Thanks all for the comments, I do like them. A bit of a long post, basically to help me get my thoughts down.
Yes C2 could be a bit higher, or I could remove it altogether (see later*). The circuit drawing is from me putting it into Kicad.
The higher cathode resistor on the EF91 has been done to reduce the normal anode current, from the typical 10 mA down to 3.7 mA. The screen is being fixed at 250 Volts to make life easier and as per the spec sheet. The screen current is at 1mA so it is not drawing the majority of the current. The stage is giving me a voltage gain of 33, if I feed it with 30 millivolts there is 1 volt peak to peak on the anode., this may be a bit high, but at the moment there is no signs of instability. *I could reduce the gain slightly by removing the C2 0.1uF as I have it now, this will then tend to give a bit of negative feedback and reduce the gain slightly but only for the first stage.
You have pointed out something I have missed and that is the HT decoupling, it is in there but at the moment not marked up, it is basically a couple of 4.7 uF caps at the ECC83 and the EF91 supply points, but there is no resistor droppers in the HT yet whilst playing.
The EC90's are in sort of class AB1 (I think) the current does rise as I input a signal. In the original circuit, the cathodes are connected to ground and there is a negative bias voltage provided to the grids of -11.2 Volts, as I do not have a negative bias available, I have used the cathode resistors to provide just under that negative bias to the grids.
So as it stands each valve with no signal is taking 4.7 mA. The cathode does rise slightly to 12 Volts when putting out a 200 Volts pk-pk signal, I scoped the drive from the ECC83 and that has a 22-23 Volt clean swing on the grids, so as one valve is being driven to zero volts the other should be shut off with around -22 Volts on the grid so at that point all the current will be in the one being driven hard on, if the cathode goes to 12 Volts then the average current goes to 11 mA, what the peak current is I would not know how to work that out, but would expect the power going in the valve to be around 2.6 watts?
Now the fact I have lost 12 Volts from the HT in providing bias this way and that the cathodes are not strapped to ground I have no way of knowing it this is the problem.
I know with EL84s in a 10 watt amp, they can sit at 40 - 48 mA, so perhaps this mod stage is not meant to give watts of output driving two screens and an anode and I may be chasing a red hearing? I have probably wrongly assumed it need 5 watts of audio to drive a 5 watt output signal?
It's fun, I think
As regards the first problem I had, I think that any engineers working on the radios had a test jig that went across various test points and the reading are what was expected on their meter, and not necessary what is there in real life, that was my conclusion anyway.
Yes C2 could be a bit higher, or I could remove it altogether (see later*). The circuit drawing is from me putting it into Kicad.
The higher cathode resistor on the EF91 has been done to reduce the normal anode current, from the typical 10 mA down to 3.7 mA. The screen is being fixed at 250 Volts to make life easier and as per the spec sheet. The screen current is at 1mA so it is not drawing the majority of the current. The stage is giving me a voltage gain of 33, if I feed it with 30 millivolts there is 1 volt peak to peak on the anode., this may be a bit high, but at the moment there is no signs of instability. *I could reduce the gain slightly by removing the C2 0.1uF as I have it now, this will then tend to give a bit of negative feedback and reduce the gain slightly but only for the first stage.
You have pointed out something I have missed and that is the HT decoupling, it is in there but at the moment not marked up, it is basically a couple of 4.7 uF caps at the ECC83 and the EF91 supply points, but there is no resistor droppers in the HT yet whilst playing.
The EC90's are in sort of class AB1 (I think) the current does rise as I input a signal. In the original circuit, the cathodes are connected to ground and there is a negative bias voltage provided to the grids of -11.2 Volts, as I do not have a negative bias available, I have used the cathode resistors to provide just under that negative bias to the grids.
So as it stands each valve with no signal is taking 4.7 mA. The cathode does rise slightly to 12 Volts when putting out a 200 Volts pk-pk signal, I scoped the drive from the ECC83 and that has a 22-23 Volt clean swing on the grids, so as one valve is being driven to zero volts the other should be shut off with around -22 Volts on the grid so at that point all the current will be in the one being driven hard on, if the cathode goes to 12 Volts then the average current goes to 11 mA, what the peak current is I would not know how to work that out, but would expect the power going in the valve to be around 2.6 watts?
Now the fact I have lost 12 Volts from the HT in providing bias this way and that the cathodes are not strapped to ground I have no way of knowing it this is the problem.
I know with EL84s in a 10 watt amp, they can sit at 40 - 48 mA, so perhaps this mod stage is not meant to give watts of output driving two screens and an anode and I may be chasing a red hearing? I have probably wrongly assumed it need 5 watts of audio to drive a 5 watt output signal?
It's fun, I think
As regards the first problem I had, I think that any engineers working on the radios had a test jig that went across various test points and the reading are what was expected on their meter, and not necessary what is there in real life, that was my conclusion anyway.
Learning as I go!
Youtube EF91 Valve
Youtube EF91 Valve
