I built an express a while back, and I have been tolerating the loud hum (not buzz) that appears when the standby is flipped - volume down or up. Now I have time to take a closer look It disappears only when v2 is pulled, the inverter is pulled, or when I check voltage on pins 2 or 7 on the PI. The voltage on those pins are 12.8 and 11.4 respectively. About half of where they should be?
What voltage should there be on pin 6 of the power tubes? I am getting 385 there.
I double checked heater wiring, transformer placement, added a hum balance pot - to no avail.
Any help is appreciated.
Express build - Hum
Moderators: pompeiisneaks, Colossal
Re: Express build - Hum
Did you try simply replacing each tube one at a time with tubes of known quality (no buzz) to see if the buzz goes away?
Check the quality and value of your PI input grid resistors (and double check to ensure tail is 470R, not 470k). Check the proximity of sensitive grid wires and their angular orientation with respect to your heater wires. Elevating the heaters to DC is an option although it should be an enhancement to a known well dressed and quiet amp, not as a band aid. Something else is at work here.
Curious, what kind of transformers are you using? To my recollection there has been a recent increase in number of posts citing buzz in Express builds and I have wondered if there has been some power transformer issues in common with these builds.
Check the quality and value of your PI input grid resistors (and double check to ensure tail is 470R, not 470k). Check the proximity of sensitive grid wires and their angular orientation with respect to your heater wires. Elevating the heaters to DC is an option although it should be an enhancement to a known well dressed and quiet amp, not as a band aid. Something else is at work here.
Curious, what kind of transformers are you using? To my recollection there has been a recent increase in number of posts citing buzz in Express builds and I have wondered if there has been some power transformer issues in common with these builds.
Re: Express build - Hum
Is the hum there when V1 is pulled? IF it stops when V2 is pulled, it is either V2 or before.
Check your grounds, input, volume and tone stack etc....
Check your grounds, input, volume and tone stack etc....
Re: Express build - Hum
It is a hum, not raspy. I tried new tubes in all positions. Re-flowed solder joints, etc.I am still curious about the voltage on the PI, and why when I check them the hum stops. Re-checked the values / connections of all pots and the board. Chop sticked everything.
It sounds like the hum from the transformer is being amplified - you can hear it before the standby it hit id you are close to the amp. They are Pacific xformers.
It sounds like the hum from the transformer is being amplified - you can hear it before the standby it hit id you are close to the amp. They are Pacific xformers.
Re: Express build - Hum
Your meter is loading down the signal. Sounds like a ground issue to me. Is the hum there when V1 is installed? It may be at the input / volume / tone if so...bcat01 wrote:I am still curious about the voltage on the PI, and why when I check them the hum stops.
Re: Express build - Hum
hums with v1 in or out. stops with v2 out.
Re: Express build - Hum
Swapped v2 with a 3rd known "good" tube - a cheap 12ax7. Problem gone!
Thanks for all the help!
Thanks for all the help!
Re: Express build - Hum
Nice! 