No such thing as good hum

[selloutrr]

The shift in CC is caused by Heat from the voltage.

SEI STACKPOLE makes nice CC new production resistors.

Allen Bradley NOS are still very stable.

[LarryLarry]

The green/yellow CT is for an 18v tap (green wires with masking tape, don't worry they are cut and heat shrinked off). I confirmed with my ohm meter.

I've tried about everything except redoing the heaters on the EL84s, so that's what I'll try tonight.

Stay tuned, and thanks...

[LarryLarry]

Ok, I moved the filiment wires on the EL84s from up in the air to down in the chassis looped up against the edge - absolutely no difference.

Resolodered all my grounds; played around with ground locations via aligator clipped jumpers - no difference.

Tried several different tubes (the hum is there with all 12ax7s pulled, just not as loud) - no difference.

Not sure if this is normal, but the artificial CT for the filiments works (sounds) the same with only 1 100r as it does with 2 100r. Is that right?

It just sounds under filtered to me, but the tone/feel/etc of the amp sounds awesome!

1. Should I suspect/test the choke? If so, what do I test for? (first build w/choke).

2. Is there a way to test or isolate the power supply filters w/o rebuilding them or desoldering them to re-spec them?

3. Is it possible that this old Baldwin PT is just out of it's league in this build? (I rechecked it and it's 285-0-285)

[selloutrr]

it's possible it's transformer placement. magnetic field.

[LarryLarry]

I did the headphone trick on the PT/OT and choke for best placement. Also, this PT and OT where oriented the same in the original Baldwin organ chassis, although they were about an inch farther apart.

It doesn't sound quite like magnetic inductance(?) to me. I had that battle with the Express I built and moved the PT all over the place to find the sweet spot (and it is silent hum-wise now).

[rooster]

Hm. Well, OK, do one thing for me? Use your DVM set to AC and measure either side of the filament supply. You should read 3.2VAC (or therabouts) on either, then measure across both of them for the total or 6.3VAC.

Does this happen as described?

[rooster]

Oh. Could you lift that 33K cc on V1 so it attaches to the pin in a vertical manner?

Then something else. I see that you have added the ground path from the 220K coupling resistors to the tone pot bus bar? This is in the build diagram - yes - but could you try attaching it to the GZ34 tube socket ground instead? (Or wherever you have grounded the two primary caps.) Just unsolder it and attach it with your jumper wire.

[M Fowler]

Take the amp to another house to test, might be noise injecting from dirty AC.

Mark

[LarryLarry]

Sure Mark, what's your address?

I have messed with changing/removing/moving the 33k resistor to no avail. I have not tried moving/jumpering the ground from the 220k's to the lug on my GZ34, will try tomorrow.

Will double check the heater AC as well...

[hans-jörg]

Hi,
I had several amps in the past where the symetry of the filament was way off the center. This you can find out by implementing a trimmer instaed of the two 100 ohms. Whatever you have: 100, 200, 250 or more Ohm. You might will see that when you turn it to the lowest point of noise, that both sides have not the same resistance.
Put the center pin of the trimmer to ground (wheres your HT CT ((red/yellow?))) and both sides to the filaments. Remove the resistors!
When you found the lowest noise point and you have stil hum, its probably not the filament by itself. Then you have to check your crossings or position of filament wireing.
You mentioned, you have this humm still without preamp tubes. So its not the preamp by itself.
You could disconnect the poweramp and hook your preamp to an other poweramp like your first Express. So you can locate step by step your humm.
You could also lift the PT and put rubber between it and the chassis.
I can hear a summ/humm when I turn the amp and put him "naked" with PT and OT downside on the table. But when there is no contact to the table its quiet. The tabe becomes a resonator.
And so on, killing the humm is a funny thing

Best

Hans-Jörg

[LarryLarry]

I checked the AC on the filiments, 3.42vac on 1 side, 3.43vac on the other, 6.89vac across them. Doesn't look like there is enough difference that a hum balance pot would fix, although any reduction in hum would be good!

I tested the ground from the 220k's to a couple different spots and it made no difference.

Hans - There is some physical vibration on this PT so I will try unbolting it and holding it above the chassis to see if it goes away.

I have to keep saying this, the amp sounds wonderful!

[selloutrr]

Just throwing this out...

Have you tried it with the bottom cover on?

[LarryLarry]

I have not attached a bottom cover, but I have taken a small sheet of aluminum and held it over various spots above (below) the chassis and could hear no difference.

[selloutrr]

I read you have resoldered your grounds have you metered them for ohms to ground? Have you tested your electrolytics for leaks?

[rooster]

Huh. I am stumped at this point, LarryLarry. Your tranny layout is good, too, so it's not that. This build is typically so quiet, too, never had any hum in the few I've built. I don't have any other ideas, sorry.

But I will keep thinking about it. I was really thinking it was the filament supply. Damn. The only wild card in the build is the PT - and if you've had a quiet build using it before then this must not be the problem. Hard for me not to look at it right now though.....

I'll check back.