27-10-2012, 04:11 PM
Hi Joe, I have to agree, the valve list and the circuit diagram are a bit in conflict with each other, doing a web search shows few internal details and the manuals that turn up all seem to be from one source, suffice to say there should be 3 EF39 types (1 RF, 2 IF)
In my experience many manuals have many mistakes..!
Valves/stages can go into unintended oscillation, sometimes the valve itself can be the problem but it's usually due to a component fault ie: insufficient decoupling etc introducing unwanted positive feedback or by the omission of screening cans/leads etc that the manufacturer originally fitted.
Most valves that are not screened internally or externally by a metalized coating can be screened if the need arises externally with a suitable valve base shroud and can, room permitting.
If a valve is in the circuit as an oscillator, then it's down to the type of oscillator, valve, and feedback arrangement, there are umpteen types, Collpits, Hartley, Clapp etcto name a few but in most radios it's the anode feedback to a tuned grid type.
Unwanted oscillations (birdies) in the RF, Mixer, IF sections can sometimes be a real pain to track down and cure, some receiviers were not designed too well and probably left the factory just on the point of producing birdies and it does not take much to tip the balance, slight loss of decoupling, leakage in switch wafers, tag strips, printed circuit boards, valve bases shifting the wiring etc.
I understand what you mean about the theory, sometimes the mere word is enough to put people off, it's best to simplify the theory in blocks down to something that is understandable.
Lawrence.
In my experience many manuals have many mistakes..!
Valves/stages can go into unintended oscillation, sometimes the valve itself can be the problem but it's usually due to a component fault ie: insufficient decoupling etc introducing unwanted positive feedback or by the omission of screening cans/leads etc that the manufacturer originally fitted.
Most valves that are not screened internally or externally by a metalized coating can be screened if the need arises externally with a suitable valve base shroud and can, room permitting.
If a valve is in the circuit as an oscillator, then it's down to the type of oscillator, valve, and feedback arrangement, there are umpteen types, Collpits, Hartley, Clapp etcto name a few but in most radios it's the anode feedback to a tuned grid type.
Unwanted oscillations (birdies) in the RF, Mixer, IF sections can sometimes be a real pain to track down and cure, some receiviers were not designed too well and probably left the factory just on the point of producing birdies and it does not take much to tip the balance, slight loss of decoupling, leakage in switch wafers, tag strips, printed circuit boards, valve bases shifting the wiring etc.
I understand what you mean about the theory, sometimes the mere word is enough to put people off, it's best to simplify the theory in blocks down to something that is understandable.
Lawrence.
