Question about voltage for detector valve

[SurreyNick]

I could do with a bit of advice about the HT and GB voltages for valve 1 in the attached circuit – a three valve battery powered TRF with reaction.

The valves from right to left are:

V3 (Output) = P220A
V2 (L.F.) = L2
V1 (Detector) = HL2

The construction notes for the receiver (a Telsen Conqueror 3) shows three HT values but suggests running the HT together for V3 and V2 and set at 120V. At this voltage the recommended GB for V3 is -9V and for V2 is -3V. So far so good, but the construction notes gives no recommendation for the voltages to be applied to V1, the HL2 detector. The maximum operating conditions for this valve are 150V on the anode and -3V on the grid. If I interpret the circuit correctly there is 0V GB on the valve, so at what voltage should I set the HT? If 120V is reasonable then why was the circuit designed with three separate HT connections!

Nick
   

[Nowhere-Man]

I could do with a bit of advice about the HT and GB voltages for valve 1 in the attached circuit – a three valve battery powered TRF with reaction.

The valves from right to left are:

V3 (Output) = P220A
V2 (L.F.) = L2
V1 (Detector) = HL2

The construction notes for the receiver (a Telsen Conqueror 3) shows three HT values but suggests running the HT together for V3 and V2 and set at 120V. At this voltage the recommended GB for V3 is -9V and for V2 is -3V. So far so good, but the construction notes gives no recommendation for the voltages to be applied to V1, the HL2 detector. The maximum operating conditions for this valve are 150V on the anode and -3V on the grid. If I interpret the circuit correctly there is 0V GB on the valve, so at what voltage should I set the HT? If 120V is reasonable then why was the circuit designed with three separate HT connections!

Nick

I lack the time to go too deeply into this but what is your detector tube? If it's a TRF, I guess it's probably a pentode with a grid leak? Is it steep slope? Sure, the bias may be around - 5 fixed, for example, but the signal if it's 2 volts will give you -3 on the grid (positive cycle) and - 7 (negative cycle). The anode voltage will be at H.T. as the amplifier but there may be a resistor leading to the anode?

Sometimes details aren't clarified in schematics which can be an obstacle. Perhaps a similar schematic of a similar radio will help you compare?

[pwdrive]

When I built regens I always found a lower voltage on the anode of the regen detector worked best for me, having said that I've never built one with a large AF choke in the anode circuit.

A lower anode voltage will reduce the anode current in the absence of regen feedback. (eg no grid bias)

Lawrence.

[SurreyNick]

I reckon somewhere between 60v and 80v on the detector would be about right. That's my experience from other three-valve TRFs, although I have seen as low as 45v. I've never used a HL2 though and the datasheet isn't very helpful. The point about the lower anode voltage reducing anode current to compensate for a lack of grid bias is helpful though. Funny, it had never occurred to me, even though it's obvious!
Live and learn

What I'll do is make a HT battery with taps at 45v, 54v, 63v, 72v and 81v for the detector. That gives me plenty of scope. I'll run the other two at 120v.

Nick

[Murphyv310]

Hi.
The detector is a leaky grid setup, the bias is derived by the 2 meg grid leak and the 0.0003 grid coupling capacitor, this circuit requires no additional bias.
The actual valve is of little importance virtually any 2 volt Triode will work even those allegedly for audio. HT on the first stage can be from 45 to 100v dependent on the choice of valve, trial and error being the best approach.
The subsequent stages, audio amplifier and output require bias but again its down to the valves in use, this is one reason the grid bias battery has so many taps as there were so many different valves around with small variations in characteristics, again trial and error is the best way, your looking for maximum output vs distortion.
Brilliant projects these little TRF's. I've now made at least two using the Russian Rod Pentodes. They work very well and the sensitivity is quite amazing.
I've built a short wave one too, hope to catch Radio Australia soon!

[SurreyNick]

Thank you Trevor. This is additional helpful information.
Going off thread, would you share the circuit for one or both of the TRFs you made with the Russian Rod Pentodes? I have a whole heap of these which I'd like a project for and this sounds just perfect.
Nick

[Terry]

What is often overlooked when considering the leaky grid detector is that the valve is actually performing as two different valves.

Although the entire valve, be it a triode or pentode, is acting as an amplifier as might be expected, the grid/cathode (or filament) section of the valve is acting as a diode at the same time!

The incoming modulated RF signal swings equally positively and negatively about the zero point which cancel out and, therefore, leaves an average level of zero. To obtain any information from the signal we need to get rid of either the positive or negative portion of the signal.

In the leaky grid detector, because there is no bias, as soon as the grid sees a positive going signal it acts as the anode of a diode and conducts, leaving only the negative portion of the waveform. The average of the negative going signal is now the effective grid bias but this will automatically track the incoming signal level, being bled to earth via the grid leak.

Note that this process leaves the AF modulation intact to be amplified by the valve in the usual way and the residual RF is filtered out in the anode circuitry.

Once the audio signal has been recovered, it is important not to distort it. In later stages, therefore, grid bias is applied to prevent any part of the signal pushing the grid positive relative to the cathode (or filament) 

   

[SurreyNick]

That's beautifully explained Terry. Thank you
Nick

[Murphyv310]

Thank you Trevor. This is additional helpful information.
Going off thread, would you share the circuit for one or both of the TRFs you made with the Russian Rod Pentodes? I have a whole heap of these which I'd like a project for and this sounds just perfect.
Nick

Certainly. I'll get drawing tomorrow

[SurreyNick]

Many thanks
N.