Light up those funky-looking loctals!

 

1    Introduction

The loctal (or loktal or lock-in) valve is a variant of the octal which was used, especially in mobile equipment, in the 1940s (I think). The shape is roughly that of an ST octal, but the glass base, which is covered by a metal sleeve, is shorter, and the pins  are shorter and thinner and moulded into the glass, as in a miniature valve. The pins do not bend easily, which is nice, and the valves sit very firmly in their sockets. Loctals can often be bought cheaper than the equivalent octals, and the sound can be just as good (see for instance Eric Barbour’s article on small-signal triodes in Glass Audio 6/97). Favourites among the few (mostly DIYers) that have tried them include the 7N7 and 7AF7. (The former is one of - if not the -  lowest-distortion small-signal valves ever made; the latter is rather less perfect, but sports a lower Miller capacitance, which helps bandwidth). Their somewhat "no bull, mil-spec." appearance would look great in an amp like the Atma-Sphere M60 (the 7N7 could substitute for the 6SN7GT with a socket change). The main problem lies in finding sockets, so make sure you have some before you buy the valves! I bought mine (which seem to have teflon insulators) from Demostenes, Norway, but they only kept a small stock. Billington also has them.
   The American 6.3-volt loctals have a designator beginning with "7", e.g. the 7A4, which is an equivalent of the 6J5GT; I don't know why, maybe just for easy recognition (few other valve types do). These are run from the same nominally 6.3V heaters as their equivalent octals beginning with "6" (but note that the rest of the designator is different - most have a single letter followed by the figure designating number of connections). Most American loctals also have 12-volt variants; these begin with "14" (e.g. 14N7=12SN7), but run on a nominal 12.6V. As with other valves, I prefer the use of 12-volt equivalents for DIYing, as these are usually half the price of their 6-volt sisters, sound the same, and can be run more easily on regulated DC, since the current requirement is half (see the Tips and Tricks page). 
   In the European types, there is as far as I can see nothing in the valve designator that suggests loctal base.
   The type designator for 8-pin loctal valve sockets is B8G.

 

2    Some Useful Loctal Valves

The following valves are all useful in various audio-related applications. The European (Eu) types may be more difficult to find.

Diodes

7A6                    double detector diode, similar to 6H6, 6AL5/EB91; 6.3 V/o.15 A; max 150 Vrms in; max 2x8 mA out.

7Y4                    full-wave rectifier with separate filament; 6.3 V/0.5 A; max 325 Vrms in; max 70 mA output.

7Z4                    full-wave rectifier with separate filament; 6.3 V/0.9 A; max 325 Vrms in; max 100 mA output.

With the rectifiers, the first filter capacitor should preferably not exceed 33 uF.

Single triodes

1LE3 (dht)            mu=14.5 Cga=1.7pF                no equivalent

7E5                        mu=36    Cga=1.5pF                no equivalent

7A4                        mu=20    Cga=<4pF                 equivalent to         6J5G/GT (oval plate)

7B4                        mu=100  Cga=1.6pF                equivalent to         6SF5GT (?oval plate)

3A-167M (Eu.)    mu=47   Cga=3.5-4pF               equivalent to         WE437A (Pa 7W, mu 47, gm 47mS at 40mA)

The first four are useful when you need a single triode, and have the same properties as their octal counterparts, but are a bit smaller and have less interelectrode capacitance, yielding a (small) benefit in frequency response. The 3A-167M is an odd one by comparison, and rare as hens' teeth. It is linear and very capable (mu=47, Gm=47mS!) and can be used as a powerful driver or alone in a single-valve ("spud") power amp (Pout=~1W).

Single triodes with detector diodes

7E6                        mu=16         Cga=1.5pF            equivalent to         6BF6/6SR7

7K7                        mu=70          Cga=1.7pF           equivalent to         6AQ7GT

7B6/7C6 (7X7)  mu=100        Cga=1.6pF            equivalent to         6SQ7GT

These are just as useful as singles without the diodes: just ground the diodes’ plates (and cathodes when applicable) and forget about them – unless you use them for a tuner detector/voltage amp or stereo decoder (unlikely, but a cool idea!). The 7E6 looks very linear and should be evaluated.

Double triodes

3B7                   mu=20         Cga=?                       no equivalent           (common cathode "Siamese twin" triode)

7AF7                 mu=16         Cga=2.3pF               ~similar to                2x6J5GT

7N7                   mu=20         Cga=3pF                  equivalent to             2x6J5GT (less Pa than 6SN7GT, same box-shaped plates)

7F8                    mu=~47       Cga=1.2pF            no equivalent

7F7                    mu=70         Cga=1.6pF               equivalent to             6SL7GT (cylindrical plates)

These are quite useful valves. The pin-out is symmetric and feels more "logical" than on the octal equivalents. The electrode structure is closer to the pins, which are moulded into the glass, sometimes resulting in somewhat lower interelectrode capacitance – a good thing. The 7F8 has a very small structure, resulting in low Miller capacitance, so might be useful as a differential voltage amp for small signals.

Pentodes

4P1L                mug1g2=10-12        no equivalent        Directly heated Russian thingie never heard of in the West! 8W (2+2V filament)

7C5                  mug1g2=9.5            equivalent to           6V6/6AQ5, etc. 12W

35A5                mug1g2= ?              ~equivalent to         35L6/35B5/35/C5 8W (35V heater)

25/50A5           mug1g2= ?              ~equivalent to         50L6/50B5/50C5 10W (25/50V heaters)

C3m/C3o (Eu.) mug1g2= 19           no equivalent           Pa=4W, Pg2=1W (20V/6V heaters)

I haven’t tried any of these pentodes yet. The 4P1L (4P1L) is an interesting Russian design I was only recently made aware of (thanks Nic!). It could be used for a number of things, from phono to driver stages. The 7C5, 35A5 and 50L6 shouldn't be inferior to the average 6V6, 35L6, or 50L6. Remember that a pentode is also a "triode" (with g2 connected to the anode). The 7C5 seems to have rather good (triode-connected) linearity with a mu of 9.5. it could serve as a GC (grounded-cathode) output stage in a xfmr-coupled headphone amp or tiny power amp. Pentodes make good CFs, according to Alan Kimmel (page currently unavailable, but there's an article by Alan that touches on this at Bas Horneman's page). Any of these could make an excellent CF Line Driver or CF driver for an OTL PA output stage, whether connected as pentode, triode, or even in ultralinear mode (see Tube CAD Jan 2000 on ultralinear circuits). The 7C5 could of course substitute for a 6V6 in its ordinary role, pushing a PP OPT (in triode, UL, or pentode mode), as well. The 35A5 might make a powerful line driver or a low-power xfmr-coupled guitar practice amp, the 25/50A5 even more so. The C3m/C3o has recently been suggested as a good driver (in pentode mode) for 300B amps by Thorsten Loesch.

Other Types

85A1             85V/5mA voltage reference valve (similar to the 9-pin 85A2/0G3/5651)

EM71            tuning indicator ("magic eye"), top-view type, 6.3V/0.3A; the 12-volt version is HM71 Pic I'm using to of these in my Göllum mid/tweeter amplifier, to good visual effect!

_____________________

Some of the above valves also come in 12-volt varieties; these are cheaper, about half the price of the 6-volters. Exceptions are the 1LE3, 4P1L, 7B4, 7C6, 7K7, 7X7, 35A5, 25/50A5, 3A-167M, and C3m/o (which is a glass valve despite appearances). 

                                              A "Loc-Soc"
                 Sylvania 7F8                                                                                            A loctal socket

 

3    Some Loctal-Valve Data (with typical op. points given)

 

Valve     Vf     If        Va       Vg1     Vg2      Ia       Ig2        Ri       Gm     ug1g2   Ra    Pa

C3m       20    0.125  220V   -4.5     150V  16mA  3.2mA  380k   6.0     18       10k   4W

                                      

                                                      C3m – a ‘post valve’

 

Valve         Vf     If      Va        Vg     Ia        Ri    Gm       u     Ra     Pa

3A/167M   6.3   0.45  175V   -2.5   40mA  1k    47mS   47    5k     7W

                                      

                                               3A/167M – a ‘repeater valve’

 

Valve     Vf     If     Va     Vg1     Vg2     Ia         Ig2         Ri     Gm     ug1g2     Ra     Pa

7C5        6.3   0.45 250   -12.5    250     45mA   4.5mA   50k   4.1     9.5        5k     12W

                                      

                                                     7C5 – a loctal 6V6GT

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