PS Hum and grounding questions

[Cygnus X1]

I'm building a Spitfire into a Gulbransen organ amplifier.
6CA4 recto, 2x12AX7, 2xEL84. The primary B+ is running low, only 280VDC to the rectifier. But that's another issue, I think.

It has a really loud hum problem.
Two places I'm thinking of looking...bad PS cap, and grounding.

Is it ok to run the secondary center tap of the power supply to the chassis for grounding? My other grounds are to the chassis also. The speaker jack is isolated. I tried running the CT to the speaker negative, with zero difference in sound.
I normally run a buss or star ground isolated from the chassis.

Or should I look for the caps passing voltage into into the preamp?
What should I look for?
AC or DC?

It does pass signal, but the hum is there no matter what, and it is as loud as the guitar signal.

So, which should I try first?

[Structo]

Have you tried different tubes to rule those out?

Does your PT have a center tap on the heater winding?

If so do you have that grounded near or to a PT mounting nut?

[chief mushroom cloud]

if you have hum as loud as the guitar signal....it sounds like you have a missing ground connection. Also, check your transformers orientation for possible inductive coupling.

double check all ground connections first

[Macshaft]

I purchased a 5751 (Westinghouse) NOS and tried it in V1 on my Rocket and it hummed as loud as the guitar input . Moved it to V2 and it is quiet . One half of this tube is bad I guess , maybe it has something to do with the "matched triodes" these tubes are supposed to have??

Cam

[Cygnus X1]

No CT on the heater.
Secondaries are red. red, an orange/red, and the green heaters.

I don't have any other tubes to try for the recto and EL84's, I do have plenty of 12AX7's though.

I'll work on the grounding.
So, it shouldn't be a problem tying the CT to ground?
(I moved it to the negative of the first filter cap...still no difference in hum).

[Structo]

Did you lift the heaters to an artificial center tap using 100 ohm resistors?

[Cygnus X1]

I found that my power supply caps weren't making ground.
I'm busy rebuilding my cap board.

Artificial CT on the heaters?
Never done it.
Should I?

[rfgordon]

For the heaters, connect each leg to ground through a 100 ohm resistor. You can also connect them to the top of the power tubes cathode resistor if you like, as that puts a DC reference voltage on them, which can help reject noise.

[Cygnus X1]

I'll look at the heaters later...right now the submarine is heading to to the ocean floor. A small leak isn't going to help or hurt this vessel!

[M Fowler]

My Spitfire was fired up today and I am very impressed with this little amp running with two Epiphone 1-12 closed back cabs she is bright, with 212 celestion 30 cab she has a lot of bottom end mixed with highs and with my alnico blues 212 cab she sounds so good. I did not try it with my 4-12 peavy cab yet.

I am interested in which spitfire schematic and layout you used.

Mark

[Cygnus X1]

Thank you, MFowler.

My layout is dated 6/20/04 by Paul Marossy.
The schem is similar.

Only difference is using a dropping resistor instead of the choke (that I don't have).

I got the B+ voltages from another place, so things are due for tweaking at that point. This is my first conversion project, so there are a lot of great opportunities for me (if you know what I mean). That particular schematic is a January 2009 Dowless 18 watt conversion from the same Gulbransen organ amp I am using. The B+ voltages are far different (lower) than the Spitfire, so I have to do some work to get it right. I like lower headroom, higher distortion amps, so I might just end up right where I am anyways.

[Cygnus X1]

Resolution:

D'oh!
The filtering caps in the power supply were not grounding.
Fixed my goof, fired it up...quietest amp I have built yet!

The sound is ok, very Vox like.
Is the Spitfire supposed to sound like a cranked AC15?

But the distortion is too "farty" for my liking.
I'll take some measurements and tweak it to taste.

Thank you, guys, this one is a success!