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The Tube Imp

The Essential

Bright Red Box
[Italian version]

Product: Tube Imp
Price: UKP 299, USD 615 (YMMV)
Reviewer: Mark Wheeler - TNT UK
Reviewed: July-October 2007

I've said it in the title: this product is an essential accessory if you own valve kit with B9a double triodes in it. That's it.

What kind of a review is that?" demand angry plebs, stage left, "That tells us nothing. He's making it up. Where's his evidence? The old fool's finally lost the plot. Or he's started accepting bribes like free gear or trips to Las Vegas..."

No bribes, no freebies, just a no brainer. For less than price of a pair of fancy wires I have reinvigorated a pre-amp and learned why certain NOS (New Old Stock) valves (tubes) work better than others.

For less than a pair of fancy wires I've also sorted out a channel imbalance problem that plagued the old Concordant Excelsior that I still have on loan.

For less than the price of a pair of fancy wires I have confirmed that I can use a rare-as-hen's-teeth rectifier diode in the B- supply of a phono pre-amp design I've been working on for 10 years. And it is this last experience that highlights both the strengths and weaknesses of the Tube Imp. The Tube Imp is very inexpensive compared to the government surplus behemoths beloved of amateur radio operators. The Tube Imp is very inexpensive compared to the sophisticated valve testers used by manufacturers and repairers.

The Tube Imp is simple to use compared with the radio ham's 1940s and 50s government surplus test bench. The Tube Imp is also very straightforward compared modern professional rivals like the US$599 Maxi-Matcher Digital Tube Tester and the Hickock 533A or even 539A or milspec TV-3!

Therein lies the limiting factor for the Tube Imp. It's simplicity is great if you have only those B9a miniature double triodes, like the ECC80s family. The almost ubiquitous ECC87 (aka 6922 and 12AT7) cluttering your Audio Research pre-amps will drop straight into the Tube Imp and you can take readings and note them down in seconds. Remember to mark the bottom of the glass envelope with an indelible number to reference to your readings list. Go through your whole stock of valves and make a big list. That'll keep the obsessive among you busy for a few long Winter evenings.

Use your list of readings to match pairs as closely as possible. If your pre-amp or power amp input and driver valves use half a valve per channel (not a good idea as crosstalk between the two triodes in an envelope can be as poor as 56 dB in my experience), choose the valves with the closest match between each triode section. The specifications of the Tube Imp are here, but don't forget to use your back button when you've had a peek.

"Well what about big bottle double triodes like the very popular 6SN7 that crops up in many esoteric Single Ended Triode amps and mounts on Octal bases?" demand plebs, stage left, "We know the old fool's got 'em in his Assemblage SET300B!"

Herein lies the first problem with the Tube Imp. In order to make it simple enough for anyone to test their popular miniature double triodes, the Tube Imp can only test popular miniature double triodes and rectifiers and single B9a triodes (they do exist). To test the weird B8a double rectifier EZ41 I had to solder up a B8 socket (rare enough itself) to flying leads with individual pins on each one. But it was worth it as this low-heater current low voltage car radio rectifier is perfect for the cathode bias I have in mind. I know hexfreds would have worked too...but to use quirky NOS bottles is more out there... otherwise we'd all just buy mass produced boxes and be happy with them.

So the lack of a B8 socket is hardly going to limit sales of a valve tester, but the lack of a ready to use, drop-in octal adapter is more of a sales limitation. It's easy enough for solderheads to make adapters, but the whole point of this tester is that it's a plug'n'play device.

Whither Octals?

"What if we've got Quad 22 & II's" shout plebs, stage left, "They've got B9a pentodes, those little EF86's?"

Extra test points for tetrodes and pentodes are also essential for collectors of old British hifi. Guitarists and techies would also welcome the Tube Imp as a go-anywhere easy to use device, perfect for identifying that faulty valve when they've set up the stack only to find a tiny distorted output where that creamy 6L6 output should be, if only they could just plug in and test octal tetrodes. So there's an obvious market for a slightly more elaborate Tube Imp, perhaps a Tube Gnome or a Tube Sprite, that sits somewhere between the Tube Imp and the suitcase sized universal valve testers of yore.

You could buy the octal to B9a adapter for US$70 from Vacuum Tube Valley who make this adapter for their own double triode tester, which itself costs US$1,199.00 and is therefore not an immediate competitor to the Tube Imp. If you're a dumpster diver you really do need something more elaborate than this. But you probably know that already. I've never owned a pukka valve tester, relying on my multi-meter and signal generator to test valves in-situ, rather like the Rats Nest 2 on the Tube Imp website.

The device itself is fed by a 12V wall-wart, which mystifies me. Why drop the volatge to 12 outside the red box, then invert it back up to over 200V inside the box to feed the plate supply? Wouldn't it be cheaper to rectify the plate supply straight from the mains with a chunky bridge? Perhaps regulation was more effective this way. The HT (high tension to plate or anode) is current regulated and limited to 12mA. When this current draw is exceeded a red LED lights. Sadly I found this current just too low for some valves when I wanted to test them under actual operating conditions, only managing to get 100V on the plate when I wanted to measure at 200V. Pin 9 of the valve base is unconnected so users have to select 12.6V or 6.3V for the heaters accross 4 and 5. Soft starting with 6.3V is always a good idea.

Testing Two Bottles

"What do we do with all this data?", demand plebs

First you need a very basic understanging of what each measurement means, and this link leads to some concise definitions. Then you need to know how the valve in your hand should measure. My favourite source of valve data is Duncan amps pages which provides links to numerous sources of data


As I said in the introduction: this is a no brainer, if you own components that feature only B9a double triodes like the ubiquitous ECC83 (aka 12AX7) and ECC88 (aka 12AT7 & 6922) you should buy one of these. If you own a $4000&eur; pre-amp you probably connected it to your power-amp with at least a $400 cable pair. Before you ever consider spending that again on next months flava cables buy a Tube Imp to keep your pre-amp in tune. If your power-amp features B9a double triodes as input valves, phase-splitters or drivers a Tube Imp will be worthwhile just to make sure those phase-splitters are spot on to minimise one of the nastiest distortions.

Then you can also use your Tube Imp to help you safely begin the noble art of tube rolling. It won't tell you why overpriced Western Electrics really are worth 4 times the price of Chinese rivals (different bases again) but it will enable you to keep all those miniature double triode components singing. Now you know what to spend your Christmas money on.

Music enjoyed in between tests

  • Alice Cooper: Live 2007 tour
  • Derby Concert Orchestra: Live in Derby Cathedral

© Copyright 2008 Mark Wheeler - www.tnt-audio.com

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