Supplying "low" plate voltage - Just reduces gain? Or damage risk?

Discussion in 'Amateur Radio Amplifiers' started by KC8QVO, Dec 3, 2019.

ad: L-HROutlet
ad: l-rl
ad: Subscribe
ad: Left-2
ad: Left-3
ad: abrind-2
ad: L-MFJ
  1. KC8QVO

    KC8QVO Ham Member QRZ Page

    I looked around for a clear answer on this and didn't come across one. Maybe one doesn't exist... I did find a reference to no plate voltage would damage a tube in attempted operation, and that dropping plate below bias (in a tetrode) voltage would likely cause damage as well, but I haven't found anything stating "low plate voltage will cause damage" across the board.

    To define my question -

    If I run a tube that, for all reference's Ive found, seems to be in the 3500-4200v plate v range at 1500v - will I damage it?

    Before anyone brings up grid and plate current - lets assume they are held to safe levels.
  2. SM0AOM

    SM0AOM Ham Member QRZ Page

    In a tetrode or pentode, if the steady-state screen voltage drops below the plate voltage, all the cathode current will divert to the screen which will almost instantly destroy it due to excess dissipation.

    If a tube designed for high voltages is run on lower voltages, and the screen and bias voltages are chosen to give the proper operating point, nothing detrimental will happen.

    However, a tube designed for high voltages usually have less available cathode emission, as it takes less current at high voltages for a given plate input.

    Most tetrodes are designed in the way that the plate current is primarily governed by the relation between screen voltage and grid negative bias.

    The plate voltage may vary considerably without changing the plate current very much, as long as the plate voltage always exceeds the screen voltage with considerable margins.

    Gain in tetrodes is quite independent in the plats vintage, but increases with raised screen voltage.

    Last edited: Dec 3, 2019
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    Correction to above.
    Should read:

    "In a tetrode or pentode, if the steady-state plate voltage drops below the screen voltage..."

  4. KC8QVO

    KC8QVO Ham Member QRZ Page

    Thanks for the info.

    I suppose I should have been more specific - the tube is a triode, coaxial type (GS-35b), and would be used in a grounded grid configuration. There is no bias - just filament and B+ for the plate/anode then the cathode (the filament terminals together) is what takes the input.
  5. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    I think you meant that if the plate voltage falls below the screen voltage, the screen will suffer. One would hope that the screen voltage would be below the screen voltage. :)
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    The GS-35B is designed to operate using a quite high plate voltage.

    Using much lower voltages, the performance will suffer considerably, and the grid current will be higher for a given drive level.

    Trying to increase output power by increasing drive may cause excessive grid dissipation which could
    damage the grid structure.

    W2VW likes this.
  7. KC8QVO

    KC8QVO Ham Member QRZ Page

    Pertinent points highlighted from my original post:

    Going back to the thread title - and the mention of "lower gain" - I think everything thus far has answered my question. The tube should not be damaged by the mere fact of the plate voltage being "low", but keeping other parameters (grid current, perhaps most notably) safe will be the limiting factor to what gain is possible.

    Thanks for the information. That gets me to a better place in my thought process on tackling this with what I have for the time being. Who knows how long that will be. The other side of this is, also - that means I can run on 110-120v mains instead of 220-240v - just at lower power levels. That is a big plus.
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    After inspection of the GS-35B characteristic curves, it seems reasonable
    that the maximum "linear" power level for a grounded-grid amplifier
    that is advisable at 1500 V plate potential lies in the 500-600 W range.

    This is governed by two factors;
    • Available cathode emission
    • Permissible grid dissipation

    The VK1OD load-line calculator
    was used to estimate the levels.

    Comparing to the 8877, which has more than twice the cathode emission available,
    and would put out over 1000 W at an 75-80 W drive level (12 dB gain) when approaching saturation
    using 1500 V plate supply.

  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    With only 1500Vdc available, I'd pick a different tube or even 2 x 572Bs or 3 x 811As, which are easier to use and socket and cool and will run the same kind of output power.:)
  10. N8FVJ

    N8FVJ Ham Member QRZ Page

    Triode tubes are ok with reduced plate voltage. 2700 volt rated 572B operate without issue in a 1500 volt Ameritron AL-811/AL811H.

Share This Page