Tube Amp connections : 0V vs EARTH vs HEAT vs CURRENT

[systemtruck]

Scope arrived and I was able to run some quick tests between WORK work. I had already reconnected the OT and ran these tests before seeing your message about current readings @CJ

Only had time to try the Grid1’s and the Plates, and the tests showed no signs of oscillation at either. Didn’t have time to test Grid2’s but will later.

I made sure to zoom in on the time span and also mV scale so i didn’t miss any high frequency waveforms. The grid’s basically are flat lined when the guitar volume itself is down, which is pretty nice to see, and on the Plates i can see the 60Hz power cycle which i understand gets canceled by the other side.

But one important thing is that no waveforms increase during the B+ collapse. So i don’t think that some high frequency oscillation is ramping up and causing the EL84’s to self overload, draw current, red plate, until rails collapse. I was really hoping to see that. I also used the Auto scaling function a number of times just to make sure i wasn’t somehow missing something. The scope does a pretty good job bringing everything into perspective with the press of that one button.

But, i was able to very clearly watch the distorting waveforms when hitting one string on the guitar heavily, and watch the clipping slowly disappear as the sound also cleared up. This happens on the plates as well as the input audio grids. Pretty much the same clipping. It’s possible that one side clips a bit more than the other, but not much i don’t think. I may measure for that later when i have more time. I did use 82k and 100k as different plate resistors for the PI, since other designs out there mention the nice sound of even harmonics that can introduce. But this is irrelevant from the issue. The clipping was the same way before i did that. But at least it might explain a slight difference in the point each clips.

It sure just looks like plain old diode clipping to me, but I’m not that well versed in the appearances of clipping waveforms. And again, this clipping does not exist in the PI outputs when the PI is disconnected from Grids. Is it possible that the EL84 stage design is just biased to only ever handle very small signals, and i just need to redesign the balance of things to re-center the waveform better?

But the AC15 design is very similar, as are other EL84 bias designs. So I suppose that’s the ticket. Maybe it’s just some bad solder joint of mine that happens to be causing clipping and also DC current draw issues, or it’s TWO sets of badly clipping tubes that also have current draw issues.

Here are the EL84 plates during clipping:



And here are the EL84 input grids during clipping:

 

[ccaudle]

The third picture shows positive grid voltage, that is always going to crunch pretty badly.

The increasing DC voltage on the grid is still the biggest concern. The bias level (i.e. voltage at the control grid, nominally 0V but possibly a few mV because of grid leakage current, and the voltage at the cathode) should be constant. You can't ever have a stable amp if the output stage bias keeps running away.

 

[systemtruck]

The third picture shows positive grid voltage, that is always going to crunch pretty badly.

The increasing DC voltage on the grid is still the biggest concern. The bias level (i.e. voltage at the control grid, nominally 0V but possibly a few mV because of grid leakage current, and the voltage at the cathode) should be constant. You can't ever have a stable amp if the output stage bias keeps running away.

Well what’s interesting about that is the fact that this clipping is present from the get go. Right after power up. So the clipping is not even depending on any kind of rising DC buildup over several minutes.
It’s almost like there is DC offset in the tube itself in some way that there shouldn’t be. And this is the case with both pairs of tubes i have. ( A brand new JJ set arrives in a few days. )

Also, i recall some measurements before… perfect 0V DC at one grid, and like 0.04V DC at the other grid. This is soon after power up, with no audio going into the grids. Doesn’t that mean there would be no clipping at that point? And clipping would only later develop and get worse if DC slowly accumulated?

I’ll do some more grid DC measurements when I can hop back into the shop.

 

[ccaudle]

The third picture shows positive grid voltage, that is always going to crunch pretty badly.

Sorry, I was interleaving doing something else, and what I wrote before isn't really correct. For the pre-amp stages that would happen, but this is a push-pull output stage, so you expect one of the tubes to be turned off during part of the cycle. If I am reading the pictures correctly the input to the grid still goes down to -25V, which seems excessive amplitude. More experienced guitar amp people will probably know right away if I am off base with that

This might be a useful reference to compare to your current configuration:
https://valvewizard.co.uk/pp.html

 

[systemtruck]

I’m leaning towards the bias issue being a bad tube, in my new matched set as well as my old set. But here are tests with my newer matched pair…

- When measuring DC at the grid, the “good” tube holds about 0.008 V indefinitely. Also, when a heavy strum is played on guitar, the grid jumps down to around -13V and promptly returns to “0”. The bad tube starts off the same way and behaves the same when playing too, dropping down to -13V, but within a minute or so it starts slowly climbing and climbing. I have been shutting it down around 3V, and it was only getting faster. At first i tried adding a new grid leak resistor to the bad tube just in case the old grid leak was intermittent under stress, but the problem persisted. So I switched the tubes, and the issue follows the tube. So it’s not the circuit nor the design.

- When disconnecting one EL84 and monitoring DC on that grid pin, voltage rises a little during power up but settles down to about 0.001 V, and flutters in the sub-mill volt range indefinitely. This happens identically at both tube locations when each is disconnected and the other is left in. (I kept the rail down at 310V by using the variac) So i don’t think there is a bypass caps leakage problem coming from the PI causing any grid issues.


As for distortion….

- Scope on just one (of either) grid to 0V visually shows clipping, which occurs rather synchronized with speaker output clipping sound. The audible and visible clipping fade away at the same time as a note fades out.

- Scope across both grids together, as in one positive/negative signal, looks perfectly smooth and balanced. Even when audible clipping occurs at the speaker output. It’s a night and day difference with visual monitoring just one or other grid… this positive/negative waveform at the pair of grids is perfectly smooth at all times, even during severe audible clipping at speaker output.

—-

I think it makes sense to wait for the brand new JJ tubes to arrive and see what happens. It is alarming how this can happen on the tube market though, with a matched pair tested for current and gm. I guess that only accounts for first power up, not after some run time.

But i do have one question… Is a Push Pull system supposed to already have the waveform chopped up at the output of the PI? Or do the EL84’s take care of that business internally and each grid should appear as if they are Class A full wave? Because I’m confused as to why the EL84 grids, ie the PI outputs, would already have chopped waveforms individually. I had been under the impression that any PI is just built to send out full waveforms just in reverse polarities.

 

[Cqwet Dbdfte]

Sorry, I was interleaving doing something else, and what I wrote before isn't really correct. For the pre-amp stages that would happen, but this is a push-pull output stage, so you expect one of the tubes to be turned off during part of the cycle. If I am reading the pictures correctly the input to the grid still goes down to -25V, which seems excessive amplitude. More experienced guitar amp people will probably know right away if I am off base with that

This might be a useful reference to compare to your current configuration:
https://valvewizard.co.uk/pp.html

-25V on negative peak seems ok for a tube biased at -12V.
and 0V at max + peak.
With cathode bias and negative supply as reference these numbers shift positive by 12.
The driver stage may be able to provide more voltage swing than the power tubes can handle
The phase splitter should provide unclipped swing on both phases up to 0V on the power tube grids.
If there is an expectation of more voltage swing, maybe a better amp is needed.
EL84s are not exactly power houses

 

[systemtruck]

-25V on negative peak seems ok for a tube biased at -12V.
and 0V at max + peak.
With cathode bias and negative supply as reference these numbers shift positive by 12.
The driver stage may be able to provide more voltage swing than the power tubes can handle
The phase splitter should provide unclipped swing on both phases up to 0V on the power tube grids.
If there is an expectation of more voltage swing, maybe a better amp is needed.
EL84s are not exactly power houses

I have faith that this is not normal for the staging. The PI and output circuit is remarkably similar to the Vox AC15, and would be pretty much identical if i just lowered the grid leak resistors (which i tried and didn’t solve anything). The kind of clipping distortion I’m getting here is not similar to tube break up ala AC15 or any amp. It happens well before that breakup, and which does also happen with this amp if turned up.

I found this article, which i suppose could be happening.
https://www.18watt.com/storage/18-watter_buzz_info_311.pdf
Some kind of self induced EL84 crossover distortion that is caused by an input signal causing extra bias upon itself due to the contact with its bypass caps or something, which seemingly can remedied with a zener+diode setup strapped around the grid leaks. Pretty interesting stuff.

Will update when I’ve put in this pair of JJ EL84’s. Who knows at this point !