Steve Bench Tester

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Steve Bench Tester

Postby msmpe » Sun Aug 09, 2009 3:44 pm

Hi All:

Has anyone built Steve Bench's RAT tube tester? It looks to be a nice unit: VR supplies, uses DMM instead of an analog meter (but you could change to a digital meter), set up for audio, uses 1k Hz signal on the grid, tests close to tube book limits, simple PSU, simple controls, easy to calibrate, only half a dozen SS parts plus diodes & zeners.

I'm working on a Mouser parts list. It needs a PCB to make life easy, so, that will be my next "design" project, the PCB. Any tips on layout? Who is the PCB manufacturer that has the software download and has reasonable cost for low quantity runs?
8>) Mike

If there's no sound in a vacuum, where'd the music come from?
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SB tube yester

Postby msmpe » Tue Aug 18, 2009 12:06 pm

As I worked on it some more (it gets simpler, so it seems :$ ), I'm going to bread board it first. The circuit breaks down into 6 fairly distinct sub circuits: PSU tripler & filter, CCS reference, plate VR, screen VR, bias supply, and grid signal, plus the switches and socket connections. So I'll use 3 or 4 perf boards, 3 each with one of the heat sinks.

I have some plexi-glas channels I'll use for the chassis: one down for the perf boards and socket, and one fixed upright for the pots, switches, test jacks. The power supply will be separate. I'm going to leave room on the upright channel for a digital readout (if I can find a good one for less than a Franklin $) ).

The bread board version will only use one octal socket (with terminal strip connections) for testing the 6L6 family of power tubes to prove the circuit. Triode testing can be added later simply enough: Mu switch, plate 1/2 switch, and revise socket connections.

If all goes well :)) it will be time for the PCB, a permanent enclosure, and expanded sockets. In the future, I'd like to put together a life test. Also a gas test would be a nice addition to my shorts/emmissions tester.

Should be a fun project. If your interested, stay tuned, although, like all my projects, it'll take awhile... :/
8>) Mike

If there's no sound in a vacuum, where'd the music come from?
msmpe
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Postby TomMcNally » Tue Aug 18, 2009 12:24 pm

Hi Mike -

What kind of scale would a digital readout need to work with that ?

I have some panel mount LED meter assemblies - I'll see what
scales they have ... maybe you could use one.

... tom
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SB tube tester

Postby msmpe » Tue Aug 18, 2009 12:46 pm

Hi Tom -

The origianl circuit uses a DMM measuring VAC across a 100 ohm resistor. Range is from 1mV to 100mV for Gm and 100mV to 10V for Mu. I was looking at a 200mV FS 3-1/2 digit LCD BY Murata for Gm. Mu would require switching in a voltage divider, maybe with a range switch.

Thanks by the way for the lead to the article on meter shunts, just what I needed... ;)
8>) Mike

If there's no sound in a vacuum, where'd the music come from?
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Postby TomMcNally » Tue Aug 18, 2009 1:35 pm

I'm pretty sure the meters I have are this one by Datel -
here is a link to a PDF data sheet ...

http://www.datelmeters.com/data/meters/dm3100b.pdf

this link is to a PDF for their range modifications ... apparently you
can do almost anything with this meter ....

http://www.datelmeters.com/data/meters/rn31-41.pdf

I have a few of these - if you get serious about the project and
want one, let me know ... I'll swap you one for something ...

... tom
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SB tube tester

Postby msmpe » Tue Aug 18, 2009 3:29 pm

Hi tom,

That's very similar to the one I was looking at, also a Datel -Murata. Without studying it, it looks like it would work.

Now, what have I got to swap?? (???)
8>) Mike

If there's no sound in a vacuum, where'd the music come from?
msmpe
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Postby TomMcNally » Mon Aug 24, 2009 5:07 pm

I finally found one of the Datel meters, it is indeed a DM-3100B.
If you want it for your project, let me know. Somewhere
I have more of them, where is the question!
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SBTT

Postby msmpe » Tue Sep 01, 2009 11:38 pm

Thanks to Tom, the tester will have a digital meter for Gm read out (y) :))
I'll post a picture of the bare chassis when it comes together...
8>) Mike

If there's no sound in a vacuum, where'd the music come from?
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Postby msmpe » Sun Nov 01, 2009 11:14 pm

Here’s an update about my Steve Bench tube tester project. I finally ordered the parts I needed. I had all the PNs from Mouser for the SS stuff and was working on the resistors, when a friend gave me not only a bunch of used tubes (to test of course!) but a bunch of NOS carbon resistors. I would never use these for an amplifier, but seemed perfect for test gear. Good, saved me a couple bucks, but I had to sort through them all and organize them, so that ate up some time.

Then, many thanks to Tom McNally, I got a digital meter for the Gm read out. In the original SB circuit, Gm (and Mu) is read on an external DVM by measuring the drop across a 100 ohm resistor (R7). I’m installing a couple jacks for the DVM to verify, but the digi-meter is way cool! Now, one of these babies is not “plug-n-play.” This meter is set from the factory for + 1.999 Vdc. However, the SBTT is looking for a range of 10 to 130mVac across R7. The meter circuit can be changed internally by changing component values, but for the SBTT range, this involved some resistors, caps, and the calibration (I think) pot. An involved process fraught with risk!

The meter spec sheet also showed that an op-amp could be used to provide gain to the low level input. Seems simple enough, and it is, if you know how op-amps work! So I pulled out my old digital electronics book and restudied op-amps…

The first step was to determine the gain required. For low Gm signal tubes like the 12AX7, Gm is around 1,250 uMhos which should be a Vdrop across R7 of 12.5mV. At the other extreme, a high Gm power tube like the 6550/KT88 should exhibit a Gm around 11,500 uMhos which should be a Vdrop across R7 of 115mV. For the 12AX7 we want the meter to read 01.25 and for the 6550, 11.50. The gain factor for the op-amp is then Vout/Vdrop = 10. Sound right?

Gain for the op-amp is calculated: m=1+Rf/Rg where m is the gain factor, Rf is the feedback resistor, and Rg is the resistor to ground. Rf/Rg = 9, so any combination of resistors such as 18K and 2K will provide the correct gain. I decided that R7, Rf, and Rg should be precision resistors, that is .1%. Well for some reason, precision resistors come in odd values (E96?). Here’s what I ended up with: Rf = 8.87K and Rga = 976 and Rgb = 10, where Rga, b are in series. The error is .05% which is less than the .1% of the resistors themselves.

This digi-meter measures VDC, for the SBTT we need to measure VAC. My question: is all I need to do is put a cap in series with the meter to block the DC? or is it more involved?

Steve’s circuit can be broken down into about 7 "modules". The PSU uses a tripling rectifier off a pair of back to back 12.6vct trannys and the filter caps. My trannys will be external to the breadboard chassis.

Module #2 is a reference CCS with Q1 and a Zener string. I’m going to put the CCS with the small filter caps on one board.

The next module is the half wave negative cathode bias with Q6, VR1 (pot), and Zener string. Q6 is on a heat-sink. This will go on a board with another module that makes up the signal oscillator which uses a hex inverter IC and VR4 (trimmer).

The last perf board will carry both the plate and screen voltage control/regulator modules. The plate control consists of Q3, Q4, VR3, and R7. There is also a Gm/Mu switch which I am not including in the perf board set-up just yet. The screen control consists of Q5, Q2, and a triode/pentode mode switch. Q4 and Q5 are on heatsinks. If there’s room I’ll put the op-amp on this board. The digi-meter supplies the +5V for the op-amp bias.

The final module is the gang of sockets. I am going to only use one 9- and one 8-pin socket, parallel wired to a 9-terminal block and then manually switch leads from the modules to connect the pins appropriately. You could use the sockets and switches from a junked emission tester instead or use a series of sockets ala the SB circuit.

I have a layout done in AutoCad. I will post a picture when it is assembled (if I can figure out to include a jpeg). I’m using Plexiglas for the chassis with the boards mounted on standoffs above the chassis. Certainly not UL, but I’ll shrink tube all the leads, which will only leave the terminal block screws exposed.

I ordered my parts from Mouser for no reason other than I’ve never ordered from them before. Their project BOM capability is pretty cool, but you have to watch how you add to the project BOM, otherwise any added parts get mixed up with your cart and added to the project BOM twice! Should be simpler than that! Also they billed for sales tax...

Here is a list of the “critical” parts with Mouser PNs [formatting is description, symbol, qty, manfr, PN]:

74HC04 (not HCT) U1 1 ON 863-MC74HC04ANG
14 pin DIP connector for U1 DIP14 1 Tyco 571-1825093-3
MPSA92 350v PNP TO-92 Q1 1 ON 863-MPSA92G
MPSA42 350v NPN TO-92 Q2, Q3 2 ON 863-MPSA42RLRAG
IRF820 TO220 VFET Q4, Q5, Q6 3 Vishay 844-IRF820PBF
Heat sinks for the FETs HTSNK 3 Wakefield 567-690-3B
51V 5% .5 watt zeners CR9-14 6 Vishay 78-1N5262B
5.1V .5 watt 5% zener CR16 1 Vishay 78-1N5231B
10V 1W 5% zener 1N4740 CR17 1 Vishay 78-1N4740A
741 Opamp for meter U2 1 Texas Instruments 595-MC33078P
Dip connector for IC U2 - 8 pin DIP8 1 Tyco 571-1825093-2
100 ohm 1/4w .1% R7 1 Vishay 71-PTF65100R00BYBF

The total cost is just over $13. The biggest expense are the heat sinks, fortunately these can be recycled. The rest of the parts you can scrounge from the “JUNK” box, although you’ll also need a recycled PST, diodes, pots, sockets, jacks, etc.

Your comments are welcome.
8>) Mike

If there's no sound in a vacuum, where'd the music come from?
msmpe
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SB TT Update #2

Postby msmpe » Tue Dec 22, 2009 4:30 pm

Hi All:
Moving forward with some actual fabrication, I got the AC power supply built. I used a pair of RS 12.6-0-12.6 trannys back to back, which net 115VAC from my 119V line (typical evening line voltage at my house), which is better than 98% efficiency. I used these units so I could tap the 25.2 voltage for the variable regulated filament supply to test some 25L6s I have. I’m using an LM317T from the junk box which is different from Steve Bench’s filament supply. Still putting that together, so I don’t know if it works, we’ll see.

I also finished the tripling rectifier and power supply filter. I built that per Steve’s plans. One thing I learned is that for a tube tester you want to minimize ripple, as that fraction of AC on top of the DC supply could impact the accuracy of the Gm readings. So don’t skimp on the uF, and that’s why the filament supply is FW rectified DC, a honker filter cap and regulated. I mounted the PS rectifier and filter circuit on the board with the trannys. The filament supply will be mounted there too. I am also considering putting the plate VR on this board as well. That way I will have a separate bench top variable regulated supply capable of 350V @ 50mA (good for testing VA/driver/PI circuits). Once the filament supply is built I can test the PSU under a load. I have some large power resistors, but do I have a 3.5K @ 35W (min)? The 25L6s draw .3A, so the filament supply load will need to be 82R @ 7.5W.

I also need to “cover” the exposed HT wiring. If I don’t fill the shop with smoke, it’s on to the electronics…
8>) Mike

If there's no sound in a vacuum, where'd the music come from?
msmpe
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SBTT Update #3

Postby msmpe » Sun Apr 11, 2010 12:11 am

Here’s an update on my Steve Bench tube tester project. I knew this project would take a while, because I have a tendency to “flit” from one project to another, so finishing projects is not timely. Since my last update, I did finish the variable filament supply, and it is mounted with the power supply. But, I got sidetracked in two directions - more lab equipment and FM tuners!

I recently fixed my wife’s Motorola FM stereo table radio (that project took over 6 months) and was I surprised at how good it sounded and picked up stations. Now I’m hooked on listening to a great local jazz station KRML 94.7 [remember ‘Play Misty for Me”? Clint Eastwood’s studio in the movie is KRML] – they also stream on the web at www.krmlradio.com, give’ em a try if you like mainstream jazz.

I bought another Motorola to learn on. This one is an AM-FM mono unit out of a console - five tubes for FM, plus tuning eye and 6X4 rectifier. Well, it plays PDG. Experimented with antennas, and did some A-B comparisons. My Sony HT doesn’t get half the stations this old tube tuner does! Next step – I bought a Fisher FM-80 to restore. This is also mono but 9 tubes so it should pick-up stations real well. It can wait though.

Of course to work on tuners you need a signal generator. So at the last e-flea market I picked up a $20 Eico 324 that looked to be in good condition. Also took a $10 chance on a Goldstar 7020 dual trace 20mHz o’scope with a broken handle. THEY BOTH WORK! So now I’m building patch cords for both, and piecing together a simple SS audio generator. With all this equipment, I had to build a shelf to it put all on – an audio lab is a-comin’.

I also took some time to rebuild my old ESS speakers – new 8” Vifa woofers, crossover caps, and added some foam along with new stuffing. They sound better than when new, at least I’d like to think so… Now all I need is a tube amp with enough power to drive them!

Which is why I have made little progress on the SBTT – flit, flit… Update #4 in a couple months - something a little more substantial I'm sure.
8>) Mike

If there's no sound in a vacuum, where'd the music come from?
msmpe
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