... consider that the EF86 is near a 6SJ7 tube ...
I
really recommend Frank's for tube data sheets.
The
EF86 and
6SJ7 have similar Gm, but the triode Mu is different; Mu
g1-g2 of the EF86 is 38, where it is 19 for the 6SJ7.
Why do you care? Because the Mu from grid to screen impacts how the screen controls plate current. As a result, if you wanted to bias the 6SJ7 to the same operating point you have with the EF86 in its circuit, while using the same cathode resistor (and so same plate current/bias voltage), the 6SJ7 screen will need to be at a different voltage.
To give an example of how different the biasing is, look at the "Typical Operation" on the 6SJ7 data sheet linked above, where 250v is on the plate and 100v on the screen. The EF86 data sheet doesn't give the same tabulated condition, but look at the graph on page 6. This graph is for various screen voltages with the plate held at 250v; Look at the V
g2=100v curve... For this condition, the 6SJ7 had a plate current of 3mA with a bias of -3v, but the EF86 with the same bias has a plate current of 0.3mA.
So you
will not be able to plug a 6SJ7 directly into an EF86 circuit using the same parts values and get similar results. You'll likely need to go through a pentode gain stage design process to select voltages & parts values which cause the 6SJ7 to perform the way the EF86 did in the stage you're replicating. Unfortunately, there are more variables & more work to designing a pentode gain stage than a triode gain stage.
the 6SJ7 has a sensibility of 1.65 mA/V and 6SH7 4.9mA/V
So ... what will happen if, in a circuit similar to 5C1, a 6SH7 is used in place of a 6SJ7 tube ?
Assuming you have accurate Gm figures for the operating point of the tube, then for any pentode gain stage the actual amplification is Gm * R
L, where R
L is the load resistance (plate load resistor in parallel with any resistance to ground which follows). So the pentode with the higher Gm will exhibit more voltage gain with the same plate load resistor.
However, Gm is not constant; it must be evaluated at the chosen operating point, especially because data sheet Gm figures may be typical of what you'd get in a specialized RF circuit and wildly different than what you could realistically get in a guitar preamp. Gm for any tube usually increases with rising plate current.
If we take the same 250v plate, 100v screen, -3v of bias when evaluating the
6SH7, then the graph on page 2 of that data sheet shows plate current at 0.4mA (rather than the 6SJ7's 3mA), while the upper graph of page 3 shows Gm is only 700 micromhos (0.7mA/V) instead of the 6SJ7's 1.65mA/V.
So a direct-swap of the 6SH7 for the 6SJ7 with no supply voltage or parts changes will result in a stage with less gain, although it is uncertain without a fuller analysis of where plate/screen/bias voltage and plate/screen current will land.
Looks like none of these pentodes are as-similar as they first appeared.
Topic: Has anyone tried a 6SH7 in place of a 6SJ7 ? (in a 5C1 or other) (Read 10001 times)