Why Does My Space Heater Only Like NOS RCA Tubes?

[Caldo71]

Okay @vigilante398 and everyone else, I need to preface this post by pointing out that I am posting build pics for you guys to examine (I know it's the first thing requested for troubleshooting, always) but you guys are going to find them to be an absolutely crazy-looking jumbled, inexplicable hot mess. And, well: it is.

So let me explain what you are looking at in the pics:

• I mount my stomp box circuits in rackmount enclosures, jutting out horizontally inside 'em, with the potentiometers facing the front face of the enclosure, extended out on little ribbon wire+pin header thingies. Back connections for in/out/power are on Molexes. So that gray plastic thing you see that looks like an oil rig? That's the armature that screws into the face of the rack enclosure and holds everything in place. it's designed and 3D printed by yours-truly.
  
  
• I found that the Space Heater was giving me weird volume drops when I put other pedals AFTER it, so you'll see a little "mini shelf" built into the 3D print on the left-hand strut? That's actually a @PedalPCB Simple Jfet Buffer....the Space Heater hits that on the way outta town, and it works to solve that problem. Impedance mismatch on the tube gear I guess? Anyhow it worked so I incorporated it.
  
  
• In tandem with that I have another little "mini shelf" built into the right-hand strut that holds a pedalpcb DC Jack Breakout Board, so I can split power as cleanly as possible to the Space heater and Buffer, and give myself some more grounding points.

Okay WHEW that's what I did...I'm no expert at electrical theory so either all of the above is complete genius or totally idiotic and causing me all kinds of problems I'm unaware of...you tell me.

SO HERE IS THE WEIRD PROBLEM:
For whatever reason, this things sounds just fine with these NOS RCA tubes I've had sitting around the studio forever. Its what I first put in when I built the circuit, and it's stayed in there after deciding to incorporate the buffer and all that. Cool, fine. Never knew there was some underlying issue...it always worked.

HOWEVER: recently I decided to try some different tubes in it, and I bought myself some new production Mullards and Tung-Sols from Sweetwater, and they don't work! So weird. The filaments barely glow at the tips, and they never get to "full glow". And no sound comes out at all.

I first suspected bunk tubes, so I tried the new ones in all my tube amps, and they were just fine! Likewise, I pulled a really rando selection of new and vintage tubes (yes they are all 12AX7s) from those amps and stuck 'em into the Space Heater, and had the same issue: literally no tube in my arsenal will pass sound in the Space Heater except for these RCAs.

So what makes these RCAs different? And is something in my "exotic" build—i.e. the buffer or the power splitter—somehow keeping my Space heater from supplying all these other tubes with the juice they need?

it's a weird issue...any solid answers or even theories would be hugely appreciated. Sorry if any of my schemes and strategies verge on mental illness.

EDIT: thought I should mention that the 9V power supply I'm using provides a full 3A, in case anyone was concerned that I was not giving the pedal enough juice. Unless (again) splitting that out to the buffer is the issue?

 

[zgrav]

have you measured the voltage on your tubes to see what they are for the heater and main voltage? after you check it for the NOS RCA tube that works, see if the voltages change when you switch out the tubes.

 

[vigilante398]

Mmkay, a few notes, in no particular order:

• Space Heater uses both triodes as gainstages, there are no buffers in the circuit, so the output impedance is relatively high (around 38kΩ). Depending on the input impedance of the following circuit, a buffer may be necessary to make everything play together nicely.
• What does "full glow" mean with regards to 12AX7 heaters? A question I get a lot is why the preamp tubes in my pedals don't seem to glow aside from a little orange dot at the tips, and the answer is that most preamp tubes just don't. They aren't pulling huge amounts of current, they're not going to give you the same glow you'll see in a power tube (EL84, 6L6, etc). The RCA tube may have a heater that was designed differently with more of the filament exposed that produces a more visible glow, but that doesn't make it better or worse.
• As for why the RCA tube passes audio while non-production tubes don't, my only possible guess would be that the RCA has a lower internal plate resistance, which would lead to a lower output impedance on the pedal. I don't have any tidbit of actual data that would back this up ("well you see in 1954 RCA used a different material due to nickel shortages, which led to a small batch of tube with slightly different specs blah blah blah"), it's just spitballing at what could possibly cause it.

As an electrical engineer, sometimes something happens where the only logical explanation is "Huh, that's weird."

 

[Caldo71]

have you measured the voltage on your tubes to see what they are for the heater and main voltage? after you check it for the NOS RCA tube that works, see if the voltages change when you switch out the tubes.

So my semi-local "tube amp builder guy that I only know through Facebook" dude asked me a similar question. Not knowing f$ckall about how to do such things, I went on the interwebz, looking to find a tutorial on how to measure such things. Everything I came up with was extremely deep navel gazing stuff that assumed an incredible depth of previous knowledge and got me nowhere. If you or anyone here could offer me a more practical dummy-level step-by-step on how to do this measurement with my multimeter, I'd be extremely grateful.

He asked me to measure plate voltage, filament voltage, and resistor value on the cathode...I dont know how to do any of those things and feel like an idiot but really want to learn.

 

[Caldo71]

Mmmkay Nathan so let me try and fire back answers in-betwixt your original questions to keep things tidy...

• Space Heater uses both triodes as gainstages, there are no buffers in the circuit, so the output impedance is relatively high (around 38kΩ). Depending on the input impedance of the following circuit, a buffer may be necessary to make everything play together nicely.

Yep so I guess that at least validates what I was hearing happening, as well as my solution of putting the Jfet buffer after it. Seemed to work a charm, seemingly with no ill effects unless this current issue I'm having is related.

• What does "full glow" mean with regards to 12AX7 heaters? A question I get a lot is why the preamp tubes in my pedals don't seem to glow aside from a little orange dot at the tips, and the answer is that most preamp tubes just don't. They aren't pulling huge amounts of current, they're not going to give you the same glow you'll see in a power tube (EL84, 6L6, etc). The RCA tube may have a heater that was designed differently with more of the filament exposed that produces a more visible glow, but that doesn't make it better or worse.

So yeah it's pretty much exactly what you just said: the newly-manufactured Tung-Sols and Mullards I've tried only have that wee bit of "tip glow" you mentioned, which from what you stated above sounds like, in-and-of-itself, is not necessarily a bad thing. Cool. HOWEVER: the obvious red flag here is that the old RCA tube has some MUCH more obvious glow happening all down the length of the filaments, and of course THAT is the only tube that actually seems to function in the pedal at all.

• As for why the RCA tube passes audio while non-production tubes don't, my only possible guess would be that the RCA has a lower internal plate resistance, which would lead to a lower output impedance on the pedal. I don't have any tidbit of actual data that would back this up ("well you see in 1954 RCA used a different material due to nickel shortages, which led to a small batch of tube with slightly different specs blah blah blah"), it's just spitballing at what could possibly cause it.

I'm not sure what you mean by "non-production tubes", so to clarify, the RCA tubes are NOS, whereas the Mullards and Tung-Sols I have are brand-spankin-new production run tubes—reissue style—purchased right from Sweetwater a week ago. So , assumedly you haven't designed a circuit that only works with weird-ass ancient tubes with low internal plate resistance, but SHOULD work with ANY new 12AX7 right off the shelf. Obvy. So then the question I guess becomes, why isn't that tube getting enough juice to make noises?

Again pardon my vast level of ignorance here...I'm just trying to pick up valid knowledge as I learn more about how all this stuff works in the process of building circuits in my spare time for a couple years now.

 

[vigilante398]

I'm not sure what you mean by "non-production tubes", so to clarify, the RCA tubes are NOS, whereas the Mullards and Tung-Sols I have are brand-spankin-new production run tubes—reissue style—purchased right from Sweetwater a week ago. So , assumedly you haven't designed a circuit that only works with weird-ass ancient tubes with low internal plate resistance, but SHOULD work with ANY new 12AX7 right off the shelf. Obvy. So then the question I guess becomes, why isn't that tube getting enough juice to make noises?

Again pardon my vast level of ignorance here...I'm just trying to pick up valid knowledge as I learn more about how all this stuff works in the process of building circuits in my spare time for a couple years now.

Whoops, meant to say modern-production there. Typing up a work document at the same time as browsing here (multi-tasking!) and got mixed up.

So have you tried the Space Heater with a modern production tube by itself with no other circuits following it to see if it's making any noise at all? Having too high of an output impedance to play nicely with other circuits is one thing, but not passing signal at all is another. 3A on a 9V supply is way more than you should need to get the tube wamred up and running happy.

 

[Caldo71]

So have you tried the Space Heater with a modern production tube by itself with no other circuits following it to see if it's making any noise at all? Having too high of an output impedance to play nicely with other circuits is one thing, but not passing signal at all is another. 3A on a 9V supply is way more than you should need to get the tube wamred up and running happy.

Yeah so this is what I really WANT to try out, but as you can see from my pics, there's just this tight little jumble of soldered-in wires permanently connecting the Space Heater PCB to the buffer PCB and the power splitter PCB, all kinda epoxied into that plastic mounting frame. So I mean I COULD tear it all completely apart just to see if the buffer or power splitter is the issue...but I don't WANT to if I can help it LOL!

Really makes me wish I'd bought TWO of the Space Heater PCBs when ya had 'em in stock, Nathan...I even have the rest of the components already. The hell was I thinkin'? That way I could keep a "pure" build in reserve to mess with.

If I get up the gumption I'll snip the output and power leads from the space heater and try to graft 'em temporarily to their own jacks so I can really see if that peripheral stuff is the issue.

I DO just now recall that before I did any of this fancy hybridization, a few months back, I'd already been experimenting with swapping out a 12AU7 for the 12AX7, and THAT experiment worked just fine...long before I piggybacked that buffer etc onto the build. So my suspicions are leaning the same way...something must be up there.