Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on April 01, 2010, 03:53:31 pm
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Just curious...
What is your favorite mod to do to that unused Normal channel on a Fender amp. Like the AB763 circuit.
I like putting the reverb on both channels and then modifying the Normal channel and using an A/B/Y switch to use either channel.
The simple stuff like change the tone stack, add a bit more gain by changing the plate and cathode resistors and changing the cathode caps to tighten up the bass.
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Add a stack defeat switch where the bright switch is or use a .047uf cap instead of the 120pf.Big boost.
Convert it to a dumble circuit.Add local negative feedback.
Use the whole channel as an effects loop for delay.
Tweedify the channel.
Remove the 12AX7 and put in a 5879.
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Here are my notes on what I like to do to Normal Channels:
Mod so the reverb is active on both channels
Change the v1 pin 8 cathode bypass cap and resistor values to a .68µF/2k2 configuration
Change the cathode resistor of v2 pin 8 to 1k5
Change the 100k plate resistors of v1 and v2 to 220k. Also add a 500pF or 750pF cap in parallel with the one 220k resistor that is beside the 0.047µF cap
Change both the normal channel caps to 0.022µF values
Change the 100k resistor that is horizontal between the normal channel caps and plate resistors to 56k
Change the 250pF treble cap on the normal channel to 500pF
Change the 0.047µF cap that connects the v1 plate resistors to 0.022µF
Add a 2M2 resistor from the 0.022µF cap over to the ground of the v1 and v2 pin3 cathode cap/resistors.
Change the v1 cathode resistor on pin3 to a 2k2
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I'm converting one of my V-Verb amps to a 5879 next week.I'm using a three position tone control and a cut control.
I'm also thinking of adding a raw control to the reverb channel as well.
I'll report back on the tones.
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Rewire so both channels have reverb/trem.
Install separate 1k5/25uf cathode resistors on V1b/V2b.
Hard wire the bright caps and use those switches to open the trem and NFB lines respectively.
Remove death cap and use ground switch to switch in extra .01 or .03 caps in the trem oscillator to slow it down.
Change the coupling cap in the trem channel to .047uf like the normal channel - makes the trem channel no longer wimpy.
Change the .01 PI coupling cap to .001.
Install a 250K or 500K raw control in the normal tone stack.
Honestly, on my Super Reverb once I wired both channels for reverb and trem I hardly ever used the trem channel ever again.
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Seems silly to do it that way.Make the normal channel into something else.Then mod the reverb channel to a Super-super channel.
Which coupling cap are you changing?
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I put the bassman/marshall tone stack on my DR's normal channel (250pf/.022uF/.022uF 56K slope resistor).. lowered the V1A bypass cap to 5uF/25.. and made the reverb/trem work on the norm channel. This channel matches up well with my humbucker & p90 guitars.
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Seems silly to do it that way.Make the normal channel into something else.Then mod the reverb channel to a Super-super channel.
Which coupling cap are you changing?
Most people never use the normal channel on an un-modded Fender amp, anyway. It's fundamentally the same thing as what I did except in reverse. I have never been really big on Fender amps. Great clean tones but not my cup of tea pushed over the edge. Over driven 6L6 finals are also not my favorites. I never did try 6V6 tubes in my Super Reverb but that might have entailed a new OT or at least a speaker reconfig. Short of gutting the circuit entirely and dropping in something else (and wiping out the resale value) I likely would not have been happy with the result. Little tweaks make Fenders more palatable to me but trying to gain them up just leaves me unfulfilled since I am a Marshall guy at heart.
I changed the coupling cap at the plate of V2B to .047uf (stock is .022uf or .02uf). Way back when I always wondered why the normal channel on Fender amps sounded fuller than the vibrato channel.
I changed the coupling cap from the junction of the two 220K mixing resistors into the grid of V6A (phase inverter) to .001uf. Mine was .01uf stock and it let too much low end through to the PI/power amp section.
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You guys haven't mentioned which conversion approach to reverb/tremolo on both channels you are using, so I'm having trouble relating to your discussion.
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You guys haven't mentioned which conversion approach to reverb/tremolo on both channels you are using, so I'm having trouble relating to your discussion.
This is how I did it. Disconnect the .047 coupling cap from its 220K mixing resistor and connect it to the input grid of V4b as shown in the picture. You'll see that the other channel's coupling cap is also .047. That was another change I made.
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Oh, another great mod to these amps: Change the reverb intensity pot from 100K linear taper to 100K audio taper. Now you really have some real adjustment in the reverb intensity instead of it going from none to wide open at 3 on the dial the way Leo designed it.
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I think that Ken Fisher had a mod where he took the tube off tremelo and sent it to the normal channel. I had it done to a quad reverb, and it did not live up to the billing.
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You guys haven't mentioned which conversion approach to reverb/tremolo on both channels you are using, so I'm having trouble relating to your discussion.
You can simply tie the plates of V1b and V2b together. If you want different value coupling caps for each channel, keep the Vibrato channel coupling cap where it is and tie a new Normal channel coupling cap to the point where the 3.3meg resistor/10pf "start".
Personally, I like the idea of tremolo and reverb on both channels. Then I'd go one of two directions:
a) Brown Fender - separate node on power rail for V1 with much lower voltage (season to taste), tapped treble pot with .047 & .022 caps plus higher value mid resistor in tone stack (for example), turn "Bright" switch into boost with cathode bypass caps, etc.
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b) Marshall - use MOSFET cathode follower after first stage, Plexi tone stack plus cathode bypass and coupling cap values.
Haven't done "b" but "a" was pretty successful. The attached schematic is for a Super Reverb, but the Normal channel was intended more for harp - especially with the "Bright" switch off. Turns out the guitar player likes it a lot with the "Bright" switch on too.
Cheers,
Chip
P.S. Do not try to put isolation resistors between the plates of V1b and V2b. You don't need them and they will kill your signal.
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I would consider the particular use to be made of trhe amp. If it will see some use "as a PA" for a solo act, or a duet, then stock is good -- you've got 2 channels w/o the need for a mixer. But for pure el guitar, different voicings are an option. Keep one channel Fender, usually the reverb channel; the other convert to Marshall/Vox. With one channel re-configured for Marshall/Vox it is possible to simulate both with some cap switching.
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snip............
Personally, I like the idea of tremolo and reverb on both channels. Then I'd go one of two directions:
a) Brown Fender - separate node on power rail for V1 with much lower voltage (season to taste), tapped treble pot with .047 & .022 caps plus higher value mid resistor in tone stack (for example), turn "Bright" switch into boost with cathode bypass caps, etc.
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........snip
Trying to think of a reference point for a brownface Fender tone.. Do you have any clips? This may be something I'd like to try....
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This is how I did it. Disconnect the .047 coupling cap from its 220K mixing resistor and connect it to the input grid of V4b as shown in the picture. You'll see that the other channel's coupling cap is also .047. That was another change I made.
OK! I can relate to this, pretty much the Torres version and similar to the one in Pittman's book but excludes the 2 220K resistors that he uses.
I used this version in my Weber Deluxe Reverb clone, it's simple and it's staying in that amp.
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You can simply tie the plates of V1b and V2b together. If you want different value coupling caps for each channel, keep the Vibrato channel coupling cap where it is and tie a new Normal channel coupling cap to the point where the 3.3meg resistor/10pf "start".
I did this with my Fender Twin Reverb and was happy with the result, but when I did this to my Deluxe Reverb clone it really increased the gain for the normal channel which was not compatible with that amp. Even dropping the plate resistor to 47K did not sweeten it up so I went with the wire to the 3.3M resistor bit for the lower power amp.
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Rich - I've been bugging the amp's owner for clips since I delivered it a year and a half ago. Half way between Tweed and Blackface sounds a bit too simple but isn't that far off. Lower voltages plus 220K plate and 2.2k or 2.7k cathode resistors adds some dirt Blackface amps don't get by themselves. That tapped Treble pot has an impact I can't quite explain but really liked in the Super Reverb - warmer, more mids???
Can somebody else point to a clip of a Brown Vibroverb or Deluxe?
Chip
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Has anyone tried a Post Phase Inverter Master Volume (PPIMV) on a Super Reverb or Twin Reverb?
Looking to get a bit mor distortion at a lower volume.
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I don't like post phase inverter master volumes. But I do like pre master volumes(250k) in mine where the ground switch would normally go. I can crank the volume and get that smooth overdrive and just use the master for the overall volume. Works great.
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pre master volumes(250k)
Pre master ???
Thanks!
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Has anyone tried a Post Phase Inverter Master Volume (PPIMV) on a Super Reverb or Twin Reverb?
Looking to get a bit mor distortion at a lower volume.
With a Blackface Fender, I'd think hard about VVR instead of a master volume (assuming new build of course). The stock preamp distortion just isn't that pretty IMHO.
Chip
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Since there is no mid control I would take out one of the inputs and add a 25KL pot instead of the 6.8K resistor(You can even use a 50KL) and change the input resistor to 33K. That gives you more control over the tone stack. Next is the treble bleed cap if I am calling it right which is a 250pf, you can change that to up to 500pf, use a silver mica. A lot of people decrease the 100K resistor on the tone stack, I have tried increasing it and that is a way to get more gain it's lowered in the cathode follower stack for a reason, try a 120k(I learned about that when this great sounding bandmaster I had had a 130K value because it had drifted that much). Also the plate resistor can be changed to anything up to 220K. Coupling cap anywhere from the stock .1 or .047 to .022. I would decouple the second cathodes from the other channel, use anything from 820 ohms to 2.7K and .68 to 25 mfd and even decouple the filtering. You would be surprised at the difference when you decouple the preamp stages, changes the sound of the amp. Use another 8-22mfd filter cap and either add another low value resistor or make a voltage divider for each stage, it depends on what voltages you want on each stage in your 12AX7, you can have one side low and one side high depending on how much drive you want and from were. The higher MFD cap on your B+ means tighter lows, a 10mfd cap might give a livelier sound more open like they talk about different PAF humbuckers, different cap values sometimes will do that. You can use the bright switch to change the values on your cathode cap and resistor on both stages if you use a dpdt. Use with different values for different guitars, it does change the gain structure a lot.
Also if you do not have those beautiful blue blobs of goodness try replacing the caps. If they are brown or dark blue I would replace them with either Jupiter or the SOZO blues which sound really nice. I A/Bd them with a bunch of different caps and they do make a difference for the better over orange drops and Mallory's which sounded better then many others that I tried including expensive PIO and audiophile ones.
Fender did everything cheap so when he could save money I guess he did so a little mod here or there is cool with most models. If you have some models that sound like dogs it's because they are or something is wrong. If you have a bad sounding BFSR then there is something wrong because I never heard a bad one unless it was broken some way..LOL.
And don't forget what Big Daddy always says....it don't mean anything what you do if your speakers suck.
AND...........
If you need a master volume that means you need a smaller amp....IMO.
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Nice Big Daddy.
Just curious about the Master but a very good point
Interesting, everybody says lower that slope resistor to like 56k or 33k. I'm with you.
I'm trying to figure out how to wedge another decoupling stage onto the board on a typical Normal channel layout.
Thanks!
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It should be easier these days with the smaller electrolytic caps. The 8mfd Spargue is so skinny and the F&T or Illinois 22mfd is small I think there's one even smaller rated to 450v which should be OK. If you remove the cathode resistor and cap from the first stage and put it on the bright switch there is your place for the big cap and use a strip for the resistor. You can try some different values there.
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If you need a master volume that means you need a smaller amp....IMO.
Or a larger venue.
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Nice Big Daddy.
Just curious about the Master but a very good point
Interesting, everybody says lower that slope resistor to like 56k or 33k. I'm with you.
I'm trying to figure out how to wedge another decoupling stage onto the board on a typical Normal channel layout.
Thanks!
Dropping the slope resistor to 82K and bumping the mid resistor to 9K at the same time did nice things to the mids w/o much loss of volume. Playing with Duncan's Tone Stack Calculator can give you a good appreciation for how the different components interact, but there's no substitute for your soldering iron!
Good luck!
Chip
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Here are my notes on what I like to do to Normal Channels:
Mod so the reverb is active on both channels
Change the v1 pin 8 cathode bypass cap and resistor values to a .68µF/2k2 configuration
Change the cathode resistor of v2 pin 8 to 1k5
Change the 100k plate resistors of v1 and v2 to 220k. Also add a 500pF or 750pF cap in parallel with the one 220k resistor that is beside the 0.047µF cap
Change both the normal channel caps to 0.022µF values
Change the 100k resistor that is horizontal between the normal channel caps and plate resistors to 56k
Change the 250pF treble cap on the normal channel to 500pF
Change the 0.047µF cap that connects the v1 plate resistors to 0.022µF
Add a 2M2 resistor from the 0.022µF cap over to the ground of the v1 and v2 pin3 cathode cap/resistors.
Change the v1 cathode resistor on pin3 to a 2k2
Hi All,
searching through these old threads, I found this one, I wanted to change the normal channel on my AB763 to give it some more gain or punch for soloing,
I did all these mods above and except for the 2M2 resistor, The change to my ears was only slight , Does anyone have any other mods that will give me a gainier normal channel for soloing , I'd prefer to use an AB box instead of pedals
regards
Watto
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Has anyone tried a Post Phase Inverter Master Volume (PPIMV) on a Super Reverb or Twin Reverb?
Looking to get a bit mor distortion at a lower volume.
Old topic, but... I typically don't use MV, but I acquired a late UL Super Reverb that had been exquisitely retro'd to FB, including tube rectifier, all new Mercury Magnetics iron (PT, OT, choke and rev), and really didn't like the look of the black plastic plugs where the Normal Channel mid control and master volume had been. I lightly modded the Normal Channel tone stack using one of Hoffman's tapped pots (which has since become my go-to tone stack), and restored the Master Volume, but used a PPI cross-line design, employing a push-pull pot to completely remove it from the circuit. I wired the switch so that in the default (in) position the MV is non functional, so it's a pull-to-engage arrangement.
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I like the "Lead Channel" mod for AB763 Normal channels:
https://robrobinette.com/AB763_Modifications.htm#Lead_Channel_Mod (https://robrobinette.com/AB763_Modifications.htm#Lead_Channel_Mod)
(https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Lead_Channel_Mod_Schematic.jpg)
(https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Circuit_Board_Lead_Channel_Mod.jpg)
Fritz mod for reverb & tremolo on the Normal Channel (keeps the Normal Channel coupling cap):
https://robrobinette.com/AB763_Modifications.htm#Fritz_Mod (https://robrobinette.com/AB763_Modifications.htm#Fritz_Mod)
(https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Fritz_Mod_Schematic.jpg)
(https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Circuit_Board_Fritz_Mod.jpg)
68 Custom Deluxe method for reverb & tremolo on the Normal Channel (removes Normal channel coupling cap):
https://robrobinette.com/AB763_Modifications.htm#Normal_Channel_Reverb_Mod (https://robrobinette.com/AB763_Modifications.htm#Normal_Channel_Reverb_Mod)
(https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Normal_Channel_Reverb_Mod.jpg)
(https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Circuit_Board_Normal_Channel_Reverb_Mod.jpg)
Other mods:
https://robrobinette.com/AB763_Modifications.htm (https://robrobinette.com/AB763_Modifications.htm)
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Hi All,
searching through these old threads, I found this one, I wanted to change the normal channel on my AB763 to give it some more gain or punch for soloing,
I did all these mods above and except for the 2M2 resistor, The change to my ears was only slight , Does anyone have any other mods that will give me a gainier normal channel for soloing , I'd prefer to use an AB box instead of pedals
regards
Watto
So why didn't you use the 2M2 resistor from the pot side of the .022uf coupling cap over to ground? Did you ever try it ? To anyone reading this .. I have implemented TubeGeeks mod (see reply #2) and have found it to work very well and gives a nice Marshall flavor the normal channel. I like it a lot.
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Hi All,
searching through these old threads, I found this one, I wanted to change the normal channel on my AB763 to give it some more gain or punch for soloing,
I did all these mods above and except for the 2M2 resistor, The change to my ears was only slight , Does anyone have any other mods that will give me a gainier normal channel for soloing , I'd prefer to use an AB box instead of pedals
regards
Watto
So why didn't you use the 2M2 resistor from the pot side of the .022uf coupling cap over to ground? Did you ever try it ? To anyone reading this .. I have implemented TubeGeeks mod (see reply #2) and have found it to work very well and gives a nice Marshall flavor the normal channel. I like it a lot.
I'm a little puzzled on the placement of the 2.2M resistor... anyone have a schematic that shows this?