DX-100/Apache Audio/Modulator Improvements

Discussion in 'Amplitude Modulation' started by AC0OB, Jul 19, 2019.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    I almost never use cathode bypass caps.
    Tim (HLR) told me he uses the value of the 1st stage bypass cap to give a presance rise, smart guy Tim is...
  2. AC0OB

    AC0OB Subscriber QRZ Page

    The Radiotron Designer's Handbook, by F. Langford-Smith, 1953, Chapters 3 and Part II ch. 7, discusses component selection for tube Audio Amplifiers and the effects of Feedback, respectively.

    So Tim wasn't the first to use these ideas.

    BTW, the whole book is now downloadable and an excellent read.


    Last edited: Jul 20, 2019
    N6YW likes this.
  3. K4KYV

    K4KYV Subscriber QRZ Page

    Not just the first stage, this can be used with several stages in tandem, each one contributing a few dB of presence rise.

    In a push-pull stage, the cathode bypass capacitor is superfluous if a common cathode resistor is used, since feedback voltages from each of the two tubes are 180° out of phase with each other and cancel. The cathode by-pass presence rise trick can be used in a push-pull stage by using a separate cathode resistor for each tube, each with its appropriate by-pass capacitor.

    If cathode bypass presence rise is used with multiple stages, the R-C time constant of cathode resistor-bypass capacitor for each tube should be the same. For example, if one stage uses a 1K resistor and 0.1 mfd capacitor, a 0.2 mfd capacitor should be used in a subsequent stage if the cathode resistor of that tube is 500Ω.
    W8KHK likes this.
  4. W7TFO

    W7TFO Premium Subscriber QRZ Page

    If using electrolytics in an audio path bugs you, try a big film cap like those from Solen. eBay lists lots of them, values up to 50uF @ 630V.

    I also use them as power supply filters when I never want to replace electros again.

    W8KHK, N6YW and KD2ACO like this.
  5. W8KHK

    W8KHK Ham Member QRZ Page

    Phil, thank you for sharing the information about Stu's simulation efforts. I would expect the voltage to divide evenly between two back-to-back electrolytic with a sine-wave AC source as he has demonstrated. But I question the results when the source is the DC plate voltage as in your example, obviously different than the AC simulation of AB2EZ.

    I would be interested in actually testing to see the DC potential across each capacitor in that application. Simulation may not provide a concrete answer, as all electrolytics have some leakage, none are perfect. When time permits, I will probably perform some measurements.

    I am still curious about the advantage, and how a pair of capacitors excel, compared to just a single electrolytic coupling the tube plate to the transformer. I do not wish to debate or contest this issue, I am simply interested in understanding the attributes, and risks, if there are any. I think your improvements to the audio in the Heath rig are excellent, and do not feel there is any need to come up with any alternative. Perhaps I should also ask if there is an advantage to just use a non-polarized capacitor, instead of a single electrolytic, in this application?

    Phil, I also find it very helpful that you define voltages and the currents through the various circuit paths on your diagrams. Thank you for any additional information that you would like to share regarding this specific section of the circuit.
  6. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    ... and the leakage isn't matched unless you're lucky. You may need very hi impedance probes to see what's really going on.

    I've often wondered how the electros find the proper polarization with asymmetrical signals and have had to repair failures with coupling caps in equipment that uses this technique. I've seen 1M 'helper' resistors pull the center of the two to a rail (up or down) to ensure that they both stay polarized.

    In situations where distortion performance is crucial or you never want to change the caps again, Solen caps have about the highest volumetric efficiency of any film caps I've dealt with. They are cost effective and great sounding too (read, cheap-n-good). These 2 replaced tantalum caps coupling to the driver stages.

    Depending on the topology, one may need to cover them in grounded copper foil to keep the gremlins out of the circuit. Of course, they take up a lot more volume than the tantalums they replaced.


    ~clicken for biggun~
    Last edited: Jul 21, 2019
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  7. K4RT

    K4RT Ham Member QRZ Page


    Thanks for the informative post. Are you saying you replaced the 1625s in your acquaintance's DX-100's with 6CA7s, or just that you changed components ancillary to the 1625s?

    This information is timely for me as I have a DX-100B with an audio distortion issue only on 80 meters I need to resolve.

  8. AC0OB

    AC0OB Subscriber QRZ Page

    Hi Brad,

    We kept the 1625's (not my choice) but modified the audio driver circuit, regulated the mod screens, added variable mod bias, and added the soft knee limiter.

    I was simply comparing the drive requirements for 1625/807's with the 6CA7's.

    Loading of the Final RF stage can greatly affect modulation percentage and distortion in these vintage rigs.

    A good stiff SStated high, low, and bias supply also helps. The 6AL5 has been a failure point in both the HeathKit and EF Johnson rigs. Failure of the bias supply really puts a strin on the Clamp tube.

    Here is the complete mods with an optional active screen regulator, but was not implemented in this specific rig.

    BTW, the idea for the capacitor coupling between the plate load of the 12BY7A and the driver transformer, and the simulation of the back-to-back capacitor coupling, originated with Stu, AB2EZ, and is not my idea. I simply implemented it because it made sense.

    The Soft knee Limiter is an idea from Bonomo in ER and as above I simply implemented it because it made sense.


    Attached Files:

    Last edited: Jul 21, 2019
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  9. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Remember when "everyone" used to say how horrible ceramic capacitors were for audio coupling....they were microphonic, non-linear, etc. The only time microphonic would conceivably be a problem is if they were in the very first stage...such as in a high gain mike preamp. I have replaced COUNTLESS guitar amp coupling capacitors with large ceramics (because LEAKAGE has far more nasty effects in tube amps than just about anything else), and everyone LOVES the sound of these upgraded amps. And I do mean upgraded.

  10. AC0OB

    AC0OB Subscriber QRZ Page

    The editor caught me before I could change a typo so here is the corrected set of schematics.

    And A good stiff SStated high, low, and bias supply also helps. The 6AL5 has been a failure point in both the HeathKit and EF Johnson rigs. Failure of the bias supply really puts a strin on the Clamp tube.

    Should read, "A good stiff SStated high, low, and bias supply also helps. The 6AL5 has been a failure point in both the HeathKit and EF Johnson rigs. Failure of the bias supply really puts a strain on the Clamp tube, " but y'all knew that already. :)


    Attached Files:

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