Thanks. Yes, it is series, of course. But I am not using those in the tube tester. As you point out, it doesn't need that kind of filtering. I am just putting a couple old oil caps on it totaling maybe 4uf.
All this high voltage stuff scares me and I try to be overly-cautious. I have a chicken stick and I use it. My primary motivation is not fear of death. It's to not give my wife the satisfaction of seeing my hobby kill me, as in "I told you this would happen..." 73, Don Merz, N3RHT -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Jim candela Sent: Tuesday, October 19, 2004 9:08 PM To: Discussion of AM Radio Subject: RE: [AMRadio] UPDATE: Testing Transmitting Tubes Don, You said, "paralleled 800MFD, 450VDC caps". I think you mean series. I wonder how clean the plate voltage needs to be for a tube tester? If you use the mega-capacitor bank, consider a suitable series resistor to limit the peak current in the event of a short circuit to something manageable for the components you use. Does each capacitor come with a shunting equalizing resistor? I would make sure your bleed down time is under 1 minute or so. Big capacitor banks scare the hell out of me... I once took a 1000uf 450 vdc capacitor charged at about 300 volts arm-to-arm. That was my 2nd worst shock, and this one knocked me out for a while. Below I am pasting something from an old post that is relevant towards your tube tester: Wow! I have been inundated with stuff from higher minds than mine! Besides your excellent replies, I got a somewhat similar, yet different perspective from Patrick Turner from the R.A.T. group, (newsgroup: rec.audio.tubes) that I paste below. The guy wants me to calculate Mu, and plate resistance as well. After all, Gm = U / Ra. Patrick's reply: > What you are doing will broaden your ideas about tubes, > but why concentrate on the Gm? > > If I want to find out how to set up a tube I find out what > all three important parameters are, the Ra, U, and Gm. > > You can set up a tube with a choke load, and strap various values > of RL across the choke. > > Then after the bias is set apply a fixed signal input voltage to get about > half the maximum anode voltage swing before clipping, so thd is negligible. > Then measure the voltage gain, and do so for at least two different RLs, > say 5k, and say 10k. > > Voltage Gain, A, = U x RL / ( RL + Ra ), for all tubes. > > So since we know what A is, and RL is, we have two equations for the > two different loads, and from these we can work out > Ra, U and thus Gm, because for all tubes, > Gm = U / Ra. > > The most constant parameter for all tubes is the U, > because its determined mainly by electrode distances, > but Ra and Gm vary widely. > > The other way to measure Gm is to simply have no choke load, > or large value RL, and just have a 10 ohm R to the B+, > and measure the undistorted current change with grid voltage changes. > This don't tell you about Ra or U, which change dramatically > when G2 is connected to the anode. > > The first method of mine involves algebra. > But from one of with two equations, U can be expressed as a factor x Ra, > and this can be substituted where U appears in the second equation so that > this then has only numbers and Ra present, so Ra can be found, and its value > inserted into either equation, and the value of U found, > and thus Gm. > > The idle conditions for the tubes can be changed, and the 3 > parameters will change. > > The tests and measurements can be done using a class AB PP pair, > which will indicate what the 3 design values are when combined. > In AB circuits, the Gm usually drops a bit after one tube cuts off, > resulting in a flattening of the amplified sign wave, > which is compression, or lot of 3H present. > But when biased to near class B, the crossover distortion indicates > that Gm is initially low, then becomes greater as more tube current flows, > and the phase of the resulting 3H is opposite that of compression. > Some tubes, like the 6DQ6, display increasing Gm > even in class AB circuits at the extreme of the amplitude, > so that the wave peaks up before clipping, and again the 3H > is opposite phase to a compressed wave, so Gm is increasing lots, > even up to just before clipping. > > In triode signal tubes, such as 12AT7, 6DJ8, > quite large variations in Gm and Ra occur for different > idle currents, and some have deliberately engineered > variable parameters by means of variably spaced grid wires. > Many pentodes have also thus been made to have variable U and Gm, > to make the gain vary a heck of a lot, which is handy for > gain control in radio receivers, or to provide a more > constant sound level from microphones, regardless of the distance away from > the person using it becomes. > > Once you have established that a given load is ideal for > a pair of say 6L6GC tubes, then quite a few others might be substituted. > Speaker loads vary between say 4 and 40 ohms, so absolutely strict > load matches are impossible. > It pays to err on the conservative side, IMHO, > ie, allow for a high value RL than a low one, > as the best fidelity is then possible, and tube substitutions > are more successful. > > Patrick Turner. Regards, Jim -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Merz Donald S Sent: Tuesday, October 19, 2004 11:49 AM To: 'Amradio (E-mail); 'Glowbugs (E-mail) Subject: [AMRadio] UPDATE: Testing Transmitting Tubes Just thought I'd throw out an update on this. I am building an emission tester for older, mostly 800-series transmitting triodes. It will be able to measure emission current at zero bias or adjustable bias (using an external bias supply) at any plate voltage up to about 2500V. The HV is full-wave, lightly-filtered DC. Filament voltage is adjustable 0-10VAC. I have an old metered, rackmount regulated supply that uses dual 6L6's for regulation. It will do 100V--so he becomes the bias supply. I gutted an old Beckman, tube-type frequency counter to use the cabinet and chassis. I just happened to have this unit laying around here. It has a unique cabinet design with sides that come off and modular chassis construction--even though, underneath the covers, it's a conventional 8.75 inch high, 19" rackmount unit. This design allowed me to remove sections of the chassis to accommodate the 2 big Variacs and the big HV iron. The top of the cabinet pops off with just 2 Dzus fasteners. I am going to put holes in the top with the tube sockets underneath, mounted on an inverted chassis bolted to the top cover. This way the whole unit won't have any protrusions or non-internal wiring and none of the tube socket connections will be exposed or accidentally accessible. I drilled and painted the panel Sunday. I then immediately discovered that the 0-300ma meter I was using did not have enough range at upwards of 2500V. I needed at least 6-800ma. So I replaced that meter with a 0-1000ma Simpson. But of course, the hole size is different. So now I have to cut a larger meter hole on my already-painted aluminum front panel. I took comfort from the fact that the black wrinkle didn't turn out that great anyway... Most of the parts for this thing came from a supply I bought at a hamfest in 2003. For $40, it had a 7.5 amp 120V Variac, 2700V transformer (amps rating unknown but probably in the 500ma range), 4 K2AN silicon HV rectifiers and a bodacious capacitor bank of paralleled 800MFD, 450VDC caps. The meters I am using are from the junkbox (note to self: Don't buy any more meters at hamfests...), as are the sockets and miscellany. But anyone seeking to build one of these with new parts would have a big bill on their hands. Check the prices on new 7.5 amp Variacs these days... I expect to be able to test to following tubes in this unit: 203, 211, 805, 808, 810, 838, 852, 25T(G), 35T(G), 100TH, 250TH and RK-65. The test is far from definitive. But it will tell me whether the spares I am saving are worth saving. The top cover is big enough that I will still have some space for tetrode and pentode sockets. Screen and suppressor supplies would have to be external. But that is phase II--or something I may never do at all. 73, Don Merz, N3RHT -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Merz Donald S Sent: Tuesday, September 21, 2004 3:27 PM To: 'Amradio (E-mail); 'Glowbugs (E-mail) Subject: GB> UPDATE: Testing Transmitting Tubes Well, here's where I am with this... -- The guy who built the tester that AES is using says that he has some information on that unit that he will pass along. -- Another guy says that he has built an adapter for the Tektronix 577 curve tracer for transmitting tubes. He says that he will write this up for Electric Radio. -- Two guys said that they built custom testers for the 4CX250 tube. One of them was expanded to test other tubes and was demo'ed at Dayton. The other was used in the guy's job. No details seem to be available for either of these. -- I checked the AWA index and found only one reference to testing transmitting tubes. It's a 50 word mention on page 21 of Volume 22, number 1. There is one diagram. The test the guy is proposing is a minimal emissions test using a known good tube as a reference. He does this for triodes only using AC both on the filament and on the plate & grid (which are connected together). It has the virtue of being simple. But it hasn't many other virtues. -- Finally, there is the e-mail below from the AWA's Ed Gable. This seems to be closest to the lines along which I am thinking--a scaled down emission tester. The GE Ham News Hartley oscillator tester is more complicated than I want to get into. And the Ham News static tester is very similar to the AWA tester described below. So this is where I am headed, though still open to better ideas. Keeping it simple, so triodes only at this point. 73, Don Merz, N3RHT -----Original Message----- From: Edward Gable [mailto:[EMAIL PROTECTED] Sent: Monday, September 20, 2004 2:11 PM To: Merz Donald S Subject: Re: Testing Transmitting Tubes Hi Don: We built a tube tester for Big TX tubes at the AWA Museum. 1. Use big filament Xfmr and variac to accomodate any FIL voltage. Use FIL voltmeter. 2. Use variace on HV supply for 0 to about 1500 volts. Need about 300 Ma, but not too much filtering. 3. Put voltmeter and ammeter in HV supply. 4. Parallel wire a bunch of convenient tube sockets and allow for clip leads for socket-less tubes like 833. 5. To test, get tube data for tube. Look at chart and see what the tube current should be for a tube with zero bias. For example, a type 810 might draw 200 Ma @ 1500 volts with zero bias. 6. Connect the grid to the filament (zero bias condition) 7. Turn the FIL on and set proper voltage for the tube under test. 8. Turn up the HV variac and watch the current & voltmeters. See if the current vs voltage is close to the spec. It varies a lot from tube to tube, but low emission, non-useable tubes really show up this way. SAFETY FIRST !!! 73, Ed Gable k2mp Curator, AWA Museum ----- Original Message ----- From: "Merz Donald S" <[EMAIL PROTECTED]> To: "'Amradio (E-mail)" <[email protected]>; "'Baswaplist' (E-mail)" <[EMAIL PROTECTED]>; "'Glowbugs (E-mail)" <[EMAIL PROTECTED]> Sent: Monday, September 20, 2004 10:19 AM Subject: Testing Transmitting Tubes > Has anyone built a transmitting tube tester? Or seen any plans on the web? The only thing I have seen on this subject is the issue of GE Ham News that covers the subject (Vol 6, #3, May/June, 1951). I have some power supplies I can use to run the static test. But I need to build something to hold the tube sockets, filament transformers and plate current meter. I hate to reinvent the wheel and would be happy to follow a pre-existing design if I could find one... > > Any ideas? > > 73, Don Merz, N3RHT > > The information contained in this e-mail may be confidential and is intended solely for the use of the named addressee. > Access, copying or re-use of the e-mail or any information contained therein by any other person is not authorized. > If you are not the intended recipient please notify us immediately by returning the e-mail to the originator.(A) > The information contained in this e-mail may be confidential and is intended solely for the use of the named addressee. Access, copying or re-use of the e-mail or any information contained therein by any other person is not authorized. If you are not the intended recipient please notify us immediately by returning the e-mail to the originator.(A) The information contained in this e-mail may be confidential and is intended solely for the use of the named addressee. 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