Cathode follower without elevated heater voltage

[Dark Mavis]

Quick question: This is the phase inverter/driver as it is actually wired in the amp:

And this is the schematic:


The second stage of that tube is wired completely differently in the amp, so I have to assume one of the following:
#1 Since the pins aren't numbered, maybe they just draw the second stage that way around to make the diagram neater, and the wiring in the amp is correct, since the amp...kinda worked/works
#2 The person who put the amp together assumed that the stage drawn on the diagram was drawn upside down for neatness, and wired incorrectly because of it, possibly this is the cause of the problem the amp had?
#3 It doesn't matter which way around the plate and cathode are wired in the case of the driver tube, for some reason.
On the diagram I drew, the orientation of that stage is the same as every other stage, with pins 1 and 6 at the top, and 3 and 8 at the bottom

[eschertron]

If I were in charge of the world, I'd make sure that the symbol for plate and the symbol for cathode were identifiably different. Truth be told, I'm not, and they are both 'electrodes' in the theoretical sense of the word. So while I understand that the cathodes of both halves are tied to the 470R bias resistor, and that the plate of the 'lower half' is drawn on the bottom of the symbol; not everyone would see that without question. It's good to ask. The plate pins, 1 and 6, are connected to the components that lead to the grids of the power tubes.

Another thing that I see 'funny', is that there's a post-PI master volume. What's funny is that it's inside a NFB loop from the speaker signal, which tries to normalize the volume at the output. In other words, makes the master volume less effective. Maybe that's why the NFB resistor was changed from 5.6k to 33k, making the NFB almost nil. Or there was a change in the output transformer secondary impedance, and the 33k is there to compensate.

[Dark Mavis]

If I were in charge of the world, I'd make sure that the symbol for plate and the symbol for cathode were identifiably different. Truth be told, I'm not, and they are both 'electrodes' in the theoretical sense of the word. So while I understand that the cathodes of both halves are tied to the 470R bias resistor, and that the plate of the 'lower half' is drawn on the bottom of the symbol; not everyone would see that without question. It's good to ask. The plate pins, 1 and 6, are connected to the components that lead to the grids of the power tubes.

So the wiring in my drawing is correct, and the schematic just draws that stage upside down to avoid having to draw wires crossing each other?

[eschertron]

So the wiring in my drawing is correct, and the schematic just draws that stage upside down to avoid having to draw wires crossing each other?

That's how I see it, yes. Not that I've ever seen it that way before...

[Dark Mavis]

That's how I see it, yes. Not that I've ever seen it that way before...

Ah, good. Then, everything is pretty much correct now. I don't like that the auxillary transformer of the preamp filaments, and the channel switching relay, bypass the power switch, but I can just unplug it when not using it, and just put it right if I sell the amp. All I gotta do now is figure out why the clean channel isn't working. I found one wire had come loose from the relay circuit board, and I think that's the problem, but I need to wait for a new soldering iron to fix that and see.

[Dark Mavis]

Well, I was going to post a full diagram of the actual wiring in the amp, as a diagnostic aid, but it seems that the amp is now fixed. I can't be 100% sure, cus I was testing with an electric toothbrush next to a disembodied pickup, but nothing sputtered or broke up like before. The overdrive channel was insanely loud. The clean channel seemed too quiet by comparison, but I remember from using a vibrating rubber penis as a sound effect, that a clean amp doesn't amplify that sort of thing very well. Basically, I can't say if it's 100% fixed until I finish the guitar. But here's the pic anyways

[eschertron]

TMI !!

[Chuck H]

I'm hopeful it was a stage prop and we're not sharing what should be private information about ones possessions.

[Dark Mavis]

I'm hopeful it was a stage prop and we're not sharing what should be private information about ones possessions.

It was in the studio with a bunch of other stuff that makes weird noises. A TV remote also makes a cool noise through a guitar pickup

[Leo_Gnardo]

a clean amp doesn't amplify that sort of thing very well.

Well no wonder. A clean amp wouldn't want to have anything to do with that sort of business.

What do the kids say these days? "TOO MUCH INFORMATION ! ! !"

Crikeys, what next?

[Dark Mavis]

So...here's a video. Yeah, the dramatic moment where the amp is turned on for the first time is totally staged for the video. I never filmed it when I actually did it, so it's more a dramatic recreation. Besides, Youtube would prefer to see a cordless blender than a vibrating D.

Skip to some time after 21 minutes to see the amp switched on.

[Chuck H]

Fun. So far so good? I do think I hear more buzz than I would generally accept for quiet playing, but I don't know how loud you have the amp set relative to speaking level for the test. That buzzy transformer is annoying, but not uncommon. I can't help but wonder if it's not the source of the buzz in the audio though.

Do all the voltages check out? It did look like you have similar enough preamp tubes throughout and the one on the left didn't visibly demonstrate the filaments glowing like the others. At least not in the video.

[Dark Mavis]

Fun. So far so good? I do think I hear more buzz than I would generally accept for quiet playing, but I don't know how loud you have the amp set relative to speaking level for the test. That buzzy transformer is annoying, but not uncommon. I can't help but wonder if it's not the source of the buzz in the audio though.

Do all the voltages check out? It did look like you have similar enough preamp tubes throughout and the one on the left didn't visibly demonstrate the filaments glowing like the others. At least not in the video.

I think that was just the way the tube was oriented. I was kinda hoping rewiring would sort it, but at least now I know the circuit and I have more room to poke around. I'll measure all the voltages again. I might as well do that in a video, to get more eyes on the problem. I really don't recall whether or not that transformer hummed that much before.
I think what I'll do is measure all the voltages, and add them to the new schematic I made.
Also, I need to get a guitar so I can demonstrate the actual problem the amp has always had

[trobbins]

Apart from checking all the valve voltages, it is worthwhile checking the specs of your voltmeter to confirm it has a high DCV input resistance (eg. 11 Megohm).

Do you have a bias adaptor to measure the idle current in each of the output stage EL34? If not then adding in a 1 or 10 ohm cathode resistor for sensing idle current would be my recommendation, as you need to be confident now (and in to the future) that all the EL34 are somewhat balanced, and the idle current in each side of the OT sum to about the same current.

Identifying/isolating the cause of noises/hums/buzzes is best done with some benchmark levels. For example, can you hear or measure the speaker AC level on your DVM when the PI stage valve is pulled out, and the MV is set to min, and then max? You can work backwards, or start from the inputs with nothing connected, and do a relative comparison to the benchmark. Apart from changing the gain, pre-amp volume, and reverb pot settings, you can add temporary shorts across valve inputs to restrict the circuitry being checked to just circuitry from the short to the output stage (eg. short the V4a grid to 1k cathode 0V node, or the V5b grid to 0V).

It's a substantial amp with lots of EL34 that could fail at some stage down the track. One simple way to protect the amp is to make each EL34 cathode sense resistor a 1 ohm 0.25W (or 0.4W) resistor in parallel with say a 150V zener, or a 22kohm 1W resistor - the aim being for the 1 ohm resistor to act as a poor man's fuse, and the zener or resistor acts to avoid damaging the EL34 if the fault wasn't from it.

[Dark Mavis]

So, I found another discrepancy while measuring the voltages again. The 3.3M resistor (At least that's the value on the schematic) that's connected to the Grid of V1, is 1M in the amp. I know it can't be the cause of the amp's problem, 'cus it's only on one channel, but what effect would this have? Would it even be noticeable? Anyways, I'll post the updated schematic with the voltages on tomorrow or the day after. Actually, I'll post the plate voltages now. I lost the light today before I could do much