Cathode follower without elevated heater voltage

[Enzo]

You are missing one factor. Yes there were two 100 ohms resistors, but they are rally in parallel to your meter. One end they are connected together and to ground. The other ends go to the winding. But the winding has half an ohm resistance. That means the two resistors are essentially parallel.

In fact when you say you measure 100 ohms to ground, I assume you have the transformer disconnected. Otherwise you'd get a reading of 50 ohms.

[Chuck H]

You are missing one factor. Yes there were two 100 ohms resistors, but they are rally in parallel to your meter. One end they are connected together and to ground. The other ends go to the winding. But the winding has half an ohm resistance. That means the two resistors are essentially parallel.

In fact when you say you measure 100 ohms to ground, I assume you have the transformer disconnected. Otherwise you'd get a reading of 50 ohms.

Yes, there was some confusion. As it is the transformer is disconnected and the false CT is still grounded. So, 197R between the branches and 100R to ground from each sounds right.

[g1]

Dark Mavis: It is best not to make corrections via edits after there have been responses.
If you must, just add the word 'edit:' and the corrections at the bottom of the post in question, or make a new post.
Otherwise, people's comments no longer make sense and it can throw a wrench into the troubleshooting procedure.

For example, Chuck's comment in post #56 about the 197 ohm reading. I know he put it there for a reason.
Same for my deleted comment above.

[Dark Mavis]

My bad. I have the amp open next to me, and the screen open in front of me. If I find I mis-measured or made a mistake, I just changed the comment without noticing that there were any new posts.

[Dark Mavis]

So then...to be clear, the 200 (approx) ohms from side to side, with the transformer disconnected is as expected? And I have continuity the full length of each winding, so...maybe it's the tubes? Is there a way to check the filament of each tube? I know the DC resistance won't tell us much when the tube is cold.
The thing that makes me think it's the winding, was just that it was the last two tubes that had glowing filaments. I have 8 preamp tubes here in total, so I can swap them around randomly a few times, and see if I get a different pattern? Or maybe just try a single tube in the last socket, then try every tube, by itself in that location?
I also need to check the cathode and grid voltages with the dmm on a lower setting. I checked a few, but not all of them.
I guess I should try plugging a guitar into the amp? I have no idea if this is part of the original problem, or something new. Remember: the last time I opened this amp, like 15 years ago, I got an accidental short with the probe, while reading the plate voltage, that caused the filament resistors to go up in smoke. BUT, that was on an output tube. The output tubes filaments have a different supply, and a different pair of resistors to ground, so I dunno how that could somehow burn out the filaments of 4 preamp tubes.
But from what people are saying, we're looking for a broken connection, rather than a short, right? Like one or more filaments are broken completely, or just not allowing current through? The tubes were sitting loose in the bottom of the cabinet, so I'll give the pins a quick wipe with contact cleaner too, and push them in and out a few times.

[eschertron]

You may try this: take a tube that 'glows' and try it in each socket. Some tubes have a heater glow that is more visible, so we can eliminate that variable with one tube only.

[Dark Mavis]

You may try this: take a tube that 'glows' and try it in each socket. Some tubes have a heater glow that is more visible, so we can eliminate that variable with one tube only.

I'll do that too. At the moment I'm just tidying the lead dress and totally rewiring the filaments, just so I can look at the amp without imagining noodles

[Dark Mavis]

Hmmm...I found one discrepancy between the amp and the wiring diagram. On tubes 1 and 3 - the ones wired as cathode followers, the drawing shows a small capacitor between pins 6 and 7, and a jumper between pins 7 and 1, but that's not what was on the actual socket in the amp, if you look at the pic

[Dark Mavis]

Okay, so I decided to go ahead and clean up the wiring, and I'm glad I did. other than two preamp tube sockets wired up incorrectly, I found one cap that wasn't connected on one end, and a washer and piece of stray wire floating around between the circuit board and the blank board behind it that's supposedly there to prevent shorts...
Also, I now feel that I know the circuit a lot better. Anyways...Here's the before and after pics.




I'll post a diagram showing how tubes 1 and 3 were wired, vs how they were supposed to be wired, too. Then I'm gonna check and re-check, that everything is connected where it should be, then I'm just gonna plug a guitar in and see if the problem even still exists

[Dark Mavis]

Here's how tubes 1 and 3 were supposed to be wired...



What's in the amp was more like this.

I could be wrong about that jumper that shorts the plate resistor. It's hard to see on the photo. It might have gone from the grid of the first stage to the plate of the second. It definitely wasn't going from the plate of the first stage to the grid of the second, like it was supposed to. Also... The order of that .22 cap and 3.3m resistor is reversed in the amp. I don't think it makes a difference in this particular case, but...
DOES it? Am i missing something there?

[jmaf]

Here's how tubes 1 and 3 were supposed to be wired...

[ATTACH=CONFIG]53034[/ATTACH]

What's in the amp was more like this.
[ATTACH=CONFIG]53035[/ATTACH]
I could be wrong about that jumper that shorts the plate resistor. It's hard to see on the photo. It might have gone from the grid of the first stage to the plate of the second. It definitely wasn't going from the plate of the first stage to the grid of the second, like it was supposed to. Also... The order of that .22 cap and 3.3m resistor is reversed in the amp. I don't think it makes a difference in this particular case, but...
DOES it? Am i missing something there?

That jumper would kill all audio. It also cannot go from the plate straight to the grid. The only correct connection for the jumper is as in drawing 1, from plate of tube 1 to grid in tube 2. Also the 500pf cap cannot go that way, the 2nd grid would not be DC biased.

The 3.3meg and .22 uf cap RC filter yields 0.2 Hz cutoff frequency, it's not doing anything the way it's set up on that drawing.

Make sure you got the drawings right If they are indeed right then whoever modded the amp before made a few mistakes.

[Dark Mavis]

That jumper would kill all audio. It also cannot go from the plate straight to the grid. The only correct connection for the jumper is as in drawing 1, from plate of tube 1 to grid in tube 2. Also the 500pf cap cannot go that way, the 2nd grid would not be DC biased.

The 3.3meg and .22 uf cap RC filter yields 0.2 Hz cutoff frequency, it's not doing anything the way it's set up on that drawing.

Make sure you got the drawings right If they are indeed right then whoever modded the amp before made a few mistakes.

Just to be clear, the top drawing is from the schematic. The bottom drawing is what was in the amp when I got it... At least I THINK it was, from memory from when I desoldered everything. I know for sure the cap wasn't from the grid to the plate like it is in the schematic. It's kinda hard to see in the photo of the amp, where it actually went.

The top drawing is from the actual schematic, so it's supposedly correct? I just took a photo of it and drew over it in photoshop, to make that drawing so it would be more clear. Either way, I re-wired those sockets to match the schematic. Is it wrong? The cap wasn't on the schematic itself; it was on a separate drawing of the tube socket in the ..."manual"

Edit: So...the cap and resistor need to be in the order they are in the top drawing? I couldn't get my head around why it would matter
Edit: So what I'm saying is: The top drawing comes from the manual. The bottom drawing is what I THINK was in the amp. All I knw for sure is that the cap was definitely not where it should have been. I can't see exactly where it was, but it wasn't between pins 6 and 7 like the drawing in the manual shows. It definitely went across the socket from one triode to the other

[jmaf]

Just to be clear, the top drawing is from the schematic. The bottom drawing is what was in the amp when I got it... At least I THINK it was, from memory from when I desoldered everything. I know for sure the cap wasn't from the grid to the plate like it is in the schematic. It's kinda hard to see in the photo of the amp, where it actually went.

The top drawing is from the actual schematic, so it's supposedly correct? I just took a photo of it and drew over it in photoshop, to make that drawing so it would be more clear. Either way, I re-wired those sockets to match the schematic. Is it wrong? The cap wasn't on the schematic itself; it was on a separate drawing of the tube socket in the ..."manual"

Edit: So...the cap and resistor need to be in the order they are in the top drawing? I couldn't get my head around why it would matter
Edit: So what I'm saying is: The top drawing comes from the manual. The bottom drawing is what I THINK was in the amp. All I knw for sure is that the cap was definitely not where it should have been. I can't see exactly where it was, but it wasn't between pins 6 and 7 like the drawing in the manual shows. It definitely went across the socket from one triode to the other

Unless I'm missing something terribly obvious, the top one looks like a typical CF.

I have no idea what the 3.3 meg resistor and 220nf capacitor are doing there. There has to be more to that circuit than just those two components?

[g1]

On the schematic in post #16 (pg.1 of thread) both cathode follower stages are drawn wrong.
What you have posted above is not the same as in post #16.
So not sure which is the original, post #70 (top pic) or post #16? Please post the actual schematic even if hand drawn.

The follower plates should go only to the supply, the tone stacks should both come off the cathode of the follower stages. (added red lines are breaks in circuit, not capacitors)

[Dark Mavis]

On the schematic in post #16 (pg.1 of thread) both cathode follower stages are drawn wrong.
What you have posted above is not the same as in post #16.
So not sure which is the original, post #70 (top pic) or post #16? Please post the actual schematic even if hand drawn.

The follower plates should go only to the supply, the tone stacks should both come off the cathode of the follower stages. (added red lines are breaks in circuit, not capacitors)

[ATTACH=CONFIG]53043[/ATTACH]

Ah, yeah. On the original drawing the .22 cap and 3.3 meg resistor are right there, but for some reason I never added them on the reproduction in post #16. So the post above is right. They're only there on tube #1, which is the cathode follower on the clean channel. i dunno how I missed them in post #16. I don't even know how long ago I made that copy. And that short from the plate to the cathode isn't there on the original drawing. I dunno how that happened. I was drawing it directly from the paper schematic. I don't even know how long ago I did it. I'll re-post it. i'll just scan the original paper schematic.