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humble clone

7/5/2000 2:08 PM
john hermsen
humble clone
Hi everybody,  
I build a dumble clone using a london city amp (a dutch plexi 100w clone) as a base. I build it 50 watt (2x el34). The OT has only 8 and 16 ohm out.  
If I am correct I now have to use the 8 ohm out for a 16 ohm load? The presence is't working correctly and the lows are not tight, the feedback comes from the 16 ohm tap. I tried the 8 ohm it's a little better, should i lower the resister in the feedback line?  
thanks john
7/11/2000 12:33 AM
john hermsen

My question was dumb? Unclear? Give the dutch guy a break.  
7/11/2000 1:09 AM
Ray Ivers

You can multiply the existing feedback resistor by .7 and connect it to the 8 ohm tap, which should give back your original presence control action and damping factor. Connecting a 16 ohm cabinet to the 8 ohm tap should also give you the same loading per tube as the original amp using four tubes.  
It's not a dumb question, Dutch Guy! :)  
Ray Ivers
7/11/2000 1:59 AM
Steve Ahola

    The presence control on many Humble clones is a bit weird... I just barely crack it open and it sounds like a "regular" pc set to 4 or 5... If I turn it up to 4 or 5 the sound is way too bright.  
    On the design I used the feedback resistor is 4k7, the presense control is 2k, the cap is a 1.0uF tantalum and the cap and pot is bypassed with a 390 ohm resistor. (The OT has a single 4 ohm tap, although I use an 8 ohm cab.)  
    Any suggestions on modifying the presence circuit so that a setting of 5 would be equivalent to a current setting of 1?  
Steve Ahola  
P.S. To the Dutch Guy... I just noticed your post last night and was going to tell you about these controls being very bright but it was time to log off... :(  
    BTW your English is better than many people who were born in America! I'd sure hate to have to post messages in the Spanish I studied in grade school ("Hola, Isabel! Como esta?") Good luck with your project!
7/11/2000 11:58 AM
Ray Ivers

Another aspect of the *umble sound reveals itself; lots of negative feedback! It's not quite up there with the 5F6A Bassman, but it's still a lot of NFB for a guitar amp.  
I would think that changing the feedback resistor from 4.7K to 10K might get you closer to what you're looking for, Steve, but then the feedback ratio is changed for low frequencies as well, changing your damping factor (you may like low damping better; I usually do).  
I might also be tempted to try wiring the 2K pot 'Marshall style', with one end grounded and the other end to the 4.7K resistor/long-tail, and then connecting a cap to the wiper sized for the frequency of treble boost you like; .1 uF is a good starting point, and then try going in each direction from there. This is the famous 'scratchy' presence control, and for a conventional NFB presence control I've always thought it worked pretty well.  
7/11/2000 3:50 PM
Gil Ayan
Weird Report, Steve... Re: humble clone
The presence control on many Humble clones is a bit weird... I just barely crack it open and it sounds like a "regular" pc set to 4 or 5... If I turn it up to 4 or 5 the sound is way too bright.  
The Dumble's presence control works like every other presence control (true presence control, that is) which uses a linear pot. That is, it does nothing until you hit the 1-2 o'clock position (so that's like 6 or 7 on a 1-10 scale) and it begins to breathe from there. The real amps do this, all three of my clones do this, Marshalls do this, Groove Tubes Soul-Os do this, etc.  
There are amps which are exceptions: MB MkI Reissue, because its presence control is simply a tone control with an audio pot dumping highs to ground; the Rivera-design Fenders, because they used reverse-audio pots in the presence positions.  
As for the amp being too bright with the presence cranked up... aren't all amps that way? They are to me; I like the presence when it's just cracked open. Anyone who's had a chance to see LC or RF with their amps will see that they set the presence control at about 1 o'clock or so, that is the threshold of where it begins to do anything at all.  
7/12/2000 5:10 AM
Steve Ahola

    Glad to hear that you're back! So how was the trip?  
    On practically all of my amps I set the presence between 4 and 6 (out of 10- no Spinal Tap tricks for me!), but at those settings my D'Clone is a bit bright... The component values used are quite different from your typical Fender or Marshall circuit, but I really don't want to mess with them blindly. And I really don't want to mess up the great sounds I'm getting from the amp... :D  
    BTW I had been using a Mesa 4x10 alnico cab with all of my heads, but recently picked up a 3/4-closed MC90 cab from Mesa; with that cab I can raise the presence control settings up a notch or two. (Both cabs together is really nice!)  
    In any case, I was wondering about the electronics theory behind the presence control/NFBL so that I could screw around more scientifically... ;)   Like how does raising or lowering the value of the presence cap effect the sound? Ditto for the value of the presence control in series with that cap? And double-ditto for the parallel resistor that bypasses the presence cap and pot? (I've already figured out how raising the value of the NFB resistor will open up the sound, but haven't experimented yet with bypassing that resistor with caps for a different effect.)  
--Thanks to anyone who cares to answer these questions!  
Steve Ahola

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