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Hi everyone - It seems us "VHFer's" know a bit more about using grounded
grid triodes than the "DC boys".
For many years we have been using active shunt voltage stabilisers to
maintain a very constant cathode to grid bias, irrespective of the cathode
current. Net result virtually no change in cathode-grid voltage with drive
(typically less than 10mV over the operating range). And I am talking
about "healthy" currents - up to 2A peak. You will need a bipolar
transistor with good gain (darlington) or better still a Mosfet, a few
resistors and diodes and something to heatsink the transistor to.
Look in the RSGB Microwave Handbook or the German magazine compendium VHF
Communications for the basic circuit. You just need to change a few values
to suit your needs. BTW the circuit is adjustable with a pot. so you can
set the cathode voltage (hence standing current) to almost any value you
need. As an added benefit the constant voltage means superior IMD
performance as the cathode voltage doesn't wobble around as the drive goes
up and down - which with resistor biasing can be quite alarming!
Also the tube can be cut-off on receive very easily by adding a suitable
zener diode and a relay contact.
Maybe Ian White G3SEK can point you to other sources for the circuit.
John ZS5JF
----------
> From: Rich Measures <measures@vc.net>
> To: sm6vvt@kungalv.mail.telia.com; amps@contesting.com
> Subject: Re: [AMPS] L4B
To: <amps@contesting.com>
> Date: Wednesday, October 14, 1998 12:57 PM
>
>
> >
> >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
>
>
> --
> FAQ on WWW: http://www.contesting.com/ampfaq.html
> Submissions: amps@contesting.com
> Administrative requests: amps-REQUEST@contesting.com
> Problems: owner-amps@contesting.com
> Search: http://www.contesting.com/km9p/search.htm
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<html><head></head><BODY bgcolor=3D"#FFFFFF"><p><font size=3D2 =
color=3D"#000000" face=3D"Arial">Hi everyone - It seems us =
"VHFer's" know a bit more about using grounded grid =
triodes than the "DC boys". <br><br>For many years we =
have been using active shunt voltage stabilisers to maintain a very =
constant cathode to grid bias, irrespective of the cathode current. =
Net result virtually no change in cathode-grid voltage with drive =
(typically less than 10mV over the operating range). And I am =
talking about "healthy" currents - up to 2A peak. =
You will need a bipolar transistor with good gain (darlington) or =
better still a Mosfet, a few resistors and diodes and something to =
heatsink the transistor to. <br><br>Look in the RSGB Microwave =
Handbook or the German magazine compendium VHF Communications for the =
basic circuit. You just need to change a few values to suit your =
needs. BTW the circuit is adjustable with a pot. so you can set the =
cathode voltage (hence standing current) to almost any value you need. =
As an added benefit the constant voltage means superior IMD =
performance as the cathode voltage doesn't wobble around as the drive =
goes up and down - which with resistor biasing can be quite =
alarming!<br><br>Also the tube can be cut-off on receive very easily by =
adding a suitable zener diode and a relay contact.<br><br>Maybe Ian =
White G3SEK can point you to other sources for the circuit.<br><br>John =
ZS5JF<br><br><br><br>----------<br>> From: Rich Measures <<font =
color=3D"#0000FF"><u>measures@vc.net</u><font =
color=3D"#000000">><br>> To: <font =
color=3D"#0000FF"><u>sm6vvt@kungalv.mail.telia.com</u><font =
color=3D"#000000">; <font =
color=3D"#0000FF"><u>amps@contesting.com</u><font =
To: <amps@contesting.com>
color=3D"#000000"><br>> Subject: Re: [AMPS] L4B<br>> Date: =
Wednesday, October 14, 1998 12:57 PM<br>> <br>> <br>> =
><br>> >Dear QRO lovers,<br>> >I am a happy owner of a =
Drake L4B . I have replaced the tubes to<br>> >new ones, RFP =
3-500ZG. I have change all<br>> >components except the transformer =
in the power supply. The amp works great.<br>> ><br>> >The =
maximum HV is abt 2500 volt from the standard Drake power supply. =
<br>> >I have order a new transformer according to my own =
specifications,<br>> >primary 2 x 400 volt 2-phase,(not ordinary =
220 volt 1-phase), <br>> >and secondary 700, 900, 1100, 1200, 1300 =
volt, 1200VA 100% duty. I will get<br>> >from 1900 to 3600 volt =
DC.<br>> >The data sheet for 3-500Z says maximum dc plate voltage =
4000 volt, <br>> >(I will use max 3500v) <br>> >Now to my =
question:<br>> >1."Typical operation" says:"Zener =
diode positive bias at plate <br>> >potentials of 3 Kv or =
more"<br>> >How do I make +10 - +15 volt positive =
bias?<br>> <br>> Put a series string of 1A rect. =
forward-conducting diodes on a perfboard, <br>> with the arrows aimed =
at HV neg. and the anode ends headed toward the <br>> Filament CT. =
This arrangement is like a cathode bias zener-diode, =
<br>> except for polarity. My guess is that 12 diodes =
would be in the <br>> ballpark for 3500v on the anode/plate. In =
any event, it takes about <br>> 75mA/no-sig. for linear operation. =
Short/unshort/add diodes in the <br>> series string to get what =
you need. <br>> <br>> >2. Can the Drake L4B circuits =
handle an output power of 1,5 Kw?<br>> <br>> easily. At 4kV, =
2kW out is typical. However, increased anode supply <br>> =
potential means more gain, and increased potential for parasites. =
If you <br>> experience signs of parasitic instability, it =
might be a good idea to <br>> decrease the Q of the VHF suppressors. =
<br>> <br>> >3. A positive voltage of 120 volt from a =
resistor divider is applied to<br>> >the cathode during standbye. =
Do I need to increase this voltage when I<br>> >increase the plate =
voltage?<br>> <br>> no. 25v will cut the tubes off. =
<br>> ><br>> - later, Tomas<br>> <br>> Rich. =
Measures. Web site: www.vcnet.com/measures<br>> <br>> =
<br>> --<br>> FAQ on WWW: =
&=
nbsp; <font =
color=3D"#0000FF"><u>http://www.contesting.com/ampfaq.html</u><font =
color=3D"#000000"><br>> Submissions: =
&=
nbsp;<font color=3D"#0000FF"><u>amps@contesting.com</u><font =
color=3D"#000000"><br>> Administrative requests: <font =
color=3D"#0000FF"><u>amps-REQUEST@contesting.com</u><font =
color=3D"#000000"><br>> Problems: =
&=
nbsp; <font =
color=3D"#0000FF"><u>owner-amps@contesting.com</u><font =
color=3D"#000000"><br>> Search: =
&=
nbsp; <font =
color=3D"#0000FF"><u>http://www.contesting.com/km9p/search.htm</u><font =
color=3D"#000000"></p>
</font></font></font></font></font></font></font></font></font></font></f=
ont></font></font></font></font></font></font></body></html>
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FAQ on WWW: http://www.contesting.com/ampfaq.html
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Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
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