Eating through Power tubes, selmer little giant MKIII clone

[Skev]

If you pull the preamp tube out is the hum still there?

Does the level of hum change with the volume control setting?

Are your output and power transformer cores at 90 degree angles to each other like they should be, or are they inline?
If this is wrong many times even with no tubes in the amp it will hum.


Sometimes the magnetic 50hz flux of the PT will couple right into the speaker with no tubes in the amp.

Are your filament wires twisted tight, are any of the preamp tube grid wires (pins 2 and 7) laying on, Or too close to the filament wires?

Are the B+ wires feeding the recto twisted as they should be?

Try a known good non humming preamp tube as this could well be your issue!

With your meter set for low ac volts how much ripple do you read at the power supply side of the preamp tube plate load resistors ?

Does the center tap of the power transformers filament winding have a solid ground ?


I list these in no set order, just a bunch of things to check out!

Hello. The amp is still working very nicely...excepted the 50hz hum, that i still can t eliminate.
The hum is still present without the preamp tube
It slightly decrease while lowering the volume pot
the two transformers are at 90degree opposite.
The "0" tap is directly soldered to the "-" of the first 32uf cap itself linked to the chassis.
I ve even change that first 32uf for a 100uf !!
I ve tried to make the "fake" center tap at el84 heaters with two 100ohm resistors.
All the wires are twisted tightly...
Very strange...i tried different el84 and 12ax7...
No difference. I have just 1 ez80 so i can t compare with another
Except all that, the amp behave really good and seems reliable.

[Stevem]

The ez80 will not be a sourse of your hum.
Any single ended amp like yours will always have some hum that can not be tamed.
If your power transformer had some extra voltage you could and another 50 uf and like a 4k resistor before the feed to the output transformer to drop the ac ripple down more than the 100 uf filter that you did try out!
When I build a amp like this that is what I do for low hum.

Please do these checks on the amp that I posted before.

1) set your meter for ac volts and tell me what you read at the power supply side of the first gain stage plate load resistor.

2). With your guitar plugged in and it's volume down hook you meter set for ac volts once again across the speaker and tell me what you read with all the controls on the amp down, and again with all the controls up.
In both of these test you will be reading voltages of less than 1/2 a volt .