Grid stoppers?

[Stuff4bikes]

Why do some circuits not use them on power tubes?

This one for instance. I built this circuit with an EL84 instead of the 6K6 and had some weird noises.

 I used values for the EL84, not the 6K6 values....

Installing a 1K resistor on the grid of the power tube solved the issue.

[pdf64]

These days, dunno 
In olden times, parts were expensive, so if they weren’t strictly necessary, why fit them?
Plus I seem to remember reading there was the viewpoint back then that the higher the equipment’s part count, the more prone to failure it was. 
It may have been a UL thing? 

[Stuff4bikes]

Why do some circuits not use them on power tubes?
…

These days, dunno 
In olden times, parts were expensive, so if they were strictly necessary, why fit them?
Plus I seem to remember reading there was the viewpoint back then that the higher the equipment’s part count, the more prone to failure it was. 
It may have been a UL thing? 

This particular circuit didn't work for me as drawn with an EL84, so did my sub for power tube cause my issue?

[sluckey]

Your layout and wiring skills may be a bigger factor than tube type.

[HotBluePlates]

... I built this circuit with an EL84 instead of the 6K6 and had some weird noises.

... didn't work for me as drawn with an EL84, so did my sub for power tube cause my issue?

6K6 data sheet
EL84 data sheet

EL84/6BQ5 has a more sensitive grid, easier to get it to misbehave.

Look at the data sheets for "transconductance" ("Gm") and you'll find the EL84/6BQ5 has a 5x higher figure.

Gm is a measure of the plate current change for a given grid voltage change.  For the same plate current output, this means the EL84 only needs 1/5 the grid-signal.

Noises/oscillation can result from electromagnetic signals in the air inducing current in circuits, which result in voltages across resonant circuits (inductance & capacitance) or circuit resistance.  The EL84's higher Gm also means these unwanted signals need only be 1/5th as-big than in the 6K6 circuit to cause trouble.

Grid stoppers aim to add resistance to the resonant circuit (wiring inductance, stray capacitance, tube input capacitance) at the grid.  The stopper makes the resonant circuit lossy, developing less voltage at the grid (which the tube sees as signal-input).

High-Gm tubes (sometimes called "high-slope" tubes) more readily create an output with small grid voltage inputs, so they will often need a grid stopper (or a higher-resistance one) where a less-sensitive tube would not need a stopper.

Your layout and wiring skills may be a bigger factor than tube type.

Indeed, good layout is even more important with high-Gm tubes.

[tubeswell]

Why do some circuits not use them on power tubes?... I built this circuit with an EL84 ... and had some weird noises ...Installing a 1K resistor on the grid of the power tube solved the issue.

You answered your own question

[Stuff4bikes]

Your layout and wiring skills may be a bigger factor than tube type.

Man , I resemble that remark.......

Turns out the 1K grid stopper didn't cure my problem. When turning the to around 9 o'clock the problem has reared its ugly head again.
I will go over my wiring again...


*********-UPDATE********   it looks like I battled my first case of a microphonic tube....the 12ax7 was bad.....