Question about Power Tube Grid Stoppers.

[psychepool]

Most of the tube amplifier's poweramp are similar in structure and have only minor difference between some value.
By the way, some amps have grid stopper infront of power tubes, and some amps dose not.
Even each amplifiers with grid stoppers have different values.

Recently, I made 2 channel amplifier based on marshall.
It is a high-gain amp that mainly uses preamp distortion, and the output tube distortions are not used well.
I omitted the grid stopper for no particular reason.(Just putting one more part in was annoying......)
But now I'm going to change my mind and try to add,
What is the role of the amplifier power grid stoppers? What happens with the different resistor values?

[roberto]

Most output stages use relatively small grid resistors, such as the 1.5K seen on the grids of 6L6 tubes in most Fenders, and the 5.6K seen on the grids of EL34 tubes in most Marshalls. In general, the grid resistor at the grid of the power tubes can be as high as 56K to 100K before any noticeable loss of high frequencies occurs. Higher values can help in reducing "blocking" distortion as noted above, and can also take some of the "edge" off of an overly brittle sounding output stage. If the resistor value is made too low, it may not be enough to prevent parasitic oscillations, and the amplifier may exhibit instability in the higher frequency range. This may or may not be audible.

source: https://www.aikenamps.com/index.php/gri ... -they-used

[R.G.]

A series grid stopper resistor forms a low pass filter with the grid capacitance of the tube. The bigger the resistor, the lower the high frequency response of the tube and vice versa.

It's a "stopper" because inserting a resistor there can stop local high frequency oscillations in some circumstances.

[pdf64]

The grid stopper resistance value should be somewhere between the minimum required to ensure power tube stability (8/gm) and that necessary to achieve the desired upper frequency response and blocking distortion mitigation, whilst also considering the limiting value of grid circuit resistance for the tube type.
Some good info on the topic (thanks to gingertube) https://music-electronics-forum.com/sho ... post413772

[gingertube]

That 8/gm value applies to triodes. I actually have never seen a formula for power output pentodes/ beam power tetrodes, However the inverse relationship with gm does still apply.

Hence you see (opposite of what was suggested above)
5K6 on Fender 6L6 Amps (6L6 gm is around 4 mA/V at typical Git Amp operating point) - 8/gm would suggest 2000 Ohms minimum
1K5 on Marshall EL34 Amps (EL34 gm is around 7-8 mA/V at typical Git Amp operating points) - 8/gm would suggest 1150 ohms minimum

The reason I wonder if the 8/gm applies for power pentodes is experience with a 6V6 push Pull amp
6V6 gm is around 3mA/V at typical Git Amp operating point - 8/gm would suggest 2700 Ohms minimum
BUT on this particular Amp I had to go to twice that to stop oscillation and in the end just put 10K in for peace of mind (x3 the 8/gm).

I'm going to suggest that for power output tubes:
you assume a gm value (at typical Git Amp operating points) of 70% of the datasheet value gm.
BUT
actually use a gm value of 50% of datasheet value for calcs to allow for tube aging (gm reduces as tube wear out)
Calculate the minimum gridstop from 8/gm as for a triode
AND then double it for peace of mind.

How would that pan out?
6L6 - 50% of datasheet gm is 2.8mA/V, 8/gm = 2K857, double and round to preferred value => 5K6
EL34 - 50% of datasheet gm is 5.25mA/V, 8/gm = 1K523, double and round to prefered value =>2K7 or 3K3
6V6 - 50% of datasheet gm is 2.1mA/V, 8/gm = 3K809, double and round to preferred value =>8K2

That is just for immunity from parasitic oscillation.
Note that while we get "typical" values for 6L6, this suggests that the typical 1K5 for EL34 might be a bit marginal, OK for new tubes but not for aging tubes.

When you overdrive output tubes and start to deliver grid current the input impedance drops dramatically. You can "tune" the power amp overdrive sound somewhat by using larger gridsdtops (than those calculated above) to limit that grid current.

This is particularly valuable for amps with concertina (Cathodyne) phase splitters like Fender 5E3 for example.
Concertina Splitters react badly to unbalanced anode and cathode loads (the 3 push pull sides) and large gridstops go a long way to equalising these load in output tube overdrive.

My own Git Amp (my design and build) uses a cathodyne splitter into a push pull 6V6. I use 39K gridstops on the 6V6s.

Hope ther is something of value in this.
Cheers,
Ian

[roberto]

This is particularly valuable for amps with concertina (Cathodyne) phase splitters like Fender 5E3 for example.
Concertina Splitters react badly to unbalanced anode and cathode loads (the 3 push pull sides) and large gridstops go a long way to equalising these load in output tube overdrive.

Thanks Ian, I think this is the reason why VHT added the cathode followers after the concertina to their PI, to be able to better drive the power tubes while in AB1, and be able to go in AB2.

[pdf64]

Schematic? Bear in mind that if there’s a coupling cap to the power tube control grids then due to grid conduction, AB2 operation generally isn’t feasible (as the grid conduction acts to charge up the coupling cap, thereby increasing the effective bias voltage).

[Roe]

this is the reason why VHT added the cathode followers after the concertina to their PI, to be able to better drive the power tubes while in AB1, and be able to go in AB2.

any schematic or layout detailing the VHT/Fryette PI?

[psychepool]

I thought it was a simple question, but I think there are very high-level answers. Thank you for all your answers.
My amplifier is of the same structure as Marshall Plexi and has only a slight mod on the NFB side of Value.
6L6 is used for the output tube. There is no particular reason, just the OT used is Hammond's fender replacement spec, so I chose it.
Among the circuits that have been widely viewed, the highest value was around 5.6K.
My amp has no grid stopper at all, so if there is no change after putting it on 5.6K, use it as it is, and If there is a noticeable change in the high-pitched range, I will try to lower the value gradually.