Grid stopper resistor required (KT66)?

[porkchop61]

I've built Mikael Abdellah's KT88 SE amp, and was wondering if I need to add a grid stopper resistor if I want to use a KT66? The data sheet suggests a value of between 10k & 50k. Doesn't this resistor control gain of the stage (along with the grid leak resistor?) I know this resistor has to be soldered right at the tube socket. Can I leave this resistor in the circuit if I want to go back to the KT88's & EL34's?
The only thing I don't like about this, is that it's another component in the signal path.

Thanks in advance.
Glenn

[Tube_Dude]

Originally posted by porkchop61
I've built Mikael Abdellah's KT88 SE amp, and was wondering if I need to add a grid stopper resistor if I want to use a KT66? The data sheet suggests a value of between 10k & 50k.

Hi Glenn

>10 K seems a little high to me. For me, 2,2 K maximum 4,7 K is about right!

Doesn't this resistor control gain of the stage (along with the grid leak resistor?)

No, the gain of the stage don't change.

Can I leave this resistor in the circuit if I want to go back to the KT88's & EL34's?

Yes, you can.

[porkchop61]

Thanks Tube Dude
I'll probably look around for similar circuits to see what others use for grid stopper values.
Glenn

[ray_moth]

With any OP tube that is capacitor-coupled from the previous stage, it's a very good idea to use a grid stopper of minimum 1k.

With a pentode/beam tetrode, the grid stopper value can sometimes be a lot higher, to minimize the effect of blocking distortion on overdrive due to transients. A screen stopper, from 47 to 470 ohms, is also a very good idea, even if the tube is triode-strapped.

There is no effect on gain, because a grid stopper does not form part of a potential divider, but there is an upper limit dictated by the tube's maximum grid-ground resistance and, with a triode, the tube's Miller capacitance.

Without stoppers, you run the risk of parasitic oscillations, which can be at ultrasonic frequency. To be effective, the lead from the tube socket pin to the body of the stoppper resistor must be very short and the resistor must not be inductive. Carbon comp. resistors are best if you can get them. Normal wirewound are the worst type and may actually make things worse.

[porkchop61]

Thanks.
I have a 100R screen stopper resistor in place already per Tubelab's suggestion. This is for Triode strapping the output tube (normally I run in UL, but it's nice to have the ability to switch to Triode & Pentode connections).

I will play around with a grid stopper resistor. What is the best type to use for this application? After all, it is now a component in the signal path. I thought I also read somewhere that this resistor changes the high frequency roll off of the circuit. I guess this is what we're after anyway to eliminated the oscillation. I guess the trick is to keep the roll off above the audio range.
Glenn

[Lowrider]

DISCLAIMER: I am a beginer, I read a lot on the net, but thats all...

I removed the output tubes grid stoppers on my EL34 PP amplifier, following the advice of a friend that is experienced, and it was one of the most effective mods I did to my amplifier...

At last, all arshness is gone, it sounds natural, very nice...

[porkchop61]

Interesting, mine is an SE amp, and I was advised to install the grid stopper resistors
Maybe SE topography has different requirements?
I didn't mean to open a can of worms here.
Glenn

[Tweeker]

Physical topology has an impact, that is to say the layout of the amp, lead inductance etc. Tube type has plays a large role, and some of the types popular in SE use might be more susceptible to it. The OPT can also play a role in some oscillations.

Grid stoppers are highly recommended in cathode followers, and in tubes designed for VHF use.

[jnb]

I think you should try yourself to get it to oscillate to confim whether it has enough (properly implemented) resistance.

Switching on/off, plugging/unplugging sources, touching the inputs, volume control up/down/centred, square waves, switch on the fridge etc...

[Poindexter]

That would be my call.  I have not used grid-stop resistors on my last several projects, and experienced no oscillation whatever.  This includes an EL34 direct-in power amp with 6dB global negative feedback, and in that (and every other) instance, I observed absolutely no resonant behavior (output peaking above 0dB, or at all) up into the low megahertz range.  The place to look, if you have a generator and meter, is for a response that (after rolling off starting between 10K and 30K) rises again and peaks somewhere. In a power amp, the usual suspect is the output transformer primary resonance, which will almost always fall between 50KHz and 120KHz.  Doesn't hurt to pan on up to a meg or so, just to make sure.

In every experiment with grid stoppers I have performed, they were noticeably audible, to the detriment of the transparency of the piece.

Try without, and test.  If you absolutely have to use them, do.  I think that those instances will be rare to none.

Aloha,

Poinz