HB 813 amplifier question

Discussion in 'Homebrew and Kit Projects' started by K0IZ, Apr 30, 2021.

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  1. K0IZ

    K0IZ Ham Member QRZ Page

    I'm beginning to design a grounded grid amp for 20M SSB using two 813's . Plate voltage about 2100 or so. I only have about 50 watts driving power, so was originally thinking about grid-drive with swamping resistor. Now leaning more towards GG.

    In researching older articles, most tetrode GG circuits have the control grid and screen grid grounded for both RF and DC. But a couple use grid bias and screen voltage comparable to that used with grid-driven amps (-90V and 750V for AB1). One article in QST (Sept 84), for example, shows a 6-meter GG with 4CX250 and normal bias and screen voltages. Author says he did this to greatly reduce the drive requirements (compared with normal GG).

    813 screen voltage is supposed to be 750V for AB1 operation. That's rather high and somewhat difficult to regulate. So I'm now wondering, what if I applied some lesser voltage, say 350V? Would the drive requirements then be somewhere between GG with grids grounded and GG with bias and full screen voltage"?

    Anyone have an comments?
  2. W9WQA

    W9WQA Ham Member QRZ Page

    you can keep it simple. ground all and see what you get,
    then decide whats next.
    you could wire it so you can get at the screens later.
    i run with all ground , no bias,3300v.
    idle current is 100 ma, a little high but nothing overheated.
    i may add bias. its an ongoing project, no rush...
    i rarely use 50 watt drive.

    btw,in an earlier amp i used a clamp tube to hold screens low. used some drive volts,neg, to let the screens go up with drive audio. made a little more power . fun to tinker with stuff...
    keep one hand in back pocket huh...

    i like 813. mine were cheap, have lots of spares
    Last edited: Apr 30, 2021
    SWL37632 likes this.
  3. K0IZ

    K0IZ Ham Member QRZ Page

    My plate voltage will be much lower, planning to use an Anteck 800VA toroid. Small size, reasonable price. With two 800V in series and bridge, 2240V no load. Looks like with grounded screen I will get about 10db gain. Figuring on 45 watt drive (50 minus a little loss), that's 450 out. If I do control grid drive with swamping resistor, I can get nearly double.
  4. W3SLK

    W3SLK Ham Member QRZ Page

    W9QWA said:
    And here I thought I cornered the market on 813's!:D Anyway, the 813 has to be the most indestructible tube ever built. Also the one where you get the most bang for your buck! Send me a PM, (my email is on my QRZ website), I will send you the email of Duane, KK4AM, (sans KN0R). He built one from scratch with 2X 813's. I thought he was running about 2500VDC. My notes, (and memory ) are very sketchy about it. I know he had to make several RFC's for the plate. One issue that came about was the development of spurs in the 144~147Mhz band. He used one of Rich Measure's, AG6K(SK), Parasitic kits and that eliminated that issue. On a side note, my good friend and elmer, Eric, WB4VVI(SK) used to run 4X813's at 4500VDC. I don't remember his power out but he did say that because you can run them safely at 2250 for AM, 2X will allow you to run them there. You just have to be very careful when tuning. A mis-tuned amp or a radical change in your antenna SWR and you will wipe out all of them.
  5. W9WQA

    W9WQA Ham Member QRZ Page

    i tried 3 but with my tank stuff,not so good.
    has been labor of love. probably less than a 100 bucks in surplus parts, note sb1000 power supply. home brew band switch.

    "And here I thought I cornered the market on 813's!"

    now up to 7 !!,6 sockets...

    Attached Files:

    K2XT likes this.
  6. K0IZ

    K0IZ Ham Member QRZ Page

    Appreciate the comments. Doing some more reading, I came across some comments by W8JI where he tried 4-1000 GG with and without bias and screen voltages. Said little increase in gain with the voltages vs just grounding, but reduced IMD. As I think further about this, I guess operating the 813's (or other tubes) in GG with screen grounded, amp is running in Class B. If GG with normal bias and screen volts (-90, 750), this would put tubes into AB1. So IMD would be better than Class B. However the negative feedback due to cathode drive would be same, so gain same. Sort of fits.

    Here's a picture of my homebrew 20M radio that I will use to drive the amp. Uses single 6146. Finished it up last P1040759.JPG summer, my Covid project.

    Attached Files:

    W9JEF, N2EY, KH2BR and 6 others like this.
  7. W9WQA

    W9WQA Ham Member QRZ Page

    oh, !! WAYYY too neat for me !!!
  8. AF5XF

    AF5XF Ham Member QRZ Page

    nice build. have you got a detailed build page somewhere to tell us exactly what you did and how you did it?
  9. K0IZ

    K0IZ Ham Member QRZ Page

    Sure. I'll email you description and schematic, and share dropbox power point with you.
  10. W1BR

    W1BR Premium Subscriber QRZ Page

    Is the Antek rated for series secondary? Most run the windings in parallel with a HV doubler.

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