Tube Amps / Music Electronics
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|8/10/1999 2:25 AM|
||Re: Dumble clone|
You don't have to do that. Look at it this way: if you merely disconnect the BOTTOM of the Treble pot from the TOP of the Bass pot, essentially you are only going through the 330pF cap, through the Treble pot into the volume control.
The rest of the circuitry, which would feed the Bass Middle controls via the 100K slope resistor will NOT have a path to the volume control, because the bass would need to return some signal into the volume control via the BOTTOM of the Treble cap. You could argue the Middle control would be still able to dump some signal to ground, even if the Treble pot disconnects from the Bass pot. IN practice, the effect is negligible, and you are left with:
1st stage --> 330pF cap --> treble control (which is essentially deactivated becasue it does NOT have a path to ground anymore) --> volume control.
To me, that works like a champ and it yields a pretty well balanced tone. To control the "degree" of boost you could, instead of totally disconecting the treble from the bass pots, connect them with a variable resisotr (maybe 1 Meg) and you could set that to your liking so that the boost effect balances volumes to your liking.
Needless to say, the lower you dial the tone controls, the more effect the boos will have when you activate it.
|8/12/1999 8:00 AM|
the boost you mentioned, works great I tried it the other way too, but your way is much less muddy thanks.
I read the article about the trim pot for adjusting the amount of highs in the overdrive circuit, and was curious
about how it is layed out, do you have any idea? I wonder if it just rolls the highs down a little or maybe boosts it a little? at any rate the amp sounds wonderful, and this one is in my old bandmaster w/o reverb this amp has the holes punched from the factory for 4 power tubes but 2 have
plates screwed in there. I'm thinking about using a bassman output trans. that I have. Well gotta go, latta.
|8/12/1999 5:31 PM|
I am glad you like the boost. There seems to have been several variations of preamp boosts over the years, the most popular the Boogie cowboy method os simply lifting the ground of the tone stack -- and that sounds muddy. Now, the one I suggested I arrived at at the same time, although in parallel efforts, with a friend of mine. We were both ahppy with the results. Very recently, I heard that other amps used the same scheme in the past, so I guess so much for the originality of the idea.
Regarding the tone control: there are numerous ways you can go about doing this, I suppose. I would favor placing the tone control at the output of the overdrive, that way, you allow the overdrive stages to generate as many harmonics as they can, and then you trim the top end. If you should put the tone control at the input of the overdrive, or perhaps between overdrive stages 1 and 2, you would probably prevent generation of some higher order harmonics. To my ears, the tone thing sounded best at the output.
How do you configure the tone control? There are a few different ways, the most intuitive of which would be to simply put a variable resistor, followed by a capacitor and tie that to ground, bery much like a guitar's tone control. One other idea I have toyed with inthe past, with good results as well, is: at the output of the overdrive, after the coupling cap, you can put in a "radical" network, like say a 1 Meg series resistor in parallel with a "large" cap, say 470pF or so. That will obvioulsy brighten up the sound a bit, and what you can do now is instead of having the 470pF cap there, put it in series with a 1 Meg trimpot, for example. Adjusting the pot will adjust hoe much of the 470pF cap the 1 Meg resistor will see, so it is a "series" type of tone control.
As you can imagine, you can milk the above idea any way you want to achieve the tonality you desire. USe a 3.3Meg with a 1,000pF cap, for example, or a 330K resisotr with a 68pF cap, whatever you want. Varying the resisotr and cap will change the bandpass of the network and you will then emphasize high treble, VS treble, VS high mids, etc.
I would probably recommend you start with a simple shunt style tone control. You have a 100K "lead master" (either pot or trimpot)? Then try a 100K trimpot in parallel with that, and use say a .001 uF cap for starters and see where that takes you. And remember, there are a great many types of tone controls out there, you can put anything you like after the OD and see what it sounds like to you.
|8/12/1999 6:08 PM|
Thanks Gil, for that Info!!!!!!!!!!! I Really appreciate it!
so now its back to ye ole tinkering (circuit) board.
p.s. I tried a few prescence control circuits, but it seems that to do that I end up changing the feedback loop resistor and the tone seems too thin and brite, I also tried putting a pot after the master volume using a cap to shunt some of the highs to ground, so I still am experiment
with different cap values and maybe end up not using one at all. also I've read that so guys like to keep the plate voltages on the 12ax7's somewhere between 220-250 volts
for a more dynamic feel or maybe it's cleaner than using lower plate volts. In this project the voltage on the 1st tage is between 220-230 depending on the time of day 2nd stage 218-225 v. overdrive 160-180 v. dumbleator circuit 280-300v. PI 230 on top and about 280 on bottom (schematic I have shows 308 top and 303 bottom.) So I'm probably just getting a little more drive in the PI I suppose, but I wonder if that will degrade the tone a bit. any way happy amping and good day!
|8/12/1999 8:02 PM|
Your voltages seem to be backwards... The highest should be the PI ones -- with as much as you can get there -- followed by the OD stages, and the lowest voltage would be the first two (clean) stages.
Regarding the presence control, there seem to be a lot of different choices there. Some prefer the Presence to be bright (use small cap), some meaty (use bigger cap), some prefer lots of negative feedback (for a constipated, Boogie-style, sound), some little feedback (for a raw, Marshall sound). That's easy to change though, and it will obviously depend on the output transformer. To me, as a rule of thumb, the Fender (Blackface types) OTS sound bad with no negative feedback (way too bright), but other trasnformers like the Hoffmans seem to sound really thick without any feedback at all.
|8/13/1999 2:15 AM|
Great post! I really like your suggestions for cutting or boosting the highs on the OD channel. I just wanted to add that even if an added OD tone control is not used, a variation of one or the other circuits you mentioned could be used without a pot to balance the tones of the Normal and OD modes. (Like if the amp is suddenly way too bright when the OD is engaged then bleed off some of the highs to ground through a 250pF to 1000pF cap, or if it is too muddy then run the signal through a contour network- like the typical Marshall one of a 470pF cap in parallel with a 470k resistor or the customized variation that you mentioned.)
My point is that still another control isn't always needed if you can balance the tones of both channels/modes... (I had added an OD tone control to my infamous Peavey Classic 30 amp and after a year I decided that I really only wanted it set to a certain position so I hardwired in an 8k2 resistor to replace the stock 10k resistor that fed a 0.015uF cap that the bled the highs to ground immediately before the post gain pot. That balanced the tone from both channels fairly well and eliminated any noises and hassles from the extra pot and cabling.)
P.S. BTW I think a better solution might be to wire up the ODS with two independent channels (with their own tone controls) one with the added OD circuitry and the other without. That way you could also fine-tune the preamp stages by running them at different voltages or using different Rp/Rk/Ck values and also use different types of boosts for each channel as appropriate. If relays (or FETs) are used for channel switching you could even alter the voltages going to the PI for each channel (for more distortion vs. more clean headroom). Or even select between two different bias currents. Thanks!
|8/14/1999 12:49 PM|
I two have been thinking of a 2 channel setup like you mentioned, that would be so much more useful I think I like the tone trim, parallel to the master vol. as Gil sugested. I used a .001 uf in series with a 1meg. pot mounted on the front panel, I tried some of the treble boost stuff, and some combinations where interesting, like 220k paralleled with 250 pf kind of a more focused sound and also as Gil sugested some other combinations that he posted, but so far I seem to just like the tone trim. I'm thinking of adding a cathode bias/adjustable - bias switch to see if that helps to sweeten things up a bit too. By the way thanks so much for the DUMBLE schematics I've been looking so long for such a treasure of info. Keep up the GREAT work you guys, your amp nerd friend Ed.
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