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MrCurwen 21st October 2011 08:45 PM

Dual grid and plate pins
I've been curious about the pin configuration on my latest find, the 8025A. I haven't yet done anything with it, but I intend to drive ESL headphones with it.

The tube has filament connections on the base (including a center tap). The top two pins on the sides are both connected to the plate, and the bottom two both connected to the grid. Now this is a single triode envelope of course.

Why did they put in double connections? Especially since you have to have caps covering the pins even if you don't use them - I wouldn't leave a metal part uncovered with the plate potential on it. So no savings really. These tubes were meant primarily for HF operation, in radio equipment and possibly radar. Is there a HF reason for double pins? How does it work?

Aesthetically I like the buggers. Rugged looks, engineer's tubes. =)

Rod Coleman 21st October 2011 10:29 PM

For UHF valves, doubling up the pins reduces inductance. The internal wiring is very long in high frequency terms - and lowered inductance makes a big difference at the frequencies they were designed for.

For non-UHF use, you can cut the pins off, if they are in your way. Just be sure to use a method that does not break the tube. OR, remove the socket connexions.

MrCurwen 21st October 2011 10:38 PM

Ah, makes sense. Thanks.

Aesthetically I like the symmetry so it's ok!

TheGimp 25th October 2012 08:07 PM

Did you ever do anything with the 8025As?

The data sheet shows a DHT whose datasheets look like it may be difficult to get linear without positive grid drive.

The 8025A is the same as a 8012A with the base with heater connections, and caps on the grid and plate terminals.

Weird looking tubes.

MrCurwen 25th October 2012 09:47 PM

I've done some crude measurements. I plan to use them in A1, with high impedance gyrator plate load.

With simple voltmeter and a couple of adjustable voltage sources I mapped out some operating points in the lower voltages, below 400 volts. Within the limits of my ordinary voltmeter it would seem to be quite perfectly linear, possibly not so nearing 0 V grid bias, but I know my results were within the (very wide) tolerance of the meter even there.

So, next step is to make a small SE amp with these, to actually test out the sonics. Heating is DC by active device, and plate voltage something low, along 300 volts or so. Gyrator load, parafeed.

If it sounds transparent and dynamic, then the next step is to make an ESL amp with it. I have an existing ESL amp with 2C26 (which is a really beefed up 6SN7 basically, and at least as linear), where the 8012/8025 would pretty nicely drop in with only minor adjustments. Plate voltage 400 to 500 volts, with voltage swing about 450 volts, directly coupled to ESL stators. Sounds very transparent with the 2C26's. In my experience DHT's often perform extraordinarily well, so I have great expectations for this one.

If this family doesn't sound as transparent as I've hoped, I have a bunch of 3C24 with similar specs that are knows to be extraordinarily good tubes for audio.

Wavebourn 25th October 2012 10:16 PM


Originally Posted by MrCurwen (
In my experience DHT's often perform extraordinarily well, so I have great expectations for this one.

This one is very different DHT, it's made not for better linearity, but for less capacitances and inductances: in class C on kilovolt B+ linearity was not needed. Anyway, good luck in your experiments! :sax:

TheGimp 28th October 2012 05:48 AM

Have you seen the Shishido 8025A amp?

Number eight on this page:

Shishido's Amplifiers Circuit

Someone posted in another thread that the designer had a book describing his amps and their design.

I think the biggest difference between the 8012 and 8025 is the change from conventional getter to zirconium getter. Yes, the 8025 has caps instead of just the wire extensions for the connections to the grid and anode, but those are superficial. The change from solder terminals for the filament to four pin base is certainly an improvement. However the getter change was probably the biggest manufacturing evolution.

Lots of info here on getters:

H.Michi 28th October 2012 07:20 AM

I built an 8025A SE amp and carried it for ETF2008.
The sound was fairly good!
In ETF2008, a German friend told me that the 8025A was used for a jamming transmitter
(XT-39). US bomber were loaded with it against German rader-site during the warld war two. The 8025A is a very special UHF transmitting tube!

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