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jon
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([email protected])
9/17/2001 12:57 AM
VibroChamp tremelo into a Blackface preamp?
A guy asked me to put a good sounding tremelo into his Bassman 100. Scrapped the first channel, subbed in a Vibro Champ trem circuit
(oscillator/cathode follower). So now I've got a standard clean Fender preamp with the 2nd triode's bias being wiggled at the cathode by the LFO
circuit.
Having a bit of a problem with volume level attenuation when the trem is on. Deeper trem, deeper cutoff, less overall volume. Great sounding trem,
but not usable in performance situation via footswitch.
Suggestions? I'd rather not rework this to use an LDR, or use a triode stage (or 2 parallelled) as a varying resistance, eg early Ampeg Jet. I'm
trying to figure an easy way (hopefully without some silicon switching scheme) to bump up the gain when the effect is on. Only immediately
obvious way to me would be to use a paralleled voltage gain stage controlled by the LFO, but then I run into the problem of switching without
running a long grid lead to the footswitch. Also, I'd rather not mess with wiggling the output stage bias.
Any and all help much appreciated...
Jon
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Don Symes
([email protected])
9/17/2001 6:04 AM
Re: VibroChamp tremelo into a Blackface preamp?
Are you up for trying to wiggle the bias at the grid?
(I'm not)
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jon
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9/17/2001 3:46 PM
Re: VibroChamp tremelo into a Blackface preamp?
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Don,
this would entail setting up that last VA stage fixed bias, with a resting (-) value on the grid, which would be cycled in/out of cutoff by the oscillator
signal....or am I not thinking this thru correctly?
The other option I'm considering is to try setting up an intervening pentode like an EF86 or 6EH7/ 6EJ7 either before or after that standard triode
gain stage and wiggle the pentode's suppressor...keep you posted.
cya,
Jon
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MBSetzer
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9/17/2001 10:36 PM
Re: VibroChamp tremelo into a Blackface preamp?
I think the volume drop on the Vibrochamp trem is just the nature of the beast. It's always noticable but not that bad, usually when I want constant
volume either way I just slightly turn down the guitar volume for the dry sound, then maximixe it when I hit the footswitch to engage the trem.
You may really want the NFB coming in to the same cathode like on a real Vibrochamp, but depending on your circuit you may have to reverse the
power tubes' plate leads to get the proper phase, plus adjust resistor values a little.
Also, even when you keep a 12AX7 as your trem tube, if you substitute a 12AU7, 12AT7, or even 12AY7 in the preamp position, the trem will be
weak to imperceptible. These substitutes draw higher current and cause a buildup of higher voltage at the cathode of that last VA stage, making the
constant LFO modulation less significant. So by changing preamp 12AX7's you might just find one that makes the knob respond the way you like it.
Still I think the deeper you turn up the trem the more noticable the volume loss.
The VC is cool because when you swamp that preamp cathode stage with guitar signal (overdrive pedal if necessary) that overwhelms the trem LFO
and it sounds dry (but distorted) until the distortion fades then the trem is noticable again, so it's a cool, living type effect.
The power tube biasing trem can also be swamped similarly but less easy on an amp like my Silvertone 1472 (same as 1482) with its cathode
biased 6V6's, you might want to take a look at the schem for that where it uses a 6AU6 as a LFO tube, this is the only single stage LFO that I like
almost as well as the VC's two triodes. Wiggling the power tubes deeply has a more noticable volume loss at low playing levels where the depth is
almost to cutoff in the troughs, but you can crank up the power tube distortion and once again it sounds dry (regarding trem) until the volume dies
a little, then it automatically grows to super depth. I'm spoiled and would rather do without trem if all I had was a single triode oscillator or those
poor sounding LDR's.
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The VC LFO stops oscillating when the footswitch makes contact, but instantly comes on reliably when you stomp again. The Silvertone originally
would stay oscillating, with the footswitch just grounding the junction of the power tubes' *bias* resistors (it is cathode biased so ground is good)
to remove the LFO component from the signal. That way either of these cheap amps would have trem knobs functional (or off when
counterclockwise) without the footswitch (available at additional cost) being required for trem functionality.
Now I screwed around with the silvertone to get it as slow as the VC, and go deeper also, plus tried a variety of 6AU6's and other similar pentodes
as LFO which are all over the ballpark in response. One of the cool tubes now does stop oscillating just from turning off the trem with the footswitch
when it it set deep & slow. Then when I kick it back in the thing does not start to oscillate for about 30 seconds and still takes another 30 seconds
to get to full speed & depth. I thought it was interesting before to just play the VC & 1472 in *stereo* with their dissimilar trems at the same
speed, but now it's quite weird to hear the 1472 gradually speed up to mesh with the VC after its already going.
Bonus if you try to combine the VC design at the preamp with the 1472 type at the power tubes so you have two trems on one amp, all it would
take is a small hole for the 7-pin socket and a couple pots and terminal strips to solder the components to :)
I've actually put a 7-pin socket sideways on a perf board to keep from drilling a chassis hole, you may have pots to spare already . . .
One possible way to solve your problem would be to include an adjustable DC bias with the LFO component, then instead of creating troughs
relative to the dry volume, you could have the LFO modulate both above and below that baseline level, maybe it would average out with no volume
change to the ear. Might even be possible with switchable changes to the cathode resistor value of that last VA stage simultaneous with the trem
footswitch, maybe that would be a low enough impedance area to allow running it along with the trem control to a DPDT footswitch.
Hope this helps,
Mike
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