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

[systemtruck]

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).

With input grids grounded to bus bar, (and with input clipped again post bypass caps so that I didn’t damage PI stage with such a low load), sure enough, it completely stabilizes and stops red plating. I let it go for a long time and it’s fine. This is using the newly aquired matched pair, where one used to red plate. Now both tubes glow less and glow evenly as well.

I’m really starting to think both of my issues, the red plating and the PI distortion when EL84’s are connected, are Fisher design issues. First of all they’re running the tubes at much higher voltages than everyone says is ok to do. So maybe that itself causing this buildup of DC because it’s simply overloading the tubes internal capabilities?

I feel like maybe a compromise can be made with the grid leak value….

http://www.r-type.org/pdfs/el84.pdf

This seems to be the Mullard EL84 datasheet that is right for these tubes but maybe it’s not the same generation. My tubes are from the 50’s or 60’s.

I couldn’t figure out from the datasheet what the recommend grid leak resistor value should be. But maybe what I can do is just try lowering it down from 490 but not so low that it causes problems either in the EL84 nor in the PI stage loading.

Maybe I’ll try adding a pair of same value in parallel via these alligator clips. That’ll half the grid leak resistance to around 250K each. I can let it sit again, and if it doesn’t red plate after several minutes, maybe it works.

 

[systemtruck]

The new “matched pair” installed, one having the red plate issue..

With PI outputs disconnected, the following grid leak resistor value was tested (parallel stuff, so weird values here..)

245k, it red plates
196k, it red plates
114k, it red plates, takes longer of course
65k, red plates, takes even longer, several minutes

Will there be a stopping point before just plain shunting to ground? Kind of seems like no, and that eventually the DC will build enough to rock the boat.

I suppose one theory is that the EL84 design is not supposed to handle this level of voltage, and that only the strongest ones will bear it but those will eventually give way.

But if that were the case, Fisher would never have been able to support this product line, so I don’t think that’s the situation.

 

[Matador]

The maximum grid resistance to cathode is listed in the "Limiting Values" section, labelled Rg1-k, and it's 300k. 220K is a common value, but keep in mind you might need to adjust the values of the PI coupling caps to keep the frequency cutoffs the same (if you halve the grid resistance, you need to double the coupling cap value, all other things being equal).

The limiting longevity factor is almost always not plate voltage, but rather screen (dissipation). Most robust designs almost always strive to limit screen dissipation instead of plate dissipation. I might suggest swapping in 470R or 1K screen grid resistors and seeing if you can stay below 300V max on the screens, even if the plate voltage is over spec.

Although I don't know of any second or third order effect where screen voltage changes or modulates grid current, but maybe you've discovered one.

 

[systemtruck]

The maximum grid resistance to cathode is listed in the "Limiting Values" section, labelled Rg1-k, and it's 300k. 220K is a common value, but keep in mind you might need to adjust the values of the PI coupling caps to keep the frequency cutoffs the same (if you halve the grid resistance, you need to double the coupling cap value, all other things being equal).

The limiting longevity factor is almost always not plate voltage, but rather screen (dissipation). Most robust designs almost always strive to limit screen dissipation instead of plate dissipation. I might suggest swapping in 470R or 1K screen grid resistors and seeing if you can stay below 300V max on the screens, even if the plate voltage is over spec.

Although I don't know of any second or third order effect where screen voltage changes or modulates grid current, but maybe you've discovered one.

Gotcha, thanks!
I already inserted some extra resistance before the second rail and the screen grid voltage now sits around 290V whenever the first rail is 340. The downstream rails were already too high also, so this helps everything.

I added another post just before yours here, with my numbers after lowering the grid leak incrementally. No luck! The tubes that want to red plate will indeed red plate eventually, they’ll just be slowed a bit. However, maybe there is a value that is low enough where it will allow audio and not be TOO low. But what is too low for a grid leak? I had it down to 65K and they still red plate slowly.

I suppose at this point I just have to throw my hands up in the air and conclude that I bought a matched pair that had a dud in it, and that my old tube pair also had a dead one.

So I’ll stick with the cold one from each set and just roll with it, and see if I can adjust the PI circuit to stop the ugly low level clipping.

 

[CJ]

One tube red plating is normal.

It is just like thermal runaway in a transistor.

Usually the tube with the higher gm will be the one to run away.

As it starts to lose its bias, it draws more current, which makes it hotter, which makes it draw more current, rinse and repeat.

New tubes can not take some of the old school plate voltages. So you will need to lower your plate voltages, or drop your screen voltage. But if you drop your screens too much, your sound will suffer. So maybe a little of both, lower B+, lower screen.

And you need to check this amp with a scope, stage by stage, to map out the gain structure.

Ultrasonic Oscillation is another topic that should be crossed off the list. But usually when this problem persists, both tubes will red plate at the sane time.

You want to hear a weird story about current hogging? One time my boss went to check the current draw on a large generator circuit. Big 1/0 cables were being used, two for each leg of the circuit, very low resistance. When he put his clamp on meter across one of the cables he got very low current. When he put it on the other cable that was in parallel, he got very low current. WTF? Turns out the insertion loss of the meter was enough to force all the current through the other cable.

This is kind of what us going on in your circuit, the higher plate resistance of one of your tubes is forcing current into the other tube. However the imbalance will not he as radical as the 1/0 cable thing.

Here us a good article by tube God Eric Barbour>