Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on March 05, 2014, 04:26:59 pm
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HI
I have a 5F11 vobrolux build that I've been enjoying
THe one thing I changed from the original build was the tone stack
I used a variation of the baxandall I'll include the schematic(courtesy of Adam's Amplifiers, Thanks). I put it in where the original one pot deal was, and it works. However I still find the amp a little too mid range focused for me.
My guitars (60's single coil gretchs') are inclined that way and I like to be able to scoop the mids out. If I could I'd use the amp a lot more.
I have room for two knobs so i was going to try the ab763 tone stack that just uses a resistor to ground for mids, I found a 3-4 K resistor in this tonestack gets me the results I like.
However the 5F11 has plate of the first gain stage going straight to the volume pot and the tone control seems to be in it's own little loop
The Ab763 is hooked up differently and usually has another gain stage
Before I yank this amp apart yet again.
-Does anone have an idea if this will work ?
-Will I lose massive amounts of volume due to the AB763 tone stack
-Is there an easier/ better alternative to get a two band EQ with some mid scoop?
-Will the original tone stack provide what I'm looking for(I didn't think so being a one knob deal)
THanks
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... I used a variation of the baxandall ... I still find the amp a little too mid range focused for me.
My guitars (60's single coil gretchs') are inclined that way and I like to be able to scoop the mids out. ...
If you turn both Treble and Bass to maximum, you still have too much midrange?
With a James/Baxandall circuit, if you turn Treble/Bass to zero, you have maximum midrange; max both controls and you get minimum midrange.
Halfway up on both controls will be nominally flat; however, you're so used to hearing scooped-mid guitar sounds this will sound like midrange-heavy to you.
Try messing with the above first, and make absolutely sure no setting of the existing circuit works for you. If you still need to mess with it, I have some suggestions.
Your parts values have a mid-dip that is not severe, and is centered at ~900Hz. You might like it more centered ~400Hz, which is more typical of blackface Fender amps.
I've got some specific circuit changes that will do the above with the basic circuit you have now; let's wait to incorporate those until you have experimented with what you have.
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Neat! an option!
Thanks
I usually end up with the both of them around 3/4 with sometimes the bass higher, depending on the cab it's plugged into. It just starts getting too bright in an unpleasant way if the treble is turned much higher.
I was aware of being able to achieve some scoop with the baxandall arangement and have used it more successfully in a couple of single ended EL84 conversions I've done.
I would be really interested in learning what components to change to achieve a more AB763 result with the same stack, also I'd learn something about the tone stacks in general.
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You used the exact values shown in your schematic?
If so, try these:
1. Change C2 to 1000pF (same as 0.001uF) and C3 to 0.022uF. That will shift the midrange dip down around 400Hz. Because of the wider spacing between the treble and bass roll-offs in the circuit, the midrange dip will also be a bit deeper & wider.
2. After trying #1 above, if you find that you never turn the controls down below half-way, replace the 500k audio pots with 500k linear pots. This will keep the mid scoop at almost all settings of the controls.
You can model these changes (and the original circuit) using the Duncan Tone Stack Calculator, on the "James" tab.
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THanks
THat tone calculator is fun
I of course (being me) had to monkey with it and I came up with a slightly different change of components that seemed to mirror a AB763 mid scoop at the settings I usually use (on an AB763) except a few dbs louder. In practice it didn't sound a lot different, It may be because it's in the circuit the same way the original 5F11 tone stack was.
Maybe I should run out of the 1st gain stage , into the tone stack, then into the volume , then into the PI. I'll like lose some volume that way though