Critical resistors for s.e.x amp?

[jrebman]

Thought I'd ask this question to get the ball rolling and as sort of a starting point for a knowledge base.  What resistors have people found to be the most critical to the sound of the s.e.x. amp?

We already know that the 499 ohm resistor in the C4S is one, but what others have people found some improvement with?

Thanks,

Jim

[Caucasian Blackplate]

Since you mention C4S, I will assume that you are building the kit with the upgrade.

You could spend some money on nicer grid stoppers.  Anything from 100-1000 ohms is just fine here.  When I built my first SEX amp, I also upgraded the quality of the 120 ohm series resistors mounted to the headphone jack.  Get six really good 120 ohm resistors and use the same part for the grid stoppers also. 

The 100k pot can also be upgraded with several different parts.  The TKD pot at Parts Connexion so far fits the best. 

[jrebman]

Paul,  I had planned on upgrading the grid stoppers, and I already have a couple of AN tantalums for the heaphone jack, as well as a pair of shinkos for the c4s boards.

BTW, I can't seem to find the instructions for the iron/c4s upgrade as they are not on the new s.e.x. amp product page (yet?).

Thanks for letting me know about the TKD pot -- I just found one today that I had squirreled away for another amp project from the past and that won't be getting built now.  This is the 100k, 270 degree rotation pot that pcx sells.  I tried a dry fit on the plate and it looks like it will fit with no changes at all, except that it points 90 degrees towards the headphone jack.

Thanks,

Jim

[dmannnnn]

Should the grid stoppers stay as carbon composition or does it really matter?

[Paul Joppa]

Should the grid stoppers stay as carbon composition or does it really matter?

VoltSecond, who should know, says metal film works just fine in this application. I'll take his word any day on this kind of subject!

[dbishopbliss]

Get six really good 120 ohm resistors and use the same part for the grid stoppers also. 

I'm just about to start soldering parts in my upgraded kit, but I don't want to go back upgrade.  What makes a resistor really good?

What is the preferred brand these days.  I thought holco's were good, but they seem to be on clearance everywhere and pretty cheap.  Does this mean they didn't sound good or do they have lead so nobody wants to use them?

[Grainger49]

Since you mention C4S, I will assume that you are building the kit with the upgrade.

You could spend some money on nicer grid stoppers.  Anything from 100-1000 ohms is just fine here.  When I built my first SEX amp, I also upgraded the quality of the 120 ohm series resistors mounted to the headphone jack.  Get six really good 120 ohm resistors and use the same part for the grid stoppers also. 

The 100k pot can also be upgraded with several different parts.  The TKD pot at Parts Connexion so far fits the best. 

Paul,

I'm confused.  I had always read that the grid stopper didn't draw any current, therefore wasn't considered to be in the signal path.  PJ had always posted that the Carbon Composition (CC) resistors were chosen for their low noise performance in this application.  That is discounted by the reference to VoltSecond.  And I too trust his opinion about anything noise related.

I'm just asking why now the grid stoppers (and I would guess the plate stoppers) are considered to be critical?

Thanks,

[JC]

I think the idea on grid stoppers is that their function is to dampen any parasitic oscillations that may arise because of the tiny capacitance between the grid and the other tube elements.  Usually, these are more of a problem at radio frequencies, but it is good practice to provide against them at AF, as tiny little RF oscillations can tend to add a harshness to the sound.  Also, tiny little RF oscillations have a tendency to become amplified in an amplifier, and create problems well beyond the audible ones!

Having said all that, then, the idea is to use a non-inductive resistor to do this job; good practice says to keep the lead of this part short where it is attached to the grid, for even a straight piece of wire has some inductance, and you are trying to avoid inductance; a tiny bit of inductance added to the tiny bit of capacitance may actually make a better oscillator, and you're trying to stop that, after all!

The most common non-inductive resistor is the plain, old carbon-composition resistor.  Film-type resistors, either carbon or metal, tend to be constructed by etching away the film in order to arrive at the desired value.  The etching commonly leaves a tiny spiral of film as the final resistive element.  IOW, a coil, which is going to have some inductance at some frequency, in theory anyway. 

So, good practice for grid-stoppers dictates that you want a non-inductive part attached to the grid to dampen any oscillation that may arise.  A carbon-comp is the most commonly available choice for this duty; certainly there are others.  Another choice, if you happen to have a fast scope, would be to leave the grid-stopper out entirely, and use the scope to see if you even have any parasitic oscillations!  Of course, you would have to check for this every time you changed tubes, and twice on Sundays, since the inter-electrode capacitance may change over time or due to operating conditions!

No doubt, Volt Second is absolutely correct in his assessment of resistors for this duty; it is highly likely that the type of part is just not that critical, especially since there may not be an oscillation to stop in the first place!  But, a facet of good design is to provide against even the unlikely occurrence, and good practice says a carbon-comp is the best choice for this duty.  Bottlehead is pretty consistent where good design and good practice are concerned!

In the past, I've used grid-stoppers up to 10 kOhms in pre-amp circuits; the amount of current running through them would generally be measured in micro-amps, if you could measure any, so the value is not all that critical, either.  Plate-stoppers are another matter, of course!  Both are "in the signal path", since the signal runs right through them, but there is such a tiny amount of current at the grid that any effect of this resistor on the signal has to be correspondingly tiny.  Again, the plate-stopper, since it has the plate-current running through it, should theoretically have a much more profound effect.

Of course, in this hobby, the only way to really find out what you like in a given application is to try everything!

[Caucasian Blackplate]

It is indeed true that there is minimal current going through the grid stoppers, although their fragile construction leads me to often replace them with some Rikens or Holcos in my own gear for their reliability and toughness, especially if they are connected between a tube socket and a terminal strip (tube socket pins move around with tube rolling).  The carbon composition resistors that come with the kits are very high quality parts, I am pretty sure it would be less expensive to provide metal films, but perhaps not as ideal.

[dbishopbliss]

I was looking around a well-known boutique part supplier and noticed that the only 499R resistors they had are Caddock MK-132 with radial leads. For slightly less money, there are 500R Shinkoh Tantalum resistor.  I assume that the 1 Ohm is within tolerance.   I've never used either of these types of resistors before.  Can someone say what I might expect from either of these choices or if there is a better alternative?

Also, for the 120R Headphone/Grid Stopper resistors, I'm considering Rikens.  Again, any guidance would be appreciated.

[jrebman]

JP,

Thanks.  My purpose in asking was so that that could be added to the knowledge base, which it looks like it has.

Thanks,

Jim

[JIPPER]

Jim,

I posted that link, and then it appeared as a "sticky" topic.  Then, seeing it above the locked topic by Doc B.  I thought I put it in the wrong spot?  My intention was a simple reply.  So I just deleted the "sticky" version and put it back here in the list of responses to your post.

In any case, if it should be a sticky, I am sure someone will put it back again?

What's the knowledge base?  Maybe that's the sticky option?

Glad to help.

JP

[jrebman]

JP,

Yes, sticky = knowledge base.  Sorry, just the computer/cognitive scientist in me using terms that burned into memory :-).

I've just been looking at this thread and will look at the whole board later when I have time, so I only saw this post and that doc had split the thread.

Thanks anyway as I think more people will want to know where the information is.

-- Jim

[Doc B.]

Yes, I felt it was an important topic for this board so I made it a sticky. Thanks for posting it.

http://www.bottlehead.com/smf/index.php/topic,61.0.html

[JC]

Doc,

Presumably, this information is on the Bottlehead server, then, and not subject to the disposition of the old forum?

Thanks!