Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Willabe on February 02, 2015, 05:13:55 pm
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While I'm waiting for a few parts to finish tweaking my GA77, I started drawing up a few ideas for a stand alone Vox/Gibson vibrato. I'm also going to finally build my 1st Champ to go with it. (I need one for it's lower volume and to use it as a tube noise tester. Plus I'd like to hear for myself why some guys love the sound of a SE amp, I've never owned one.)
I don't really want to build a Vox AC15 at this time and I really want to hear/play through it's Vibrato, so..........
Here's a few drawings I did, what do you think?
Should I move the volume pot on V1b to V1a? (On the stand alone vibrato.)
The vibrato unit will probably need a 2nd volume for it's final output.
Champ will have a simple passive FX loop. Will the passive FX loop be a problem if I put it in between the treble pot wiper and the x end of the volume pot on V1b? Do I need coupling caps there?
Brad :think1:
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Gibson used a fixed voltage divider at the output to knock down the signal.
Here's the link for Gibson's stand alone vibrato and AC15;
http://el34world.com/charts/Schematics/files/gibson/Gibson_GAV-1.pdf (http://el34world.com/charts/Schematics/files/gibson/Gibson_GAV-1.pdf)
http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-83S_STEREO-VIB_(PWR_AMP).pdf (http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-83S_STEREO-VIB_(PWR_AMP).pdf)
http://el34world.com/charts/Schematics/files/Vox/Vox_AC15.pdf (http://el34world.com/charts/Schematics/files/Vox/Vox_AC15.pdf)
Brad :icon_biggrin:
Edit; Added Gibson GA83 vibrato schematic.
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Are you sure about the LFO on/off switch?
It leaves the grid floating. Off-state grid voltage and tube current is undefined. While no harm is done, switch-on action may be inconsistent, especially with tube changes (and maybe humidity).
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Are you sure about the LFO on/off switch?
It's from Vox's AC15 schemo with the speed control from Sluckey's AC15 build schemo.
I was probably going to go with a Fender LFO and driver. I just put in the Vox to start with what they used.
Brad :icon_biggrin:
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This is the LFO/driver I'm thinking of using. The LFO is from Sluckey's Warbler (with an LED for K R), I like that on the front panel for a time sight reference. And the vibrato/trem on/off foot switch leaves the LFO always on for a quick start, very good way of wiring the full circuit. So when you kick in the vibrato/trem it starts instantly. :icon_biggrin:
The driver is from the Vox AC15.
I'm wondering if a few of the R's/C's that connect the LFO plate coupling cap to the drivers grid can be eliminated? (2nd drawing.)
Does it realy need 2 capacitive dividers? And maybe move the depth pot to the grid leak position?
Brad :think1:
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I'd leave those Rs and Cs alone. The Rs isolate the oscillator from the driver and the Cs smooth the LFO signal.
Your driver circuit is drawn wrong. The 560K grid resistor does not connect to ground. It connects to the top side of that 22K cathode resistor. Connect the footswitch between the wiper of the Depth pot and ground.
I had the same reservations about the original AC15 footswitch and considered moving it but cannot remember if I actually did. I'll look tomorrow and let you know.
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I'd leave those Rs and Cs alone. The Rs isolate the oscillator from the driver and the Cs smooth the LFO signal.
Ok.
Your driver circuit is drawn wrong. The 560K grid resistor does not connect to ground. It connects to the top side of that 22K cathode resistor. Connect the footswitch between the wiper of the Depth pot and ground.
Missed the boot strap connection, fixed, thank you. Fixed drawing posted below?
I had the same reservations about the original AC15 footswitch and considered moving it but cannot remember if I actually did. I'll look tomorrow and let you know.
Ok, thanks.
Brad :icon_biggrin:
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How about this? #4?
Or, how about #5?
Brad :icon_biggrin:
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Ciao Brad
Some time ago I had a similar idea
for my documentation I used also this
(http://i81.photobucket.com/albums/j207/merlinblencowe/Vibrotron/ValveVibrotron.jpg)
http://s81.photobucket.com/user/merlinblencowe/library/Vibrotron?sort=3&page=1 (http://s81.photobucket.com/user/merlinblencowe/library/Vibrotron?sort=3&page=1)
Vibrato Sample
http://www.mediafire.com/download/b25k4tkacac9had/Vibe.mp3 (http://www.mediafire.com/download/b25k4tkacac9had/Vibe.mp3)
Tremolo Sample
http://www.mediafire.com/download/ehx9k3qz8ate053/Trem.mp3 (http://www.mediafire.com/download/ehx9k3qz8ate053/Trem.mp3)
I hope this can be of your interest
Franco
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Yes, thank you K. :icon_biggrin:
That's Merlin's version, I have seen that.
It's interesting that he got rid of the 5 stage hi-pass filter at the output and the R/C network between the LFO and driver.
Brad :think1:
Edit; Added Gibson GA83 vibrato schematic to reply #1.
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I had the same reservations about the original AC15 footswitch and considered moving it but cannot remember if I actually did. I'll look tomorrow and let you know.
I never actually followed through with moving the footswitch. It's still wired IAW the original 1960 Vox AC-15 schematic. Works very well. No pops and starts fast. That's probably why I never changed it. :icon_biggrin:
Your redrawn #3 looks good. I don't like your #4 and #5 drawings because you must run two ungrounded conductors to the footswitch.
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I never actually followed through with moving the footswitch. It's still wired IAW the original 1960 Vox AC-15 schematic. Works very well. No pops and starts fast. That's probably why I never changed it. :icon_biggrin:
Ok, good to know that. Not good to have to wait for the LFO to get going if your only using the vibrato for certain parts, not the whole song.
Your redrawn #3 looks good. I don't like your #4 and #5 drawings because you must run two ungrounded conductors to the footswitch.
Ohhhh, I see, 1 is from the grid and the other goes to the top of the 22K at the boot strap junction.
Thank you.
Brad :icon_biggrin:
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Then either of these 2 should work?
Edit; #7 was wrong, correct #7 added in next post.
Brad :think1:
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Sorry, #6 and #7 are the same, here's what I meant for #7.
Brad :think1:
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Sorry, #6 and #7 are the same, here's what I meant for #7.
Brad :think1:
Maybe I'm thinking off in left field, but when bootstrapped as in #7, wouldn't turning the depth control down to nothing result in a direct positive feedback on the grid? Did the original have that grounded, and could that be the reason? How about a large value cap from bottom of pot to ground, for AC stability at that point, and a fairly large value resistor between the pot "ground" lug and the bootstrap point.
OR. move the depth pot to before the .1 coupling cap, and ground reference it, then add a normal bootstrap resistor to the grid?
OR, if the depth pot is ground referenced, as in the first drawing, no need for the 1K5R.
Maybe I'm over thinking a non issue? For sure wouldn't be the first time for that.
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Maybe I'm thinking off in left field, but when bootstrapped as in #7, wouldn't turning the depth control down to nothing result in a direct positive feedback on the grid? Did the original have that grounded, and could that be the reason? How about a large value cap from bottom of pot to ground, for AC stability at that point, and a fairly large value resistor between the pot "ground" lug and the bootstrap point.
I think your right.
OR. move the depth pot to before the .1 coupling cap, and ground reference it, then add a normal bootstrap resistor to the grid?
That's what Sluckey liked, drawing #3, in 6th reply. I'll go with that.
Brad :icon_biggrin: