thoughts on building a tube test fixture

[supawah]

I'm thinkin of building a test jig for all my used tubes, basically taking a chassis, wiring three circuits into the preamp section, one for 12ax7, 12at7, and 12au7. basically whatever the three most common fender style circuits and maybe a Gibson 12au7 style, any way, wire them all in parallel, stick a calibrated 1k tone in , DMM measure the outputs of each triode section through test sockets, then have a line out to a powered speaker to listen for microphonics. my question is , how to decouple the outputs of the tubes to interface to a line level out? would there be an impedance matching issue which would affect testing? I'm just looking for a quick real world test for guitar amp conditions to test my 300 pulled preamp tubes. I can't afford the Orange vt1000 right now. thoughts?

[Leo_Gnardo]

I'm thinkin of building a test jig for all my used tubes, basically taking a chassis, wiring three circuits into the preamp section, one for 12ax7, 12at7, and 12au7. basically whatever the three most common fender style circuits and maybe a Gibson 12au7 style, any way, wire them all in parallel, stick a calibrated 1k tone in , DMM measure the outputs of each triode section through test sockets, then have a line out to a powered speaker to listen for microphonics. my question is , how to decouple the outputs of the tubes to interface to a line level out? would there be an impedance matching issue which would affect testing? I'm just looking for a quick real world test for guitar amp conditions to test my 300 pulled preamp tubes. I can't afford the Orange vt1000 right now. thoughts?

Done it. Started with an old Heathkit W-4 power amp that has insert jacks next to each output tube to measure bias current. I gave each output tube separate cathode resistors, bought a couple cheap ammeters 100mA scale from Antique and I was off and running for an output tube check that was more revealing than just stickin' em in my old tube tester. The W-4 has an octal socket on the side, with hi voltage & filament power to run a preamp. Made a cable & ran this to a preamp I built into a discard reverb can. Decoupling the preamp plates with a 0.1 uF 600V cap with a DC bleeder 1M on the measurement side. Some switches to pad the input 0, 20, or 40 dB, I typically use a 100 mV AC signal input, and measure the output. Ratio of output/input is gain. I don't worry much about tweaking impedances because in the real world the tubes' output is loaded down some & IMHO the gain I read is pretty close to what you would see in-circuit. Typical 12AX7 gains run from 55 to 75. The preamp stages are standard Fender-like circuits. I have a switch to change the plate resistors to a lower value @ 30K for testing 12AU7 and 12AT7. After a successful gain test, I run the output to the Heath amp and listen for noises. A switch on the preamp output so I can listen to one triode or the other, and another to switch between voltmeter and audition amp. I've tested hundreds of pre tubes and managed to cull out the worst of them. And most of the time I'm listening to FM radio thru this rig for entertainment whilst working.

Good luck on your project!

[trobbins]

Supawah, if you're good at mechanicals, you can set up a fixture to measure the 'signal-to-clang' ratio to indicate microphonics, perhaps using a VU meter, or VU plus peak leds from a cassette/tape deck.

[silverfox]

I think this product and schematics set is really cool.

The uTracer, a miniature Tube Tester / Curve Tracer.

[Enzo]

Wow, that is really cool. I just read through the entire thing. I wish it had come along 10 years ago, I'd have built one. Now alas i am winding down looking to shrink the shop.

I enjoyed seeing all the pictures of the many ways the various folks made theirs, some crude like my work, others very slick and professional. Some minimalist, some with every socket imaginable.


So more people see it, you might give it a title in its own thread up in Music Electronics section, 200 Euros including shipping anywhere sounds like a deal.

[silverfox]

I'll take the hint

As I recall everything needed to build one is on the site including the programing burn for the chip. The price is what dissuaded me. But, like I said if one wants to build it at home that is an option.

I would build one myself but don't have the kind of future in building tube amps that would justify it. Plenty of tubes of known good status to work with for the time being. I'll take your advice and start the thread.

I've posted/contributed controversial subject matter on another Forum and unfortunately some of the members there were too immature to discuss controversial topics or even read them without a corrupting influence bleeding over to the technical side. In the end I was subjected to a "Shunning". Nice to see we can hold heated discussions and still appreciate and contribute towards the technical good here at MEF. That's what R E S P E C T does.

Silverfox.