Re: Autolite capacitors

[Gerry O’Hara]

I have attached the schematic for the 11/95 model. As Phil suggests, locate the Autolite caps on the schematic by tracing the circuit, identifying the correct part(s) and then replace them with similar values as per the schematic. If ever you cannot find the value of a part (capacitor or resistor), it is possible to make an educated guess at approximate values unless they form part of a tuned circuit (then it is rather more problematic, though you will find you don’t have to change these too often anyway as they are almost always silver mica types that tend to be much more reliable than paper or electrolytic types). The part(s) selected by ‘rule of thumb’ (see below) may not be perfect but it will most probably get the set working to a point you can experiment a little to optimize the values – most tube circuits tend to be fairly tolerent of resistor and capacitor values that are not exactly as designed (most parts used had a tolerence of +/-20% anyway), however, exercise some caution as component values in more innovative circuits can vary significantly from the ranges given below:

Capacitor ‘rule of thumb’ for tube sets:

– Power supply smoothing and audio frequency (AF) decoupling: 10uf electrolytics, 450vw (or at least 10% higher than the plate supply voltage).
– Decoupling of plate/filament supply, screen or cathode at radio frequency (RF): 0.01 to 0.05uf, at intermediate frequency (IF): 0.05 to 0.1uf, Mylar, polycarbonate or polypropylene, 630vw (or at least the plate supply voltage of the set plus 10%, unless used as a cathode decoupler, then it can be much lower, though at least 50vw, though if in doubt use 630vw). 0.1uf caps may also be used for AF stage cathode decoupling if the original cap was not an electrolytic.
– Inter-stage coupling: at RF: 0.0001 to 0.005uf, at IF: 0.005 to 0.05uf, at AF: 0.01 to 0.1uf Mylar, polycarbonate or polypropylene, 630vw.
– Tone: 0.01 to 0.25uf (use a range if there are several different switched tone positions – higher values give more bass cut), Mylar, polycarbonate or polypropylene, 630vw.
– Only use silver mica caps (or polythene caps if you can find high-voltage ones if needed) in IF an RF tuned circuits. IF tuned circuits at ~460kHz often use 100 to 150pf.

Resistor ‘rule of thumb’ for tube sets:

– Cathode: 150 to 470 ohms, 1w.
– Screen dropper: 47k to 100k, 1w.
– Plate (if used): 1k to 4.7k (RF and IF stages), 47k to 270k (1st AF stage).
– Grid bias: 470k to 1M, 0.5w.
– Volume control: 0.25M to 2M, audio (log) taper.
– Tone control: 50k to 250k, linear taper.

Download Rogers Majestic 11_95 small.jpg. (Caution: This file may not be virus scanned.)