joulupukki
Tele-Holic
The bad hum and buzz is finally gone in the first version of my B-Verb amps (single-channel AB763 w/ bias trem)! I finally traced it down to a noisy power transformer. To confirm my suspicion, tonight I took the Mojotone Deluxe Reverb Power Transformer that was in Mk II (version 2 of this amp) and moved it into the Mk I after cutting out a hole in the chassis for it. Turns out, the Musical Power Supplies PT330M was the source of the bad hum and buzz this whole time! Grr!
Earlier, when troubleshooting the Mk I, I found that disconnecting the 6.3V heater center tap from the MPS PT and using an artificial center tap with two 100R resistors reduced some of the hum the amp had. But it still had way too much hum and a nasty buzz with the volume knob in certain positions.
I also noticed, and finally realized again late last week, that even with all the tubes removed, I was picking up noise on the chassis as soon as I would turn the MPS PT on. The noise was pretty easy to spot using an oscilloscope connected at the input and also directly to the chassis. After doing all kinds of things to trace the buzz and hum, that was the last straw. I’ve got another Mojotone Deluxe Reverb Power Transformer on order that I’ll put into the Mk II.
It is late so I wasn’t able to really crank up the amp and test out the reverb circuit, but I’ll do that tomorrow and return to give a report on that. I’m hopeful that the reverb circuit will also be significantly cleaner than it was before.
What a journey!
Last week, in hopes of squelching the buzz and hum with an improved layout and grounding scheme, I replaced the cap can with some high-quality Nichicon radial caps. I moved the bias circuit off the main board. I also modified the grounding of the different stages so that they each ground to the negative terminal of the filter cap for that stage (I had good luck with that design when building Mk II). Without redoing the entire board in Mk I, I drilled a number of holes to route many of the wires underneath like I did in Mk II. It’s a bit cleaner now but Mk I is definitely my hacked-up amp. I’m just really excited to finally have this solved!
Here’s an updated gut shot of Mk I:
Earlier, when troubleshooting the Mk I, I found that disconnecting the 6.3V heater center tap from the MPS PT and using an artificial center tap with two 100R resistors reduced some of the hum the amp had. But it still had way too much hum and a nasty buzz with the volume knob in certain positions.
I also noticed, and finally realized again late last week, that even with all the tubes removed, I was picking up noise on the chassis as soon as I would turn the MPS PT on. The noise was pretty easy to spot using an oscilloscope connected at the input and also directly to the chassis. After doing all kinds of things to trace the buzz and hum, that was the last straw. I’ve got another Mojotone Deluxe Reverb Power Transformer on order that I’ll put into the Mk II.
It is late so I wasn’t able to really crank up the amp and test out the reverb circuit, but I’ll do that tomorrow and return to give a report on that. I’m hopeful that the reverb circuit will also be significantly cleaner than it was before.
What a journey!
Last week, in hopes of squelching the buzz and hum with an improved layout and grounding scheme, I replaced the cap can with some high-quality Nichicon radial caps. I moved the bias circuit off the main board. I also modified the grounding of the different stages so that they each ground to the negative terminal of the filter cap for that stage (I had good luck with that design when building Mk II). Without redoing the entire board in Mk I, I drilled a number of holes to route many of the wires underneath like I did in Mk II. It’s a bit cleaner now but Mk I is definitely my hacked-up amp. I’m just really excited to finally have this solved!
Here’s an updated gut shot of Mk I:
