Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Searing on June 01, 2010, 08:55:37 pm
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I am looking for a way to get reverb on both channels of my twin. Is this possible? Thanks!
Chris
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Yes. Totally.
The easiest way it to connect the plate of the normal channel (V1B) with the plate of the Vibrato channel (V2B). Then just remove the coupling cap on the normal channel and it's done.
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Is this not the same as cascading the channels? I thought this would cause phase cancellation? Let me know.
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Not cascading - channels in parallel and same phase. This is going to increase the gain on the Normal channel so it will be just like the Vibrato channel.
You can also keep the separate coupling caps (.047 for Normal and .022 for Vibrato) by cutting the connection between the .047 and the Normal channel's 220K mixing resistor, then attaching the .047 to the junction of the .022 and the 3.3meg/10pf resistor/cap combination.
Why are you making this change?
Depending on your goals, this might be a good time to experiment with different caps in one of the tone stacks, cathode bypass caps, plate resistors, etc. There's a lot more going on in this example, but it shows how the two coupling caps would connect (except this Normal channel's coupling cap is 0.10uf) http://s222.photobucket.com/albums/dd298/Chipster457/Super%20Reverb/?action=view¤t=UberVerbSchematic-Preamp.gif
Cheers,
Chip
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Well, I am looking to play my electric thru channel 2, and my acoustic thru channel 1. I want to have reverb for both though. So, I do what exactly? Connect the Plates? Is that pin 1 to pin1 on each socket. Then removing the coupling cap on channel 1? Which one is that exactly? Do I remove it, then put a jumper there? Thanks!
Chris
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If you just want reverb in the normal channel this is the way i had donne:
Put a 500pf cap between the imput of the reverb (V3-pin7) and the output of the Normal channel (just after the .047uf coupling cap).
This is the way how is donne in the vibrato channel and will work too in this channel. No extra gain and no tremolo.
Un saudo,
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Well, I am looking to play my electric thru channel 2, and my acoustic thru channel 1. I want to have reverb for both though. So, I do what exactly? Connect the Plates? Is that pin 1 to pin1 on each socket. Then removing the coupling cap on channel 1? Which one is that exactly? Do I remove it, then put a jumper there? Thanks!
Chris
Chris - exactly which Twin are we talking about? If you post a link to the layout, that'll help a lot. Or we can try to find it for you...
Acoustic through Normal channel sounds like an opportunity to tune the frequency response & gain in that channel for your acoustic guitar.
David (Choco) has an interesting alternative. The reverb signal through the Normal channel is going to get the added gain of V4b (after reverb recovery stage) while the clean signal is not. Does this mean the reverb will swamp the clean signal? I don't know.
I've taken the liberty of attaching sections of the schematic and layout for the Twin Reverb AB763 showing the approach I described above. Not gospel, but it worked well in a Super Reverb. In your case, I might change the coupling cap (0.047) on the Normal channel to a lower value for the acoustic guitar depending on its bass response right now. Also, I drew the new jumper so it runs perpendicular to other components & leads - if you use solid core wire you can keep it 1/2" above the board too and further out of the way.
Cheers,
Chip
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+1 on the way Chip described. This is the way I have done it many times and it has no negative effect on the tone that I can perceive. Also, it essentially makes the two channels exactly alike with the exception of saide coupling cap, so they are in phase and the same including tremolo on both.
Dave
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Thanks Chip! Just did it and it sounds great!
Chris
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I used the Chip way in a stock AB763 amp and work really well.
Them i modded de Normal channel to a 5F6A (using a IRF820 as cathode follower) and the problem with the Chip way is that add too much gain (5F6A + extra gain stage= too much gain...), so i think in it for a while and tried the "mixig cap" method without any audible problem.
I you are gonna use the normal channel for acoustic guitar, maybe you dont want/need the extra bump that the new gain stage add.
I had test too the direct plate connection method, and it work. The problem is that in high level settings there are some interactuation between the channels.
un saudo!
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This article has floated around the internet for eons.
http://www.blueguitar.org/new/schem/fender/modnotes/both_rev.gif
(http://www.blueguitar.org/new/schem/fender/modnotes/both_rev.gif)
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I have a Hoffman style AB763 board and I just removed the 220k for the 1st channel. I did this because the wire tying the .047 and the 220k is under the board. I then ran a wire from the lug that the 220k usd to be on to the 3.3meg/.022 junction. Anyways, like I said seems to have worked great. I am just curious, by removing that one 220k, am I losing something? Thanks!
Chris
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That 220K resistor you removed is called a "mixing resistor". The purpose is to isolate the two channels and minimize interaction. Look at the schematic and you'll see that there's a total of 440K between the plate of V1a (2nd triode Normal channel) and the 50K Intensity pot. That's a good thing if you don't want tremolo on the Normal channel.
You are right to always ask "what am I losing/changing" if you remove (or change) a component. For example, the Hoffman-style AB763 uses bias vary tremolo so it needs a 47K resistor to replace the Tremolo pot. If you don't have it, there's way too much gain (for most people). Unintended consequences are not limited to politics.
Toprent - where were you and that article when I was stumbling through my Super Reverb modifications?
Cheers,
Chip
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I am a bit confused. :rolleyes: I removed the 220k entirely that was for channel 1. So now both channels are going through one 220k, then on to the PI cap. Is this OK? Should I have left the 220k there even though it would not go to anything? Sorry if this is redundant.
Chris
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What you did is electrically identical to the pic Chip posted. You're fine.