Quote:
Originally Posted by bikerhifinut
for further completeness, the EF40 has identical characteristics apart from heater draw to the earlier EF37 and later EF86 the difference mainly being the base........................
Which suggests that Mullard/phillips got it right before WW2.
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As I understand the chronology, the EF40 was part of the initial Philips Rimlock valve range and introduced the bifilar heater for lower hum transfer than was the case with the EF37, which originated as a lower microphony version of the EF36. Then somewhat later, circa 1950, the EF37A, with the bifilar heater, was introduced, initially supplementing but later supplanting the EF37. It was described in Mullard Outlook Volume 1, Number 3, 1950 November. The EF86, as said essentially a rebased EF40, followed circa 1952. Data on hand suggests that the EF37, EF37A, EF40 and EF86 all had 6.3 volt, 200 mA heaters. So they may have been suitable for 200 mA series strings as well as for 6.3-volt parallel circuits. The 200 mA concept was pretty much dead (except perhaps for Mazda) by the time the EF86 arrived, so its 200 mA heater was more happenstance carryover. But both 100 mA (UF86) and 300 mA (PF86) versions followed, although possibly their very low hum attribute were somewhat wasted in series-string circuits.
Cheers,