Philips 834A and 274A. Substitute for B3 994V.

[FERNSEH]

The high mu triode used as the anode bend detector in the Philips Superinductance models 834A and 274A is almost impossible to find these days.
Fortunately, for me the 994V is OK in the Philips 834A I have in the collection.
In the 1958 Mullard Maintenance Manual no replacement for the 994V and 904V is listed.
Today, I made up an adaptor to find out if the common PC900 triode would work as a substitute for these valves. The 994V has a mu of 125, a gm of 3.5mA/V and an ra of 35Kohms. The 904V has a mu of 72. The E499 used in the continental versions of the model 834A has a similar mu but different gm and ra figures.
Well, the PC900 does work but certainly no way as good as the 994V. Also, there is some distortion which will be a result of the PC900 having vari-mu characteristics.
Maybe the UHF tuner valves PC86 and PC88 might work instead? Both have 3.8 volt heaters.
There's plenty other high mu triodes available but have 6.3 volt heaters, the 6AV6/EBC91 could be considered. mu is 100. ra 62.5Kohms gm 1.6mA/V

DFWB.

[Sideband]

I thought it was the grid base (cut-off point) that was the important characteristic? The valve as I understand it is biased near the cut-off point when used as an anode bend detector. The 994V is also good in my 274 but I have tried other valves as well, notably a triode strapped EF80 (what else??) with a doubler for the heater and a resistor to bring the voltage down to 6 volts and a 164V which I happen to have. Both worked after a fashion but gave lower gain than the 994V. They all gave a degree of distortion on strong signals.

[FERNSEH]

According to the circuit diagram and parts list for the continental versions of the Philips 834A the value of the cathode resistor of the anode bend detector is 16,000 ohms and the anode load resistor is 330,000 ohms. The latter seems to be a high value for any triode valve.
The ra of all the triode TV tuner valves is low being about 5,000 ohms.
The 994V has an ra of 35,000 ohms and the 904V is 36,000 ohms.

DFWB.

[turretslug]

There is a 6AX7 version of the familiar 12AX7 that has a centre-tapped 6.3V heater, i.e. it can be run at 3.15V- whether there are many around is another matter! Similarly, a 7AU7 version of the 12AU7 with the same heater arrangement.

[FERNSEH]

The 6AX7 is available from the Tubestore.com for $19.00. Also, the Russian equivalent 6N2P from an UK seller for £19.95p.

Might have a 6AV6/EBC91 somewhere in the shop.

6AV6 characteristics similar to 12AX7. mu = 100

DFWB.

[turretslug]

Beware that the 6N2P is only usable at 6.3V though, as it forsakes the heater centre tap so that a base connection to a screening plate between the two anodes is available instead.

[FERNSEH]

Thanks for the info and the warning about the Russian valve. There seems to be plenty 6N2P valves available. Pin-out will be the same as the 6/30L2.

DFWB.

[murphyv310]

Hi.
You may have to change the cathode resistor and substitute a pot and adjust for best results especially on strong signals. When I made a radio with an anode bend detector I initially fitted an EF91 with good results, then I moved onto an EF80 with no real change but then I tried an EF184 with improved output but distortion on strong signals, I then went down the pot route on the cathode, once adjusted I got a good balance between strong and weak stations and then fitted a suitable fixed resistor.