Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: misterc57 on January 01, 2026, 01:21:06 pm
-
I've been rehab this old Mace 212 6 output tube combo that had probably been idle for years.
Replaced the original filter and bias caps. Over a short period of time of testing reverb issues and wrapping things up a hum started to creep into the mix and quickly became a full blown 120 cycle hum.
There are two main filter nodes. Each using 2 100uF/350V in series. The first node is putting out around 445 VDC to the plates via the output transformer. I measure about 4.4 VAC ripple at the first node, (is this normal?), this is sent to the output transformer for further smoothing and this is where things get funky.
One side of the transformer is reading around 0.6 VAC ripple going to half the plates and the other side is reading 7.5 VAC ripple going to the other half of the plates. All 6 plates have consistent VDC around 445.
Primary center tap (red wire) to the blue wire reads 21.7 ohms resistance. This has the 0.6 VAC
Primary center tap (red wire) to the brown wire reads 26.5 ohms resistance. This has the 7.5 VAC
Any thoughts on this? I am thinking it cannot be the new filter caps but maybe the output transformer?
Thank you.