Drake L4-B Resto

Discussion in 'Amateur Radio Amplifiers' started by WB1E, Sep 27, 2017.

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  1. AA2AS

    AA2AS Premium Subscriber QRZ Page

    L4B: I have the grids grounded. So the grid bias resistors (a.k.a "toaster ovens") in the PS are gone. Out! (what the heck was Drake thinking when he did that?) as well as the L/C strange network on the grids (???); added a resistor, keyed by the relay to cut off stby current. Since all that is gone (a) the amp standby current is now very very low; (b) no more heat from the PS and (c) you will need less power form the exciter to drive the amp. But the interesting point is that since that crazy circuitry is all gone, the ALC pot is now free. SO... time permitting I want to add a small negative PS inside the rf deck and use it to control the exciter power via that (now) unused pot ( DC offset fed back to the the ALC radio's jack) I drive the amp with a FT1000MP which requires 0 to -4V for power control. I am hopping to control the output of the linear via that pot. Doin it in the 1000MP was a PITA to adjust power because of the size and location of the RF out pot in the radio. Is anyone out there doing something similar?
  2. WA2LXB

    WA2LXB XML Subscriber QRZ Page

    Thanks Alberto. If I have enough room I'm going to mount it on the outside of my Square D 30A fused disconnect that I have under my desk for the L-4B. That way if I want to re-orient the power supply some day I won't have to worry that the display is oriented away from view. Great idea for moving the decimal and improving significant figures on the display!

    I've heard of the mods to the PS that you mention above. I'm not quite ready to ground the grids...I need to do a little more reading first. I agree...as it sits right now it is a toaster oven!
    Last edited: Mar 29, 2018
  3. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    That's your "winter" amp! :p
    WA2LXB likes this.
  4. W1QJ

    W1QJ Ham Member QRZ Page

    Couple of things to point out and clarify. Just so all L4B owners are clear. Those "toaster" oven 50K resistors serve 3 purposes in the L4b and they cannot be arbitrarily removed. The 3 purposes they serve are 1. Fast bleed down of HV after amp is turned off. The 100K resistors across the caps will serve as bleeders if they are removed however. 2. Together with another 7 watt resistor on the bottom of the PS the 50K resistors form a voltage divider where the voltage is derived to cut the tubes off in stand by. If the resistors are removed you will have to rewire the cut off bias supply so that the amp will "self bias". Simple change but absolutely necessary. 3. The same voltage divider is used as a source for ALC. Once again an ALC voltage supply will need to be added or not if you will not be using ALC anyway. Removing these heat generating resistors are a good idea but that has no effect on the directly grounding the grids. Just be advised that by removing those resistors the PS will no longer be "universal" and will NOT be able to be used on another L4b that has not been modified to accept that power supply.
    W0BTU and WA2LXB like this.
  5. WA2LXB

    WA2LXB XML Subscriber QRZ Page

    Thanks Lou. I'll leave my L-4PS bone stock...I don't mind cracking a window in January. All of the resistors including the glitch have been replaced with identical units, except the 7 watt was replaced with a 10 watt.
  6. AA2AS

    AA2AS Premium Subscriber QRZ Page

    Lou, you are absolutely right. I should have been more clear. Used a position of the relay to switch in/out a power resistor for self bias. Experimented with a string of diodes as well and opted for the resistor (I think 20K - value not super critical). The PS was retrofitted with a harbach (with bleeding resistors). In my case marrying the PS to the rf deck well justified the changes. It's a day and night difference (for me) operating the rig. As for controlling the power of the transceiver via alc - I internally mounted a small meanwell irm-03-5 module where the alc pot now acts as a voltage divider for the 0 to -4v. It works like a champ. The rf out pot in the ft series is really difficult to reach, and I now have an use for the pot in the front of the l4b. The ALC time constant and values of the l4b are not near the requirements of today's solid state radios. The fact that drake also derived the +120V off the HV made me always nervous if R11 (the 7w bottom resistor) fails and opens up. Lastly I need to do some testing to confirm the theory but I came across several forums where there are claims that the lc network used in the 3-500z grids increases the IMD figure. By a lot. But I have not seen real numbers so I am not sure that is true.
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Oh I agree. any of the those 3 resistors, either 50K or R11 underneath opened up and the tubes would just sit there and roast. I never left my L4b unattended for that reason. As far as the "floating grids" well that is probably #2 most popular debate after the Nichrome wire debate. Since Rich Measures went SK a few months ago that issue has been a bit silent but not before it caused a major disruption among a few fellow hams. It has been noted that the AG6K web page will remain on the Internet in spite of his passing. The issue of "floating grids" still gets hashed out among a few and like the Nichrome issue there are two camps. When I get asked about it I take a neutral stand, I report both sides of the issue and I let the customer make the final decision. I tell them what I know from both stand points and let them decide. as for myself and my personal equipment I tend to go with directly grounding. I find either way acceptable. As the debate goes there are pros and cons on both sides. I have never gone into any in depth experimentation other than what is noticeable without valuable equipment assessment. It was interesting to see the development of the Swan Mark II in particular. Swan came out with the Mark II after the Mark I. The Mark II (version 1) was basically a Mark I with a separate PS with a bit higher plate voltage and a bit heavier transformer. The RF deck design was similar to the Mark I with directly grounded grids. Then came the Swan Mark II (version 2) which had some RF deck changes and showed up with "floating grids", no longer directly grounded like version 1. The cut off bias was a bit strange and the way they "floated " the grids with a DC return was unusual and unlike any other "floated grid" design. Cut off bias was applied to the grids and therefore directly grounding them requires a bit more work to restore cut off bias. Not as simple as just removing parts and grounding like in a Drake or Heathkit SB-220. Floating grids was picked up by several companies and only a couple resisted jumping on that band wagon. Amp Supply and Dentron were among those who I believed raised a middle finger to that idea.

    I recently did a Swan Mark II conversion to 6 meters and had to ride through all the necessary changes to directly ground the grids. Basically restoring it to a version 1 style. The power supply got a full overhaul in the process.
    KD2ACO likes this.
  8. AA2AS

    AA2AS Premium Subscriber QRZ Page

    my fear with the divider going belly up is not the tubes - that's the easy part to fix (with money). My fear is that there is a bunch of people using the alc feedback connected to their SS radios. When and if the divider fails the +120v line now may become 3+ KV going back to the radio and possibly frying everything on its way - alc potentiometer (poor radio...) as well as anything touching it (including hands). So, as much as I love Drake I always though that was kind of negligent. At least they should have used a 150-200 volts crowbar circuit to blow the +B fuse!

    Loved your "nichorme debate" quote though. Anyway and just to close the subject I am posting an excellent link to Eimac's 1973 design note. Of special interest are: page 8 - Eimac's recommended cathode driven amp. Note the grounded grids and RY1/10K 10W circuitry - the pnit you made. We agree on that. But the most interesting is page 6. If you look at the interpolated grid current lines - look at the one corresponding to a near zero grid voltage - perfectly linear! and a mere 10 milliamps grid current in stand by. THAT sold me on grounding the grids. I was a bit skeptical at first - after all, those charts are under lab conditions - but once I made the conversion... sure enough the grid current meter of the deck, on standby is near zero! From that point on - every piece of gear I get with 3-500z's get their grids grounded! but I agree with your approach - give the facts, let the end user decide.

    WB1E likes this.
  9. AA2AS

    AA2AS Premium Subscriber QRZ Page

    by the way on the same graph (page 6) linearity is maintained up to 26 Volts. The net is full of debates also in using diodes in series versus resistors. I did try a string of diodes and (as I said) did not see a single advantage. The opposite all you accomplish is increase the grid current. S I am sold on using resistors.
  10. WB1E

    WB1E Ham Member QRZ Page

    Good conversation Gents.

    Well today, I drove my beautiful L4B with my de-tuned Valiant II at a mere 45 watts (rms) input. 1200+ pep output (845 watts carrier (rms) woke up a few S meters around the state and my Murch turner produced some photons and ozone. So.....time to tackle the SA2060a's issues, like the roller inductor cleaning and whatnot. Having fun, that is all the matters. And YES my grids are grounded directly!

    Howdy KG2DT!


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