>
>Dear QRO lovers,
>I am a happy owner of a Drake L4B . I have replaced the tubes to
>new ones, RFP 3-500ZG. I have change all
>components except the transformer in the power supply. The amp works great.
>
>The maximum HV is abt 2500 volt from the standard Drake power supply.
>I have order a new transformer according to my own specifications,
>primary 2 x 400 volt 2-phase,(not ordinary 220 volt 1-phase),
>and secondary 700, 900, 1100, 1200, 1300 volt, 1200VA 100% duty. I will get
>from 1900 to 3600 volt DC.
>The data sheet for 3-500Z says maximum dc plate voltage 4000 volt,
>(I will use max 3500v)
>Now to my question:
>1."Typical operation" says:"Zener diode positive bias at plate
>potentials of 3 Kv or more"
>How do I make +10 - +15 volt positive bias?
Put a series string of 1A rect. forward-conducting diodes on a perfboard,
with the arrows aimed at HV neg. and the anode ends headed toward the
Filament CT. This arrangement is like a cathode bias zener-diode,
except for polarity. My guess is that 12 diodes would be in the
ballpark for 3500v on the anode/plate. In any event, it takes about
75mA/no-sig. for linear operation. Short/unshort/add diodes in the
series string to get what you need.
>2. Can the Drake L4B circuits handle an output power of 1,5 Kw?
easily. At 4kV, 2kW out is typical. However, increased anode supply
potential means more gain, and increased potential for parasites. If you
experience signs of parasitic instability, it might be a good idea to
decrease the Q of the VHF suppressors.
>3. A positive voltage of 120 volt from a resistor divider is applied to
>the cathode during standbye. Do I need to increase this voltage when I
>increase the plate voltage?
no. 25v will cut the tubes off.
>
- later, Tomas
Rich. Measures. Web site: www.vcnet.com/measures
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