MULLARD MUSTARDS OR C296 POLYESTER CAPACITORS

Well, I have broken off my series of Mullard historical blog posts to answer a query I have received from a customer about the so called Mullard Mustard capacitors.   These capacitors, introduced to the radio market in 1958 have a polyester dielectric which exhibits very low dielectric loss, high insulation resistance coupled with good temperature and moisture resistance.   In all these parameters they far exceed the efficiency of paper dielectric capacitors and impart a very similar audio signature I am told.

Throughout the 1960’s Mullard Mitcham conducted a whole raft of accelerated storage and use testing and found these are ultra stable, ultra reliable capacitors. Indeed ask many service men and they will tell you that never in their career did they need to replace one of the Mullard mustards due to failure in use. It is because of this reliability and  intrinsic audio characteristics that these are sought today for use by audiophiles and guitarophiles ( - is there such a word?).     Actually, the serviceman probably would not refer to them as Mullard Mustards but rather Mullard polyester capacitors  C296A.

I have been asked what these codes mean, well, the prefix C stands for capacitor, 296 is the series type of the capacitor, the series type notation is followed by either  AA for 160V rating or AC for 400V rating.    The coding will then continue in the form C296Ay/xxx. where the first (post slash) letter that indicates the tolerance of the capacitor.  In the case of the 296 series, A = +/- 10%. The numerical component denotes the capacitance value of the device.    Let us use the lowest value available of 0.001uF which can equally be expressed as 1nF.    

This will appear on the device as 1K giving, for a 400V device the full code C296AC/A1K.   Similarly for an 0.033uF 160V device, the full code would be C296AA/A33K.    In the same fashion for a 0.22uF 400V device, the full code would be C296AC/A220K.

In summary, one can always easily read the capacity of the device by considering the letter K to equate to the unit nF and the numerical component to be the value.  To translate an nF value into other units of capacitance, the following conversion factor may be applied.

1uF = 1000nF = 1000000pF

Anyhow, because I am a top geeza, I have produced a table showing the ENTIRE range of C296 capacitors ever made with their corresponding codes and capacitance values in various Farad units - pretty good huh?

The C296 series of capacitors also carry a three digit batch code of the form A8B

Where digit 1 is the quarter code: - A = January - March; B = April - June; C = July - Spetember; D = October - December.

Where digit 2 is the year code: - 1 = 1961; 4 = 1964 etc....

Where digit 3 is the manufacturing plant: - B = Blackburn; D = Hamburg: S= Barcelona;