Golborne Vintage Radio

I am putting this in this section as I am planning something for the future but the circuit I am basing things on has me stumped.

This will be for a modulator, the plan was to use part of the circuit as used in an old military radio as I have the mod driver transformer out of it, unfortunately the interstage transformer was toast.
Anyway here is the circuit from the available online manual.
[attachment=21661]

The area that has me baffled is 2V2, the HT is 250 Volts, The cathode bias resistor is made up of a 1K2 and a 2K7 to give 3K9 total, with the grid bias taken off at the junction of the two resistors via a 330K resistor.

In a later section of the manual it lists the voltage on the Cathode of 2V2 as being 10.5 Volts.
[attachment=21662]

So this would put the grid bias at -3.2 Volts with respect to the cathode?   The valve is a CV136 or EL91 and OK with 240 Volts across the valve rather than 250 I would not expect to see much difference in I_c but according to the spec sheets at -3.2 grid bias, 250 Volts on screen and anode, the anode current I_a should be around 55mA and not the 2.7 mA suggested by a cathode voltage of 10.5.
Considering the screen current I_g2 should be around 2.3 mA, then there is only 0.4mA of Anode current in the manual?

Now the manual also states using a meter with 1,000 Ohms per Volt and I am guessing a 25 or 30 Volt range to take the measurement, so the true Cathode Voltage may be around 12 Volts with -3.5 Volts Grid bias but the spec sheet for the valve would still put that at 50mA anode current.
[attachment=21663]

I can not build the circuit as the transformer 2T2 is duff. But even with a 1K resistor in there I could not get the values to work out.
So I am confused as to what I have wrong, or, if the manual is wrong, if I have neglected something like space charge across the 330K resistor adding some more negative bias.

So please assist if you can and say what I have wrong/missed etc.

Adrian

Hi Adrain
The 10.5V is just measured across the 2 resistors but there is D4 between the resistors and the cathode of the valve. Is D4 a Zener? If so the negative grid voltage will be the 3.2V + whatever the zener voltage is.

Frank

Hello Frank.  Unfortunately they are not diodes but test points, although in the manual I have can not not found any references to them.  I trimmed the end of the circuit picture off to save image size  This is a bit of text from the end. and to show that other diagrams have similar test points but using A or B etc.
[attachment=21666][attachment=21667]

Adrian

Hi Adrain
That shines a different light on it. I am afraid i am out of ideas.

Frank

Actually thinking about it the triangles may not be test points as I can see test points in dashed boxes. I wonder if they are pointers to locations either physically or something in the chassis. Not that it helps.

Adrian

Well I have had a bit of time and more importantly the inclination to patch the circuit around the EL91 together.

I have tried replacing the anode transformer with a resistor, used two values one of 220 Ohm and one of 2K2 Ohm. Nether made much difference to the values with the cathode at around 40 Volts, the junction of the three resistors being at 27.5 volts and the grid a bit lower at 26 Volts, all measured with my DVM. The two different values made little difference to the 10+mA through the cathode resistors.

I guess the issue is mute as I need to drive the output stage in another way so thinking of using a Phase Inverter
using a CV4031/ECC91.

Adrian

Things have moved on a bit, I have a stage with EF91, ECC83 and two EC90's driving the mod transformer from the above circuit..  i still have to add a bit to the circuit, as in a LDR, two LEDS and two Zener's and a few resistors to provide a form of level control, that lot courtesy of discussions on the other forum.
I am not sure if the mod stage will actually work in real life, but it is fun building and trying things out.
[attachment=21716][attachment=21717][attachment=21718][attachment=21719]
It is being powered by 12 Volt to 250 and 6.3 V switching PSU's while I play, the end idea was to mod a small valve 5 Watt TX for either 80 or 40  metres, which I would still have to build.
This is the circuit so far before the compression bits are added.
[attachment=21720]
I learn from building and figuring so for me it is good.

Adrian

Hi.
I was under the impression that a 2V2 was a rectifier!
Have you built the circuit and done practical tests?

These days I only use low level modulation to save the cost of modulation transformers, the EF50 & 6F33 can be modulated on the G3, IMHO less is best.

The odd thing here is with 50ma cathode current and 10v between anode and cathode the circuit would clip like hell. I personally think you are missing something and will work perfectly well.

2V2? There is a 3V2, just how I put voltage readings on the circuit a bit like 2K7 for a resistor. What is in the pdf has been built, should be adding the simple compressor bits today.

Adrian