12DT8 Valve

[Danndad]

Would anybody have a good 12DT8 valve they want to sell? Unfortunately after cleaning the chassis off my Rincan Jap radio I seem to have caused a valve to go to air

They appear to be plentiful and cheap in the States etc. but the postage to the UK is the deal killer. I could use a ECC85 with a dropper resistor across the heater? Or a ECC81 with some modification to the wiring. I would rather have a 12DT8 though. Price to include postage to Cornwall please.

   

Danndad

[Geordie McBoyne]

12DT8 has a 12.6V 0.15Amp heater. Pin 9 is the intersection screen connection.

From the Radiomuseum: https://www.radiomuseum.org/tubes/tube_12dt8.html

12DT7 characteristics are more similar to 12AT7/ECC81 than 12AU7.

Disconnect pin 9 from chassis.

Geordie McBoyne.

[Mike Watterson]

12DT8 has a 12.6V 0.15Amp heater.  Pin 9 is the intersection screen connection.

From the Radiomuseum:  https://www.radiomuseum.org/tubes/tube_12dt8.html

12DT7 characteristics are more similar to 12AT7/ECC81 than 12AU7. 

Disconnect pin 9 from chassis.

In fact the 12AT7 /ECC81 seems identical except that 12DT8 screening is to pin 9 and has no centre-tap heater, but 12AT7 has the heater centre-tap to pin 9 instead.
https://www.radiomuseum.org/tubes/tube_12at7.html

The 12DT7 may only be preferred in a very few applications where additional RF isolation between the two triodes is needed. The two triodes are well spaced apart in the 12AT7/ECC81

Do what Geordie writes, simply cut pin 9 connection and use an ECC81. Even if it's in the VHF it's unlikely to be any more effect than changing a 12DT7.

[Geordie McBoyne]

Kolster-Brandes employed the 12AT7 as the FM tuner valve in many models such as the FB10FM and the NR30.

The attachment shows the circuit of the FM tuner in the model NR30.

Geordie McBoyne.

[Geordie McBoyne]

Evidently there is a 0.1amp heater version of the ECC85 or 6AQ8.

The HCC85: https://www.radiomuseum.org/tubes/tube_hcc85.html

Geordie McBoyne.

[Mike Watterson]

Evidently there is  a 0.1amp heater version of the ECC85 or 6AQ8.

The HCC85:  https://www.radiomuseum.org/tubes/tube_hcc85.html

Geordie McBoyne.

Not 0.1A
That's the 0.15A for USA/Japan 110V markets, usually actually a 17EW8 (Click on 16=1963 etc)

The 0.1A for 220V-250V is the UCC85
https://www.radiomuseum.org/tubes/tube_ucc85.html

I had one of these. Any Series heater AM/FM Radio set in UK from 1955 would likely use UCC85. They'd all be 100mA (TVs P series, 0.3A, so there is a PCC85).
https://www.radiomuseum.org/r/ekco_u243_u_243.html

The total heater voltage gets too high for 110V countries using 100mA series on AM/FM radio. The 200mA was pre-WWII C series for AM only sets.

[Danndad]

Thanks for all the replies, shame nobody has 1 for sale though.
I will see if i have a ecc81 here and do the necessary mods.

[CambridgeWorks]

Just had a look. Despite my having a box containing around 30, mostly nos valves, not that one unfortunately.
Mine are all b7g and versions of the more common 6*** types.
Rob

[boater sam]

I have many thousands of valves but never had a 12DT8, sorry.

[roadster]

Langrex, who are my first choice for buying valves, are showing 11 12DT8s in stock. It is worth a phonecall: 01403 785600.
Rod