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vibroclone flatuance

11/15/1999 10:19 PM
vibroclone flatuance
I've got a vibroclone that I made out of a '68 Pro Reverb. I managed to squeeze a Eminence 15CG in there. My problem is the bass is too mushy and I can't turn the bass above 3 without if farting all over the place. What can I do to tighten that bass up? I put in a SS Rectifier and had a larger bassman tranny in there, but that made the bass too overwhelming so I put the original back in. What can I change to tighten it up? I like running my fenders volume around 6 or just around the point of breakup. I still want it to breakup, I just want that bass to be tight. Any help would be appreciated.  
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11/15/1999 11:02 PM
Michael Cameron

About the easiest thing to try would be to go to smaller value coupling caps. If you are using .047 try .02 or .01. this ought to give you less mud and also have the benefit of making the amp more touch responsive.  
11/16/1999 12:44 AM
John Stokes
Kid, as Mike mentioned, Fenders use rather large coupling caps since they were never originally intended to be used as distortion machines. This can make them fart at high volumes. Read some of the excellent articles on grid blocking on some of the Ampager's web pages. Anyway, this is a well-known issue with Fenders. The typical solution is to turn down the bass the more you turn up the volume. Something that most Fender players are used to doing.  
11/16/1999 12:58 AM

I do turn my bass down as I turn up, but then it gets too shrill or thin. I've heard that its possible to change things around so that your low end remains fat and clear and the high end breaks up smoothly. When I try to get that with a stock fender I either get icepicks or mud to deal with. I've heard that Cesar Diaz has accomplished this with his CD-100 amps. Any body got a schematic?  
11/16/1999 6:00 AM

Naw! We don't need no stinkin' scheamtics!  
Try some of this:  
A 12AT7 driver tube or,  
.047uF coupling caps to the power tubes,  
120K grid load resistors instead of 220K,  
bias those tubes to about 17 watts at idle.  
Try a 39K and 1k combination on the phase inverter instead of 22K or 27K and a 470 to 820ohm as seen on other amps,  
and use nothing more then a .01uF cap to the phase inverter but try smaller and smaller.. don't go below 500pF.  
DON"T get sucked into the Torres poop using big .022uF caps here unless you are going to be playing at one third to half volume all the time.  
I use a .0075uF/630v polystyrene here frequently.  
Move over to the tone controls and install a .0047uF cap from the treble wiper to the top of the volume control instead of straight piece of wire.  
And finally on the cathodes of the tone recovery triode sections make sure the 12AX7 has it's cathode's split so each section has it's own cap and resistor and make it something like, 1K5 to 1K8 but no more then 5uF to 10uF cap.  
That will still give you a nice low freq bump.  
You don't have to do all of this, but pick one and start tweaking.  
11/16/1999 7:37 AM
R Hudson

Kid: a straight to it way is: change the caps on pin 3- to a .47uf and 5uf on pin 8 of the pre amp tube. You will get great tight bass at all volumes. I have done this for hundreds of Fender owner's with the same trouble.
11/16/1999 9:31 PM

I'm just wondering, does your amp have negative feedback around the output stage, or are you running those 6L6s wide open? You do need some NFB, even if it's less than stock amount, to help control the speaker cone when operating in pentode mode.

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