[AMPS] 8877 optimum operating conditions,bias etc

[km1h@juno.com]

On Sat, 05 Sep 1998 15:53:04 +0100 Conrad Farlow
<conrad.farlow@virgin.net> writes:
>
>Hello I am just about to start building a 8877 amplifier for 6m.I have
>looked at lots of designs for 6m all of which are using the standard 
>Pi
>tank.This part is no problem.What I find confusing is the correct bias
>voltage and optimum eht volts.Eimac give the max as 4kv I am using all
>vac variable so I was thinking of runing about 3800 off load which 
>will
>probably drop to 3500 on load.I can run any EHT volts up to about 5kv 
>at
>1.5 amps.
>Now comes the bias bit.In Europe it is very important to have a clean
>signal.The density of amateur population is at times incredible
>particularly in IO93 and IO91 in the UK.6m is a very popular here and 
>I
>wish to happily co-exist with my fellow amateurs,so I am after optimum
>IMD performance.Here are my concerns.
>1.Do variations in the bias voltage severly degrade the IMD 
>performance
>of a 8877?


With most GG triodes the higher resting current usually means better IMD,
the difference is in going between AB and B modes of operation. 
Using the Ameritron AL-1500 as an example the resting DC is 3500V. With a
7.5V 50W zener the idling current is approximately 160ma.


My experience with tetrodes has taught me the importance of
>keeping the screen regulation absolutely rock solid (look at G3SEK's 
>WEB
>site for an explanation).How does this translate to Triodes?I have 
>never
>run one and therefore don't know.Is a 50 watt zener adequate or should 
>I
>use a shunt stabaliser like the OE9PMJ one?


The Zener will be perfectly fine and a new 8877 is capable of almost
-40dB 3rd order IMD in the AL-1500.


This is also variable so 
>you
>can set the bais for the optimum standing current.


This is OK for experimenting with various tubes and voltages but the 8877
is/was  used in the most demanding medical services where linearity is
important and a Zener is usually used.


>2.What is the optimum standing curent for AB1 operation on a 8877?Most
>designers say use a 12 volt 50 watt zener and say that the idling
>current will vary depending on EHT volts available!Surely that can't 
>be
>right can it?

Many years ago when the USA power limits were different the CW-SSB switch
added another zener in series. This made the tube harder to drive by
bringing it roughly into Class B and slightly increased efficiency. The
AL-1500 switches in a second 7.5V Zener for CW and RTTY modes altho most
hams use the SSB position for CW at 2500W out!


 
>3.What is the maximum grid current that should be expected in normal
>operation with 3500 volts on the anode.I intend using a grid trip and 
>I
>would like to know where to set it?

My experience with good 8877's is that maximum output typically occurs at
60-80ma of Ig for 2500W out, 100W of drive and 3200V key down DC. With
new medical versions I have obtained full output at 50ma.
In any event I would not exceed 150ma for the grid trip.


Good luck.

73  Carl  KM1H
Amplifier Repairs and 6M Conversions Our Speciality




>
>73 de Conrad G0RUZ
>
>
>
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