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Hi all,

As seen here (http://tdsl.duncanamps.com/show.php?des=417A), pin 4, 5, 7, 8 of the 417A are all connected to the grid. Do I just run a single piece of wire through all those pins? I also heard that the tube is somewhat prone to oscillation and it is recommended that a 100-ohm grid stopper be wired in series with the grid. If I wanted to follow this recommendation, would I "star" the grid pins then wire a 100-ohm in series or would I wire a 100-ohm in series with each of the pins?

Thanks,

Min

I've seen a million (well almost) variations. One is to connect ALL grid pins. Another is to use a 100 Ohm resistor connected to the common grid point and another using one 100 Ohm resistor for EACH grid connection.

Separate grid stop resistor for each grid pin ( 4 off) back to a common connection point is the RECOMMENDED way.
This thing has huge gm and will just about develop parasitic oscillation in its storage box so best to follow the recommendation.
Cheers,
Ian

The 417A was designed as a grounded-grid UHF and low-microwave RF amplifier, the multiple grid connections were in order to reduce the inductance of the connections from grid to RF ground. As Gingertube says, the thing has scarey-crazy transconductance, and it WILL oscillate at the slightest provocation

http://frank.pocnet.net/sheets/049/5/5842.pdf

For audio frequencies, I'd just connect all four grid pins(4, 5, 7 and 8) together, and use a single 1K ohm, carbon grid stopper resistor. Might not hurt to use a plate stopper (100 to 220 ohms) too, small enough so it won't adversely affect the circuit gain, but sufficient to reduce the probability of spurious RF oscillations, in conjunction with the tube's internal capacitances. Connect it between the plate, and the plate load resistor / coupling capacitor junction.

If you do choose to use five separate grid stoppers, make each one approximately four times the normal resistance for a single GS resistor (think parallel resistances) , otherwise their efficacy could be compromised. Or just "cheap out" and use pins 4 and 7, that should be more than sufficient.

With its relatively low internal plate resistance of 1600-1700 ohms, and its 4.5W PD, the 417A / 5842 could be used in a single stage "spud" amp, with a 5K primary winding, with 750-900 mW power output.

/ed B

As Gingertube says, the thing has scarey-crazy transconductance, and it WILL oscillate at the slightest provocation

...does this have any correlation to the 12at7 falling out of favor in guitar amp phase inverters, after the 1960s....?

EWBrown wrote:The 417A was designed as a grounded-grid UHF and low-microwave RF amplifier.....

This 417A?

http://www.r-type.org/exhib/aab0137.htm

Klystron!

OK, this is the FIRST time I've ever seen one used as an audio amp. This oddballness is SO up my alley, I might just get one to see what-the-heck

Cheers!

I'd just connect all four grid pins(4, 5, 7 and 8) together, and use a single 1K ohm, carbon grid stopper resistor

I think this is what I will end up doing Thanks!

I've seen the 417A is used in phono preamps and sometimes as a driver for filamentary triodes in SET amps. In both case it is the low rp which is what the designers were looking for to best do the job.

nyazzip,
No - the 12AT7 fell out of favour as splitter in guitar amps because it was cheaper for manufacturers to just purchase just 12AX7 rather than having to stock 2 types of tubes. No other reason. It has nowhere near the transconductance of a 417A and is not much more susceptible to parasitic oscillation than a 12AX7 (both are trnsconductance wimps compared to the 417A).Technically the 12AT7 is a better choice for guitar amp phase splitter than the 12AX7 used these days. I use 6SL7 in my guitar amps which is similar to a 12AT7 in the older octal bottle. I have had problems with parasitic oscillation in a guitar amp phase splitter (the Industry "standard" stacked schmidt splitter or whartever more accurate name you want to give it) with a 12AT7. Putting in a 12AX7 made a rodents hind quarters difference, it oscillated too. Grid stops on both triodes fixed it.

You should ALWAYS use grid stops regardless of what tube or what circuit function.

Cheers,
Ian