Cathode follower without elevated heater voltage

[Chuck H]

We'll help where we can, but I don't envy you. I knew Dan Torres before he ever had the amp shop. He was just working out of his garage at his home. He was an early pioneer in the amp mod DIY genre in that he made efforts to bring it to the public rather than pretend it was wizardry And in truth I thought most of his circuits sounded good. He really only get's bad mouthed by other amp guys. Players typically like his stuff. And this from a guy who had somewhat of a falling out with him. I owned a couple of amps with his mods inside and another that he built in a Traynor chassis before he was making his own amps. I remember things getting a little crowded sometimes, but nothing like your amp. If you remember gigging with it then I have to assume it pretty much worked as it is (when repaired). Probably best to stay on track with that for now.

The added filament transformer for the preamp was likely used because EL34 tubes draw more filament current than 6L6 tubes. So the existing PT filament winding wasn't up to the whole job by itself and needed a friend.

[eschertron]

I thought I'd mention that the filament voltages at the transformer looks OK. I'd expect a broken connection somewhere. We now return you to your regular troubleshooting.

[Dark Mavis]

We'll help where we can, but I don't envy you. I knew Dan Torres before he ever had the amp shop. He was just working out of his garage at his home. He was an early pioneer in the amp mod DIY genre in that he made efforts to bring it to the public rather than pretend it was wizardry And in truth I thought most of his circuits sounded good. He really only get's bad mouthed by other amp guys. Players typically like his stuff. And this from a guy who had somewhat of a falling out with him. I owned a couple of amps with his mods inside and another that he built in a Traynor chassis before he was making his own amps. I remember things getting a little crowded sometimes, but nothing like your amp. If you remember gigging with it then I have to assume it pretty much worked as it is (when repaired). Probably best to stay on track with that for now.

The added filament transformer for the preamp was likely used because EL34 tubes draw more filament current than 6L6 tubes. So the existing PT filament winding wasn't up to the whole job by itself and needed a friend.

Most of the negative stuff I heard was more about the customer service at his store, and bad repair jobs and such that could have been done by some random employee for all I know. I like how the amp sounds, and I've not even been able to turn it up far enough to get it really singing.
I dunno why I'm seeing 7.2 volts across that transformer. I just measured across the lugs where the resistors to ground are mounted. I never measured at each tube. It got too dark to be trying to poke the probes among all that mess. So...I gotta try to get it back to where it was, which was broken, but less broken than right now. Tomorrow, I'll check the pins for shorts. In some places, I could barely get a cigarette paper between one lug and another. I'll also swap out the 12ax7 in socket 5, with one of the four that have no glow, just to rule out 4 tubes with dead filaments, coincidentally ending up in the first 4 sockets. Since they're all wired in parallel, I guess it's the wiring rather than the tubes, unless 4 out of six tubes had the same problem and all ended up in the first 4 sockets, by sheer coincidence.

[Dark Mavis]

I thought I'd mention that the filament voltages at the transformer looks OK. I'd expect a broken connection somewhere. We now return you to your regular troubleshooting.

My thought exactly. The 7.2 volts seems a bit weird, but I'll worry about that later. I think there's a short or bad connection on that fourth or fifth tube. Maybe it's in the tube itself. I'll try subbing them, too. But that's gotta wait. it's too dark now

[Chuck H]

FWIW if there is a fault*** in the filament wiring then that could be the reason for the high voltage. Filament windings need to handle a relatively high current. The voltage will drop some when current is being pulled through the whole circuits resistance (winding, wire, tubes). I doubt it'll drop to 6.3V, but it'll drop some.

*** I prefer to call an unintentionally open circuit a fault and an unintentionally closed circuit a short. I do this for clarity because a short is ONLY an unintentional connection and not an open circuit. Though it's common for people to call any circuit flaw a "short", it's technically wrong and confusing for troubleshooting and repair efforts.

[Dark Mavis]

FWIW if there is a fault*** in the filament wiring then that could be the reason for the high voltage. Filament windings need to handle a relatively high current. The voltage will drop some when current is being pulled through the whole circuits resistance (winding, wire, tubes). I doubt it'll drop to 6.3V, but it'll drop some.

*** I prefer to call an unintentionally open circuit a fault and an unintentionally closed circuit a short. I do this for clarity because a short is ONLY an unintentional connection and not an open circuit. Though it's common for people to call any circuit flaw a "short", it's technically wrong and confusing for troubleshooting and repair efforts.

I think I'll check continuity along the filament string, and check there isn't continuity where there shouldn't be. I only suspect a short 'cus ridiculously crowded and badly wired lugs shorting seems more likely than a complete break in pretty thick stranded wire. Or maybe something is up inside a tube, and filament current is going somewhere it shouldn't. In which case, I'd suspect the fourth tube, since the filament transformer connects at the sixth, and the fifth and sixth have working filaments? Just guessing here. I'll test the wiring first. If that seems okay, I'll sub some tubes. I don't know much about tube amps, so it's just educated guesses. Though...educated by what, I have no idea. All I remember from school, is what happened on the lunch breaks.

[g1]

A short will cause excess current consumption, resulting in lowering of the winding's voltage and heat. A short will not make the voltage rise up at the winding. An open circuit will.

[eschertron]

...make the voltage rise up at the winding. An open circuit will.

I thought I was in the presence of Yoda for a second

[Chuck H]

I thought I was in the presence of Yoda for a second

Yoda... John Candy. It's easy to get them confused.

[Dark Mavis]

Sooo...a new day. I'm thinking, it's gotta be the wires, right? A fault in one tube's heater, wouldn't stop the other tube's heater's working, surely?

Hmmm...with all the tubes out, I get continuity the full length of both sides, BUT also from one side to the other. The resistance is 0.4ohms from one side to the other. Is that right? With the tubes out, he only things between those legs should be the 100 ohm resistors to ground, and the transformer. Think I'll disconnect the transformer and check again

With the filament transformer disconnected, and no tubes in place, I'm getting 197 ohms between the 2 sides of the filament string. Kinda what I'd expect. So now I can check the wiring without that confusing me. Should the secondary on the transformer ven have such low resistance? I guess it's gonna be pretty low cus...stepping down from 240v to 6.3v, so maybe.

[Chuck H]

With the filament transformer disconnected, and no tubes in place, I'm getting 197 ohms between the 2 sides of the filament string. Kinda what I'd expect. So now I can check the wiring without that confusing me. Should the secondary on the transformer ven have such low resistance? I guess it's gonna be pretty low cus...stepping down from 240v to 6.3v, so maybe.

BING BING BING BING BING!!!

That's what Enzo was talking about with "seeing" what's there to see. You nailed it. That filament secondary will be very low resistance. But do set your meter for a low range and see to it that there is some resistance. Maybe also check for continuity between the primary and the secondary (should be none) and each winding to the chassis (should be none).

Does that transformer feed any tubes that ARE lighting up or just the tubes that aren't?

Make sure the false CT (the two 100ohm resistors) is actually grounded. That 197 ohm reading would indicate either that they are not or that you lifted the ground before measuring.

[Dark Mavis]

Secondary resistance on that trans is .5 ohms. I can't be any more accurate, cus the meter doesn't do hundredths. There is no continuity between primary and secondary. Well, between the lugs of the power socket and the secondary. No continuity between either secondary and ground - They're disconnected, still, and sticking up in the air. Resistance to ground is about 100 ohm on every filament pin, as, I guess you'd expect. There's continuity to ground, from the false center tap. I also checked for continuity between every filament pin, and every adjacent pin. There was none, BUT only a resistance of around 1.5 ohms between the filament pins on tubes 4 and 5, and the cathode pins next to them. (again, with no tubes in the amp) The rest show no reading at all.
Isn't around 200 ohms what you'd expect the resistance from one side of the filament windings to the other to be? Since you'd surely be measuring those two 100 ohm resistors in series, right? Form either side to ground, it measures 100 ohms.
Maybe the filaments are working fine but just not glowing?

Edit, yeah the extra transformer powers the filaments of all the preamp tubes, two of which were glowing. The last two. The driver and - I think - the stages before and after the reverb send and return.

[Chuck H]

I also checked for continuity between every filament pin, and every adjacent pin. There was none, BUT only a resistance of around 1.5 ohms between the filament pins on tubes 4 and 5, and the cathode pins next to them. (again, with no tubes in the amp) The rest show no reading at all.

I can't come up with any reason for that. It's odd so I thought I might be missing something obvious, but that's a problem.

[Dark Mavis]

I can't come up with any reason for that. It's odd so I thought I might be missing something obvious, but that's a problem.

I see 1.5k resistors from the cathode to ground, on the schematic, and I think it's the right tubes. If they put in 1.5 ohm resistors by mistake...ah, no, cus then it would read 101.5ohms or more from the filament pin, down the filament wire, through the 100 ohm resistor to ground, through the chassis, through the cathode resistor, and to the cathode pin. I'll check those resistors anyways.

My bad. It's 1.5K from the filament pin to the cathode. 1.57, actually.

[Chuck H]

It's 1.5K from the filament pin to the cathode. 1.57, actually.

That's better.

What's weird is that there's no current through half of V5 as well as the first four tubes.?. I would look for a wiring fault at pins 4 and 5 (typically joined) on V5 where the filament lead connects. I know you already tested for continuity across the sockets, but do it again and wiggle the probe on the pins. It might be an intermittent fault that you're closing when the you press the meter probe into it.?.