SLO 100 grid and screen resistors

[makepeace]

Hi guys,

The grid and screen resistors of the Soldano SLO100 (from Weber's schematic) are 1k each. What would the effect be if the grid resistor is increased to 1.5k and the screen resistor reduced to 470ohms (the conventional 1x push-pull pair setup). Would it be a bad idea? Also if you were to change to cathode bias instead of fixed bias would a change in those resistors be problematic?

Thanks!

Jono

[defaced]

You could take the grid resistors way up and not notice a difference.

The schem I have lists 500R for the screens (SLOClone forum), so I don't see why you couldn't use 470R. Personally though, I'd keep it at 1K. The sonic/power changes you will see are minimal but you're screens will be happier with 1K screen resistors.

[makepeace]

thanks defaced. have just done some reading about grids and screens, and have gained some vital understanding! i understand that unless you're not using 'good' KT88's, you should stick with higher screen resistors, BUT, there is a noticeable tonal variation by decreasing them.. Genalex Gold Lions it is

i'm busy thinking up my next amp and this will be of much use..

thanks a lot!

[defaced]

I run 3k screen resistors on my KT-88s. They're too expensive (for me) to risk them going up in smoke. I'm also running the screens at 530v, so there's another reason to use them.

[makepeace]

yoh hectic. yes after doing some research i realise they are rather expensive. not really worth the chance hey? i'm going to be running mine at around 450V. do they go quite often? cause thats quite hefty if they do.. 1 more question if you will , screen resistors on a 5881 push-pull pair.. will it largely affect tone if they are increased from the conventional 470 ohms to 1k or so?

[defaced]

I have only killed KT-88s by things unrelated to normal operation. Translation: my f%&k ups while working on the amp.

"Largely affect the tone"? IMO, no. But you must consider the source, I'm a 95% sorta guy. I'm not willing to put in the extra effort to chase the last 5% of magic pixie mojo dust. I want an amp that will both sound good and work, not sound mojo amazing and fry tubes all the time. Those who say they hear a difference say it's "smoother". Given what's actually happening, I interpret that to mean power amp compression. To what extent that is going to happen and whether or not you'll like it is up to experimentation. I'd fit the amp with 1K resistors then if you notice you don't like the effect this has on the output section, change them.

[makepeace]

Thanks defaced, great advice. As I haven't been into guitar and building all that long, I'm still in the process of "placing myself" so to speak, in either the realist or the magic pixie mojo dust camps. I really do like the sound of the second one, but my experiments in the sphere haven't been all that fruitful.. My guess is that i'll fall in to obscurity somewhere in between

Will go for 1k screens to start with. Is there a way to test for when you're getting close to the limits of the screens?

[J M Fahey]

Dear makepeace.
May I humbly suggest you start by actually building some classic, time tested amp, your choice?
Then you plug your guitar, test it in 1000 different ways, get well acquainted with it, and only then start modding it, (one at a time) always comparing "before" and "after" to check whether you actually improved something .... or not.
You seem to be pre-modding it before actual building.
I think going slow and steady will be much more satisfactory for you.
With best wishes.

[makepeace]

YGPM.

Thanks defaced for the useful posts.

[defaced]

You're welcome, but if you sent me a PM, I did not get it.

[jmaf]

If you add a too big screen resistor you start getting lots of compression from the power tubes. It's "harder" with no resistor, but then again that screen grid is way more fragile than a plate and it can be destroyed by relatively low power.

[Zozobra]

Just wondering if anyone has experimented with mixing screen resistors in an amp running a quad, say using 2k on one pair and 3k on the other? Are there any reasons why this is a stupid idea?

[jmaf]

Just wondering if anyone has experimented with mixing screen resistors in an amp running a quad, say using 2k on one pair and 3k on the other? Are there any reasons why this is a stupid idea?

IMO it's not a stupid idea. It's the kind of thing that sets boutique amps apart from those made in a factory, the kind of thing I'd expect in a Trainwreck or Dumble, but not in a Peavey. Little details that add that sweet tone to some amps, you analyze the circuit and can't tell what it is and it was just a different resistor on the screens...a pair works more compressed, the other works more loosely, in the meantime both are protected because 2k for example is a good resistance for most tubes(el84's may require more, from strange cases I've seen).

Edit: by "good resistance" I mean in terms of protection. In my case I use the cookbook recipe of 470 for 6L6 and 1k for EL34, 2k2 for EL84.

[Zozobra]

Ah cool, thanks! I had thought about the tonal effects of this but was slightly worried that their might be some deeper issue that might result in an expensive failure as I'm really still getting to grips with power amps. When I finally fix my AOR I might try this out.

[jmaf]

Ah cool, thanks! I had thought about the tonal effects of this but was slightly worried that their might be some deeper issue that might result in an expensive failure as I'm really still getting to grips with power amps. When I finally fix my AOR I might try this out.

The expensive failure risk is from a too small resistor. For instance, if you use a 470R resistor on a EL34 it'll get really really hot and may fry. (The resistor, not the tube. I've fried some of both trying to learn this.)

So err on the side of bigger screen resistors. 2K for EL34 or 6L6 is safe, perhaps even too large for 6L6. I've tried 10K, it really compresses the output.

To fry a KT88 plate you need to do some heavy damage, but the two grids are very delicate things....so you oughta naturally protect the first grid from excessive current and the screen grid from excessive dissipation when the plate swings lower. That is all I can think of as precautions for the tubes, the rest is up to experimentation.