Tube Amps / Music Electronics
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|5/23/2000 8:26 AM|
|Dominik||Old Styl Dumple OD: ears ringin!|
with interest I followed the thread about Dumb-le clones. Someone stated that putting a 35 pF cap across the last plate resistor of the OD would cure this. Does anyone else have an idea about taming the harsh sound of the 70s OD??
One more thought about including the Dumble-ator in side the amp:
I have juste recently done this. Two preamp strages and the dumbleator in one rack chassis. I must say that things will get crowded the more stages you put into an amp and that noise will go up. Altho I donīt have a problem with oscillations, I have some trouble with hum that is very hard to locate. When I will build my real Dumb-clone I will keep it as simple (as Gil says) as possible. This is a great design and should be kept simple to yield the best sound me thinks.
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|5/23/2000 3:12 PM|
The cap value you mentioned is .0035 or 3500 pf not 39 pf. I also had a 500pf on the first stage plate resistor.
As far as the dumbleator goes, my feeling is if it's going to be in the signal path anyway I want it inside the amp. Basically it's just a tube buffered effects loop. Lots of amps have them. I think putting the dumbleator inside is actually making the circuit "simpler" isn't it? The signal isn't going out of the amp thru cables & there's one less power supply. It will be easy to take the existing B+ and feed the extra tube. I guess I want to do it to show it can be done neatly and sucessfully at this point.
|5/23/2000 5:19 PM|
a .0035uF cap is simply immense. A few months ago Andy Fuchs had reported than when you start approaching 750pF (plate to cathode puffs), the sound begins to get too muffled, like in a Boogie amp.
I have done some (read: too much for my own good)experimentation here as well, and I agree with Andy 100%. I would say 500pF, 390, 350, etc. But 3,500pF is way too much, and that's more of an upper midrange cut than just a treble cut. You will lose too much of the harmonics that way, and your sound will be way too compressed.
If you think your amp sounds harsh, turn the gain down, and soften up your touch, and turn the master volume up a bit and listen again. Still harsh? Maybe, but now try and go play with a drummer and listen; then put the .0035uF cap, and play with the drummer again... I bet you will lose 100% of your highs all together.
At home we want our amps to sound bassy, rich, full, smooth. Such an amp is useless on a bandstand, no exceptions, unless -- I made this remark as a joke just a few days ago -- you want to do a Cream tribute gig... without a bass player. A quote from Dave Funk about gigging musician who asked him to mod their amps: "Make it brighter, and make it louder." I couldn't agree more.
|5/23/2000 6:22 PM|
Keep in mind the circuit I'm working with is in a 71/2 x 5 metal box and the master for the OD citcuit is about 1/4" from the plate resistor. If this circuit was in a full size amp I think longer lead lengths and other factors would add capacitance and change how we are tone tweaking.I'm also using a regulated supply which may add to the high end. With the .0035 cap in place I still need to turn the tone control down on the guitar on the treble pickup to keep my ears from bleeding. This is playing into a princton reverb with treble and bass on 5 with an EV 12" speaker. Believe me there is plenty of high end!
I before I installed the regulated supply I had a .001 cap on the plate of stage two. I usually don't use a larger value than that. In this case however the .0035 works well.
|5/23/2000 7:30 PM|
Ahhh... I was referring to the "actual" circuit, of course, if there are some other changes there, anything could be "right."
|5/24/2000 6:02 PM|
Gil: As we had previously discussed, the issue of caps on the OD stages is an interesting one. I don't think they were put there to stop oscillations (unless they may be momentary signal related transient oscillations), but the circuit without them simply sounds spitty, brittle, and brash. The range of values is more according to taste than anything else. 330 to 470 pf seems to be a value that most people like. It's absolutely imperative that the builder has a circuit thats totally free from any stray oscillations to begin with. These caps will "fix" oscillations, and the resulting sound is drastically different that a circuit that was clean and stable to begin with. If you build a circuit and put them in place without checking for stability first, they could actually mask other problems and the resultant sound will not be good. These caps should be mounted right at the tube socket pins, and hi-voltage ceramics (to my ears)sound best. The best stability test is all controls on ten in overdrive mode. You should hear hisssssssss and only hiss. If it whistles, oscillates, activates your garage door opener, or makes the metal plate in your head warm up, it needs to be fixed first !
|5/24/2000 7:35 PM|
I must say that I will try say 390 pF first. My preamp clone did not have any oscillations in the 70s OD channel to begin with. Apart from some hum it is rather quiet. The skyliner channel was more of a problem with oscillations, but that was due to lead dress. The 70s OD is indeed very harsh. It might be the best thing for a band tho. I remember playing thru my Fender Blues DeVille that had a horrfying lead sound (way to harsh and undefined), but in a band situation it sure would cut through. The skyliner channel is much more pleasing to the ear IMHO. I have it almost stock and the 70sOD is totally stock. Funny thing is that I find the Deep switch not very usable at all. The Jazz Rock makes some tremendous changes, but the Deep switch really is only slightly audiable. One thing about the Skyliner is that the OD sound could me more Midrangey in my opinion even with the mid pot almost all the way up. Maybe one should play with different values here (any experiences?)
One more thing that needs to be fixed:
There is some hum that bothers me in both channels. I poked around some, but could not find the source really. And the 70sOD has much less gain than the skyliner so the skyliners Masters (Clean Master and OD Master) have to be run rather low.
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