36w Lite, hum - even on standby - fixed!

[LarryLarry]

Howdy all - been gone a few months due to a move from Calif. to Georgia. Now I'm back up and running

I built a 36 watt Lite, basically combining the Lite IIb preamp with Richie's 36 watt power section. I used a Baldwin organ chassis that had the right tube compliment already. I tore it down all the way - except for the heater wiring. I figured it worked before, it should work again. The heater wires are not quite as twisted as I normally twist them - maybe that's my problem. All my voltages look really close to those of the 36 watter in the download section.

Here's my problem, I have a fair amount of hum and it's the same on standby as it is off standby. I've elevated my heaters to the cathodes of the EL84s using 2 100 ohm resistors. This is the first amp I've heard that has the same amount of hum on standby as it does off standby. If tried different tubes so that isn't the problem.

Any ideas?

[dartanion]

Hey Larry,

Glad to hear the move was successful.

Do you have the filaments referenced to ground?

[morcey2]

Howdy all - been gone a few months due to a move from Calif. to Georgia. Now I'm back up and running

I built a 36 watt Lite, basically combining the Lite IIb preamp with Richie's 36 watt power section. I used a Baldwin organ chassis that had the right tube compliment already. I tore it down all the way - except for the heater wiring. I figured it worked before, it should work again. The heater wires are not quite as twisted as I normally twist them - maybe that's my problem. All my voltages look really close to those of the 36 watter in the download section.

Here's my problem, I have a fair amount of hum and it's the same on standby as it is off standby. I've elevated my heaters to the cathodes of the EL84s using 2 100 ohm resistors. This is the first amp I've heard that has the same amount of hum on standby as it does off standby. If tried different tubes so that isn't the problem.

Any ideas?

Larry,

Welcome Back! Hope the move went well. (Do they ever?) Our last move was a nightmare and it was only about a half mile.

If you're hearing the hum with the amp just in standby, you're probably getting magnetic coupling between the PT and OT. I've got that on my P1Ex, but it's low enough that it's not a problem. If you can move one of the trannies, you can probably get rid of it.

Totally off-topic. How's the Mark Durham JTM45 treating you? I finally got the JTM45 kit that I won finished and in a cab. Awesome amp.

Matt

[LarryLarry]

Yes, filaments are running through 100 ohm resistors to ground (cathodes of EL84s). Before I did this the hum was really loud!

My OT and PT are indeed fairly close together. I tried holding a small sheet of metal between them to see if it reduced the hum but there was no difference. Attached is a photo for reference. This PT has some extra taps, one for sending 117vac out and 2 18v taps. I guess for Leslies, or an external preamp or something. I just have them shrink tubed off.

I put in a 1/2 power switch that is just an on/off switch between pin 3 of one pair of EL84s and it's 150R/100uf capacitor. This didn't seem to make a difference on the hum issue.

Hey Darin, where did you get the grillcloth on the 2x12 cab you built?

[teletroy]

I had a very similar issue once- I was rewiring my 5e5 pro to have a normal standby instead of lifting the center tap for the HV, and forgot to move the center wire for my OT- so when it was in standby the OT was still getting voltage. Hummed like a mother. Once I moved that wire to after the standby switch worked great, I originally had that wire coming straight off the rectifier.

[LarryLarry]

Thanks TR, I'll check and see if that is my problem. I don't think it is, but I hope it is

[BackwoodsJoe]

I've run into that & ended up moving the OT.
Joe










i

[dartanion]

Hey Larry,

The grill cloth is the Fender oxblood with gold. I just used it with the stripes going vertically as I like that look better. I got it through CE Dist. I think I still have some if you need a piece.

[LarryLarry]

Thanks for the info Darin. I thought it was kinda familiar looking! I'm going to build a small head cabinet for this 36w lite and I thought it might be nice if they matched since the 2x12 cabinet sounds so good! I have a Celestion Vintage 30 and a Celestion G12H in it btw...

Ok, well I seem to have either a flakey speaker jack and/or a flakey speaker cord. When I wiggle it a bit to a certain position while the amp is on, the hum almost goes away completely. I also found that some of my EL84s were causing the hum to be louder. I'm getting somewhere at least!

[LarryLarry]

Resolved

I did indeed have to rotate the OT by 90 degrees. Got rid of all the hum on standby and there is barely any there at all when off of standbay. I knew the PT and OT needed to be 90 degrees offset from each other but since this was a build from a working Baldwin organ amp I assumed it would be ok as is.

One more issue left - new topic on the way...

[tarzanalog]

Makes ya' wonder how good the Baldwin sounded before...