Relays And Popping

[Paul Marossy]

Well, it's for sure not right. The plate voltage can vary all over the place, but a 12AX7 with 5+ volts on the cathode and its grid at 0V can't pass signal. I think you found a better lead on the issue than I did.

1. Have you replaced V2 with a known good 12AX7? I have to ask. Swapping with known good tubes is always the first thing to do in a tube circuit.
2. Do the voltages on pins 6, 7, and 8 change when you change the channel and drive switches?
3. What *are* those voltages, anyway? Particularly is pin 7 at ground and does pin 8 vary when you flip the switches?

I did swap all three preamp tubes around many times, and had more or less the same problem every time. So I never really thought the preamp tubes could be the problem, especially since they are pretty new. But when I switched V1 with V2, the +5.4V on the cathode problem followed the tube around. Then I stuck a preamp tube in there that I knew was good and the amp works like it should now.  

So now I feel like the biggest idiot in the world! Although, looking at this schematic, this has to be one of the screwiest channel switching schemes I have ever seen in an amp and I would have never suspected a bad preamp tube in this shell game-like schematic, ha ha. So I learned a few things I didn't know before in the process, I guess it's not all bad. Man, I think I'm sticking with the old Fenders from now on. I just use this at church and keep it there, I'm not really married to it for any reason. But it has been a decent & reliable amp for me since 1998.

[Paul Marossy]

Only one problem... I still have a pop when switching between the normal and drive channels.  

Seems like as the amp warms up it starts to happen more. I must have a leaky cap somewhere I guess.

EDIT: How loud the pop is proportional to how strong the signal is when I hit the switch. With no signal at all it doesn't pop, so that is making some progress I guess compared to when I started.

[R.G.]

EDIT: How loud the pop is proportional to how strong the signal is when I hit the switch. With no signal at all it doesn't pop, so that is making some progress I guess compared to when I started.

I don't know that this is your problem, but it may be.

Imagine a sine wave, simple, rounded signal wobbling up and down. Now image that you have a metal-contact switch that can conduct that sine wave to some amplifier. If the switch happens to close when the signal is near zero, the signal seen by the output amplifier starts near zero and continues on, as you'd expect. But if the switch happens to make contact near the peak of the signal, the output sees the signal jump from zero to maximum instantly. This is a problem organ manufacturers have wrestled with for about a century. Simple metal-contact switching simply cannot be completely transient-free, by the nature of the beast.]

There is another possibility. It is possible that some DC level inside the circuit is being increased by bigger signal levels. This may be too slow to hear when it's continuously varying, but switching the signal may suddenly connect the DC level where it shouldn't go. This can be an issue in too-simply-designed compressors and other amplitude-sensitive circuits.  Again, don't know if it's your problem, but it might be.

[Paul Marossy]

Maybe that could be it. I'm beginning to wonder if these preamp tubes are my problem. Going to have to mess around with that some more to rule that out.

I do notice that relay K1 makes a noise you can physically hear when it switches, whereas K2 you can't hear at all when it switches. Not sure why that would be. This whole thing is odd to me as my old 1984 Seymour Duncan Convertible never pops when switching, but it's also nowhere near as whacky circuit-wise as this Hot Rod DeVille of mine.

[R.G.]

I do notice that relay K1 makes a noise you can physically hear when it switches, whereas K2 you can't hear at all when it switches. Not sure why that would be.

Since you socketed them, swap the relay positions just like you would with a tube so you can find out whether it's the part or the socket it's in.

The schemo shows both relay coils in parallel, so they should be getting the same drive. However, I am distrustful of the soldering what would make "parallel" be strictly true. If it follows the relay, OK, different vintage of relays. If it follows the socket, there's an oddity about the connection to it on the PCB.

This whole thing is odd to me as my old 1984 Seymour Duncan Convertible never pops when switching

Design finesse and completion. Maybe luck. There aren't many situations where a part does exactly what you want without you tinkering and compensating for its quirks.

[Paul Marossy]

I do notice that relay K1 makes a noise you can physically hear when it switches, whereas K2 you can't hear at all when it switches. Not sure why that would be.

Since you socketed them, swap the relay positions just like you would with a tube so you can find out whether it's the part or the socket it's in.

That K1 relay always makes the clicking noise no matter what relay is that particular socket. I also went over the soldering on the sockets carefully to make sure all those solder joints were solid. It's like that particular relay gets slammed when it switches and the other one doesn't. Must be some quirk in the circuitry somewhere. I still need to try different preamp tubes, the other two in the amp are currently under suspicion now.

[Mike Burgundy]

That is *really* odd. Aren't  K1 and 2 coils hooked up in parallel? That suggests it's either some kind of physical resonance thing (they both make the same physical click but one gets dampened by the surroundings and one resonates freely) or a bad electrical contact slowing one down.

[Paul Marossy]

That is *really* odd. Aren't  K1 and 2 coils hooked up in parallel? That suggests it's either some kind of physical resonance thing (they both make the same physical click but one gets dampened by the surroundings and one resonates freely) or a bad electrical contact slowing one down.

Yeah, they are in parallel. All I know is that when I push the "drive" and "more drive" button, I can not hear any audible noise coming from the relay itself. But when I switch between "normal" and "drive", it's a click you can definitely hear with everything turned down.

I would lean towards a bad electrical contact, but where the heck it is so far have been a mystery. I can not find the source of the problem to save my life.  

EDIT: It looks like maybe I was erroneously thinking that one relay was for the "drive" and "more drive", but I think that just happens with a switch and not a relay. So what I must be hearing is both relays clicking when switching between "normal" and "drive" channels. At first I thought one relay switched between "drive" and "more drive" and the other one switched between "normal" and "drive" channels. I'm not sure exactly.

[Mike Burgundy]

*Both* relays switch at the same time when changing between normal and drive. more drive engages some solid state switching, nothing mechanical, so the behaviour is perfectly normal. I though you heard a distinct difference between the two relays, but it turns out it's two different circuits. I can't say anything about the hotrods at our studio (they're never switched, just used for poweramps and speakers) but I'll have a look how noisy/quiet their switching is.

[Paul Marossy]

*Both* relays switch at the same time when changing between normal and drive. more drive engages some solid state switching, nothing mechanical, so the behaviour is perfectly normal. I though you heard a distinct difference between the two relays, but it turns out it's two different circuits. I can't say anything about the hotrods at our studio (they're never switched, just used for poweramps and speakers) but I'll have a look how noisy/quiet their switching is.

Yeah, I eventually figured that out.  

Anyway, here is the inside of the relay just for kicks.



There's about 1/2 millimeter between contacts and the white assembly swings to the left or the right. So you would think that it would be pretty quiet with such a short distance for the contacts to travel, switching would be quite fast. I must either have a bad cap somewhere or some crappy preamp tubes. I'll have to do that preamp tube thing later today. It's probably these preamp tubes. I'm surprised, I thought J&J tubes were supposed to be pretty good.  

[Gus]

FWIW

Using  sockets for the relays and ICs is a recipe for failure.  An amp vibrates and the parts could/will fall out or have contact issues.  I do not use sockets for anything in an amp or an effect except for tubes (I sometimes solder in tubes in microphone builds).

Did you changeV1 (V1b) and V2 (V2a, V2b)?  If there is charge built up on the grids you might be getting pops from the triodes.

[Paul Marossy]

FWIW

Using  sockets for the relays and ICs is a recipe for failure.  An amp vibrates and the parts could/will fall out or have contact issues.  I do not use sockets for anything in an amp or an effect except for tubes (I sometimes solder in tubes in microphone builds).

Did you changeV1 (V1b) and V2 (V2a, V2b)?  If there is charge built up on the grids you might be getting pops from the triodes.

I don't agree. This isn't the space shuttle, things aren't going to just fall out of the sockets from a little speaker vibration. I don't ever have volume knob up past aboout "4" anyway.I have to use needle nose pliers just to get the opamps out of the sockets. And the relays and transistors take considerable force to pull out too. And my Seymour Duncan Convertible built in 1984 never had any of its ICs fall out of their sockets. Nor have I ever had a tube fall out of any sockets in any of the several tube amps I own. 

Anyway, I need to get a couple of new preamp tubes. I'd be willing to bet it's these preamp tubes I have in the amp.

[Mike Burgundy]

FWIW I had a quick peek at the ones we have -  the channel switching is quiet when not playing, it might pop when audio is present but that's not uncommon. Tiny physical click audible at the chassis. There is a slight delay going from drive to clean which is a bit odd, but no real problems. This amp is developing (after a week or two of being quiet after a capjob and reflowing the output tube sockets) some more hum, so I'll have to hunt for more bad joints again. Maybe I'll just reflow the whole damn thing and be done with it. Hope you find your problem - wouldn't be surprised if it's also solder related.

[Paul Marossy]

FWIW I had a quick peek at the ones we have -  the channel switching is quiet when not playing, it might pop when audio is present but that's not uncommon. Tiny physical click audible at the chassis. There is a slight delay going from drive to clean which is a bit odd, but no real problems. This amp is developing (after a week or two of being quiet after a capjob and reflowing the output tube sockets) some more hum, so I'll have to hunt for more bad joints again. Maybe I'll just reflow the whole damn thing and be done with it. Hope you find your problem - wouldn't be surprised if it's also solder related.

Thanks for getting back to me on that. 

So far so good on mine. The popping problem appears to have for the most part went away after changing out the faulty preamp tube. If there is anything audible in the speakers when switching, it happens only after the amp gets warmed up. Now I'm getting a smell like something is overheating, but so far nothing has failed. The power tubes seem to be running quite hot even though I've set the bias to factory specs. These hot rod Fenders sure do seem to be finicky. Mine's been pretty bulletproof since I bought it in 1998, but lately I'm beginning to wonder about it... 

[Mike Burgundy]

To be sure - check the output tubes for solder cracks (ie poke it with a wooden stick when on). If there is no hum (which might indicate a severely misbehaving tube which might cause the other to take most of the current and fry), and the bias measures ok (60mV for 30mA across each tube) across the 1 Ohm cathode resitor you shouldn't have hot running tubes. 30mA isn't that hot for these voltages- although I can't remember what the plate voltage was (always use plate voltage, not B+ for bias calculation). If memory serves you're well below 50% plate dissipation. Make DAMN sure the tubes seat well and the bias voltage is reaching the tube - if it's off, or encounters a bad contact (increasing series resistance) the tubes might intermittently run very hot. If bias voltage fails, there's nothing stopping the tube from running wild. The stick poking bit (BE CAREFUL) helps find problems that occur not when you're measuring but only when playing/vibrating/the amp gets really hot etc.
That said: tubes get hot, really hot, but I don't think a hot-running tube causes enough difference in outside glass temperature to be noticable, let alone cause a smell. If that were the case, I think your plates are glowing and the tubes are write-offs.
If it's not just you being extra perceptive (and noticing little stuff that's been there before for the first time) because you're hunting problems, and there really is a distinctive smell of magic white smoke trying to escape, you'll find new clues soon. I don't want to talk about how often I've been suspicious about smells, sounds, etc that turned out to be modus operandi that I just hadn't noticed before, or dust getting hot that was disturbed by the previous repair ;P

[Paul Marossy]

To be sure - check the output tubes for solder cracks (ie poke it with a wooden stick when on). If there is no hum (which might indicate a severely misbehaving tube which might cause the other to take most of the current and fry), and the bias measures ok (60mV for 30mA across each tube) across the 1 Ohm cathode resitor you shouldn't have hot running tubes. 30mA isn't that hot for these voltages- although I can't remember what the plate voltage was (always use plate voltage, not B+ for bias calculation). If memory serves you're well below 50% plate dissipation. Make DAMN sure the tubes seat well and the bias voltage is reaching the tube - if it's off, or encounters a bad contact (increasing series resistance) the tubes might intermittently run very hot. If bias voltage fails, there's nothing stopping the tube from running wild. The stick poking bit (BE CAREFUL) helps find problems that occur not when you're measuring but only when playing/vibrating/the amp gets really hot etc.
That said: tubes get hot, really hot, but I don't think a hot-running tube causes enough difference in outside glass temperature to be noticable, let alone cause a smell. If that were the case, I think your plates are glowing and the tubes are write-offs.
If it's not just you being extra perceptive (and noticing little stuff that's been there before for the first time) because you're hunting problems, and there really is a distinctive smell of magic white smoke trying to escape, you'll find new clues soon. I don't want to talk about how often I've been suspicious about smells, sounds, etc that turned out to be modus operandi that I just hadn't noticed before, or dust getting hot that was disturbed by the previous repair ;P

Yeah, it's probably me just being extra perceptive about everything now. There is no hum and the power tubes are not "red plating". I did put some leather cleaner stuff on the whole amp exterior, so maybe part of it is that it is getting "burned off" the backplate thing on the chassis (that was my suspicion right off the bat). Anyway, I know that the bias at 30mA is running the tubes relatively "cool", I just don't remember the power tubes getting quite so hot before. I'm sure it's nothing to worry about.

[Mike Burgundy]

The glass is not a good indicator of running "heat" - but it does get hot. Very hot. Like up to 200 degrees Centigrade.

[Paul Marossy]

The glass is not a good indicator of running "heat" - but it does get hot. Very hot. Like up to 200 degrees Centigrade.

Yeah, I've noticed that. 

[R.G.]

Even more interesting - even a mild airflow over the glass will make the glass much cooler, only slightly above ambient. But this changes the plate temp hardly at all because of the extremely high insulation value of the vacuum between plate and glass. It's an almost exact analog of a voltage divider, where the top resistor is huge. Lowering the bottom resistor changes the current (heat) flow almost not at all.

[PRR]

Power tubes, at any decent dissipation, will run very close to their glass-limit. That's around 415 degrees F for the usual guitar-amp tubes.

They tend to run near max temp because heat from plate to glass is all Radiation, and (without fan) a considerable part of glass to ambient heatflow is also radiation. Radiation goes as the cube of temperature difference. A large change of Watts is a small change of temperature.

12AX7 and kin run far cooler, less surface but far less heat. 200 deg F IIRC.

While I'm typing: _I_ do not like the one cathode resistor nor the stupid diode. _I_ would want to convert to two 1 ohm resistors and no diode. With one resistor, if one tube is dead you can't tell (except by extreme grid voltage and redplate in the other tube). With two resistors you can verify each tube is at a safe dissipation. However I sure know that PCB construction can foil such mods.