[R-390] Gain drift

[ROSS HOCHSTRASSER]

It probably wouldn't be a bad idea to replace these resistors with either more modern carbon or maybe even metal film resistors. It is possible that these resistors have drifted far enough out of value that their effectiveness has been compromised. The audiophile world has realized that carbon comp resistors do contribute noise of their own , especially if excessive heat was used when "modifying" the circuit.It may also be advisable to re-tune the coils after replacing the resistors. Very interesting thread!! W1EKG
> On October 20, 2018 at 9:41 PM Roy Morgan <k1lky68 at gmail.com> wrote:
> 
> 
> 
> >On Oct 20, 2018, at 4:58 PM, dog <agfa at hughes.net <mailto:agfa at hughes.net>> wrote:
> >
> >Jacques,
> >
> >Yes, those T502 resistors are still in there. So far I haven't heard the gain drift since I resoldered the joints but I need to give >it some hours. 
> 
> Dog and others,
> 
> There was a BAD modification published a long time ago - folks were advised to clip those Q-spoiling resistors.  This increased the gain of the set, making it “much hotter”.  This of course was a big mistake.
> 
> Don’t do it.  If you find a radio with that mod applied, put the resistors back into the circuit.  Then do an alignment on the affected stages, and set the IF gain.
> 
> 
> By the way, I recall discussions about the use of modern carbon film resistor (some with spiral tracks of resistance material on ceramic tubes), and how they exhibit unwanted inductance in RF circuits.  I have long held that many uses of these resistors do NOT involve RF voltages on the resistor, such as in bypassed screen dropping or bypassed cathode use. And blindly assuming that these modern resistors will act badly in all situations is a mistake.
> 
> BUT, these resistors in the IF (or RF?) cans of the R-390A really do experience RF voltages.  (If I remember correctly, the R-390/URR - the “non-A” - may use such resistors in different stages than the R-390A.)
> 
> HOWEVER, it is another matter whether or not the actual inductance that may be present if you use modern carbon film resistors matters much if at all.
> 
> If anyone can point us to measured data on the inductance of such resistors, please do.  I remember that in the past, such data was posted or referenced on the R-390 list.  (I may have it but can’t locate it now.)
> 
> A final note: the technology involved in 30 megacycle IF amplifiers used in WW-II era radar systems involves the use of resistances to set the Q and stagger tuning the various stages to achieve much desired bandpass and phase characteristics.  This is covered in Valley and Walman “Vacuum Tube Amplifiers” of the MIT Radiation Laboratory Series.  (The math involved is not for the faint of heart.)
> 
> 
> Roy
> 
> Roy Morgan
> k1lky68 at gmail.com <mailto:k1lky68 at gmail.com>
> 
> 
> 
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