Grid Stoppers

[ToolGuyFred]

Did a quick forum search before posting, sorry if this has been covered elsewhere.

Have recently completed an original Foreplay (using Foreplay 2 assembly instructions, so I'm not sure what that makes it...). I have tried quite a variety of tubes and so far have seen a bit of a pattern emerge. Either I get a good sound and the tubes are microphonic (not so good with loud volume) or I get a good sound and the tubes go noisy. Current production (JJ and Gold Lion) are less microphonic and more noisy. NOS can go either way. Electro Harmonix Gold didn't impress.

I see many designs use grid stoppers but the FP2 doesn't have them. I think I read somewhere that they can reduce noise. I have some nice 330 ohm carbon resistors I thought I might try. Any thoughts? Should they only go on the voltage amplifier section or on the cathode follower too?

The amp has the current source and stepped attenuator upgrades if that makes any difference.

[Grainger49]

The grid stopper is to stop high frequency oscillations (think RF).  It has no effect on microphonics.

I have had only a few microphonic tubes all others have been fine.  Microphonics are the fault of the tube itself, not circuit design.

Reading the post the only question is how you wired the heaters.  The FP 1 was a 12.6V heater winding the FP 2 was 6.3V.  Plus there is a heater biasing circuit that keeps the heater to cathode voltage reasonable.  I'm guessing the rest was the same.  I haven't seen a FP 1 schematic. 

[Paul Joppa]

Grid stoppers are never a problem, and always good practice. They are usually more important with tubes that have higher transconductance - the 12AU7 is nominally 2200 micromhos, vs. 5500 for a 2A3 or a 12AT7. Though very similar, the Foreplay III is the first one that I designed, and I always use stoppers.

For noise and microphonic problems, you may want to adjust teh gain budget of your system. See the notes on signal and noise levels on the Community page, if you have not already.

[ToolGuyFred]

The grid stopper is to stop high frequency oscillations (think RF).  It has no effect on microphonics.

I have had only a few microphonic tubes all others have been fine.  Microphonics are the fault of the tube itself, not circuit design.

Reading the post the only question is how you wired the heaters.  The FP 1 was a 12.6V heater winding the FP 2 was 6.3V.  Plus there is a heater biasing circuit that keeps the heater to cathode voltage reasonable.  I'm guessing the rest was the same.  I haven't seen a FP 1 schematic. 

Thanks Grainger. I understand the function of a grid-stopper, I was just moaning that my only noise-free tubes are microphonic.

The mains transformer on my amp is from Black Art Audio (Australian) as Bottlehead didn't supply a transformer for European voltages at the time. Heaters are 12.6V and I have the heater biasing circuit which measured correctly at the time of build.

Grid stoppers are never a problem, and always good practice. They are usually more important with tubes that have higher transconductance - the 12AU7 is nominally 2200 micromhos, vs. 5500 for a 2A3 or a 12AT7. Though very similar, the Foreplay III is the first one that I designed, and I always use stoppers.

For noise and microphonic problems, you may want to adjust teh gain budget of your system. See the notes on signal and noise levels on the Community page, if you have not already.

Thanks Paul. I have way more gain than I need so I'll look into reducing that.