistor installed? The picture looks like you had 110 ohm installed, but it could just be a discrepancy in photo color or my monitor.