Grid Stopper resistor

[Neil Purling]

In a radio. A 100K resistor between the junction of the grid leak resistor & grid-coupling capacitor and g1 of the o/p valve. But surely the correct value is 10K wired very close to the g1 pin on the valve socket?
The radio has some issues & I am wondering if this component is the wrong value what the effect is going to be.

[kalee20]

Depends on the valve, Neil. Small battery output pentodes may well use 100kΩ as a grid stopper. Yes, wired very close to the grid pin is best. And yes, for mains valves, 10kΩ is more usual. But what is the 'correct' value? Anything between so small that it is ineffective, and so large that other effects (treble cut; bias instability) start to kick in. Which can be a wide range!

[Neil Purling]

Just trying to understand what is going on in a radio I am dealing with at the moment.

[G8HQP Dave]

100k grid stopper might suggest that the designer was trying to keep IF out of the audio circuit, having failed to filter it properly at the detector?

[ukcol]

In the Pye Black box record players, both the BBH model which uses 2 x EL42s in push-pull strapped as triodes and the BBH MK2 which uses 2 x EL42 in ultra linear configuration use a 100k grid stopper. What is interesting is that the grid stopper is in the lower EL42 only, the upper one having no grid stopper at all.

[turretslug]

Not familiar with the circuit, but does it use a single valve "concertina" phase-splitter, with the "upper" output valve fed from the anode resistor- perhaps it's an attempt to even up the source impedance seen by each output valve's input capacitance?

[ms660]

In the schematic that Neil posted in the other thread it shows a 100pf decoupling the HT rail, I wonder if that should be decoupling the anode of the detector?

The detector looks like an anode (bottom bend) detector judging by the value of the cathode bias resistor, I wonder if the high value grid stopper is to reduce the chances of the output valve drawing grid current and affecting the operation of the detector during strong signal reception?

Lawrence.

[ukcol]

Yes, the phase splitter has a load resistor in the anode circuit across which the signal is developed for one of the push-pull output valves and a load resistor in the cathode circuit associated with the other output valve.

So this 100k resistor is not a grid stopper?

EDIT: this is a response to post 6.

[kalee20]

Originally Posted by turretslug

Not familiar with the circuit, but does it use a single valve "concertina" phase-splitter, with the "upper" output valve fed from the anode resistor- perhaps it's an attempt to even up the source impedance seen by each output valve's input capacitance?

Not a good idea if so. The concertina phase splitter relies on the anode load and the cathode load being identical, for identical outputs. So if one output has significant capacitive loading, then the other should too. Putting a resistor in series with one output and not the other will upset the balance.

[ukcol]

Yes, the resistors in the BB amplifiers I was referring to are the usual 20% types but the 2 load resistors in the phase splitter are closer tolerance high stability components.