No such thing as good hum

[dartanion]

Hey Larry,

This is a bit off topic, but I would change how you have the bypass cap setup on the cathode resistor for the power tubes. With it sitting right on top of the resistor is a good way to cook that cap.

As for your hum issue. Are you sure you want your plate leads running directly on top of your filament wiring for the power tubes? Take a look at the real Liverpool photos to see how the filament wiring was done by Ken. Doing the filaments that way gets them out of the way of the plate leads.

[LarryLarry]

Hey Darrin!

Do you basically mean run the plate leads to the EL84s toward the inside of the chassis instead of toward the outside where my filiment leads run?

But the chassis is upside down, so the resistor will be on top of the cap, that should be ok I would think. I can move it to the side though, or perhaps rearrange it.

I'll take a look at the Liverpool wiring, if I can find it. Maybe in a .zip file here somewhere?

[LarryLarry]

Will this work for a hum balance pot? It is 500 ohms

[dartanion]

Look at the official layout done by Ron Worley. This is based off real Liverpool amps and works great. I am recommending this style of filament wiring versus moving the plate lead runs.

[LarryLarry]

Looks like a real lazy loop towrds the inside of the chassis for the EL84s. I'll switch them around tonight.

BTW Darin, I'm using the cab you built for this amp. 1 Celestion Greenback that was reconed to 50 watts by Weber (a long time ago) and 1 Mesa Celestion MC90; wired in parallel to 4 ohms.

[LarryLarry]

Tried the hum balance pot thing, but didn’t really make a difference. It made no difference at all with the volume on zero, when I turned up the volume all the way (no guitar plugged in) I could hear the affect the pot was having but it was not really increasing/decreasing hum, it was slightly higher pitched than hum, kind of buzzy (but really faint). So I found the sweet spot, pulled it out and measured each side and I got 114 on one side and 116 on the other so I assumed that would be dead center so I put the 2x100r back in.

I re-routed the filiment wiring on the EL84s as per suggested, can't really hear any difference in the hum, which is slight now anyway. But it looks better! I'll post a couple photos tonight...

[LarryLarry]

A couple photos of the re-routed filament wires on the EL84s, and I found the original 6BQ5s that came with the organ chassis. RCA maybe?

(The speaker cab was made by Dartanian)

[hans-jörg]

Hi,
looks good now with the heater filament. Small suggestion: don´t lay the cathode all the way parallel to the heater wireing so close.
Evan a wire wich goes direct to ground (lets say from the cathode) can absorb the AC hum of the heather or mains.
If there a slight hum remains when you switch the power on but stay in Standby you have to find out for yourself if you can live with it. There are not much possibilitys from where it comes. Heater or iron or mains wireing - the layout of the AC wires. the rest is after the Standby - if I´m right.
Whats the effect in Standby when you turn the Hum-balance-pot. You dont have a small plastik trimmer which you can mount there between the two heater wires-connection? Something between 200 and 500 ohm. Thats the best way. Sometimes the hum changes, so you are allways able to corrct it. I make this heater symetry only by trimmer. Thats the securest way.
Sometime you have to choose if you want a layout which is closest to origin, or that it works proper. I choosed for myself the second.
At least drill your anode wires and move the heater wires away from them. You could shorte the a little bit, it will fits more and they will not lay together which causes definitly noise problems.
Every hum problem is home brewed - hiss is an other story.

But I think you are allready happy with your amp and thats the main thing.

The last: try the amp with and without the alu tube shoes. I could imagine there is a big different in sound and clarity of tone. They are only neccessary when you have problems with let say neon light or so.

Best

Hans-Jörg

[LarryLarry]

Great suggestions Hans, thank you! I will put the hum balance pot back in, it certainly didn't make it worse and you are right, it gives easy access when power tubes are changed.

There is no hum when the amp is in standby mode.

I didn't know/think the aluminum tube shields could affect tone, but I'll give it a shot.

I may hunt down some Zener diodes to drop the PT a bit and see if that gets rid of the rest of the hum. I've not used them before, I'll have to hunt around to see how they should be used. 2 in series on the CT for the PT from what I understand.

I see some Zeners on eBay, they are IN5347B, are those correct?

[LarryLarry]

Hans, what do you mean by 'drill your anode wires'?

Sounds like the Zener diodes needed for dropping overall voltage are not the common type (NTE5125A)...

[LarryLarry]

To see if the hum I have left could be filtered out, I decided to jumper in an additional filter cap (47uF 450V) at B+ right off the standby switch. It definitely helped a ton - I had to walk over to the speaker cab to hear if it was on! Still sounds awesome, still running cool and with 3 10V 5W Zeners off the PT CT, my EL84 plate voltages are 316vdc and the voltage across my 100 ohm 25W cathode resistor is 12.3vdc.

Can I leave the additional filter cap in? Does this indicate my choke may not be working correctly, or that I have a bad filter cap?

I'm starting to think that this PT was running in AB mode in the Baldwin amp (that ran a 5U4GB rectifier) and it's there's just more current to deal with in this build...

I noticed that as I turn up the bass pot, the remaining hum gets louder. guess that make sense since it's a low frequency!

[Darkbluemurder]

Hans, what do you mean by 'drill your anode wires'?

Sounds like the Zener diodes needed for dropping overall voltage are not the common type (NTE5125A)...

He meant "twist".

Cheers Stephan

[LarryLarry]

Ah ha, as in twist using your drill!