ECC83 and Germanium diode to replace 7C6
 

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

Re: ECC83 and Germanium diode to replace 7C6
 

Nice job, Epoxy pot it.. Thats miles better then our mod for the PL802 for all them CT205s in the 70s. Well done.

Bob.

Re: ECC83 and Germanium diode to replace 7C6
 

Ingenious. Did you consider the 12SQ7GT?

Re: ECC83 and Germanium diode to replace 7C6
 

To be honest I had not heard of the 12SQ7GT. Checking just now I see it has a 12v heater, it is the correct current, 0.15A so probably would not make much difference in the series heater chain.

I just used what I had to hand.

Peter

Re: ECC83 and Germanium diode to replace 7C6
 

I did a similar thing but in my case I used a 6AT6 (EBC91) which is a double diode triode just like the 7C6 but a B7G base. That works just as well and I was later able to replace it with the correct valve.

7C6 should be still available although I haven't checked the usual valve suppliers recently.

That's a neat job you've done.

Re: ECC83 and Germanium diode to replace 7C6
 

Langrex are showing 163 of them in stock.

Cheers,

GJ

Re: ECC83 and Germanium diode to replace 7C6
 

Very neat job Peter - glad my original thread has proved helpful.

My EL84/EBL31 substitute was in use for some time and gave excellent service until I was able to obtain a sensibly priced EBL1 side-contact valve to convert to an EBL31 by fitting an octal base and correct sized 1/4" top cap. (All the so-called 'EBL31' valves sold by UK dealers are converted EBL1s - too tall to fit in the Ekco A22, and with a 3/8" top cap).

As you say, one of the benefits of this approach is that the chassis and valve-holder wiring is left intact so if the correct valve comes along at the right price, it will plug straight in.

Re: ECC83 and Germanium diode to replace 7C6
 

The 14B6 Loctal valve is a later issue of the 7C6. It's a 12.6v, 150ma version of the same basic valve. It's a perfect plug-in replacement.
Of course, it's an American thing to come up with a range of valves, where the heaters add up to our mains voltage to save the cost of a dropping resistor.
BTW, the 7volt and 14 volt loctal valves, the heater voltage is 6.3 and 12.6 volts respectively.
Another Dave, chiming in.

Re: ECC83 and Germanium diode to replace 7C6
 

That's a neat adaptor! I remember there used to be commercially-available adaptors like that - one version was supplied with the 8 Octal pins wired to short flying-leads with ready-tinned ends, so you could wire them to the tags on the B7G/B9A socket in the right order for whatever valve-substitution you were undertaking.

As to the valves used in US radios, yes the 'all american five' was cunningly aranged so as not to need a mains-dropper when supplied from US 110/120V supplies. There were some really-odd valve-voltages availabe over there though: I remember a 19AQ5, being the 19V version of the 6AQ5 and used in a RCA communications-receiver.

Re: ECC83 and Germanium diode to replace 7C6
 

Ingenious and very neat! Well done.

Re: ECC83 and Germanium diode to replace 7C6
 

I did a similar fix once on an old Murphy - the octal-based rectifier had failed, and in such an old set I didn't want to subject the smoothing capacitors to the high HT voltage from s.s. diodes during warm-up of the rest of the set. I simply removed the octal base from the old valve, soldered into the hollow pins some 2.5mm cores stripped from some T&E, sleeved them and wired them to the appropriate pins on a B9A holder for an EZ80 to sit in. The finished structure was very rigid and stable, though not quite as neat as Electronpusher's

Re: ECC83 and Germanium diode to replace 7C6
 

Thanks to G4XWDJim the correct valve has now turned up.....
Swapping between the correct valve and my adaptor shows no difference in performance but for the sake of keeping the radio original my adaptor is now in the spares box :(

Peter