bad hum in my 2 x el84 liverpool

zambo · Post by **zambo** » Sat Mar 26, 2011 8:41 pm

I built the half power liverpool and followed the layout pretty close. i have sheilded input wire on v1 a and v1 b . the original grounding has been changed to a star ground as i thought the ground was bad and causing hum. nothing has changed. Iused a steel chassis. would that cause a hum?

M Fowler · Post by **M Fowler** » Sat Mar 26, 2011 8:47 pm

I completely rebuild one amp I had two times before I realized it was a freaking Helical 25w light bulb not even near my wok bench that was the problem with hum and buzzing.

You could have a ground issue but you could also have a dirty power line in the house/building your in.

zambo · Post by **zambo** » Sat Mar 26, 2011 8:55 pm

i thought that as well and i know my house is ungrounded ( working on that) but when i plugged into a known grounded house the hum was just as bad. This is a transformer set out of a blues junior. i used the blues jr power supply scheme as well. I dont know if thats causing the hum or not. I have heard reversing the wires on one of your tubes heaters can help but am a little intimidated to try it. any thoughts?

zambo · Post by **zambo** » Sat Mar 26, 2011 8:58 pm

also the hum changes as i roll the volume up or down on my guitar but is quietest with the guitar volume at about 2 or 3. dont know why that would be. When i play bass notes it goes away a little to. dont know if its from power supply lag or speaker overload etc. just trying to figure out where it comes from. this is my 2nd rebuild of it.

fishy · Post by **fishy** » Sat Mar 26, 2011 10:05 pm

Maybe you can post some pics which might highlight something.

Gargoyle · Post by **Gargoyle** » Sun Mar 27, 2011 1:51 am

I had a similar experience a while back, it ended up being an EI 12ax7 that was causing the problems.....

zambo · Post by **zambo** » Sun Mar 27, 2011 1:58 am

what does EI mean? Is that the brand? Sorry I am kind of a noob and cant speak acronym yet...

I also thought of this. my speaker jack is a metal jack connected to the chassis. everything else is connected to star ground thats connected close to the input jack. Have i just made a ground loop out of the whole chassis?

statorvane · Post by **statorvane** » Sun Mar 27, 2011 11:29 am

Have i just made a ground loop out of the whole chassis?

Could be. I isolate my jacks (speaker, input, reverb...) and run a separate ground wire for them to a star point. Works very well.

breakfastbuddy · Post by **breakfastbuddy** » Sun Mar 27, 2011 2:16 pm

EI is a brand

M Fowler · Post by **M Fowler** » Sun Mar 27, 2011 2:20 pm

What transformer did you use to power this amp?

zambo · Post by **zambo** » Sun Mar 27, 2011 4:42 pm

I used the stock blues jr. transformers for power and ot. pt is a flat mounted job. not to familiar with those. I also isolated the speaker jack last night. didnt seem to help. I cut out one of my first filter caps abd want from 80uf down to 40 uf for the first power stage filtering. the hum goes away when the power supply sags and as it comes back it changes from lower freq to higher.

M Fowler · Post by **M Fowler** » Sun Mar 27, 2011 9:43 pm

Okay here is the bluesjr schematic showing the PT wiring.

Which looks like 130.5vac per leg did you use the type of rectifer on the schematic or the TW design?

Mark

zambo · Post by **zambo** » Mon Mar 28, 2011 1:17 am

i used the blues jr rectifier and power section. i elevated my heaters off of the pi like a liverpool. made the heater center tap from 2 100k resistors. center tap of b+ is grounded at star ground.

zambo · Post by **zambo** » Mon Mar 28, 2011 3:42 am

ok good news is this. i moved the ground for my heaters to the star ground rail and the hum got quieter! Its not gone by any means but its less. i also swapped the wires on my first preamp tube to see if that would give some hum canceling. it didnt as far as i can tell. I added a 100k resistor in paralel to v1b plate. less gain but VERY COOL TONE comin out of it! Not near as much gain but the glassy almost clean is beyond cool. for what thats worth.

passfan · Post by **passfan** » Fri Apr 01, 2011 8:43 pm

zambo wrote:i used the blues jr rectifier and power section. i elevated my heaters off of the pi like a liverpool. made the heater center tap from 2 100k resistors. center tap of b+ is grounded at star ground.

Most people use 100 ohm and not 100k. If they're 10% you could actually create more of an imbalance than having none at all.

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