Cathode follower without elevated heater voltage

[Dark Mavis]

The original schematic was really dirty and torn in half and some of the middle was missing. (Just a bit of the tone circuit) so I recreated it. I made a few mistakes, it seems, so I've just checked it thouroughly against the original and corrected them. So THIS is the actual schematic

[Dark Mavis]

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?

Nope. They're connected from the grid of the first stage, to the cathode of the second, kinda bypassing the whole tube. I don't get what the idea is, either. It's not on the lead channel, nor any other drawing of a cathode follower I've seen. They are on the circuit board drawings in the "manual", though. (again, just on that one tube) There's even an arrow pointing at the 3.3m resistor, and the following text: "If you want ONLY the clean channel to be louder, change this resistor. A higher value will be louder, but will have more distortion"
Surely that's gotta be a mistake, right? I don't see how that resistor would have much of an affect on anything

[Helmholtz]

This is different and now the 0.22µ/3.3M network makes sense. It is a NFB path from CF output to grid input.

[Dark Mavis]

This is different and now the 0.22µ/3.3M network makes sense. It is a NFB path from CF output to grid input.

I take it the order doesn't matter, in this case? In the actual amp, the resistor is the other side of the cap. I don't see how that would matter, though. Anyways, the real discrepancy between the schematic and the actual amp, was the location of that small cap on the tube socket - a cap I also don't see on other diagrams of cathode follower circuits - and possibly the jumper between the two triodes. Doesn't matter now, though. I've wired it to match the schematic and drawings.

The only other odd things I've found is that the power switch is after the PT primary, rather than before, which I guess doesn't matter, cus it's a switch, and the auxiliary transformer for the channel switching circuit, and the reverb transformer, are wired straight to the mains, bypassing the power switch (but not the fuse) which I guess is no big deal, but I might change it anyways

[jmaf]

This is different and now the 0.22µ/3.3M network makes sense. It is a NFB path from CF output to grid input.

Sure looks like NFB but it's tuned to 0.2 hz which is why I thought it was strange.

[uneumann]

Sure looks like NFB but it's tuned to 0.2 hz which is why I thought it was strange.

I think it's "all pass" NFB. It's not tuned to produce an audible roll off. The amount of NFB does vary with the vol pot setting. At low volume, the NFB is also low (shunted to ground) so it has little impact. At high vol settings, there will be more NFB to keep the whole stage clean. It's unusual to use a DCCF with this kind of feedback. It sort of negates the whole point of the DCCF circuit which is to get some distortion as the signal level through the stage increases. If you want a clean buffer, you could just use one CF triode. There is also use of local NFB in two other amp stages so maybe NFB was part of the overall amp design approach.

[jmaf]

I think it's "all pass" NFB. It's not tuned to produce an audible roll off. The amount of NFB does vary with the vol pot setting. At low volume, the NFB is also low (shunted to ground) so it has little impact. At high vol settings, there will be more NFB to keep the whole stage clean. It's unusual to use a DCCF with this kind of feedback. It sort of negates the whole point of the DCCF circuit which is to get some distortion as the signal level through the stage increases. If you want a clean buffer, you could just use one CF triode. There is also use of local NFB in two other amp stages so maybe NFB was part of the overall amp design approach.

Sure but if whoever modded this wanted a full band NFB why didn't they just use a resistor and make it a divider with the vol pot?

[Helmholtz]

Sure but if whoever modded this wanted a full band NFB why didn't they just use a resistor and make it a divider with the vol pot?

??
The cap is there to decouple signal AC from DC. It is necessary in this configuration.

I don't think Leo Fender used the DCCF (or better a gain stage followed by a DCCF) in his early amps because it distorted nicely. Rather it was an economical solution (compared to AC coupling) providing gain and a low source impedance to the tone stack.

Seems the designer wanted the upper channel to be cleaner and lower gain than the lower one.

[jmaf]

??
The cap is there to decouple signal AC from DC.

Sorry, overlooked that it's a CF. You're right.

[Helmholtz]

I take it the order doesn't matter, in this case?

Right, the order of cap and resistor doesn't matter.

[Dark Mavis]

Aight, well tomorrow I'm going to check everything against the schematic one last time and then fire it up. I think I'll try plugging a guitar in immediately. If it works, then the problem was either the mistakes made in the wiring of tubes #1 and #3, or the filter caps, since I changed those. If the problem still exists, I'll measure all those DC voltages again and post them

[Dark Mavis]

Hmm, so I just powered it on, and first thing I noticed was that it was humming. Not through the speaker - nothing came through the speaker. The amp itself was humming. I guess that's gotta be a transformer, but they're all bolted up pretty securely. I don't remember if it used to do that when it "worked" or not. I used to have an Orange that hummed a little bit. I'm getting nothing at all through the speaker though.
At least all the tubes are glowing now.

[Chuck H]

Have you replaced ALL the power supply filters? At that amps age it's possible that any old filters could be shorted or partly so causing a heavy load on the power supply (I would expect a blown fuse). You've powered it up without power tubes in already, but did you ever take it out of standby like that? and if yes, did it not hum?

[Dark Mavis]

Have you replaced ALL the power supply filters? At that amps age it's possible that any old filters could be shorted or partly so causing a heavy load on the power supply (I would expect a blown fuse). You've powered it up without power tubes in already, but did you ever take it out of standby like that? and if yes, did it not hum?

All but the last one. The one off by itself near the input jacks, where B+5 is taken from. The fuse is still fine. I just checked it visually and with a continuity tester. Before I rewired it, I also powered it up with tubes in. I got the usual hum from the speakers, so I guess I messed something up while rewiring. I was really careful to check every connection. Maybe I broke a signal wire free while I was poking around. I'll follow the signal path and check. also...I think I'll plug in the footswitch and see if the other channel is silent, too

Edit: No guitar was plugged in, but I'd still expect to hear some hum. Less than before, cus I no longer have filamen wires flopping all over the place, but I wouldn't expect dead silence

Edit: Seems like both channels are silent - Or the footswitch doesn't work. Also, the humming is pretty quiet and i really don't remember if it was always there or not. It's not coming from the output transformer though, it's definitely a stronger vibration at the otherend of the amp

[Chuck H]

Since you can't remember if the hum was there or not you should err on the safe side and do some current checking before going any further. We already checked for grid bias negative voltage, right? If not then you need to do that and also check bias current. There are a few ways to do that and perhaps you already know one or more. Otherwise we can help. After that you'd test voltage at each power supply node. If anything starts getting too hot or if any tubes start to glow at their plates you'll want to power down immediately.