Grid stoppers, clamping, blocking and crossover distortion. - diyAudio
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Old 13th September 2009, 11:02 PM   #1
jjman is offline jjman  United States
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Default Grid stoppers, clamping, blocking and crossover distortion.

In my homebrew guitar amp Iím using a LTPI to drive 2- 6v6 tubes in PP. They are cathode biased running ABx at 12watts idle each. The plates are at 350VDC. Iím wondering about the grid stoppers and if they are adequate at 1.5k. I sometimes think Iím hearing a bad distortion in addition to the good distortion. No doubt the PI is driving them very hard. The PI doesnít clip on itís own until after the output is clipping.

Iíve read these to get educated:

http://www.aikenamps.com/BlockingDistortion.html

http://www.freewebs.com/valvewizard/gridstopper.html

http://www.tubelab.com/powerdrive.htm

I understand the anti-oscillation reasons and the possible reduction in higher frequencies from the miller effect of grid stoppers. I don't fully understand blocking distortion yet. I do understand that the grid stopper curtails grid current that can occur when the grid is positive.

I think I understand the clamping that occurs when the grid is driven positive. (I know I see it on my scope.) But Iím just barely understanding the references to ďtimeĒ and the charging/discharging of the coupling cap and how that is related to all this.

I definitely see crossover distortion on the speaker output when driven somewhat past the onset of the output clipping, but is that normal? Does a strongly overdriven PP output have to experience crossover distortion?

Can blocking distortion be recognized on a scope? Is the crossover distortion the manifestation of the extreme clamping? Is that essentially the ďblocking distortionĒ revealed? If not, does anyone know of a link to blocking distortion on a scope?

I will switch to 5k or 10k resistors on the grids as a test next time I open it but Iíd like to fully understand 1st.
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Old 14th September 2009, 12:03 AM   #2
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As I understand it, with blocking distortion, grid current causes the coupling cap to charge, biasing the tube more negatively and causing crossover distortion.

The amp I built has DC coupled source followers and it was strange that I could see the clipping on a scope before I could hear it (playing music). I just figured clipping on a hifi amp would sound really bad, but it is amazing how much clipping can be tolerated if you can get rid of the blocking distortion. I don't know exactly what makes good guitar amp distortion, but I would imagine blocking distortion would not be it.
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Old 14th September 2009, 12:15 AM   #3
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Hi,

Schematic of your amp please?

We can help you better

Cheers!
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Old 14th September 2009, 05:16 AM   #4
jjman is offline jjman  United States
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It's starting to all make sense because I leaned towards higher value caps in an attempt to maximize potential bass. I'm thinking this is contributing to the chain reaction and crossover distortion. I recall watching the period of cutoff increase on each 6v6 when overdriving harder. Now that mystery is starting to make sense if increased grid current leads to the bias cooling. I need to get back in there!

It's a modified Trainwreck Express. There is more than shown but it is about time I started to make up a proper schematic. Here is my quick and easy schematic:

Click the image to open in full size.

Last edited by jjman; 14th September 2009 at 05:28 AM.
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Old 14th September 2009, 05:49 AM   #5
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I just built two Trainwreck Express clones with tweaks

Those coupling caps to the 6V6... need to be 0.022uF.

A Trainwreck is a bright circuit to begin with, so changing the cap between your fist and second 12AX7 (the 0.0025uF one) is where you want to look. Try 0.01uF.

Cheers!


(PS - your NFB resistor is too big... try 100K)
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Old 15th September 2009, 12:55 AM   #6
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If you "maximise" the bass in a guitar amp, it's possible you may produce too much at low frequency for the OP transformer to manage, causing distortion.

Blocking distortion is caused when the OP tube grid is driven by cap coupling and tries to go positive (i.e. overdrive).

Crossover distortion is a separate thing, normally due to running the OP tubes too lean and is unlikely to happen in your case because you have caqthode bias.
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Old 15th September 2009, 01:02 AM   #7
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Quote:
Originally Posted by ray_moth View Post
Crossover distortion is a separate thing, normally due to running the OP tubes too lean and is unlikely to happen in your case because you have caqthode bias.
The two phenomenae are related since the extra grid bias can push the finals more towards Class B, if not Class C. That will indeed lead to lots of nasty x-over distortion until the excess negative charge leaks off the coupling capacitors. That's bound to be hideous.
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Old 15th September 2009, 02:41 AM   #8
jjman is offline jjman  United States
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Quote:
Originally Posted by Miles Prower View Post
The two phenomenae are related since the extra grid bias can push the finals more towards Class B, if not Class C. That will indeed lead to lots of nasty x-over distortion until the excess negative charge leaks off the coupling capacitors. That's bound to be hideous.
I'm getting closer to understanding. Sounds like the coupler discharges somewhat during the clamping time then recharges thru the grid during the "recovery" time. (The whole concept of grid current is not something I've thought about much.) Since the recovery (recharging?) time is "long" it pushes the bias colder during that time which moves the tube towards class B or C. I guess because the grid is made “more negative” while it is supplying current back to the coupler for it’s recharging.

I'm not 100% clear on all of it but I'm close enough I guess. The bottom line for me is that increasing the grid stoppers should/could reduce the crossover distortion I get when driving hard. I’m hoping this is the “bad” distortion I’m hearing with the good distortion. From what I’ve read, crossover distortion is bad sounding, even in guitar amps. Once I've finished with the stoppers I'll see if I need to reduce the couplers. I don't want to sacrifice bass if not needed. There is not a surplus of bass going on as it is.

Thanx for all the feedback.

Here's a pic of it:
Attached Images
File Type: jpg Out-DSCF0014.JPG (52.2 KB, 421 views)

Last edited by jjman; 15th September 2009 at 02:47 AM.
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Old 15th September 2009, 08:42 AM   #9
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The waveform is distorted, but symmetrical, so both 6V6s seem to be pulling & pushing OK. That is very nasty crossover distortion though.

does it dispappear at lower power output?

If not, is the impedance match correct (not an 8 Ohm speaker on a 4 ohm tap)? Is the transformer correctly rated? - adequate power handling, and 5000 to 8000 ohm anode-anode load?

The 500 Ohm resistors on the 6V6 cathodes should have about the same 12V on them. If one is wildly different, change that valve.

I agree with Geek that 0.022nF should be used for gate coupling. "Blocking" happens when the grid starts taking current. Normal operation means they take only 1uA or so, but when the driving signal comes near to the cathode voltage (say, grid = +11.5V) mA-level current is taken and this charges up the coupling cap! As this grid voltage gets above 12V the current really flows, if not limited by a stopper. Now, when the signal (grid) voltage starts going down again, the charged cap causes the grid to be more negative than the driven side of the coupling cap, and effectively biases the 6V6 in the OFF direction! As Miles says, this bias shift can result in crossover distortion. But if it's there without overdriving the amp, suspect transformer or 6V6 parts trouble.

This is one reason you use a small coupling cap in a guitar amp. You can eliminate blocking completely by using a power MOSFET to drive the grids. Circuit have been put up on here I believe.
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Old 15th September 2009, 10:24 AM   #10
Svein_B is offline Svein_B  Norway
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Your scope trace is almost identical to what I get when i try to "push" a cathode biased EL84 PP beyond the clipping level.


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