Tube Amps / Music Electronics
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|9/17/1999 2:50 AM|
||Re: "Best guess" schematic of 1997 Dumble ODS|
Any and all tips/corrections on the current D-ODS models are appreciated! I will probably be editing that drawing on a daily basis as more information filters in. I'd just heard that they are using EL34's these days (getting more and more like a Marshall- eh?) but fill me in on what stages should have the 500pF plate to cathode caps (I left it off the 2nd OD stage figuring that the post-OD tone stack would handle any filtering needed).
P.S. I will also be drawing up an "advanced" version that will include the Rock/Jazz, Mid-cut and Boost switches for anyone who wants to build a more exact clone amp. TBA
|9/17/1999 3:35 PM|
My apologies, your schem. is so clean,
I just flat missed that 500pf. looking
forward to the "final version"
Thanks for your work.
|9/20/1999 1:54 AM|
You most certainly want the 22Meg/.05 on the second AX7 stage. You also must have (sorry, should have) SOME capacitor on the plate to cathodes of the OD stages. Ayan suggested that they may have been for oscillation control, but I've built the circuit (stable without the caps), but theres a bad sort-of edge in the distortion when the OD is pushed. I've gone from 500 to 470 to 330 and ended up at about 100 pf (ceramic 500v mousers), which is fine. It seems that the minimum you can get away with seems like the best OD sound without being rolled a la Mesa.
|9/20/1999 6:21 AM|
Thanks for your suggestions! (Personally I think that H.A.D. has taken some wrong turns in his latest ODS designs, and has "thrown out the baby with the bathwater" in searching for a smoother sound like a modern Marshall.)
We were discussing the small caps between the plates and the cathodes a few weeks back and I was wondering how it differed from running the cap from the plate to ground. It was then brought up that the cathode is at the same ac potential as the ground, and the dc potential is only a few volts above ground.
Perhaps they put the cap between the plate and the cathode so that it can be added right on the tube socket itself (to run it to ground might involve an 8" length of wire if you were trying to follow a strict star-grounding scheme). Or have you noticed if there really is a difference running that cap to the cathode instead of to ground?
Adding that cap to ground AFTER the coupling cap would be quite different- right? It seems like that would be like a treble-cut tone cap on a guitar...
|9/20/1999 5:50 PM|
Not having two Dumbles (an old one and a new one) to compare (hell, not having ANY Dumble at all), I can't say which sounds better. But, considering the schematic on your page is from the early 70s, I am sure there was an evolution from that design to the 1997 Skyliner you're putting together (schematic-wise).
The thing is, all of those Dumbles that made it into famous recordings during the 80s are most lilely NOT the same design as the old schematic. Put it this way: none of us that has wired that old circuitry has like it! Yes, the clean sounds good, but the OD sound is lacking life, for lack of a better work. It's like you need to feed it some more treble for it to shine (I am currently experimenting with a 470K//470pF network in front of the 100K input trimmer) with good results. Of course, the OD becomes brighter, but in a good way... Then, if you kill some of the excessive highs at the back end of the OD, you end up with a sound quite reminsicent of a Mark IIC+ or Mark III Boogie. It is a small world.
Now, if you will be putting a tone stack at the back end of the OD, perhaps you can afford to do away with the 500pF caps around the OD stages. I won't know until I wire a "Skyliner" myself, but perhaps those 500pF caps were needed because the earlier Dumbles had no treble gate after the overdrive.
I have played with that -- running the caps to ground VS the cathode -- and there is no difference (BTW, I was the one who mentioned that the cathode was at AC ground anyway). Furthermore, you can also run those caps in parallel with the plate load resistors (as done by Boogie, Marshall, etc.). Same difference, they AC-shunt to ground from the plate via the power supply caps in this other instance, but it is the same effect.
Well yes, for one the coupling cap is smaller than either the power supply cap or the cathode bypass cap, so it's not quite like the (treble nuking) cap is tied to the plate anymore (this may have an effect if the coupling cap is "small," like .0022uF or so). Also, the impedance of the circuitry that follows the coupling cap begins to play a little bit of a more predominant role (it always has an effect on EVERYTHING though). In the Dumble schematic, there is a 100K pot after the OD2 stage, so to kill some treble you neeed quite a hefty capacitor to bypass that pot (in a guitar, for a 250K pot typically a .022uF cap is used).
However, if you use a 1 Meg pot like, then the cap could be 10 times smaller for the effect to be similar... So I have experiemtned with caps in the .002 - .02 range for the low impedance pots (100K), and maybe 100pF to .001uF with 1 Meg pots.
Well, that is the way I look at it.
|9/21/1999 12:18 AM|
I think we all have different ideas of what we want from an OD channel, but I bet that the solution for many people will end up being some "missing link" between the 1977 and 1997 designs (with some variation of the tone shaping components you've mentioned after the OD stages of the earlier preamp). And it probably will be different from anything that H.A.D. built himself and possibly could be even better...
P.S. I've been trying some of the "Dumble" tricks on my other amps but have been reluctant to add a 100pF to 500pF from the plate to the cathode of a 5F6A/Marshall cathode follower (since I don't recall seeing something like that used on any schematic). Of course with a cathode follower having no gain, oscillations may not be a problem at that stage of the amp...
|9/19/1999 5:17 AM|
||1st revision uploaded.|
I just uploaded Rev 1.2 (with some corrections you guys have been sending me):
P.S. BTW to view PDF files on-line and off-line you need to download Adobe Acrobat (click on the link on my home page).
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