Findings on implementing some tweaks

[Structo]

Yes this has become very confusing with all the schems and layout variations.
Here is one from Normster that is called the final layout.
It doesn't have the jumper or bass mod.
So there is a little more to it then just severing the jumper on 2 & 3.
http://bad-domain/DumbleLite/D-LiteLayoutFinal.jpg

[Structo]

What is the purpose of this 10M resistor on the tweaked schematic?
Thanks

[Deric]

I believe the 10M resistor is just a pull-down to prevent popping when turning the OD on and off.

[kleinm]

I believe that the 4.7K in the NFB loop is a partial/incorrect implementation of the BM power section. The BM has a 4.7K across the presence pot among many other changes if you are comparing it to the standard D'lite plan.

Check out our gracious moderator's JCM800 schematic at mhuss.com for a BM launching point for the D'lite. Starting from the PI input grid through the OT if using 6L6'ish tubes. Tweak to taste from there.

Cheers,
Dave O.

The 4K7 resistor is the NFB resistor tying the 4ohm tap on the OT back into the PI. My understanding is 4K7 off the 4ohm tap is a very common value for Dumbles with the standard phase inverter with the 820R/24K long tail. This is neither incorrect nor partial implementation of the Bluesmaster PI.

In the Bluesmaster PI, this resistor is a 100K.

The 4K7 you are likely thinking of is the tail resistor on the Presence. The standard Dumble phase inverter/Presence uses a 390R tail resistor. In the original D'Lite, the NFB resistor was a 6K2.

Hope this clarifies things a bit better.

[heisthl]

What is the purpose of this 10M resistor on the tweaked schematic?
Thanks

It provides a ground reference for the signal path at that point.

[ampgeek]

Thanks for the clarification klienm!

I keep trying to coral off all of these D'lite mods into "regions" of mods that should be done all at once to attain the desired outcome (or at least a good starting point for further research). One of those being the BM mod. In which I include the 100K NFB resistor and 4.7K presence tail resistor.

I was being thrown by the Tweeked D'lite schematic (which I *ass*umed was the template for this thread) which partially incorporates the BM and believing that this was the direction it was being taken. Hence, the *ass*umption that the NFB resistor (coincidently, 4.7K in your mod and the BM presence mod) may have been confused. Sorry if my comment mis-lead anyone.

Do you and others look to the Marshall type power section that I referenced as a good starting point for one wishing to try the BM mod? If so, would you also include the NFB/presence pot regions in that mod?

I have and it is working very well for me and a pair of NOS TungSol 5881's at the moment. It also sounded very nicely with a pair of Sovtek 5881 WXT's. Not to mention....just as nice with some used RCA 6V6's!

Thanks again,
Dave O.

[Structo]

I believe that the 4.7K in the NFB loop is a partial/incorrect implementation of the BM power section. The BM has a 4.7K across the presence pot among many other changes if you are comparing it to the standard D'lite plan.

Check out our gracious moderator's JCM800 schematic at mhuss.com for a BM launching point for the D'lite. Starting from the PI input grid through the OT if using 6L6'ish tubes. Tweak to taste from there.

Cheers,
Dave O.

The 4K7 resistor is the NFB resistor tying the 4ohm tap on the OT back into the PI. My understanding is 4K7 off the 4ohm tap is a very common value for Dumbles with the standard phase inverter with the 820R/24K long tail. This is neither incorrect nor partial implementation of the Bluesmaster PI.

In the Bluesmaster PI, this resistor is a 100K.

The 4K7 you are likely thinking of is the tail resistor on the Presence. The standard Dumble phase inverter/Presence uses a 390R tail resistor. In the original D'Lite, the NFB resistor was a 6K2.

Hope this clarifies things a bit better.

OK, trying to make some sense of this all.
On my stock D'Lite 44 I have a 390R across the presence pot (legs 1 & 3).
Then I have the 6K2 on the board, then to the tap.

So the "tail" resistor is the 390R and the NFB resistor is the 6k2?

[ampgeek]

So the "tail" resistor is the 390R and the NFB resistor is the 6k2?

That is correct!

A reduction in the NFB resistor value (e.g. to 4.7K) results in a higher injected voltage and a reduction in gain and increased dampening at the PI as greiswig reported.


Dave O.


[/quote]

[Structo]

So what effect will that have on the tone, say on the clean and OD?

[odourboy]

So what effect will that have on the tone, say on the clean and OD?

Hard to say. Increasing NFB does at least three things:
1) Reduces the gain of the gain of the power section (so it won't be as loud)
2) Increases the damping to the speaker; which is going to give you a 'tighter' sound
3) Decreases the effect of signal errors introduced by the power section. This is the wild card as far as tone is concerned. The influences introduced by the power section are reduced by NFB. But since you don't know exactly what they are, you won't know for sure what the changes are going to be until you try it. Chances are the audible effect will be minimal.

It wil also have some small impact on the precense circuit since it uses the NFB loop to work as an active tone control.