813 in grounded grid

Discussion in 'Amateur Radio Amplifiers' started by VA3DXV, Nov 8, 2019.

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  1. VA3DXV

    VA3DXV Ham Member QRZ Page

    Hi folks. I brought home someone's homebrew amp project. It uses a pair of 813's, cathode driven, with grids/screen (pins 3, 4, and 5) tied directly to chassis ground.

    It did not come with a power supply but I have a 1400 vdc supply with -130 vdc for bias. I realize this isn't quite as much plate voltage as the 813's want but it should work.

    How would you suggest I bias 813 tubes with the grids at DC ground? All designs I have found for this tube bias and feed the grid, not the cathode. Can I apply bias to the cathode?

    My supply has the center tap of the HV transformer going through a choke and getting rectified to 500 vdc, that could be regulated for screens but I'd prefer the simpler construction of grounded grid, and I'd rather just ground that center tap.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Easiest thing might be to lift each pin #4 connection (control grids) off ground, while leaving pins #3 and #5 grounded, so you can bias the tubes via the grids and use your negative output power source, although you'd have to adjust this to about -85V for 813s. You can still use "cathode drive" this way, just feed bias via good RF decoupling system (RF choke, bypass caps) as shown here:


    I would not expect much output from a pair of 813s at 1400V...probably 250W output or so, maybe 300W if you're lucky. The "higher powered" 813 linear amplifiers normally ran ~2500V on them.
  3. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    If I were in that situation, I'd reconfigure it as a grid drive amp. 813s really shine in that mode. 5 watts in/500 out.
  4. K2XT

    K2XT Premium Subscriber QRZ Page

    In grounded grid like you described the tubes run zero bias.
    That is, although I forget the exact current (maybe ~30 ma ?), the tubes will pull a safe current with no bias. I have tested several tubes I have at 2200 volts and all drew the same current. I was making sure they were not gassy before starting an amp project.
    N8ZL, W2VW and KK4RSV like this.
  5. W9WQA

    W9WQA Ham Member QRZ Page

    i run 2x 813 all grids grounded, 3300 volts. will do 1kw.
    no bias issues. drive at about 50 watts.

    btw i once put a 2nd supply "under' the main supply to lift it by 450 v. if the main insulation can handle the boost!
    AH7I likes this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Will it do a kW on 10 meters, or only lower frequencies?

    That's waaay more than they're rated for, but I know those are very "robust" tubes.
    W9WQA likes this.
  7. N8FVJ

    N8FVJ Ham Member QRZ Page

    813 tubes makes for a great HF amplifier and new tubes are only $50 each. At 1400 volts, it is not enough voltage to get more than perhaps 400 watts out. Also, your amplifier PI (matching) network inside is likely made for 2500 to 3000 volts. A low voltage PI network at 1400 volts is a very different design and may have issues on 20 meters and higher frequencies.
    If your power supply is a bridge rectifier, can you modify it to a voltage doubler? Or if full-wave center tap, change to a bridge rectifier and the center tap is not used. I am assuming the transformer is large enough for the current needed at the higher voltage. If existing PS is a bridge rectifier with capacitor input, a voltage doubler would produce about 2800 volts no load that 813s handle with no issues. If existing is full wave center tap. you would get the same voltage at 2800 volts going to a bridge. If existing is a voltage doubler, you are out of luck increasing voltage.
  8. W2VW

    W2VW Ham Member QRZ Page

    Hi Mu triode connected 813s work almost exactly like a large 572B.

    They have more plate dissipation and twice the cathode emission.

    The 125 watt plate dissipation figure seen in most spec sheets most likely refers to the rectangle plate types which are not often seen. The more common graphite plate models have a plate structure almost identical to tubes rated at 175 watts plate dissipation. RCA spec sheets show a picture of graphite types but still call out 125 watts plate dissipation. Not all tube ratings are an equal reflection of ability.

    People with hands on experience will tell you the plate dissipation is even more than that.

    Zero bias works fine up to the low 2KV range. Above that some zener bias can be used to set the idling current.

    Find a way to increase plate voltage if you wish to use the amplifier closer to the tube's potential and the tank circuit looks robust enough.

    Grid driving any tube requires decent screen regulation and an effective screen bypass. Grid driving takes a back seat to cathode driving in terms of distortion products all other things being equal. It also matches closer to available driving power with most xcvrs.

    If one has Pure Signal or another way to make up for the increase in distortion then grid driving might be a consideration.
    K2XT likes this.
  9. N8FVJ

    N8FVJ Ham Member QRZ Page

    And the 572B is over rated at 160 watts Plate dissipation. The 572B are more like 125 watts plate dissipation. The 572B tubes will start to show some red plate about 125 watts plate dissipation. As for voltage the 572B is max'ed out at 2700 volts, I read the 813 can take 3200 volts.
    Being a somewhat cheap ham, I use three Cetron 572B in an AL-811 for 800 watts PEP out. The second inexpensive high quality (not Chinese) tube I would use is a pair of vintage RCA carbon plate 813 for the same 800 watts out. I do not know of any commercially built 813 amps with a tuned input though.
    Last edited: Nov 9, 2019
  10. N8FVJ

    N8FVJ Ham Member QRZ Page

    BTW- you can build a new power supply using an Antek AS-8T800 supplying 2100 volts ($105), but I would use the Antek AS-15T950 for 2500 volts and more reserve capacity ($195). You can wind #12 wire over the Antek core to get the 10 volts @ 10 amps required for the 813 tube filaments. About 1 volt per turn. Transformer has two 6.3 volt windings for auxiliary voltages for the antenna relay and use a 6.3 volt or 12.6 volt @ 1 amp transformer with secondary connected to Antek transformer low voltage winding for 120 volts AC to make the bias voltage after you convert to DC. This is shown in the schematic below. Article to build the power supply is here:

    Last edited: Nov 9, 2019
    W1TRY and W9WQA like this.

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