Dumble tone stack explained (in 60 seconds)

[Merlinb]

I thought it may interest people to see the fundamental differences between the Dumble tone stack in Rock and Jazz mode. Freq response plots correspond to a 40k source resistance.

Here it is in rock mode:

DumbleRock.jpg

As you can see, it's basically a Fender tone stack, except the Mid control has been separated from the Bass control.
Treble freqs are fed directly to the Treble pot via the 330pF cap. All freqs pass through the 100k slope resistor, and the upper+mid freqs are then bled off via the Mid pot. This leaves only the lower freqs which are always fed to the Bass pot. Therefore the Bass pot really controls bass and middle; the Mid pot simply decides how much mid actually gets fed to the Bass pot. Also, the more you turn up the bass, the less you can cut the treble, because the two pots are stacked.

Here it is in jazz mode:

DumbleJazz.jpg

Look vaguely familiar? It should; it's more-or-less a James tone stack plus mid-shift resistor (that's the 100k resistor between the Treble and Bass pots). The operation of the Mid and Bass pot's is the same as before. The Treble pot, however, is now pretty much independent of the Bass pot, so that more treble cut is available even with the bass pot set to max. Lowering the value of the mid-shift resistor to zero ohms will shift the whole response up by about one octave (it actually has very little affect on the separation of the controls), so there is something you can play with.

[exocet]

Spooky, I have been thinking about this very subject recently but didn't have the brainpower to work out what the rock/ Jazz mode did to alter the tonestack configuration.

Thanks very much, really useful information.

[Max]

Thanks, Merlinb, for these info concerning the "classic" tone stack.

Could something like this perhaps be done by someone here concerning the other tone stacks, too?

Here's the "pre-classic" stack of most of the known 2nd generation ODS amps for a start:

Treble: 500KL pot / 510pF treble cap
100k slope resistor
Mid: 250KA pot / .01 mid cap
Bass: 1 MA pot / .1 bass cap / .02 cap between terminal 1 and 2 / .002 cap between terminal 2 and 3 / 10k tail resistor

Thanks again and all the best,

Max

[mlp-mx6]

Merlin, if you would be willing to model the "Skyline" stack from the ODS-101 schem that would be great too. Thank you for what you provided already.

Thanks also for your book. It is great.

[ChrisM]

Lets not turn this into another lengthy discussion of what pots and caps HAD may or may not have used over decades.

The values will just be changing how the frequency cutoff and the range/extent of the controls. Basic function of the TS is the same.

Thanks Merlin, very helpful. This LTSpice?

[BobW]

Thanks MerlinB. Great info here.

[Max]

Lets not turn this into another lengthy discussion of what pots and caps HAD may or may not have used over decades.

ChrisM, I agree. We should only discuss the pots and caps that we know first hand or from clear pictures HAD has used and not speculate about what he "may or may not have used". Good suggestion IMO.

Cheers,

Max

[Drumslinger]

Very nice info. Thanks Merlinb.

[Structo]

What about the .001uF cap across the bass pot and the 10K to ground on the CCW?

[ChrisM]

The 1nF cap scoops the mids a tiny bit. The 10K resistor just sets the limit of the bass pot.

[talbany]

Tom
Take a look at the James stack in the TSC you can run those and that will show you the response curves of the bass.. You can do the 70's with both caps or just the 1 I think..

FWIW I recently go a stash of the early Mallory 6PS caps NOS and stuck them in my MM.. Nice cap and noticeably smoother with a bit less of the midrange honk to them.. The SBE's are a really good replacement very similar.. After some time on the SBE's they will smooth right out as we all know so I personally wouldn't sweat going through all the trouble in digging these up... Pots on the other hand the CTS are worth the trouble IMHO...If you can just find the CTS for Vol/Treb/Drive/Level/Master.. Those are the most important
The more I use these CTS's the more I want to get away from the Alpha's
It would also be nice to see exactly how the stack works with the PAB wired in Skyline style..
Hope some of this helps..

Tony

[Structo]

On the CTS pots, are you talking NOS pots or what is available now?
I get confused on the CTS because it seems there are several versions made and since they custom make them for companies it makes it even more confusing.
You have the ones with the nylon shaft, with a brass shaft with a dimple back, and I have even seen them with a grey metal shaft.

You can order them with whatever taper you want, whatever turning resistance you want, bushing width and length as well as the shaft differences.
So I kind of gave up ordering them because I was never quite sure what I was going to get.

[talbany]

Tom

These.. NOS or even used in good shape!!

[LPSGME]

Pots on the other hand the CTS are worth the trouble IMHO...If you can just find the CTS for Vol/Treb/Drive/Level/Master.. Those are the most important
Tony

You said to use the CTS on the 'treble' pot.
But in the photo you provided, isn't it on the (middle) Mid Pot and not the treble.... ?

I was just playing around with different types of resistors, and how different they sound, and was wondering about the pots. How noticeable are the sound differences between Pot BRANDS compared to the differences between resistor 'types'?

http://ampgarage.com/forum/download/file.php?id=13268

[massygt6]

Great post, thanks for the practical and clear explanation!
I'd like to remove some "honkiness" and darkness form the clean channel of my clone, it has a lower B+ than the standard values, around 420V.
it's better to work on tonestack, or should I lower the value of the anode resistors to let more voltage come into the preamp stages?
Thanks!