Single Ended Hum

[leadfootdriver]

I remember reading that Matchless would run their EL84's at 400dc..

[J M Fahey]

SCOPE it, only definitive answer.

Hum waveform shape tells A LOT.

[Tom Phillips]

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...

There are several circuit factors that affect the voltage that reaches the first stage preamp tube plate. To assist people to respond to these type questions it would be helpful If you would post the schematic of your existing circuit.

[leadfootdriver]

I found this on EL34 World with a number factor to calculate voltage..

So, if you used a PT that has 275-0-275 with:

5Y3GT 275 X 1.1 = 302.5v
5V4 275 X 1.2 = 330v
GZ34 275 X 1.3 = 357.5v
solid state 275 X 1.4 = 385v exceeding the EL84 max plate voltage

https://el34world.com/Forum/index.php?topic=16664.0

[Chuck H]

I found this on EL34 World with a number factor to calculate voltage..

So, if you used a PT that has 275-0-275 with:

5Y3GT 275 X 1.1 = 302.5v
5V4 275 X 1.2 = 330v
GZ34 275 X 1.3 = 357.5v
solid state 275 X 1.4 = 385v exceeding the EL84 max plate voltage

https://el34world.com/Forum/index.php?topic=16664.0

That registers the voltage drops relative to different rectifiers and is basically correct for SUPPLY voltage, but... A little arbitrary and out of context. Operating class and OT impedance also affect ideal plate voltage for any power tube. Trust me, you really DON'T want to run an EL84 at 385Vp in class A. Only the PT ratings AND circuit values can tell us anything valid for what the tubes will see. We do not have a schematic. No offense intended at all but if you're over your head on this then at least give us the information we need to help.

[leadfootdriver]

I've been playing with this thing. I'll draw up what I have and post it.

[leadfootdriver]

I don't write so good..

[Helmholtz]

Nothing wrong with your schematic drawing skills.

Some comments:
- the cathode wiring of the power tube is missing,
- one end of the OT secondary should be grounded,
- you can't have different B+ values after the 1k dropper except if there is another resistor,
- the NFB from the output to the MV wiper is strange and will load down the power tube input signal.
Please add the power supply wiring.

Did you "design" the circuit?

What is the remaining problem?

[leadfootdriver]

Yup forgot to bypass the EL84 Cathode.. 150R with a 220uf..

I had 2-10K Dropping Resistors for the Preamp after the 1K, but I think this PT is weak. It sounds way better, bigger, fatter.. after I removed them. I want a different PT..

[Helmholtz]

You can't have 257V as well as 267V on the same B+ bus.

Again: Who "designed" that strange power stage NFB? Doesn't make sense to me.

[leadfootdriver]

Design probably isn't the proper term..
I just tried inserting at different points and rolled in some pots and resistors, tried a Presence arrangement.. until it sounded descent.
It's tighter and faster now, and not shriekingly bright. The Tone Controls have a good range of adjustment.
The volume dropped a bit but no more than any other point in the circuit I tried..

[Helmholtz]

Again: Is one end of the OT secondary grounded?
If so, what happens if you disconnect one end of the 33k NFB resistor? Especially at higher MV settings.

Please answer all of my questions and show the power supply circuit with voltages.

[The Dude]

Design probably isn't the proper term..
I just tried inserting at different points and rolled in some pots and resistors, tried a Presence arrangement.. until it sounded descent.
It's tighter and faster now, and not shriekingly bright. The Tone Controls have a good range of adjustment.
The volume dropped a bit but no more than any other point in the circuit I tried..

I'd suggest it was "shriekingly bright" because you have low value cathode bypass caps throughout the entire preamp. Again, just suggesting: I'd try different cap values there instead of some oddball NFB arrangement. I also wouldn't run 4 triodes and screen supply on the same node. That's asking for oscillation and/or other odd interaction issues. And, as Helmholtz pointed out, you can't measure two different voltages on a single node.

[leadfootdriver]

Again: Is one end of the OT secondary grounded?
If so, what happens if you disconnect one end of the 33k NFB resistor? Especially at higher MV settings.

Please answer all of my questions and show the power supply circuit with voltages.

Yeah, back on post 20.. The OT has it's own Ground Wire..
I can put in 2-1K Resistors on the B+ to separate the Gain Stages if you think that would be enough.
I'll have to try disconnecting it tomorrow.

[leadfootdriver]

I'd suggest it was "shriekingly bright" because you have low value cathode bypass caps throughout the entire preamp. Again, just suggesting: I'd try different cap values there instead of some oddball NFB arrangement. I also wouldn't run 4 triodes and screen supply on the same node. That's asking for oscillation and/or other odd interaction issues. And, as Helmholtz pointed out, you can't measure two different voltages on a single node.

Tried ALL Kinds of combinations.. But I'm trying to keep the lowend tight with a crisp attack while maximizing gain given only 3 Stages.. A fully bypassed Stage at any point turned the sound into mud.. A higher Slope Resistor offset the balance of the Tone Controls IMO..
The NFB wasn't exactly random and smoothed out everything making it pretty balanced.. I tried a lot of values to get the sound and response I have. It's the best I've come up with so far..

I want it to sound as big and tight as possible, but it IS only 1 Cathode Biased EL84.. Still open to suggestions tho..