Working my way back in time - Drake TR-4

Discussion in '"Boat Anchor" & Classic Equipment' started by K0OKS, Feb 16, 2018.

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  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Have you ruled out one or more bad tubes and or something like a cracked parasitic suppressor or as you've said one or more out of tolerance screen resistors?

    The three final tubes are in parallel so if one goes South you'll only be able to draw 2/3 of the expected current which applies to bias current just as much as full output power current. Same thing if one of the parasitic suppressors is cracked at a solder joint or otherwise not supplying anode voltage. That one's a double whammy as you'll lose some of your current in all operating states but also fry the tube as the screen starts functioning as the anode.

    Screen resistors way out of value is a bit of a different beast but if they've drifted far enough it can impact your ability to draw expected current through one or more tubes.

    You can check for current balance or current at all through each of the three tubes independently by measuring the voltage across the cathode resistors one at a time. They may not be exactly equal due to variations between tubes, but assuming the resistors themselves measure equally the voltage drop across each when keyed in SSB mode should be close to equal with each tube carrying a third of the bias current. One of the cathode resistors is a bit tough to meter but not too bad if you turn the rig off, attach some clip leads to your voltmeter then turn the rig back on, warm it up and make the measurement.
  2. K0OKS

    K0OKS Premium Subscriber QRZ Page

    As far as tubes go, I don't have spares of many of them. I have (yet) no spares of the finals.

    The 68 Ohm screen resistors were between 67 Ohms and 74 Ohms I believe.

    I will give this a shot.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Checking the individual current draw through each tube may tell you what you need to know. But another approach is to remove all three tubes then plug each tube into the same socket in turn and check bias current for one tube at a time. You should be shooting for roughly 33mA of bias current with a single tube installed and they should all be similar. The rig won't be properly neutralized with only a single tube so do this test on a lower frequency band like 80m or 40m where oscillations aren't likely.
  4. K0OKS

    K0OKS Premium Subscriber QRZ Page

    K7TRF, another good trick. Thanks...

    Here are the numbers on the screen resistors when doing the bias procedure (sideband switch counter-clockwise)

    Back: 16.67 Ohms 0.35V drop = ~21 mA
    Mid: 16.90 Ohms 0.26V drop = ~15.4 mA
    Front: 16.01 Ohms 0.43V Drop = ~25.42 mA

    So definitely looks like an issue with the middle tube, if not the back as well.

    I checked all the plate chokes from the center. They are all less than 0.02 Ohms to the tube. I re-seated the final tubes as well. No change on the total output.

    It's OK to test this out with the unit on its side, right?
  5. K0OKS

    K0OKS Premium Subscriber QRZ Page

    NOTE: I have not neutralized this. Is there someplace a procedure listing for starting from scratch with a radio of unknown pedigree? The Drake service instructions seem to already assume most things are aligned correctly.

    Would also be nice if anyone had made a manual detailing aligning the TR-4 with modern equipment (frequency counter, scope, etc.)
  6. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Those were of course the current through the cathode resistors, not screen resistors. I wish QRZ let you edit old posts...
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yeah, no problem there.

    The neutralization instructions don't really care whether the rest of the rig is well aligned or not as long as you can generate a CW carrier and tune the finals to peak output.

    Poor neutralization won't lead to reduced bias current so I wouldn't chase that red herring right now. Sure if you actually had an oscillation then neutralization could increase idle current (because the finals wouldn't actually be quiescent) but it wouldn't lead to low idle current.

    Drake recommended the old, peak power with plate tuning matches minimum plate current method of checking bias. When you do get around to checking neutralization, especially if you replace one or more tubes, then I'd do this first on 20m where self oscillation risk is fairly low and then repeat it on 10 meters. IOW, rough set neutralization on 20m and then fine tune it on 10m. There are other, perhaps more precise, methods where you disconnect the screen voltage at the RF cage feed thru and measure for minimum RF output with a scope, RF millivoltmeter or test receiver.

    All of those are low, but yeah tube 2 looks like it's in trouble. You should be seeing ~500 mV across each 15 ohm resistor (if it is 15 ohms). Your resistors seem to measure a bit high but some ohm meters struggle with lead resistance when measuring low resistance so I'd verify with some known good low value resistors or at least zero out lead resistance before rushing in to swap the cathode resistors but if those measurements are accurate at least two have drifted up more than 10% which isn't terrible but you might think about replacing them. They're not far enough off to be the root cause of your bias adjustment troubles but it can't help either.

    You might try swapping tube positions or doing the one at a time tube test to see if the current issues track the tube or the tube socket (and related resistors, connections, etc.). IOW, swap each tube into what looks like your best result above (Front socket) and see how they compare.

    I have to head off to work, but if you want I'll take a few readings this evening to let you know what I see in terms of bias voltage vs idle current at various points in my working TR4-CW.
  8. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Cool. Thanks for the help.

    Understood on the neutralization, and why I had not attempted it yet. I didn't think it had anything to do with this.

    The resistors values measured are accurate. I did them with Kelvin clips :) The plan was to replace them, but I wanted to figure out the issues first.

    I was going to play musical tubes as, like you say, a way to make sure the tubes are the issue rather than the socket or supporting circuittry.

    Last night (it was late) I did try one tube at a time. The sideband indicator lamps no longer lit, and I got no receive. I assume this is a filament voltage thing, but maybe I don't fully understand the filament schematic yet. It looked like all three filaments are in parallel and that a single tube should work. I will have to take another look. I know with a single tube the filament was glowing much brighter. I did not check the voltage, but I didn't leave it on for long.
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yeah, the final tube filaments are wired in parallel but the set of three are in series with the 6v tubes. Apparently a single tube filament has too much resistance and drops the filament voltage on the other tubes too far. This shouldn’t impact the bias current testing but I wouldn’t run it that way too long.
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    PS. Best bet if you’ll test for extended periods would just be rotating the tubes so all three are installed but in different sockets. Then repeat the cathode current measurements to see if they track the tubes or the sockets.
    K0OKS likes this.

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