Weber 6M36 Blocking Distortion?

[aaronlowther1993]

Hello lads,
I've been looking over the sight trying to find the answer to my question here and I can't seem to find a solid answer. I did come across <a href="https://tedwebers-famous-loudspeakers.vanillacommunities.com/discussion/4764/6m36-issues-still">this</a> thread which seemed to be dealing with a similar issue to mine. When playing my newly built 6m36 I'm getting what I suppose could best be described as a harsh clipping over the guitar signal. It's not part of the signal, but alongside it. At first I thought I'd blown a speaker. That wasn't the case. Then I assumed I must have mucked something up, but after opening it back up and checking through everything it's honestly one of my cleanest builds. Thus, I was flummoxed

However, the more I read about the peculiarities of these amps, the more I'm beginning to believe that this is perhaps an issue of blocking distortion? I encountered it way back with my first tweed builds, but it sounds a bit different here. There's no issue of volume cutting out or down, and the amp gets royally loud. But something isn't quite right. I know in the past people had issues with the 5.6ks needing to be on the tubes and not on the board. That's already been done here, so that doesn't seem to be it.

So, could someone point me in the direction of resolving this issue? I honestly think this amp would be my new live amp if I could sort that. If it is blocking distortion, where and what value of resistor would you place?

Anyhow, thanks lads! I appreciate the help!

 

[aaronlowther1993]

https://tedweber.com/media/kits/6m36h_layout.jpg

https://tedweber.com/media/kits/6m36h_schem.jpg

Layout and schematic!

 

[Jon Snell]

I would see where it is coming from by using an oscilloscope and monitor the signal path.
It may be the unbalanced phase invertor but we will not know until we take a look.

 

[aaronlowther1993]

I would see where it is coming from by using an oscilloscope and monitor the signal path.
It may be the unbalanced phase invertor but we will not know until we take a look.

I’m going to show the gaps in my knowledge here. I’m assuming an out of balance phase inverter would be clipping only on one half of the waveform significantly sooner than the other half. What would cause that? And why would that manifest as such unpleasant clipping?
Thanks for pitching in to help me out!

 

[Jon Snell]

We are talking complete theory here as we have no oscilloscope diagnosis but if one half of the phase invertor was clipping, it would affect the other half of the cycle as well, and without an oscilloscope, difficult to tell the difference between total and partial clipping.
I cannot remember what I used to do before I had an oscilloscope, I think I just inserted a known clean audio signal at various stages until I found where it was comming from. I do remember however, I wasted many hours and lost a lot of hair over it.
Start from the output stage and work backwards towards the input stage.

 

[aaronlowther1993]

Luckily I have a scope. So, still speaking in theory, how does one correct this unbalanced phase inverter?

 

[andrewRneumann]

Hello,

Interesting situation. Unfortunately Weber's site is not able to bring up the schematic and layout for the 6M36 head or combo. I did find a combo schematic here https://www.tdpri.com/threads/debugging-weber-6m36.666752/ in a thread that sounds similar to yours--alas it ends with the problem unresolved.

Could we get an audio clip of the problem? A phone recording (video or just audio) is just fine. My experience with blocking distortion is that it is more of a volume pumping of the overall dynamics of the notes as they play and cut-off. So if you are hearing additional notes added to your signal, then I would guess we are chasing something else.

A couple questions for you:
Does it occur in both channels and both inputs?
Do the tone knobs affect it?
Does changing the volume/tone settings in the unused channel affect it?
In the schematic in the link I posted, there is an optional R37 for attenuating the TMB signal into the LTP. Did you install something like that?

 

[Jon Snell]

Luckily I have a scope. So, still speaking in theory, how does one correct this unbalanced phase inverter?

Have a look to see where it comes from first. You will see the source staraight away.
If unbalanced, changing the anode load resistance of the second anode is the usual way but check before the phase splitter first.

 

[aaronlowther1993]

Hello,

Interesting situation. Unfortunately Weber's site is not able to bring up the schematic and layout for the 6M36 head or combo. I did find a combo schematic here https://www.tdpri.com/threads/debugging-weber-6m36.666752/ in a thread that sounds similar to yours--alas it ends with the problem unresolved.

Could we get an audio clip of the problem? A phone recording (video or just audio) is just fine. My experience with blocking distortion is that it is more of a volume pumping of the overall dynamics of the notes as they play and cut-off. So if you are hearing additional notes added to your signal, then I would guess we are chasing something else.

A couple questions for you:
Does it occur in both channels and both inputs?
Do the tone knobs affect it?
Does changing the volume/tone settings in the unused channel affect it?
In the schematic in the link I posted, there is an optional R37 for attenuating the TMB signal into the LTP. Did you install something like that?

Unfortunately the cost of gas will be prohibitive to when I can get the amp. I'm hoping tomorrow. My rehearsal space is a half county away lol. Did the links for the schematic I posted above not work for you? They still work for me.
1. Both channels and both inputs
2. Not that I can hear. It doesn't really have enough tonality to it for me to tell.
3. No. The background noise of the amp changes when doing this, but not the distortion.
4. R37 is in place, and is a 470k.
5. The issue is made worse, more apparent, when you dig in/hit the amp harder.

 

[andrewRneumann]

Unfortunately the cost of gas will be prohibitive to when I can get the amp. I'm hoping tomorrow. My rehearsal space is a half county away lol. Did the links for the schematic I posted above not work for you? They still work for me.
1. Both channels and both inputs
2. Not that I can hear. It doesn't really have enough tonality to it for me to tell.
3. No. The background noise of the amp changes when doing this, but not the distortion.
4. R37 is in place, and is a 470k.
5. The issue is made worse, more apparent, when you dig in/hit the amp harder.

I can't even get the main page at tedweber.com to come up. Can you post the actual image files for the layout and schematic?

 

[Snfoilhat]

I'm curious about how people resolve phase inverter problems when it's in this Vox/Matchless kind of arrangement with both grids being used as a mixer, rather than one grid AC grounded, if that difference matters.

There could be a sticky in the amp forums encouraging everyone seeking help to make sure as much of the documentation as they can get is hosted right on TDPRI instead of links to other sites, to lower the barriers to people offering help. Different browsers, old caches, etc.

 

[andrewRneumann]

Thanks @Snfoilhat .

I agree with @Jon Snell to check out the LTP PI. Finding a LTP in a guitar amp that was even is probably more the exception than the rule though. This particular LTP is different in that there is some negative feedback because the 2nd grid is not fully grounded to AC signals. I'd have to crack open the books, but I'm going to guess that's going to increase the headroom of the LTP and probably decrease the unevenness.

A second line of investigation for me would be uneven load sharing between the 4 power tubes and crossover distortion. Crossover distortion could be seen on the OT secondary with your scope. One of the drawbacks (to me) to this design is that all the tubes share one cathode resistor and one screen resistor. We have no way of telling which tube is the "current hog." We could maybe narrow it down to 2 using the resistance of the OT primary as a reference to measure current. But how to fix it, besides replace the tube?

If it were me, I would divide that 50Ω/10W screen resistor (R33) into four 200Ω/3W screen resistors mounted on the individual tube sockets. I would mount 1Ω/1% resistors on all four cathodes (prior to the 50Ω/10W shared resistor R32) to monitor the current in each power tube. If it turned out that there was one tube hogging all the current, then I would have to go back and consider ways to adjust the bias for each tube individually. Obviously this is not fun on a 4 tube cathode biased power amp.

Also, while troubleshooting, you could probably just disconnect that 100uF (C16) to verify it wasn't part of the problem. Maybe this is some kind of crazy interaction of 4 power tubes sharing one cathode capacitor? That's a shot in the dark for sure, but easy enough to test out.

Do you have a signal generator to go with that scope? Your going to need some way to inject a clean signal. And a dummy load unless you can put the speaker in a different room with a blanket over it to keep from going insane. Good luck!

 

[aaronlowther1993]

@andrewRneumann @Snfoilhat @Jon Snell @robrob Tagging Mr. Rob since he is usually super helpful here.

Here's what it sounds like.
I put a 440hz sinewave into the input, and then with the help of a friend started at the output with the scope. The signal at the output looks like a nice healthy sinewave, aside from the fact that it's modulating. However, If I muck about with the treble or tone control on either channel, eventually it goes into some rather nasty ringing. Eventually, if you play about with them again, you can get it to stop. All of this was measured from the low input from the Volume and Tone channel. The weird interaction of the two channels is present regardless of which jack one uses.

 

[andrewRneumann]

I’m not quite sure what to make of your results. I wonder if the lack of response from others means the same thing?

Let’s start with that PI. Do you feel comfortable measuring voltages in a live amp? It would be nice to see voltages (to chassis, heaters in Vac) on all pins + measure the voltage at the junction of R5 and R14. Also what is the B+ voltage supplying the PI (upstream of the plate resistors)?

If you want to scope the PI, set up your signal at the guitar input and then measure the results at the grids of the power tubes. The sine wave should be intact if you don’t have the volume up too high and you should be able to measure the peak voltage on the grids for each side of the power amp. Compare peak voltage and that will tell you if your PI is way out of balance.

For some reason that I can’t give a reasonable explanation for, I wonder if you have a bad coupling cap somewhere. Possibly C2, C3, C4 or C12?

Holiday weekend so expect delayed responses from our friends.

 

[Lowerleftcoast]

I’m not quite sure what to make of your results.

The imbalance of a power tube might be able to be found by swapping tubes around. I can't tell how the tubes may be wired because the schematic does not match the layout. You would have to determine which pair shares plates and control grids on the actual amplifier. (The non-matching schematic and layout makes one wonder if these could be wired wrong but I think it would be more noticeable weirdness happening.) Anyway, the pairs of tubes could be swapped and if the scope didn't change, it would rule out a wonky EL84. (The problem would not follow the tube...)

Next. Try a different tube in the PI, just to rule that out.

 

[aaronlowther1993]

My apologies, I haven’t abandoned this thread or project, just suffering from Covid.

 

[aaronlowther1993]

The imbalance of a power tube might be able to be found by swapping tubes around. I can't tell how the tubes may be wired because the schematic does not match the layout. You would have to determine which pair shares plates and control grids on the actual amplifier. (The non-matching schematic and layout makes one wonder if these could be wired wrong but I think it would be more noticeable weirdness happening.) Anyway, the pairs of tubes could be swapped and if the scope didn't change, it would rule out a wonky EL84. (The problem would not follow the tube...)

Next. Try a different tube in the PI, just to rule that out.

So, I have a half-ass answer. Since covid I've had no energy or will power to finish working on this. Today I summoned enough to just say **** it and do a full tube swap. NOS EL84s and NOS RCA 12AX7s. Aaaaaaand now it works like a charm. I'm going to go back through and try to find the bad tube. But that's for another day when I have the energy. Thank you all for the help!

 

[Lowerleftcoast]

Take care of yourself. Hope you feel better soon.