I am restoring a Rent-a-set model 551 which is based on the Strad 553 Chassis and needed to replace 3 of the valves as they had gone (very) low emission, 14S7, 35A5 and 7C6.
The first 2 I was able to get NOS on ebay at about a fiver each but the 7C6 proved more elusive, I could only find used “from working equipment” ones at a much higher price.
I found David’s excellent post about the use of germanium diodes to “substitute for the two diodes in the rare EBL31 valve, when it is substituted for a more common valve such as the EL84 output pentode”:
https://www.vintage-radio.net/forum/...ad.php?t=49848
and decided to try and substitute the 7C6.
The 7C6 is a triode / dual diode construction with the triode described as “high mu”.
I have several ECC83 dual triodes also described as high mu. The characteristics seem very similar so this was my starting point.
An added complication is that the valve heaters in the Rent-a-set are in a series chain with a current of 0.15A, the 7C6 is 6.3V.
Fortunately each half of the ECC83 is constructed entirely separately with a separate heater for each triode. Each heater is also 6.3V, 0.15A.
In my set the triode section is grid leak biased with a grounded Cathode and a 10M resistor from Grid to ground, I needed to check if the ECC83 would work if biased this way so I set up a quick test rig. Anode resistor of 220K connected to a stable 150V and measured the Anode volts. I tested a selection of valves. The results were encouraging:
Mullard 83.3V
Mullard 79V
Brimar (12AX7) 81.2V
Out of interest I also tested an ECC82 19V!
My next problem was to make a LOCTAL plug. I wanted to have the substitute valve plug into the existing valve base so that the set could be returned to original condition if ever a 7C6 turned up.
I broke up one of the duff valves from this set to obtain the metal base. To mount the pins at the correct spacing I utilised the base of an old OCTAL relay, of course the pins are too large and the centre spigot is wrong on Octal but the spacing is correct.
For the pins I used 1.2mm semi hard brass wire which I soldered into the pins of the Octal base UPSIDE DOWN. I reduced the Octal base diameter so that it fitted the metal base of the Loctal metal base and glued it in place using 2 part epoxy.
The centre spigot of the Octal base which now points up was drilled and used as a convenient mounting point for the B9A base, I used a ceramic pcb version. The pins of the Octal base also point up and are convenient points to solder the links to.
Hopefully the photos illustrate what I mean.
The connections are
LOCTAL plug B9A base
1 9 heater
2 6 Anode
3 7 Grid
4&7 8 Cathode
8 5 heater
Pins 1,2,&3 of the B9A are connected together and linked to pin 8 of the B9A, this links the connections of the unused triode to cathode (0V in my set)
A germanium diode is connected Anode to pin 5 and Cathode to pin 4 on the LOCTAL plug. I only used one diode as only one is used in my set, if 2 diodes are required the other diode is connected Anode to pin 6 and Cathode to pin 4 of the LOCTAL plug.
I am happy to report that the substitute works perfectly.
I will probably find some sort of skirt to cover the links and tidy it up.
Of course if I costed my time it would not be cost efficient but that’s the beauty of a hobby, you never cost your time.
Peter