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

[ccaudle]

The addition of the 1M to the PI did nothing at all to the sound. Same distortion.

Have you tried disconnecting the phase inverter from the power stage to see if the power stage is loading the PI in an asymmetrical way to cause early clipping?

 

[systemtruck]

Have you tried disconnecting the phase inverter from the power stage to see if the power stage is loading the PI in an asymmetrical way to cause early clipping?

I suppose just kill the connections after the C39 and C40 bypass caps and monitor the audio of either one of those leads? Do both those leads need to be dummy loaded down with some specific value so that the PI performs as it should? Or will the PI outputs perform just fine without any load other than an audio connection? And if so, what input impedance will i need for the audio input? My interface has 15k by default, but maybe i can add something more substantial like a series 200k so that I’m not loading down the PI output. It’ll create a voltage divider but it’s probably still plenty loud.

 

[systemtruck]

Have you tried disconnecting the phase inverter from the power stage to see if the power stage is loading the PI in an asymmetrical way to cause early clipping?

Ok i made a setup that seems to function pretty well for this test, and i think i can conclude that the EL84’s are somehow causing the PI to self clip FAR FAR FAR before it normally would self clip on its own.
I disconnected the PI’s bypass cap outputs and patched the anode’s and cathode’s respectively into a pair of 150K series resistors and connected those to + and - of a balanced line input to audio interface. I chose 150K so that it was a not heavy load, just in case i was going to load it down too much with a 15k line input. I connected the local 0V bus bar point to the ground of the audio input. Seems like great audio, like a solid DI signal.

There are two volume pots.. one within the preamp stages, and one just prior to the PI. Before, i had to keep both knobs at about 15% to avoid this particular clipping. Now, i can keep them both at about 50-60% before the clipping starts. They are linear pots too, so there is a lot of extra juice there between 20%-50%. Surely these levels would be blasting the great sounding overdrive of the amp, before this PI clipping sound would even occur.

But with this current setup, if i crank them enough, i will get a somewhat similar clipping. However i do have to say it is a lot less ugly somehow.

So is the PI getting badly loaded down by the EL84’s or something?
Should i maybe also measure my pair of 470K resistors at the EL84 inputs just to make sure somehow it’s the wrong resistor? I simply used the ones from the old amp.

What else can cause a PI to dramatically self clip only when connected to output tubes? Shouldn’t my 82k grid stoppers that i currently have in place help to stop this too?

 

[Matador]

I think you may end up chasing your tail quite a bit until you get your output tubes operating together with two known good (matched) tubes.

It's also possible your 'hot' tube may be getting driven into significant grid current. The cathodyne will behave strangely when one side is overdriven because the output impedances aren't equal between the two driven sides.

 

[ccaudle]

So is the PI getting badly loaded down by the EL84’s or something?

Or the imbalance in the output tubes is causing one tube to hog all the current, and the other tube is almost cut off. Again, I don't have much experience debugging power tube stages, but that is what would happen in a long tail pair of transistors, so I would definitely not trust any behavior that occurs with one of the power tubes almost red-plating. Getting the output stage working properly is the first job.

 

[merlin]

I don't think you've posted a schematic of the whole circuit yet? That would probably be useful.

 

[systemtruck]

I don't think you've posted a schematic of the whole circuit yet? That would probably be useful.

I did in post# 39, but here it is again with the new grid stoppers, as well as one correction to a rail cap leg that was missing.
If it doesn’t appear properly, I’ll re-up shortly with different file type.
But I believe if you click on this it will expand well enough.

View attachment Fisher R-20 Amp Feb 14 2025.jpeg

 

[systemtruck]

I ordered a pair of seemingly well matched EL84’s, pretty sure a close generation to the 1950’s British Mullards that my current pair is. Very modest price, so a low risk here.
I’ve thought about trying to keep these Mullards i have in there because buying a pair of these exact ones is very high priced, and I considered attempting to adjust each tube’s bias to see if i can get them to work better and not cause this clipping. But from what i am learning there are several factors in a well matched pair and i guess individual biasing might not help my situation at all if one tube is truly messed up. (Which seems likely at this point)

Also, i checked my first recordings of this Fisher amp when i first got it and performed a simple mod to play guitar with it, and sure enough this clipping is very much there. i just didn’t notice it because i was playing in a certain way. So i think this one tube has been faulty from the start, and it’s not something i caused.

I’ll report back when I receive and pop these replacement EL84’s in.

 

[systemtruck]

@merlin
@ccaudle
@Matador

All right, I’ve hooked up a matched pair of vintage Mullard EL84’s.
As far as I can test, the ugly distortion is gone. It sounds perfect and I can’t wait to get to play it… one day.

Issues….

1 )
I think perhaps the “busted” EL84 was somehow refusing / rejecting its normal amount of current draw. Because now my 360 B+ is more difficult to attain, it’s much lower. I recall someone here doing math and pointing out that I should only have to add around 150R to get my voltage, but I ended up needing to add 800R. So i think that explains that. I eliminated almost all extra resistance and that fixed this problem and I achieve 360. I think now this pair of EL84’s is drawing the current that it should be.

2)
After some time, say 30 seconds after the B+ has been stable, so let’s say that’s around 1 minute in total since power up… my B+ starts radically dropping and one (fresh) EL84 starts red plating again and my sound disappears very quickly.
Again, this is the just installed matched pair of EL84’s.

So I tried hooking back up the NFB, but that changed nothing. I thought maybe the output stage was becoming unstable without it, but it is the same.

Then I thought.. maybe it’s the components around it. Let’s try swapping the EL84’s “left to right” , just like old times. And just like before with the different set, the red plating tube followed the move. It’s the same tube red plating, but now in the other half of the waveforms. So what are the chances that I had a red plating imbalanced set before, and another red plating imbalanced set even though they were a tested matched pair? 1 in 1000?

One more crazy thing… when I first tried this swap from “left to right” of the new tubes.. the red plating one sparked internally. It made a bit of a pop sound in the speaker, and was a blue lighting bolt inside. This was only around 20 seconds after power up, so well before it started red plating. What does an internal spark mean?

I suppose this could all just be dumb luck and I need to order a third set of tubes??

I would love for this to be the components somehow. I worried at first that maybe the output transformer was to blame, and that would be terrible. But the red plating doesn’t remain in correlation with the transformer primary legs.

 

[merlin]

Some photos of your wiring may be helpful.
Remove the power tubes, then tell us what voltages you measure on the EL84 socket pins at idle.

 

[Cqwet Dbdfte]

Glowing red tube? With tube out of the socket measure its grid 1 pin with a DMM. The voltage should be next to zero. A typical DMM is 10 megohm, so divide that voltage with 10,000,000, that is the leakage current thru the coupling cap (with the grid to ground resistor disconnected). High probability of coupling cap needing replacement.
Any voltage on grid 1 pin is death for the tube.
Caps may be OK at room temp but leak when warmed up.
If the EL84 have fixed bias rather than cathode bias, there may be a screwdriver adjustment.
The cathode (bias) resistor voltage shows the tube current. Check with data sheet for correct values.
Old capacitors don't improve with age, replace with modern variants, film caps should be as rated for 2X powersupply voltage, for long life and happiness

 

[systemtruck]

Thanks! I actually am using new caps already, the MKT1813 series for all audio / bypass, the Nichicon UCY elec series for all rails, and the ATOM elec series for the cathode bypass caps.
I also just replaced these tubes with two good matched ones and still one will red plate.
And further more, just like with the prior old tubes…. (which surely had a damaged tube caused by this over time probably, which ended up causing audible clipping and this clipping does NOT occur with these fresh tubes)…. one of the fresh tubes will red plate and then if i flip them around the same tube will red plate again but now on the bottom half of the PP waveform. So i don’t suspect the red plating is being caused by a specific component on one side or the other of the PP output or PI. But like i said, i suppose there IS a chance that one tube is bad in this pair that i just bought and received. Things happen. But it would just be slim chances that that is the case, since this is a reputable seller.

Side note, I realized i had too much capacitance on the B+ after deleting the 800 ohm resistor, and this may have caused the one spark that i saw after a bunch of testing of these new tubes. A total of 160uF was at the B+ as opposed to 40uF. I’m powering up slowly with a variac but I learned from reading around that a spark is often a sign of too much capacitance. So i just deleted the excess and now the B+ has just 40uF and then i powered it up earlier today and no spark occurred. But side note to the side note… the red plating I’m getting was already happening before that spark ever occurred, so it’s not that i damaged a tube and it’s now causing red plating.

The Fisher design has just one single resistor/cap on the two EL84 cathodes which are combined. So there is no individual bias adjustment. Maybe it’s time to start considering implementing this? I do lean that direction, because…. If the same tube is still red plating when I flip them, to me that says that maybe this amp is VERY VERY sensitive to tube differences, even if the tubes test as NOS and also test as a matched pair. That would explain the fact that the issue follows the tube, not the circuit necessarily. But im still just learning about this tube stuf so i really am stabbing in the dark.

I’ll take pics of wiring when i get back to the workbench, and will also run some more DC readings at tube points with EL84’s out.

 

[Matador]

A properly wired cathode bias should be next to impossible to red plate, because as the current through the tube increases, it shuts itself off "even harder".

Red plating over a short period shouldn't damage the tube, so I wouldn't worry too much ... yet.

What I would do: place a DMM on the shared cathode resistor, and monitor the voltage there. You should be seeing somewhere between 10 to 20V there under normal operation. If at some point it collapses to 0V and then the red plating begins, that will be a clue.

 

[systemtruck]

A properly wired cathode bias should be next to impossible to red plate, because as the current through the tube increases, it shuts itself off "even harder".

Red plating over a short period shouldn't damage the tube, so I wouldn't worry too much ... yet.

What I would do: place a DMM on the shared cathode resistor, and monitor the voltage there. You should be seeing somewhere between 10 to 20V there under normal operation. If at some point it collapses to 0V and then the red plating begins, that will be a clue.

I did some more DC testing….

When red plating occurs, the cathode voltages rises accordingly while the B+ collapses. That happens in tandem. i was measuring cathode and red plating started on one tube and the voltage rose quickly from its formerly stable 9.5V upward and I shut things down around 16V before it got carried away. Its pace was quickening so I’m sure it would have launched way past 20V. This is how the B+ collapses during red plating too, slowly then quickly. So i guess for the EL84’s the voltage span is shrinking together and that’s when we lose sound completely.

Curiously…. I put the two EL84’s that did NOT red plate ever, into the unit as a pair. Guess what happens… the red plating never occurs. Also, all voltages remain stable. So, this is one old tube, and one fresh tube from a matched pair. So with everything seeming perhaps solved, i went and played guitar… it clips just as badly. A brittle ugly clipping. The timbre of clipping seems to have changed a good bit with some changes to the circuit, so that actually leaves me hopeful that it can be solved.

I’m starting to think i have to change some of the original design of the output stage.

I tried to take a good photo but it’s pretty tightly packed in there so any pic is seeming pointlessly cluttered to look at… unless there is a specific location i should be most focused on about wiring? Like the cathode bias of outputs i suppose? I have short extensions there… my cap/resistor sit between the tubes and hover close to the ground bus wire, and i added about 1.5” of heavy wire going to each tube outward from that center point. The way Fisher did it is they had a lead connecting the two cathodes, and then on one tube’s pin 3 they put the cap to earth on the socket, and on the other tube pin 3 they put the resistor to earth on THAT socket. It seemed sloppy since then you’re throwing earth into the mix and in difference places to boot. Lots of stuff about the factory wiring seemed weird and it was very noisy, where as my build is very quiet. Anyways, for this cathode bias situation I decided to locate the cap/res hovering in the middle together and have even length short wires going to either tube. But if you look at a photo it just looks hard to follow.

 

[systemtruck]

Some photos of your wiring may be helpful.
Remove the power tubes, then tell us what voltages you measure on the EL84 socket pins at idle.

I forgot to add that i did tests without the EL84’s in place…
Both plates: 456V
Both grid-2’s: 446V

These voltages remain infinitely stable. And as noted, they are perfectly symmetrical tube to tube.

Side note, i corrected the grid-2 voltage range recently by adding a higher value dropping resistor within the rail. I had forgotten to lower this rail stage to match another radio current draw, this one a lesser secondary radio draw downstream. So now if i run this test again, the grid-2 will be lower perhaps like 410V. But it will be stable and symmetrical again. I then considered that this rail may have been causing distortion, so ran all tests accordingly and with diffferent tubes. Doesn’t change anything.

 

[Matador]

Looking at the EL84 load lines, with -16V grid to cathode bias, even at 500V there shouldn't be much more than 20mA of plate current, but with a 125R cathode resistor you are flowing close to 70mA. There must be some interaction with the grid causing it to go positive. What is the grid voltage when the current starts to increase? It should be rock steady at 0V.

 

[systemtruck]

Looking at the EL84 load lines, with -16V grid to cathode bias, even at 500V there shouldn't be much more than 20mA of plate current, but with a 125R cathode resistor you are flowing close to 70mA. There must be some interaction with the grid causing it to go positive. What is the grid voltage when the current starts to increase? It should be rock steady at 0V.

I think you’re onto something..

I put in the new “matched” pair for this.

At the audio input grid, the red plating tube starts fairly close to 0V but not perfect, lingering around 0.020 V to 0.050 V but then when red plating it rises quickly. I shut it off when it reached around 12.5V. It would have continued upwards. I’m guessing it is in perfect timing with the cathode voltage rising.
The other “cold” tube kind of hovers around 0.012 V at first, and then when the hot tube red plates this cold tube’s 0.012 V drops down to true 0V and even slightly below and then back up again.

I also rotated the tubes for this test again, and the audio grid voltage performance follows the tube itself, not the circuit around it.

When you say interaction “with the grid” are you talking about plate or grid-2 or cathode? And what would cause this?

I’m betting if i put in my old pair, which also has one tube that red plates, that it will be a similar situation.

Gosh this is such a learning curve hahaha.

 

[merlin]

I tried to take a good photo but it’s pretty tightly packed in there so any pic is seeming pointlessly cluttered to look at… unless there is a specific location i should be most focused on about wiring?

Are you absolutely sure your grid leak resistors are connected properly? Because this smells like a lack of grid leak.

 

[systemtruck]

Are you absolutely sure your grid leak resistors are connected properly? Because this smells like a lack of grid leak.

I visually re-inspected all points in the PI and EL84 pins…all looks good.

New tests:
Removed EL84’s, tested all pins to ground.
Pin 2 measures 510K to ground on both sockets. This is my grid leak resistors.
Pin 3 is 225R, and that is because i recently added a 100R to the 125R to see if that helped anything. Maybe that’s a bit high in the long run, so will lower. But this has the cathode bias sitting at 12V instead of 9.5V.
Pins 7 and 9, the hot DC pins, each have very high 1-2M impedance and slowly sinking impedance due to the capacitors.

Then I put the EL84’s back in, and I snipped the connections from the pair of bypass caps coming from the PI, added a pair of 470k resistors in series coming out from each cap, connected this balanced signal to a TRS input on audio interface, and powered everything up as normal and measured DC post caps just to see if there is some leakage coming through the bypass caps that had nothing to do with the EL84’s. There may be a little bit but i don’t know enough about cap technology to say it’s a problem… they both rise up to around 20mV during power-up surging, but settle back down quickly to around 5mV to 12mV. Both caps do this. And they remain there for a while. I thought maybe that would be enough leakage to feed the grid and cause a surging DC situation, but even with the connection from PI to EL84’s completely severed, the one EL84 still red plates. And when it red plates, interestingly these detached cap outputs settled down a bit further to almost 0. I think one side settled lower than the other, in fact in went negative maybe -0.005, and the other cap remained positive around 0.005. These are brand new MKT1813 caps. Is that sufficient? I am guessing that the reduction here during red plating is due to the overall B+ collapsing during red plating.

But again, this test means the EL84 red plating is not directly related to any tiny DC output from the PI. Maybe it’s not helping and maybe it’s slightly exacerbating it, but it’s not the root cause. And yes, the 470k grid leak resistors were still in place when i ran this EL84 test that red plated. The only thing detached from the ELI84’s was the audio input lead post the bypass caps. My currently existing 41k grid stoppers were just floating mid air.

I also listened to the audio of the PI output. Just like the last time i did this, the sound seems excellent. Mind you this time i had 470K resistors feeding a 15k audio input, so this made quite the audio level drop due to voltage divider there, so I had to turn up the preamp and output volume of the amp to get a good level. Just like last time, it was pretty much perfect until it does eventually start clipping in the PI itself but this in theory is WAY beyond the point where the EL84’s would be driving with their amazing drive sound so it is not even close to the same clipping level. The normal clipping happens even at low volumes.

The only thing i can possibly conclude right now is that not only is one of my old tubes bad, one of my new “matched pair” tubes is ALSO bad, just by some bad stroke of luck. Like can a tube cause it’s own DC surging/leaking?

But even using both “cold” tubes as a pair, although it stabilizes the situation and nothing red plates, it does NOT solve the early clipping level….

As I mentioned this yesterday, i hooked up the one OLD cold tube and the one NEW cold tube, and by that i mean the tube from each pair that didn’t ever red plate, and the system becomes stable and no tube red plates even after sitting for several minutes (red plating has been occurring around 1.5 minutes i would estimate). So….

The PI distortion is still there. But this has me wondering, maybe i have two usable tubes on hand now, and maybe i need to adjust the circuit that somehow sucks by design? Like if I have two tubes that i can pop in the EL84 sockets and the DC situation doesn’t red plate, I’m one step closer to this functioning. However, what would then be adjusted to stop the PI from clipping at low levels?

Or maybe i just need to abandon this PI design and change it to a long tailed pair or something else? I hear cathodyne PI’s are known for clipping ugly. But my audio test proves that the cathodyne early clipping is caused by the load the EL84’s are putting on it, or something like that.

Here is the latest schematic and i think i got GroupDiy’s platform to insert it visually now..

 

[Matador]

That tube is sourcing grid current into it's grid leak resistor, if you've disconnected the coupling caps and it still happens. If you are saying the pin 2 of a "bad" tube is charging upwards of 12V then it's sourcing north of 20uA which is a LOT.

If you want to be triple sure this is the root cause, I would hard ground pin 2 of both tubes with an alligator clip lead from pin 2 directly to your audio 0V bus bar. The tubes should DC idle under those bias conditions all day, as no amount of grid current can overwhelm your 0~0 ohm wire.

I don't know of any underlying mechanism for this (generally grid current is a function of running the cathode and grid at similar potentials, but that isn't the case here).