23-11-2015, 04:40 PM
(20-11-2015, 09:49 PM)Terry Wrote:(20-11-2015, 04:53 PM)Nowhere-Man Wrote: Assuming this is indeed the control grid resistor it measures about 53 K. The schematic gives I think 48 K ...
Something that is frequently overlooked by novices such as you - please don't take this personally! Just read on - is the subject of tolerance. These days we have digital meters that measure resistance very accurately (or purport to) whereas when the set was actually made, even the best meters around relied on looking at what was a very cramped scale and trying to interpret where the meter needle was pointing!
Originally, manufacturing components to a high accuracy was very expensive, so a scheme was devised whereby components with a ±20% tolerance would satisfy any need. This resulted in a range of 6 values spanning each decade which neatly butted together so that there no gaps. The range was; 1, 1.5, 2.2, 3.3, 4.7 and 6.8, known as the E6 series.
If you look at the markings on resistors on your radio you will see the value marked by 3 coloured bands (which superseded an earlier method known as body, tip, spot). It is extremely unlikely that you will see a fourth band, which means that the tolerance is ±20%.
The resistor on the oscillator grid, R5, is shown as 47k. This can measure anywhere between 37.6k and 56.4k and still be within the specified ±20% tolerance!
Forums like this are full of threads with posts such as 'I replaced Rx (100k) because it measured 103.6k ...', yet it could have been anywhere between 80k & 120k and still been within tolerance! Don't fall into this trap!
Over time, as manufacturing procedures improved, the range of values expanded to 12 per decade (the E12 series) although the vast majority of resistors were still sold as ±20% types. Gradually, though, ±10% resistors became available, indicated by a fourth, silver ring.
If you dabble with much more recent kit, you may find resistors with a gold band - this indicates ±5% tolerance. Yet again, the number of values doubled to give the E24 series.
I produced this graphic some time ago to demonstrate how these various values but together or marginally overlap for these three ranges.
I suggest you keep it handy or even print it out - you will find it very useful when trying to decide if a resistor is within tolerance or not ...
Depends what you mean by "novice"? I do have some background in boat engineering which also includes work on diesel engines or cylinder heads, alternators, water-pumps, inverters and so. This is different in as much as thicker cables and larger currents are normal. Maybe it's even harder in some ways as the technology keeps moving onwards whereas the tube radio work is frozen in time.
I became involved in radio engineering somewhere around 2 years ago. Most of this was basic theory till around a year or more when I started actively working on sets. I'd say the bulk of my initial knowledge was taken from Chas Miller's well-known Radio and Audio book but now I'm using a lot more American textbooks aimed at HAM engineering.
There was never any apprenticeship for me naturally as I'm self-taught and also isolated on my boat. This is a good thing I think as I have to work much harder.
In terms of where I want to be, it does seem radio construction is very much on the horizon and probably using a lot of American circuit design. Also seems probable I'll be sticking to tubes and not semiconductors.
