Good evening,
repairing my PYE FV1 I had some problems with the line output
stage. With a not good tested valve the picture was ok but
after a few miutes the brightness und H with changed with a low
frequency.
First I thought it is a inteference of the V frequency and the mains
frequency. Running the set with DC mains made no difference.
I replaced the PL38 by a new one.
The picture was not that bright as with the old valve.
I decided to measure out what is going on.
Please have a look at trader sheet1023.
With the new valve I measured 2500Vp at the plate
and 8Vp at the katode. 8V at 33 Ohms meanes 240mA
.
The new valve can handle this but it is too much.
The neccessary current is much lower so the current goes
over to g2 and loads the 300V booster voltage.
This causes a higher plate current and so on.
That is why the old valve had trouble and the efficiency was
bad with the new one.
I solved this problem in the following way:
I connected the katode to ground because I don't want to
loose voltage for the output stage.
I placed a 4700 ohms resistor from g2 to U boo.
and blocked g2 with 10nF.
The result is a peak current of 100mA (240mA bevore)
2700Vp at the anode.
The voltage at C96 rises from 185V to 200V.
So I don't have to replace the MR5 !
BTW I placed a 10 Ohms in series with F2.
Note that there is no current limit resistor in some positiones
of link 2 and 3.
Kind regards
Darius