Single Ended Hum

[stokes]

If the output is cathode biased, try connecting that "fake Center Tap on the Heater with 2-100R's" to the cathode of your power tube instead of ground.

[Chuck H]

If the output is cathode biased, try connecting that "fake Center Tap on the Heater with 2-100R's" to the cathode of your power tube instead of ground.

I think we need to make sure the filament CT is not in use first? It's probably not but I'd like to be sure.

[Tom Phillips]

Isolate the problem.

Try pulling the preamp and driver tubes and power up the amp with only the EL-84 power tube.
Is the objectionable hum still there?
If so, then that will indicate that the hum is most likely due to inadequate B+ filtering for a single ended power amp.
This issue is present in many small commercial amps such as the Fender Champ series.
Mozz in post #7 suggested the solution.

[Mick Bailey]

I suggest taking a measurement of the B+ ripple and then setting up the PSU in Duncan's PSU designer. See if your reading is close. I find this software to be remarkably good at predicting PSU output. You can also make changes to the circuit and see the results before committing to physical changes. Proper filtering with RC stages is much better than just increasing capacitance. I'm playing through DIY SE amp right now that I can't tell is switched on - no hum at all. It runs an EL34 at 500v and is plenty loud enough.

[leadfootdriver]

Thanks everyone for the help!

Connected the Fake Center Tap directly to the EL84 Cathode.. No change.
Pulled all the Tubes, powered it up to a quiet Amp. Put in the Preamps to no noise and then the noise came back when I put in the Power Tube. The noise stayed with the Preamps back out Power Tube still in..

I did find one error on my part so far... I had the Power Connection to the Output Transformer connected directly to the Rectifier with no filtering. I moved it after the Filter Caps so the Tone should be a bit better!

Rechecking the Transformer Wiring.. The PT is on the left. OT on the right.

PT:
Black and White, Primary's..
Red, Secondary's..
Orange, Filaments
Yellow Center Tap

OT:
Black and Yellow to Output Jack
Green Ground
Red Power in
Blue output to Plate..

I'm getting 314DC off the Rectifier with no Tubes, 272DC with Tubes in. Right now I have 2-100uf Caps in parallel right after the Rectifier..
I'll need to check out the Duncan Calculator..

[Helmholtz]

Connected the Fake Center Tap directly to the EL84 Cathode.. No change.
Pulled all the Tubes, powered it up to a quiet Amp. Put in the Preamps to no noise and then the noise came back when I put in the Power Tube. The noise stayed with the Preamps back out Power Tube still in..

So we can exclude heater hum.

I did find one error on my part so far... I had the Power Connection to the Output Transformer connected directly to the Rectifier with no filtering. I moved it after the Filter Caps so the Tone should be a bit better!

Confused. So there is no cap connected to the rectifier output?

Please post a drawing of your power supply wiring!

[leadfootdriver]

Figured it out.. I didn't have Filtering for the Screen Supply of the Power Tube.
I connected a 100uf directly to the Screen and the Amp is silent!
I learned something..
Even tho the Screen was connected to the 1K Resistor after the 1st 100uf, it didn't do a thing until I put a Cap right on the wire.

[leadfootdriver]

Got this thing sounding alright for what it is, a little gain monster.. Sounds okay as intended for a little 8" Speaker.. Sounds like shit hooked up to a 412..

Removed the Input Jack and Line Out and installed Pots. It goes Gain, Treble, Mid, Bass , Master Volume..
I put the Input on the back and even put a Cliff Jack to go out to an external cab but it sounds better as a little practice Amp..
What a mess of wiring. I burnt everything.. LOL But it works and sounds alright.

[Chuck H]

PT:
Black and White, Primary's..
Red, Secondary's..
Orange, Filaments
Yellow Center Tap

There's still one thing you never addressed. Even though it's been brought up twice and is important. The issue with your filament wiring center tap. It seems you have the filament center tap grounded AND you're using a pair of 100R resistors to ground as a false center tap. You should only be using one or the other. NOT both. Looking at the schematic you'll see that the PT HV secondary has no center tap. The PT center tap you indicated is the filament winding center tap. You need to lift it and cap it off or remove the 2x100R false center tap circuit.

More could be done for improvement in the ground scheme but if if's quiet enough that you're happy I don't suppose it's necessary.

[Helmholtz]

It doesn't hurt to leave the 100R resistors with the heater CT grounded though they won't make a difference.

[g1]

It doesn't hurt to leave the 100R resistors with the heater CT grounded though they won't make a difference.

Maybe not significant, but does this not add to the load on the heater winding?

[Helmholtz]

Maybe not significant, but does this not add to the load on the heater winding?

Sure, adds 32mA. But no matter if the CT is grounded or not.

[leadfootdriver]

I went ahead and removed the 2-100R's from the Heater..
It did nothing and there's no reason to drain current..
Those Transformers were spec'ed for 1-12ax7 and 1-EL84..

[Chuck H]

I went ahead and removed the 2-100R's from the Heater..
It did nothing and there's no reason to drain current..
Those Transformers were spec'ed for 1-12ax7 and 1-EL84..

That was actually my thinking. Being as this is a purpose spec'd transformer and you've already added a tube anything at all extra can only be detrimental. 32mA is mice nuts compared to what tubes draw but it all adds. And...

If you DO decide to idealize for even greater hum reduction you'll need to change the daisy chain arrangement and add a "humdinger" type circuit. To implement a humdinger you absolutely would need to eliminate the filament CT. Since this was the train of thought on this thread I chose to prioritize it.

[leadfootdriver]

I've long thought about a new Power Transformer for this thing. I was thinking a Hammond 260 series..
With the one I have, I'm only getting like 175dc to the 1st Triode.. This has to make for a thinner sound?
It would be 4-Triodes and one EL84 for around 53ma's of current..

How do you figure the Rectified voltage from the Center Tap specs?? Am I over thinking this?