Luix Dual Channel EL34 50W Power Amplifier

[luix]

UPDATE:

I've changed the transformer as I've used to think that commutation spikes are due to its leakage inductance, but I was wrong, the transformer has about 100mH of leakage inductance but this is not the problem.

Diode commutations spikes are still there.

NO-snubber_1.png

Those spikes are attenuated with 100ohm+1uF snubber on the HV secondary of both transformers (transformers are from two different productors).

I dismantiled the whole project and left only the phase inverter + final board, the spikes was there.
I removed the feedback and the output transformer ground, spikes always there.
I've disconnected the power tubes ground from the signal tubes ground and wired two ground cables to the star ground, the input ground is connected to the first tube grid resistor (not directly on the star ground), noway, the spikes are there.

So the problems can be two:
1) PCB errors that I'm not able to see
2) course on the chassis, transformers or me (I'll call Wanna Marchi and I'll let you know )

EDIT the problem was solved using a humdinger and the PCB was modified to accomodate the relative trimmer.

[luix]

Ah, one thing I forget to say, I was thinking of a transformer problem because using a second transformer for the anodic power supply the spikes completely vanish!

I have three transformer:
1) 320V@200mA, 3.15V+3.15V@6A, 40V@100mA
2) 330V@200mA, 6.3V@6A, 40V@100mA
3) 300V@50mA, 12.6V@2A.

Any combination of the three transformer above that permit (a correct) filament power supply separated from anodic will solve the problem...

So, 1) for the anodic and 2) for the rest, is OK
2) for the anodic and 1) for the rest is OK
1) for the filament and bias 3) for the anodic is OK
2) for the filament and bias 3) for the anodic is OK

[TUBEDUDE]

Will xfmr 3 source enough current for your anode supply?

[luix]

Will xfmr 3 source enough current for your anode supply?

no, it was used without input audio signal, I've used to see if with this transformer the spikes are there, with 80mA bias there was 390VDC at the anode of the EL34 and no spikes.
Perhaps knowing the man which buildt transformer 3 I bet it can deliver much more than 50mA.

I've buildt at least 10 tube amplifier since 2003, never had such a problem! bah?!?

[TUBEDUDE]

1608304993177738149723530752305.jpg

I hear ya. I've built many amps with no PSY issues and this one has problematic rectification spikes also.

[luix]

Did you found any realistic solution?

[TUBEDUDE]

I was just investigating when I got the 'rona so I'm slowing down to conserve energy as I isolate.
I've never had these using tube rectifiers before. So i removed the safety diodes off the 5AR4 plates, suprisingly no joy. Re-routed wiring, shortened some leads, and very little improvement. Moved some sheet metal around to shield off the output/PI area, extra capacitance on the PA n PI nodes.. no or minimal change. I was coming to the conclusion it was a mismatch of the 5AR4 plate resistors. That's where I stopped to order resistors and slooo down. I'll post findings after. If.

[luix]

Is the amp you're talking about present in this forum? Have you posted schematic and PCB? I'd like to see them

[TUBEDUDE]

20201217_171115.jpg

Here's the "PCB".
I haven't done a schematic yet. My process is a little different. I use some circuit blocks repeatedly and have them memorized and I adapt them to the topology I'm building. The rest i create/mod on the fly. After it's perfected I draw up the schematic.

[luix]

Woow! a bit disordered

[TUBEDUDE]

To the naked eye, yes. Every proto build differs. At the start, the power tranny, choke and output tranny are positioned for minimum coupled hum using the "headphone trick". That, with thermal and signal level considerations, define where the tube sockets should go. From there the paths between them are selected with the straightest path possible evaluating the possible thermal and noise immunity aspects/physical size of each component/wire in the path. That will establish the needed positions of the ceramic standoffs to interconnect them. That done, all is assembled and tested to evaluate function. Then the fun begins. Components are changed, repositioned, added or replaced to get the desired response. Then additional tie points and adhesives stabilize their positions.
The many component substitutions, added components, reroutings etc. will make for some sloppy joints and an overall random look compared to the physically organized appearance of turret board or PCB construction.
This will serve as a template for a production build, which will vary slightly, depending on the positioning of the iron.
Each set of choke/transformers, even of the same exact model, will have a slight to substantially different magnetic radiation pattern. This will require some change in their position and angle at step one. Which changes things slightly for the following steps.
So basically the test bed/prototype is the ugliest iteration of the design. Builds from that template will be cleaner, but to the uninitiated, have a random look. To borrow a phrase: His design results can best be described as "There appears to have been a struggle".

[Phil_S]

I feel compelled to comment. There is something inherently satisfying in a build like this. The challenges have been met and conquered. The result speaks for itself. Just like people, looks can be deceiving. I like what Tubedude showed us. I seems to be fine example of how to allow the imagination drive the build. I want to say, "great job on that one!"

[pompeiisneaks]

I feel compelled to comment. There is something inherently satisfying in a build like this. The challenges have been met and conquered. The result speaks for itself. Just like people, looks can be deceiving. I like what Tubedude showed us. I seems to be fine example of how to allow the imagination drive the build. I want to say, "great job on that one!"

I fully agree. There's a reason I've never dared to a Point to Point like this, it's intimidating to keep in my mind how to make all that work. Anyone that does is a next level builder IMO.

Well done!

~Phil

[R,i,c,k]

I'll chime in.....

You have a method.

It's not following the herd.

It works.

What's not to like? $-)

(And it's not "if" it's when, have faith and ride it out....all the best for a speedy and complete recovery)

[TUBEDUDE]

Thanks guys, I have several factors allegedly against me, mainly being obese with Sarcoidosis, but I feel relatively well at day 5 and think this will be in the rear view mirror for me soon. My wife got her first vaccine dose yesterday so that concern is relenting. Best wishes to all who deal with this plague (all of us).