Now, this below might be very important. Maybe this is the key to
success.
The DAF´s that I consider ok, sounds good and have normal SSB
bandwith, all use QBL tube (or tubes).
I have also heard DAF amps using Russian tubes (GU84´s I think)
that did splatter, don´t know if that was a tube problem or a
poorly
designed DAF.
I really dont care about this dB talk and whatever you measure in
the lab. If a radio station (on the air) sounds good and have
correct
bandwith for the mode used it must be considered OK. If sutch a
station is using a G2DAF amp design I must rate that piece of
equipment being OK, one of these days I must put something
together
with a QBL tube. However I need a few vacum variables to pull it
off,
or at least one, anybody got a good deal?
73, Jim SM2EKM
----------------------------------------
-----Ursprungligt meddelande-----
Från: k7fm@teleport.com [SMTP:k7fm@teleport.com]
Skickat: den 16 mars 2000 01:49
Till: 2@vc.net; Jan.E.Holm@telia.se; amps@contesting.com
Ämne: Re: [AMPS] Re: New G2DAF amplifier information
Not wishing to discuss the G2DAF circuit any further, I still want to
question your statement regarding substantial grid current causing
distortion. I do not think that is necessarily true.
The tube used by Lars is a Philips QBL 5/3500. If you go to his web
site,
the data sheet indicates that the tube is designed to draw substantial
grid
current in linear service. Grid current alone would not cause
distortion.
Distortion would arise only when the grid current causes the plate
current
to saturate. In most amplfiers using normal screen voltage, the maximum
normal plate current would be designed to occur at zero grid current.
Therefore, by design, grid current would cause the plate current to
exceed
the design of the tube. In the case of the QBL 5/3500, plate current
curves
show the tube can draw 5 amperes. So, if the power supply could handle
this
substantial current, grid current would not saturate.
One method to allow the flow of grid current in tubes not otherwise
designed
for grid current, would be to lower the screen voltage. If the screen
voltage is lower, the gain of the tube would decrease and grid current
would
not cause the plate current to flatten out. An interesting experiment
would
be to install a tetrode amplifier with a stabilized and variable screen
voltage. I expect that you could pick the grid current you wanted to
run
and then adjust the screen voltage so that the desired maximum grid
current
would cause the plate current to raise to the desired level. Less grid
current would cause a corresponding decrease of plate current. If the
resulting curve is linear, the amplifier will be linear and no
distortion
will be caused by grid current.
Read about the tests conducted by G2DAF, which are included in the
article
posted by Ian. G2DAF stated that by adjusting the screen voltage
downward,
it is possible to eliminate the bias supply. Perhaps there is merit in
further research.
In a circuit and tube where zero grid current yields maximum output,
your
statement would be a true statement. But there is more than one way to
run
an amplifier and they certainly merit discussion.
An amplifier running less screen voltage than specified by the
manufacturer
could have undesireable characteristics and I would not recommend doing
so
on the air without adequate testing under all conditions.
Colin K7FM
<<Fil: Re_ [AMPS] Re_ New G2DAF amplifier information.TXT>>
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|