Tube ducking on the attack

[Lynxtrap]

This is related to my amp build discussed in another thread, but I thought this might be worth its own thread.

I know this phenomenon indicates a particular condition in a gain stage, but if I ever knew what it is, I've forgotten.

It happens when I have the preamp gain turned up to "metal" amounts of overdrive. The signal "ducks" on the attack of a note or chord, and there is also a slight wobble to the sound almost like there was some phase shifting going on. As the initial attack fades, the signal recovers to a normal state.

I'm not likely to use that much overdrive anyway, but I'd be interested in knowing what it is and what to do about it.

This is the schematic for reference.

Lil Bumble-B_Rev 13.png

[TUBEDUDE]

Sounds like you're overdriving a tube grid into cutoff. As the level drops, the tube recovers and signal returns.

[Lynxtrap]

Sounds like you're overdriving a tube grid into cutoff. As the level drops, the tube recovers and signal returns.

OK! Cutoff...or clipping/grid current limiting? I looked it up in Merlin's book and it seems that hot biased gain stages are prone to grid clipping? I have two hot stages in a row with the OD channel engaged.

[Reeltarded]

At a glance HOLY SHIT

You need to do some interstage attenuation! Ok I see.

And I will get a link to Rob's micro bassman that might help you develop your idea that won't cook the lil booger with 2x the gain required to drive the tail.

Obviously not a high gain design but if you listen to the outcome, the schematic and comparison to HG designs might help you.

https://robrobinette.com/Bassman_Micro.htm

[Lynxtrap]

At a glance HOLY SHIT

You need to do some interstage attenuation! Ok I see.

And I will get a link to Rob's micro bassman that might help you develop your idea that won't cook the lil booger with 2x the gain required to drive the tail.

Obviously not a high gain design but if you listen to the outcome, the schematic and comparison to HG designs might help you.

https://robrobinette.com/Bassman_Micro.htm

Thanks, appreciate it. This thing has its own 4 page thread and I have spent 2 years on and off "developing my idea"
https://ampgarage.com/forum/viewtopic.php?f=6&t=35731

I know having two threads on the same amp is confusing. I figured this one could be just about the clipping/cutoff thing in general applying to any high gain circuit.

I think if you take more than a glance it might make more sense. The OD channel has its own master volume called "OD Level" on the schematic.

Rock'n roll amounts of overdrive happen at about 11-12 o'clock on the gain pot, and with the "OD Level" at noon the gain level into the CF matches that of the clean channel with its volume at noon.

The ducking happened with the OD Level close to dimed, and that is not really intended to be a usable sound. I just tried it and noticed the phenomenon and got curious.

You are more than welcome to chime in on the OD circuit (I have a feeling I'm still not completely done with it), but perhaps we should keep that to the existing thread

[Reeltarded]

Turn the high gain pre down until you are a couple of degrees below the blocking. Play for a while and make sure.

Measure the pot. That will tell you what the splitter values need to be.

[Lynxtrap]

Turn the high gain pre down until you are a couple of degrees below the blocking. Play for a while and make sure.

Measure the pot. That will tell you what the splitter values need to be.

Yeah, might do that or just put a grid stopper on the CF (which I guess is the prime suspect). But as mentioned, having the OD Level turned up that much is not really how the amp is supposed to be used. There is no blocking or grid clipping in the OD channel.
The clean (Bassman) channel sounds so good that I don't want to mess with it.

Anyhow, I guess the question has been answered, this is the sound of grid current limiting/blocking distortion.

[Stevem]

This.

B9C359EB-1BA7-4A25-B664-628E41C77338.jpeg