Grid Stopper Question

[surfsup]

I wanted to ask you guys what was going on here, whether my ears were deceiving me. In the pic below, the Treble pot is on the face of the amp and the tube socket is on the other side of the board. So I have the 220k mounted on the treble pot lug, with the 56k hanging off it to ground, with the screen lead running across the board to the tube. I was getting a little bit of blatty bass distortion. Thinking this was "blocking distortion" I added a small resistor to the pin2 of the tube (3k3). THis seemed to clean it up enough for my ears to hear a difference.

Some questions,

1) Shouldn't the 220k provide insurance against blocking distortion even though its not at the tube?
2) The 3k3 added to 220k doesn't seem to be much considering it is only 1.5% more than the 220k already in series, so are my ears deceiving me?
3) Does the 3k3 provide additional filtering while acting alongside the input impedance of the second tube, because its right on the tube, as opposed to the 220k which is, say, 6 inches from the tube?

[img:830:564]http://chicagocadcam.com/ChrisHahn/xen10/V2Rgs.jpg[/img]

[guitarmike2107]

Why do you think its blocking distortion and why do you think it's blocking distortion in this position in particular?

The plate driven tone stack will be attenuating the signal quite allot, and then the 220k/56k voltage divider will again be attenuating it.

I would suspect there is not enough drive to distort V2a very much all? Have you got a scope to check it out?

Your blatey distortion is probably from V1B because is has a (fully) bypassed cathode.

Pity you didn’t show us the rest of the schematic so we could properly understand what is going on.

Mike

[Colossal]

you didn’t show us the rest of the schematic so we could properly understand what is going on.

Mike

Mike,

The schematic is from the X10 thread:
https://tubeamparchive.com/files/x10_sc ... v2_147.pdf

Please note that the 56k grid leak is from the first production version of the X10 which used a 12AX7 in V1ab and V2a positions. The 56k grid leak was dropped from the Version 2 (latest production) and the Treble wiper is connected directly to the grid of V2a. V2ab (b is unused) was changed to a 12AU7 to reduce gain in the production amp. I believe Surfsup added the voltage divider and left a 12AX7 in his amp. I left the 56k on the schematic but it is greyed out just to show the evolution of the amp's design.

[surfsup]

Right now I have 12at7/12at7/12ax7/EL84s

I edited the Schemo to show what I did (220k/56k at the pot) and then added a 3k3 at pin2

I don't have a scope. I am wondering what this 3k3 is doing because its so small, it seemed to make a difference.

Another theory I have is I am wondering if the 220k is too small and "overloading" V1b to greatly? But the tone stack should provide some load as well making it 220k+TS resistance (though these may be paralleled which will result in an overall R of less than 220k?).

[rhinson]

Right now I have 12at7/12at7/12ax7/EL84s

I edited the Schemo to show what I did (220k/56k at the pot) and then added a 3k3 at pin2

I don't have a scope. I am wondering what this 3k3 is doing because its so small, it seemed to make a difference.

Another theory I have is I am wondering if the 220k is too small and "overloading" V1b to greatly? But the tone stack should provide some load as well making it 220k+TS resistance (though these may be paralleled which will result in an overall R of less than 220k?).

put the 220k/56k combo right up at/on pin 2. this in reality is where they belong. it will probably help get rid of some of what you're talking about and will decrease noise level at the same time (even with shielded wire). i never put grid resistors of any kind on the board anymore. rh

[guitarmike2107]

Mike,

The schematic is from the X10 thread:
https://tubeamparchive.com/files/x10_sc ... v2_147.pdf

Thanks! I will have a look

[guitarmike2107]

Ok I had a look, There is almost no bass attenuation /treble boosting

Most TW amps are quite bright, this keeps them clear when overdriving, first thing I would do on this is drop the first coupling cap to 0.001uf, thats in keeping with the rocket/voxeness of the tone stack and EL84's

Mike

[surfsup]

Guitarmike, regarding the 1n0 I already have a 1n0 there.

Rhinson, I had them there, but the extra ground lead off the pin to the board prompted some suggestions to move the 220/56 to the pot to remove grid leads hanging off. On the pot is cleaner. Maybe I will up the grid stopper to a 10k or something more to test it.

Do you think that will make a difference?

What about adding a cap in series with the 3k3 at the tube, or off the 220k/56k at the pot?

[guitarmike2107]

Is there a thread where your up to date schematic is available then?

I don't see 3.3k making a big difference in terms of grid blocking, but it is a 5% increase in the grid leak value that the valve sees, so that may account for it

[surfsup]

Layout is here, with 1n0 as coupling on v1a/b

https://tubeamparchive.com/viewtopic.php?t=14565

You can see the 220k on treb pot to the 56k on a switch. I do have a 47k there that switches in if i want it.

Another thing about the 220/56 being on the tube, i read its not important as the chances to amplify radio signals is less here as opposed to the input stage. Also i dont have high freq oscillation.

Anyway, back to the 3k3...main question is if the proximity to the tube makes a difference w blocking distortion.

[rdjones]

Anyway, back to the 3k3...main question is if the proximity to the tube makes a difference w blocking distortion.

The closer physically to the grid the more effective.
But 3.3k and even 10k are small values in terms of grid stoppers (on preamps, drivers and PIs)
Go up to 50k or 100k if you like.

rd

[surfsup]

I replaced the 220k with a 22n cap and put a 33k on the tube pin 2, and its better. I read in merlins book about using a cap as a blocking cap to stop dc. I figured a 200hz freq would be a good start. Not sure if my thinking and the solution matched but its a noticeable improvement.

But, merlin states the grid leak should be 5x the plate resistor. So should i crank up the 56k to a 470k due to the 100k R at the plate of the prev stage? Or does the tonestack in parallel reduce that R value considerably where 56k is ok?

[azatplayer]

Ahh the secrets of the X10 still hiding themselves.
I spent some time when i started the X10 thread, trying to get that damn amp sounding right. I gave up. The tonestack just does not work with the info we can glean from pics. I still maintain the pots must be a different value from assumptions and there are secret pixxie dusts everywhere..

[guitarmike2107]

I replaced the 220k with a 22n cap and put a 33k on the tube pin 2, and its better. I read in merlins book about using a cap as a blocking cap to stop dc. I figured a 200hz freq would be a good start. Not sure if my thinking and the solution matched but its a noticeable improvement.

But, merlin states the grid leak should be 5x the plate resistor. So should i crank up the 56k to a 470k due to the 100k R at the plate of the prev stage? Or does the tonestack in parallel reduce that R value considerably where 56k is ok?

Have you got Duncuns tone stack calculator installed, you can adjust the output impedance after the Vox stack to see how it affect the gain of the stage.

Being plate driven the stack you’re using is already very lossy , then with your voltage divider afterwards your throwing away loads more signal, or with just a 56k resistor afterwards you are loading the previous stage very hard and the gain suffers more so, this is why I wondered whether you even had enough drive to distort your v2A stage never mind reaching blocking distortion.

I would think that having a very small load after the tone stack will also affect the frequency response of the stack and basically make it sound $h!T£

[surfsup]

I still maintain the pots must be a different value from assumptions

The TS seems to have a wide range of adjustment.

Have you got Duncuns tone stack calculator installed, you can adjust the output impedance after the Vox stack to see how it affect the gain

I do. Picking the vox and setting 56k puts the curve very low. Putting in a large value puts the curve higher. Hence my question about Merlin's statement about "overloading" the previous stage. In the book there was no tonestack. When I used a 100k, the cleans were tougher to dial in. Admittedly I was testing with the gain knob in the upper range... Maybe I'll increase that resistor again but when I went higher, it seems the cleans were lower on the guitar volume, too low. I removed the 220k thinking the same thing, why get rid of gain?

Honestly there is a lot of play in the T/B pots and the amp is very loud. Everything sounds good except playing a power chord on the low strings there's a touch of perhaps too much bass. Playing double stops it is a bit too overdriven. I'll tweak it some more. Here's an updated schemo:

[img:830:564]http://chicagocadcam.com/ChrisHahn/xen10/V2Rgs-2.jpg[/img]