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

[systemtruck]

Do you read 0V on both cathodes, when measured right at the tube socket? The 125 ohm cathode resistor I assume measures fine by itself?

Yes and yes. The cathodes are connected so it’s just a wire between the pair of socket pin 3’s.
Also, somewhat surprisingly, the 125R to ground measures fine WITH the 50uF cap in place too. I measured that before I snipped the cap. I thought for sure it would read as a short circuit since I figured the cap was dead short, but it measured 125R before and after snipping the cap. So I guess the cap is fine.
So the very notable thing here is that both tubes have 0V cathode bias somehow, but only one tube is red plating. I have basically zero knowledge about what could go bad inside a tube, let alone to cause this, so I’m basically in the dark. If I had a spare pair of EL84’s I would just pop them in and take a chance to see if that solved it.

 

[Matador]

What do you read on the other nodes (plate, screen, and grid)? If you remove the tube, do you read 125R between the actual tube socket cathode connection and ground?

Seems highly suspicious that you read exactly 0V at the cathode, as there's aren't any other internal tube elements which would be grounded directly in a typical beam pentode, unless you happen to be running DC heaters.

You said 'pin 3' so this is an EL84 amp?

 

[systemtruck]

What do you read on the other nodes (plate, screen, and grid)? If you remove the tube, do you read 125R between the actual tube socket cathode connection and ground?

Seems highly suspicious that you read exactly 0V at the cathode, as there's aren't any other internal tube elements which would be grounded directly in a typical beam pentode, unless you happen to be running DC heaters.

You said 'pin 3' so this is an EL84 amp?

Yeah the fact that I’m getting a precise 0V makes me think something is shorted mechanically, like a wiring mistake somewhere or something is rubbing up against another joint. But i’ve been through a visual inspection a number of times now. I’ll do it again though.
Yes EL84 amp.
It’s AC heaters.
And yes, i successfully read 125R between the pin3 situation and ground. That ALSO is confounding, because I’m sitting here thinking pin3 is accidentally shorted to 0V but I’m reading a correct 125R between pin3 and 0V, so clearly it’s not shorted.
So it’s like it’s own little floating 0V, not the main 0V. Super weird.

 

[ccaudle]

And would this one glow red where as the other EL84 would not?

The circuit configuration you describe sounds like what is known generically as a long tail pair, or differential pair, which is very sensitive to device balance unless there is DC feedback to force the devices to balance. Maybe pentodes are different, but a transistor long tail pair will very easily switch between all current through one device or all current through the other device without DC feedback, or degeneration resistors in the emitter/cathode to reduce gain and make the devices easier to balance.

By way, I find this section of the schematic a little confusing:


Typically a schematic will use a dot to make crossing nets unambiguous regarding whether they actually connect or not, like this:


Also, EL84 is a pentode, but I only see 4 terminals on your power tube symbol. Shouldn't there be another grid?

 

[systemtruck]

The circuit configuration you describe sounds like what is known generically as a long tail pair, or differential pair, which is very sensitive to device balance unless there is DC feedback to force the devices to balance. Maybe pentodes are different, but a transistor long tail pair will very easily switch between all current through one device or all current through the other device without DC feedback, or degeneration resistors in the emitter/cathode to reduce gain and make the devices easier to balance.

By way, I find this section of the schematic a little confusing:
View attachment 145604

Typically a schematic will use a dot to make crossing nets unambiguous regarding whether they actually connect or not, like this:
View attachment 145605

Also, EL84 is a pentode, but I only see 4 terminals on your power tube symbol. Shouldn't there be another grid?

Ah, i meant to add NODES! Yes it is terrible to read without nodes, my apologies. Will add in next version.

Hmm, that’s really REALLY interesting about imbalancing and it sounds like that’s what is happening…
I deleted the audio NFB in this circuit because the sound/drive is a huge improvement without it. So perhaps i need to implement one of the balancing solutions you mention. But…

I’ve attached the original schematic. That modification is not mine. It must be someone’s old modification that got uploaded. My 125R 7W / 50uF 25V cathode setup is what was installed in the amp when i took it apart, with pin 3s combined. I got new replacement parts but kept those values. But should i try doing what they did and have independent cathode setups?

And as you can see, Fisher drew only 2 grids for the EL84, so that’s why i only drew 2 grids.

One other difference in the hardware VS the schematic is that they used 68 R resistors to feed the rail to pin 9 of each EL84. The Fisher had them installed and they looked factory, so i kept them in mine and that’s a difference you’ll also see in my schematic vs this one.

How would DC feedback or degeneration resistors be implemented into this amp? And what are the benefits or either vs the other?

 

[Matador]

Also, EL84 is a pentode, but I only see 4 terminals on your power tube symbol. Shouldn't there be another grid?

In an EL84 the suppressor grid is internally tied to cathode.

 

[ccaudle]

In an EL84 the suppressor grid is internally tied to cathode.

Thanks, my pentode inexperience shining through.

 

[Matador]

As a test, you can replace your tube with ~6.8K, 10W resistor, placed directly in each tube socket between pins 3 (cathode) and pin 7 (anode). If your anode is at 300V, and you're expecting ~10V at the cathode, then you want to draw 40mA through your 'tube', so (300V - 10V) / 40mA = 7.2K. 6.8K is close, meaning you'll draw a few more mA than 40mA, but you should see something close the typical bias voltage at the cathode if everything is working correctly (except the tube).

Dissipation is ~12W, so you'll need to make your measurements fairly quickly and not leave it running forever.