Variable cathode resistor

[Rutger]

Hi,

This is my first post here. Since I can't find hardly any decent information about this idea/mod I thought I might ask you for some input on this subject.

I'm planning on building a SE guitaramp with only two gainstages. There are several common ways of controling the amount of overdrive (at lower volumes), but I was wondering if this would do the trick as well:

Why not make the cathode resistor of the second gainstage variable? In theory the higher the value of the cathode resistor, the less gain and the more overdrive you will get from the gainstage. But will this work properly in real life, or are there certain side issues you have to consider?

Has anyone experimented with this idea?

[Enzo]

No reason you can't try it, but having the DC tube current in the pot you'd have to have would result in a scratchy sounding pot.

[Steve Conner]

I used a variable cathode resistor in the first stage of my old Ninja Toaster amp. Even though it used a capacitor to remove DC from the pot, it was still scratchy, and changing the capacitor out didn't seem to make any difference. I replaced it with a mini rotary switch and network of fixed resistors.

[Rutger]

Thanks! Good idea, I will use a rotaryswitch with fixed resistors

@Steve: I had in mind to vary the actual cathode resistor (which is typicaly 1,5k) and put a 22u cap in paralel. In theory the gainlose would be minimized by the cap while increasing the value of the cathode resistor. Should that work properly?

[Steve Conner]

That doesn't make sense. In your first post, you said that you wanted to decrease the gain by increasing the cathode resistor. Now you say that you want to avoid gain loss when you increase the cathode resistor. Bypassing it with a 22uF capacitor would certainly avoid gain loss, but it seems to me it would defeat the purpose of the whole thing.

It's negative feedback across an unbypassed cathode resistor that lowers the gain. If the resistor is bypassed, the gain won't change, except in so far as the tube's own gain is a function of its bias point. But that's a second-order effect.

My circuit does the opposite, it changes the AC gain of the tube without moving the DC bias point.

[Rutger]

If the resistor is bypassed, the gain won't change, except in so far as the tube's own gain is a function of its bias point. But that's a second-order effect.

My circuit does the opposite, it changes the AC gain of the tube without moving the DC bias point.

I'm sorry I've not explained my idea well enough, I'll try to make myself clear:

Actually the purpose of my idea is to move the DC bias point, so the tube will distort easer. I understand there will be some loss of gain by doing this, but that's more of a side effect. By keeping the gain as large as possible by bypassing the cathode resistor, the loss of gain will be limited.

I just want to make a simple SE design a little more versatile by doing this.
I'm wondering if that makes sence or if it's just a stupid idea...

[tedmich]

I used a variable cathode resistor in the first stage of my old Ninja Toaster amp. Even though it used a capacitor to remove DC from the pot, it was still scratchy, and changing the capacitor out didn't seem to make any difference. I replaced it with a mini rotary switch and network of fixed resistors.

Merlin does this in his chap 1, the 1uF will boost above ~150Hz right? Still scratchy eh? Merlin says w/ DC blocking it shouldn't be...good idea the ohmic quanta...

I built a little variable parallel cap (w series resistor) to try on my cathodes.


tants but they're not the antichrist some say they are.

[PaoloJM]

I used a variable cathode resistor in the first stage of my old Ninja Toaster amp. Even though it used a capacitor to remove DC from the pot, it was still scratchy, and changing the capacitor out didn't seem to make any difference. I replaced it with a mini rotary switch and network of fixed resistors.

That's a very interesting first gain stage. I'd imagine that it has close to infinite input impedance with the boostrapping. Is it a AU7 you're using, the load value would seem to correspond. With the Baxandall it nearly looks more like a HiFi stage!!
What's the idea behind this design? Wide, flat frequency responce? How does it sound, any clips?

[Prairie Dawg]

tants but they're not the antichrist some say they are.

sorry Ted that looks like the mother of all noomies.

[Steve Conner]

Rutger: I'm sure that would work, if that's what you're trying to achieve. In another amp I built, I applied combination cathode and fixed bias to one of the preamp tubes. I used the main power tube bias supply, reduced by a zener, and a little pot to adjust the bias on that stage. It certainly did change the character of the overdrive.

paolo: I used a 12AX7. Never thought to try the stage with a 12AU7. A 12DW7 would probably be best, because the second stage needs all the gain it can get to make the tone controls work. I also never thought of the first stage operation as being a kind of bootstrapping, but I guess it is.

tedmich: The idea behind the undersized cathode bypass cap is to get a kind of Tube Screamer effect. As you turn the gain boost up, it boosts mids and treble more than it boosts bass, so it doesn't get so farty.

This amp was built in 1999, so it predates Merlin's book by a long time. It also has a 2-band parametric EQ and a switchable gain structure with 3, 4 or 5 stages. I was deliberately trying to make something abnormal.

You can hear it here:
http://scopeboy.com/music/mp3/Way%20...%20Worrier.mp3

and here: http://music-electronics-forum.com/a...lve-test-1.mp3

It's hard to say what it sounds like because it's so configurable. When I built it I was into metal, and I would use all 5 gain stages and use the EQ for a mid scoop, to get that kind of nasty Boogie grind. Now I'd tend to use 4 stages and boost the mids instead.

The little tantalum guy looks scary! :-O

[Rutger]

Thanks Steve, great help!

[Rutger]

Hi,

Actually a different question on the same subject.

I would like to try a sort of 'powescaling' on my SE design. I have a EL34 in the powersection (10W) and want to reduce the power by switching to a different cathode resistor value. The normal biaspoint is 350V/70mA, by switching the resistor it will become, lets say 350V/10mA.

I think it will work, but my main question is: will it still sound/play nice? Are there any experiences with this kind of powerscaling?

Thanks

[Steve Conner]

"Mooreamps" on the forum has a power scaling method where he varies the screen resistor. If that works, I guess varying the cathode resistor might work too.

But my feeling is that the screen resistor will give better sounding distortion at low power, because the clipping will still be quite symmetrical.

If you use a high value cathode resistor, the tube can turn ON a lot further than it can turn off, and that will make the clipping very asymmetrical.

[Rutger]

Thanks, thats true. But it's not that I'm searching for a spectacular sound at low volumes. I just want a decent sound so I can play without anoiing the neybours. I also want to use it as a practice amp during my students' lessons.

[Rutger]

One thing about Mooreamps powerscaling method. I couldn't figure out the details on this forum, but it seems to me that he achieves the same result (lower the platecurrent) with his method as I do with dailing in another biaspoint (at a lower platecurrent)...?