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7788 Triode Curves

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I don't think John Atwood would mind if I post his picture of a curve trace of triode-connected 7788.
 

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Interestingly, neither John Atwood's nor Jim Hagerman's 7788 triode curves come very close to the spec. Not surprisingly mu is about right in either case (John's mu was about 53 to Jim's 50), but in both cases gm is low and rp is therefore high, even with John's tube measuring significantly better than Jim's sample. For example:

At 100 volts on the plate and 20mA plate current, John's tube requires -1.3V grid bias, gm is about 30mS and rp is about 1800 ohms.

At that same operating point on the plate, Jim's tube requires -1V grid bias, gm is about 12.5mS (about what a 6DJ8 can do) and rp is about 4000 ohms.

At that operating point I'd have expected a gm value of roughly 40mS and an rp value of roughly 1300 ohms. I never quite expect to attain full data sheet values in all tubes I measure, especially with these "hot rod" types, but I would have expected better than these numbers. I have a number of samples of 7788s, and in the crude checks I've done for gm, I've achieved gm values that were close to expected values, except for well-used samples. John's measurements are low but almost in the range of acceptability, while Jim's sample seems off. Jim, how did you connect the screen grid - large resistor? Worn-out tubes? Can't be the VacuTrace, I'm sure ;)
 
What`s the reason for stopping the production?

I ran out of chassis. When I went to reorder, the manufacturer had lost my cad files and silkscreens. Didn't wanna pay for those again. So I am designing a new chassis that is way more cool. Will cost a little more, though. I expect to be back in production by the end of the year.

neither John Atwood's nor Jim Hagerman's 7788 triode curves come very close to the spec

Are we sure? The spec is written for pentode only (except for that last graph). Not the same. Things changes in triode mode.

In my test I connected screen through a 100 ohm resistor. Don't know what the history of these tubes are. They look great, but bought them on eBay. Boxes were not original. They could have come from anywhere.

What I got was roughly mu = 50, gm = 20, rp = 2.5k. Bias at 200V, 20mA is -3.0V.

I could have me a worn out tube, in which case rp rises. Nonetheless, it looks like a great piece with superb linearity. I'm trying it out as a 2A3 driver at constant current 20mA.

jh
 
hagtech said:
Are we sure? The spec is written for pentode only (except for that last graph). Not the same. Things changes in triode mode.

Yeah, pretty sure. See my post from 5-27-07 above. Those are triode-connected data from the manufacturer. If a pentode has a plate gm of, for example, 10mS with 10mA of plate current, while the screen current is 2mA, then the triode gm (screen connected to plate) should be pentode gm * (10+2)/10 = 12mS. In others words, triode gm should be better than pentode gm under the same operating conditions since the usually "wasted" screen gm is added to the plate gm.

In my test I connected screen through a 100 ohm resistor. Don't know what the history of these tubes are. They look great, but bought them on eBay. Boxes were not original. They could have come from anywhere. ... I could have me a worn out tube, in which case rp rises.

That was probably the case.

Nonetheless, it looks like a great piece with superb linearity.

Yes indeed, a very nice tube.
 
7788 in triode

I just stumbled upon this thread and have a few comments.

The scope picture was made on a Tektronix 570 tube curve tracer, which I keep fairly well calibrated. I haven't recalibrated it in a few years, but it is quite stable. The 7788 that was tested was N.O.S. and I think it was a Dutch Amperex.

The 7788 is indeed a very linear tube. However, be careful about VHF oscillations, due to its very high gm. Grid-stopper resistors on the control grid and screen grid (mounted right at the socket) are essential.

- John Atwood
 
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