Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Alistar on June 08, 2016, 09:48:13 am
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I'm trying to find the best way to bridge the channels internally.
Vibro Channel - I tapped the output of the first stage (inverted phase) with a 0.1uF cap to the input of Normal channel input, using the Bright switch to activate.
It works, but is too gainey and has more Hiss than I like - I was going to try dropping the signal with a 100k, but wanted to know if anyone has done this and has recommendations.
Thanks!!
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I'm trying to find the best way to bridge the channels internally.
Vibro Channel - I tapped the output of the first stage (inverted phase) with a 0.1uF cap to the input of Normal channel input, using the Bright switch to activate.
It works, but is too gainey and has more Hiss than I like - I was going to try dropping the signal with a 100k, but wanted to know if anyone has done this and has recommendations.
Thanks!!
Take a look here:
https://robrobinette.com/AB763_Modifications.htm#Normal_Channel_Reverb_Mod
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Thank you for the link, that's a great page - however, not exactly what I'm looking for.
Specifically, I am trying to set an internal bridge after V2a (After the 1st triode in the Vibro channel, but preferably before the tone stack), and rout that signal to V1a, normal channel 1st stage.
I would like the Normal channel to have 1 extra gain stage and adjust V1a to get significant, but useable 'Dirt'.
I also would prefer to not have reverb on the Normal Channel.
As I'm typing this I'm thinking Ill start to look at some schematics like Soldano for ideas.
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You'll definitely be out of phase at that point, the original is inverted. That way when they come back together at the PI they'd cancel.
I'd guess you want to tap after the second half of the input, but you'll be a bit hotter at that point, and may need a dropping resistor, but better may be to just jumper from the input jack itself over, maybe with a switchable 'off' jumpering for normal operation as well? That way you get clear direct identical input from the guitar to both inputs. (make sure its before the resistors on both sides, or identically after them either way).
~Phil
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On the BF Reverb amps, you can't really jumper at the input because the two channels ARE inverted at the PI. The reason I want to jumper internally after V2a, tying back to the grid on V1a is that both channels will then be in phase at the PI and ultimately output. Using that I was thinking out the 'extra' triode stage to use by getting some dirt outta it.
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Not sure i follow? If you put the signal through both halves of each dual triode, they both come out in phase with one another, the output of the vibrato goes split part way into the second stage for the vibrato itself, but the other half goes into the PI just like the normal channel. Maybe I'm just misreading it.
another way to say it is that if you put an input at the vibrato, at the point after the 220k resistor where it hits the PI, the signal is in phase from the first input, as it would be at the output of the normal channel. (You could, in theory, just use a jumper cable).
Or at least that's the case on the BANDMASTER-AMP AB763 schematic I'm looking at:
http://www.thevintagesound.com/ffg/schem/bandmaster_ab763_schem.gif (http://www.thevintagesound.com/ffg/schem/bandmaster_ab763_schem.gif)
~Phil
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In fact, just to keep my sanity I used paint to run lines for where the signal goes, in my head, and the light blue woudl be vibrato, red normal, and dashed darker blue is what feeds into the vibrato part of the circuit.
Let me know if that seems 'off' and what I'm missing.
~Phil
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Or at least that's the case on the BANDMASTER-AMP AB763 schematic I'm looking at:
That's true. For that Bandmaster. Or any other non-reverb AB763. The normal channel has the same number of inverting stages as the Vib channel. So, you can jumper the inputs with no problems.
But, the situation is different for a reverb AB763 amp. In this case, the normal channel has two inverting stages but the vib channel has three inverting stages due to the wet/dry mixing stage. Jumping inputs on these amps will result in out of phase signals cancelling and sounding kinda thin.
It would help if we knew exactly which AB763 amp we're talking about.
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This is a Super Reverb model.
Does what I'm saying make sense? Taking the signal after the first triode, V2a, and
Tagging it into the input for the Normal channel.
I've got it working with a 0.1uF cap from V2a plate into Normal channel input via the bright switch. It definitely works in terms of phasing, but comes off as too loud with a lot of hiss.
http://el34world.com/charts/Schematics/files/soldano/soldano_supercharger_gto.pdf (http://el34world.com/charts/Schematics/files/soldano/soldano_supercharger_gto.pdf)
I was thinking of trying to use a network as in this Soldano Supercharger from the output of the first triode to the input of the 2nd, bias the SR V1a as the 2nd stage in the Supercharger, and using an internal trim pot for the gain adjustment.
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Does what I'm saying make sense? Taking the signal after the first triode, V2a, and
Tagging it into the input for the Normal channel.
I've got it working with a 0.1uF cap from V2a plate into Normal channel input via the bright switch. It definitely works in terms of phasing, but comes off as too loud with a lot of hiss.
Yes, it makes sense. But as you found, there's a ***LOT*** of gain. Hiss (noise) naturally goes along with gain. You will have to knock the gain down in the normal preamp. You can use a signal voltage divider between V2A plate and V1A grid to knock the signal down. You can also use a voltage divider (split load resistors) on the plate of V1A. Or you can remove (or greatly decrease) V1A's cathode cap. All these things will help tame the extra gain. Any one of these changes may get what you want but it will likely take a combination of these ideas to make it happy.
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Wow that's definitely confusing to me, I didn't realize that fender had multiple AB763 layouts. I thought the naming AB763 was one specific layout. Learned something new :) (in the naming and in what sluckey's suggestion is, very cool!)
EDIT: typo of AA instead of AB
~Phil