tone stack gain recovery stage (v1b) fdbck loop explanation

[pula58]

I have perused through a bunch of Dumble schematics. They all (or at least, most) of them have a parallel R/C at the input (grid) of the second triode in V1, and also, the same triode has feedback in-between the plate and the grid through a series RC.

What does this stuff do to the sound and feel of the amp? I know what mathematically happens...but, in terms of describable tone/feel, what does this do?

Or, one way of putting might be like this: If the plate-to grid feedback and parallel R/C input at the grid were eliminated, how would the sound of a Dumble-type amplifier change?


Or, put in another way: If I take my AB763 circuit and modify the second triode in the way that the dumbles have, how would the sound of my Fender amplifier change?

Thanks!!

p.

[ayan]

Bypassing the grid stopper resistor (220K) on V1B will let more high end through the stage. The plate-to-grid feedback network reduces the gain of the stage, as expected. If you elimintate that network you will get more gain out of the stage which will translate in a louder clean channel with the possibility to be pushed into some grind, which may be hard to achieve otherwise. The OD "channel" will sound gainier as well.

I think most of us are used to having the V1B feedback loop in place since, with the exception of 183, most other schematics showed that feedback network. Some folks like K&M out that on a switch (with a clever twist, actually), so that you could either have the network activated or deactivated. To me, on a Skyliner amp for example, the feedback makes the OD channel sound a little more to my liking. The clean channel, however, always sounds better without the network. The lack of network and the Fender tone stack on a BM, for example, is what we all probably like so much about that amps' clean sound.

Cheers,

Gil

I have perused through a bunch of Dumble schematics. They all (or at least, most) of them have a parallel R/C at the input (grid) of the second triode in V1, and also, the same triode has feedback in-between the plate and the grid through a series RC.

What does this stuff do to the sound and feel of the amp? I know what mathematically happens...but, in terms of describable tone/feel, what does this do?

Or, one way of putting might be like this: If the plate-to grid feedback and parallel R/C input at the grid were eliminated, how would the sound of a Dumble-type amplifier change?


Or, put in another way: If I take my AB763 circuit and modify the second triode in the way that the dumbles have, how would the sound of my Fender amplifier change?

Thanks!!

p.

[ampdork]

The clean channel, however, always sounds better without the network.

Don't let Max hear you say that! ;O) However I agree and I think without exception everyone who I have ever shown this too also preferred the cleans without the network.
I am sure not everyone would agree but I think the vast majority would.
Built a few on relays to switch out on the cleans... That works really nice.
I started with HRM amps so maybe I just got my ear inculcated to it.

That said with tag's 6l6 183 it seemed to demand it...
Do we know any other non hrm skyliners with el34's?
If so does it have the network?

[ayan]

S/N 147 uses 6CA7s, has the feedback loop and is a non HRM Skyliner.

G.

The clean channel, however, always sounds better without the network.

Don't let Max hear you say that! ;O) However I agree and I think without exception everyone who I have ever shown this too also preferred the cleans without the network.
I am sure not everyone would agree but I think the vast majority would.
Built a few on relays to switch out on the cleans... That works really nice.
I started with HRM amps so maybe I just got my ear inculcated to it.

That said with tag's 6l6 183 it seemed to demand it...
Do we know any other non hrm skyliners with el34's?
If so does it have the network?

[stelligan]

To me, on a Skyliner amp for example, the feedback makes the OD channel sound a little more to my liking. The clean channel, however, always sounds better without the network.

Agree 100%. Best cleans without the network.
How does K&M implement switching this in and out?

[butwhatif]

Never liked the local fb , either on v1b or loop return. i think the only use it had was to remove a slight bit of preamp tube microphonics from being further amplified downstream.

[ayan]

To me, on a Skyliner amp for example, the feedback makes the OD channel sound a little more to my liking. The clean channel, however, always sounds better without the network.

Agree 100%. Best cleans without the network.
How does K&M implement switching this in and out?

If I recall correctly, they have two 22 Meg in series in the loop and they either ground the middle point, to eliminate feedback, or not.

Gil

[Joost]

....I think most of us are used to having the V1B feedback loop in place since, with the exception of 183, most other schematics showed that feedback network. Some folks like K&M out that on a switch (with a clever twist, actually), so that you could either have the network activated or deactivated. .............

My curiosity is piqued. Can you tell me more about the clever twist on the feedback network on a switch?

thanks

[exocet]

....I think most of us are used to having the V1B feedback loop in place since, with the exception of 183, most other schematics showed that feedback network. Some folks like K&M out that on a switch (with a clever twist, actually), so that you could either have the network activated or deactivated. .............

My curiosity is piqued. Can you tell me more about the clever twist on the feedback network on a switch?

thanks

The hand drawn TR Custom Reverb that is posted on this site somewhere shows the LNFB switch grounding the signal or not as the case may be - it is positioned after one of the 22M resistors.

[Structo]

Yep, that is how I have mine switched.

The switch goes to point between the .047uf cap and 22M or 2x10M on the grid side of the network.
The other lug of the switch goes to ground.

[talbany]

I've too prefer the cleans without the LNFB like most here but if you like that sort of growl (Dumbley in your face) low end breakup in OD flavor which IMO is really kinda cool, go without it all together.. You can really hear it in this clip of 183..Matt

http://www.youtube.com/watch?v=HGGkAvgl_hI

You can see he has the OD drive/level set fairly low and yet the amp snarls nicely at him when he rides and double stops the lower registers..This IMO is due mostly to no LNFB..The amp will still have that quality with the LNFB just not as much..This is why I like 183
On the flip side you have to be careful if the amp is too bass heavy or not voiced right it can get away from you..If your a smooth jazz guy leave it in in OD
These are my findings..


Tony

[Structo]

Are we sure that is 183?


I have to say that for the most part I leave the LNFB in the off position.
I just like the more raw sound, not as refined, especially in OD.

[talbany]

Are we sure that is 183?

Tom
Have some faith my brother..

https://tubeamparchive.com/viewtopic.php?t=14145

Tony

[Structo]

Oh I wasn't doubting you Tony I just didn't see it mentioned which Dumble it was on the video.
Now it all comes back to me.......man my short term memory is shot!

Thanks for the info!

[pula58]

thanks for all the replies folks..I am glad I asked about this..learning a lot! Keep it coming!

P.