PYE FV1 line output stage efficiency
 

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:o.

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 !;D
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

Re: PYE FV1 line output stage efficiency
 

Hi Darius,

Have you looked at the conditions at the PL38 in your TV24? Is there
scope for efficiency improvements there too?

Kind regards,

Peter.

Re: PYE FV1 line output stage efficiency
 

A safety warning to all of you who are tempted to try this sort of thing. Darius has a proper high voltage scope probe for making those measurements. Unless you are lucky these are rare and/or expensive. An ordinary scope probe will be rapidly fried, along with the scope's input stages. You will probably get a nice shock but may live to tell the story. A simple resistive dropper, as used for EHT voltage measurement, will be safe but stray capacitances will ensure that the waveforms you see are totally false.

The other useful device for this sort of measuremnt is a current probe. This will clip round a wire and enables you to display the current waveform on a scope. Please be careful when probing in high voltage circuits. The probe's own insulation may not be adequate when clipped round the anode lead of a line output stage.

I don't have either a current probe or a high voltage probe but I have used both types in the past.
Again. Please take care. Darius and I probably know what we are doing when mucking about with this sort of thing and we don't want to to see too many toasted TV enthusiasts.

Re: PYE FV1 line output stage efficiency
 

Hi Jeffrey,
Yes I am using a special probe for these measurements.

Hi Peter,
I did not have a look at my TV22. But with my new knowllege
I will do.


Maybe you noticed C59. It is there to keep the H-def. coil
free from the DC drop of the LOPT.
Before the changes the picture moved ca. 4cm to the left
direction when I replaced it.
After, there is no movement because the DC component is much
reduced!!! So you can bridge this electrolytic you don't need
a replacement there.
In my set there was a modern one and I replaced it compleatly.

Kind regards,

Darius

Re: PYE FV1 line output stage efficiency
 

Can't find the edit button???
In this post it is there in the old posts it is not possible to edit.

Here is a picture of the line output stage.

Kind regards,
Darius

Re: PYE FV1 line output stage efficiency
 

Have you looked at the conditions at the PL38 in your TV24? Is there
scope for efficiency improvements there too?

Hi Peter,
Yes, definatly. I tried out the best values. 2200 Ohms and 47nF.
The DC voltage for the RF deck increased ca.10V.
I am sure the PZ30 likes this too!

Here a few pics of my TV22:

Kind regards
Darius

Re: PYE FV1 line output stage efficiency
 

Hi Darius,

Did you do a before and after check on the Line Output cathode
current on the TV22?

If there is a general pattern of drawing heavy currents from the
screen grids in these early sets it would be interesting to get
some comments from some of the forum's set designers.

Kind regards,

Peter.

Re: PYE FV1 line output stage efficiency
 

Hi Peter,
I measured the g2 current with a 390 Ohms shunt and I got 60mA.
Without shunt maybe more. This is definatly too much.:o

It is a mistake to leave out a g2 resistor. Have a look at the
trader sheets. Some sets have one, others not.:-/

With the PL38 you must take the screen grid voltage from the
booster stage. I don't like this and now I know why the
P50 (Wehrmacht LS50, now russian GU50) was used in the Rembrandt
line output stage.;)

Kind regards
Darius

Re: PYE FV1 line output stage efficiency
 

Compareing the PYE FV1 and the Bush TV22
I noticed that I got a marked maximum in H-width and brightness
by adjusting the H- drive cap C52 at the FV1.:)

In the Bush the maximum is not that strong marked and the cap
is nearly adjusted to its minimum. I don't like this.:(

I did not think about this before because the set worked.

Before I got the idea with the screen grid RC I experimented
with the H- drive. I noticed the lower the resistance of R54
the more capacitance at C52 is needed to reach the maximum.
But the result is the same.
I did not change the values in the H- drive of the FV1, it is perfect!
But I think about the Bush:-/

Kind regards
Darius