Dual Rectifier 2 Channel 120 (100) cycle hum

BesaMoogie

BesaMoogie

Well-known member
Hi there, maybe someone can help me sort out this one.
My RevF Dual Rectifier has a constant 120 cycle hum, or to be precise it`s a 100 cycle hum as I life in the EU.
It is an 120 Volt version and I run it with an TAD step-down transformer at 120Volt. The hum is definitely louder on this one as one my other 2 Channel Dual Recto.

Here is what I did so far:
1. Changing all the tubes --> no change in hum
2. using another step-down transformer --> no change in hum
3. turning up the master volume --> no change in hum
4. pulling preamp tubes V1-V4 --> no change in hum
5. pulling V5 (PI) --> hum disappears completely!
6. putting amp in settings with lowest plate voltage (spongy, tube rectification) --> hum gets lower with every lower setting

One thing noticeable in this amp is the strong vibration coming from the PT. The whole chassis vibrates at the same frequency as the hum coming from the speakers. Putting the amp at lowest plate voltage settings decreases this vibrations.
I tightened the screws on the top of the PT (I can`t reach the nut on the bottom) which improved the vibrations a little bit. But on the other side, I actually don`t care for the vibrations so much, it`s the hum that annoys me.

Any Ideas would be appreciated.
 
Last edited:
I’m not a recto guy but anytime I’ve had a PT vibrate or hum a lot it was because I was drawing a ton of current or had done something improper. Does the PT get hot? Have you checked the power tubes? I wonder if they’re on their way out and are causing excess noise and or current draw. Does the PT vibration stop when the PI is pulled?
 
What’s the bias for the power tubes set to and what’s the plate voltage in and out of standby?

Have you verified the correct size fuses for 120V are in the amp?
 
Chubbs said:
I’m not a recto guy but anytime I’ve had a PT vibrate or hum a lot it was because I was drawing a ton of current or had done something improper. Does the PT get hot? Have you checked the power tubes? I wonder if they’re on their way out and are causing excess noise and or current draw. Does the PT vibration stop when the PI is pulled?
Click to expand...
glpg80 said:
What’s the bias for the power tubes set to and what’s the plate voltage in and out of standby?

Have you verified the correct size fuses for 120V are in the amp?
Click to expand...
The PT gets a little bit warm. It`s at 93F after 15min (92F at the other DR). PT is still vibrating when PI is pulled.
PV sits at 450 Volt and Bias is, depending on which tubeset I use, either between 32-29 mA (new set) or 24-22 mA for 6L6. So Bias for both sets is under 50%.
The fuse is 4A and is what should be used for the amp (2A for the EU version).

Using a wrench, I was able to tighten the screws even a bit more. The chassis rattling is thankfully gone now. There is still some vibration going on, but not that much more as in the reference DR.
What I discovered is, that the same set of tubes is biasing about 2mA higher in the reference DR. PV is 450 Volt vs. 459 Volt in the reference DR.

The hum went away a tine bit with the "new" set of tubes. But all in all, it`s still hummier as the reference DR.
 
You must log in or register to reply here.
Back
Top