Planning ahead but stumped by a circuit.

[Murphyv310]

Thinking out loud here.
I can only assume that the original valves characteristics are such and the more modern valves are not suitable as you've no doubt found.

[Murphyv310]

I'm referring to the original circuit.
Sorry for the confusion.

[AdrianPH]

OK, I understand. The numbers 2V1, 2V2, 2V3 etc were just valve numbers, there is a list of them on the first diagram for unit 2, which is the modulator.
2V1= CV131 = EF92
2V2= CV136 = EL90
2V3, 2V4 = CV133 = EC90.

The transformer 2T2 ended up being faulty as open circuit, probably due to the wire being very fine and corroded. So I went similar to the classic Mullard amp circuit with an ECC83 Schmitt Phase circuit driving the two EC90's. Actually using CV4058 which is a more modern replacement for the CV133.

In the aircraft radio that this is from the mod stage drives the anode and screen of the TX output valve and the screen of the driver valve. I had assumed that the output of the mod stage would have been approx 5 Watts as that was the output of the radio at 130 MHz but I can not achieve anything like that, although I am driving the output into 11K resistive load I can only get a 200 Volt swing. So I need to do more reading and figuring out.

But I am still playing with the circuit as I have it built, still to complete the compression part.

Adrian

[Murphyv310]

Hi
If you are getting 200v pp output that's correct. With 250v HT how can you achieve more? What is your anode and cathode and anode DC voltages with no audio input?

[AdrianPH]

If you look at the pdf I put in post #7 it has the DC voltages marked, these are what I have. I was expecting a larger output swing, even if I tie one end of the output to 250 Volts the other end voltage swing makes it down to 150 Volts and up to 350 volts and not as I was expecting nearer to 400 - 500 Volts pk to pk.

Adrian

[PerdioPal]

Is C2 a bit small?

[Murphyv310]

Hi
We are talking at cross purposes here.
I'm more interested in the circuit that was uploaded in post 1.
Perhaps the thread should be split to avoid confusion.
I'm only trying to help you get your head round the problem you had initially.
I'll bow out for now.

[Murphyv310]

Ok.
Looking at the PDF I assume this is a PC generated circuit?
There's a few anomalies IMHO, the EF91 V1 anode load is too low a value, the cathode resistor of 680 I'd say is low but ok if you stick with the 33k which I personally wouldn't, C2 should be a higher value as a 0.1uf will boost HF not a good idea as you want roll off above 3kHz for an amateur radio TX. It also might be better to strap the G3 to ground, you want more swing to the phase splitter and replace the ECC83 with an 81 if you leave in the 83 then the anode load resistors will need to have unequal values, ECC83s don't make good phase splitters.
Valves 3&4 may need a lower cathode resistor to achieve 5 watts drive, the EC90 gm is quite low, I'd use EL84s which will have loads of headroom.
Other than that Id say the circuit is good.

EDIT: V1 could be replaced with a 6BR7 or an EF86 and run in starvation mode with an anode load of 1 meg, this will give you a large peak to peak drive to the phase splitter, the EF91 might work in that mode but they I've never tried it. You want plenty of drive to the phase splitter and, less gain in that part of the circuit.

Also it's possible to directly couple the anode of the first stage to the grid of the phase splitter and retain the. ECC83, have a look at the Mullard 5-10 circuit.

[Murphyv310]

Hi
Addendum. Strapping the G3 to ground isn't always a good idea so discount that train of thought. One thing I missed was the G2 directly connected to HT, not a good idea as ideally we want the G2 at a lower potential than the anode, so you need to feed it via a suitable resistor and decoupled. Also you'll need to add a resistor in the HT line between the output stage and before the ECC83 of about 2.2k decoupled with a 10uf, it may also be prudent to also do the same between the ECC83 & EF91. The amp has no NFB so the decoupling is even more important to reduce any possible instability.
Hope all this helps.

[AdrianPH]

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.