Purpose of 5842 tube?!?

[mctavish]

Just got my first pair for an amp build and looked at the pinout, Why so many pins to G1? Oscillation is not my friend! What was this tube originally created for?

[Miles Prower]

Originally Posted by mctavish

Just got my first pair for an amp build and looked at the pinout, Why so many pins to G1? Oscillation is not my friend! What was this tube originally created for?

It's a UHF triode designed for grounded grid service (the only thing that'll work at frequencies north of 300MHz if you don't go for more exotic designs). It includes so many connections so that by connecting all of them to the ground plane you cam minimize the impedance of the ground return to prevent instability.

[bob91343]

http://tubedata.milbert.com/sheets/009/5/5842.pdf

This was intended for rf use and with a grounded grid, it is desirable to return the input and output circuits to separate grid pins.

[mctavish]

Thanks for the replies. Is it wise in that case to put a grid stopper on all grid pin connections if used for audio in grounded cathode config?

[Miles Prower]

Originally Posted by mctavish

Thanks for the replies. Is it wise in that case to put a grid stopper on all grid pin connections if used for audio in grounded cathode config?

It might make for less noise pick-up on unconnected grid pins.

[Eli Duttman]

Originally Posted by mctavish

Thanks for the replies. Is it wise in that case to put a grid stopper on all grid pin connections if used for audio in grounded cathode config?

It's essential, not "merely" wise, that a Carbon composition stopper be placed on each grid connection. Arrange 4X 10 KOhm CC stoppers into a "cone". Apply the driving signal to the construct's apex.

The 5842 and the similar Russian 6С45П (6s45p) come annoyingly close to oscillating in the cardboard box. Stoppers on ALL grid connections are the 1st, but not the only, precaution necessary. Put ferrite beads on the heater power wires as close to the socket as can be obtained. Put a 100 Ω Carbon film stopper on the plate connection. While CC makes the "best" stopper, there is the issue of noise.

[kevinkr]

Not trying to be contradictory but I have been using the 5842 in designs for over 10yrs now and have found there to be absolutely no difference in the tendency to oscillate whether all 4 grid pins are used or not. Good overall layout is the most critical thing. Given this finding I use a single grid pin with a grid stopper resistor mounted right at the pin and leave the other 3 pins open. I have never had a problem with oscillation with this type, the D3A or 7788 (or any other high transconductance type for that matter) and have not been able to measure any difference in performance at audio frequencies for any possible combination of grid pins provided that oscillations are not present. (I have >100MHz scope and access to even faster ones.)

Keep components in the cathode circuit short, place the grid stopper resistor right at the grid pin used, and in cases where the output signal (Plate) is going to the outside world I place a 100 ohm resistor in series with the signal lead right at the plate. Careful planning to minimize required circuit lead lengths is a good idea.

Many people also employ common mode chokes on the filament mounted right at the socket, not a bad idea, but I have not found this to be necessary either and generally don't bother.

Metal chassis and proper grounding design help a lot. Note that I live less than 1 mile from a 10kW AM radio transmitter and have no rfi issues in my system. Wish I could say the same thing for the B*se home theater system that shares the same space..

[Enzo]

I don't know this tube, but I should think the central thing here is that using it in an audio circuit is a lot different from using it in RF circuits.

[Eli Duttman]

Originally Posted by Enzo

I don't know this tube, but I should think the central thing here is that using it in an audio circuit is a lot different from using it in RF circuits.

Look at the data sheet. The type will oscillate if you look at it cross eyed, given its VERY high gm of 25 mA./V.

[kevinkr]

Originally Posted by Eli Duttman

Look at the data sheet. The type will oscillate if you look at it cross eyed, given its VERY high gm of 25 mA./V.

Eli is correct, special precautions beyond what is typical for most audio types are required.