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Thread: deluxe reverb amp with extra gain stage on chanel 1 to make it more like channel 2

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    deluxe reverb amp with extra gain stage on chanel 1 to make it more like channel 2

    Hi All,
    Curious if any of the amp tinkerers out there added an extra gain stage on channel 1 of a Fender Reverb amp (most schematics are fairly close), to get the gain of channel 1 up closer to channel 2 (reverb channel). When I switch between the two channels, on my deluxe reverb clone, they both sounds good, but with reverb off (or set to 1 anyway), channel 2 sounds better, a little more 'gainy', don't know how to word it.
    I read that channel 2 has an extra gain stage to make up for the losses in the reverb portion of the circuit but that it more than makes up for the loss, which is what I experienced as well.
    I spent a few days searching for a schematic, but couldn't find one. Some discussions here and there, but more like guitarists asking how to add a gain stage on channel1, and less technical discussions that get into the design parameters.
    Going through Merlin's book as well.
    Thanks

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    Don't forget about that "loss" stage, at the dry/reverb mix point. Consider the mix circuit together with the following triode as a total gain block. You can dial the gain up or down by selecting different resistors in the voltage divider formed by 3M3 resistor in series with 220K. As well as the brightness by leaving out the 10pF cap bypassing the 3M3, or selecting different brightness caps across whichever resistor you may select in this leg. Lots of choices available here. You can leave the 3M3/10pF alone, the 470K from reverb pot too, but increase the 220K to 470K or 1M for more overall gain - actually less loss. You're certainly not stuck with the values Fender selected. Take a look at some early Traynor schematics like YGL3, you'll see variations on this circuit. And back to Fender, they leave out the 220K ground leg in Princeton Reverbs to conserve overall gain so the amp doesn't get wimpy. Plenty of room to tinker here, and find the combination that suits your ear. Have fun!

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    Last edited by Leo_Gnardo; 10-21-2018 at 09:15 PM.
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    Thanks Leo!

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    Or, you can mod the amp to habe the third gain stage shared between both channels, sp you dont HAVE to add a stage. Also, I think that would put the channels in phase, which is another bonus!

    Justin

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    Quote Originally Posted by mikepukmel View Post
    I spent a few days searching for a schematic, but couldn't find one.
    http://el34world.com/charts/TubeAmpSchematics.htm

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    Quote Originally Posted by Justin Thomas View Post
    Or, you can mod the amp to have the third gain stage shared between both channels, sp you dont HAVE to add a stage. Also, I think that would put the channels in phase, which is another bonus!

    Justin
    Thanks Justin, there's a big space where tremolo would normally go, that I did not drill out the turrets, and there's the unused tremolo socket.

    I haven't tried Leo's suggestion out yet, do have the parts, have vacation time, will try adjusting the gain and 'bright cap' on the reverb channel tomorrow.

    Over the weekend, curiosity got the better of me, I tried out wiring in 1/2 a 12AX7 setup just like the reverb ch has (820R resistor + 25uf 25v, 100k plate resistor) right before the 220k mixing resistor (after the dc filter cap) on channel 1. As you gurus would expect, it did add quite a bit of gain. Lots of gain. Wayy plenty gain. And a huge amount of noise. It was pretty late, and I could only get the volume for ch1 on about 2, but I could only hit 2 notes, it was too loud. i think some more resistors haveta be put in there someplace. Ain't fer sure where, jest yet.

    Shut the amp off, and disconnected the experiment for now. No sparks or smoke.

    At the drawing board now, going through pertinent parts of Merlin's book, and some other amp schematics looking for how they shoehorned in a gain stage. Think the final thing will be a combination of an extra 1/2 tube on ch1, and the gain/tone mod's Leo suggested.

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    Cool, thanks for the link Raybob.

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    Last edited by galaxiex; 10-23-2018 at 01:49 AM.
    The world is full of people that are right.

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    I vote for modding to have channel 1 share the third stage as noted above. Did this on my brother's Pro Reverb among quite a few other BF Fenders and everyone I did that mod for loves it now. You can voice Channel 1 to be slightly different or lots different and since they are now in phase it makes for a more versatile amp. On some SF amps it can get noisy due to layout issues but you can mod that too and have a great and versatile amp. no need to add another gain stage either.

    Greg

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    Quote Originally Posted by galaxiex View Post
    Though his site is a fantastic achievement, be aware that Rob doesn't seem to have any EE training, and as a consequence there's a fair amount of nonsense there.
    Lot's of great ideas but take any explanations / analysis with a pinch of salt.

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    Quote Originally Posted by mikepukmel View Post
    Over the weekend, curiosity got the better of me, I tried out wiring in 1/2 a 12AX7 setup just like the reverb ch has (820R resistor + 25uf 25v, 100k plate resistor) right before the 220k mixing resistor (after the dc filter cap) on channel 1. As you gurus would expect, it did add quite a bit of gain. Lots of gain. Wayy plenty gain. And a huge amount of noise. It was pretty late, and I could only get the volume for ch1 on about 2, but I could only hit 2 notes, it was too loud. i think some more resistors haveta be put in there someplace. Ain't fer sure where, jest yet.

    Shut the amp off, and disconnected the experiment for now. No sparks or smoke.

    At the drawing board now, going through pertinent parts of Merlin's book, and some other amp schematics looking for how they shoehorned in a gain stage. Think the final thing will be a combination of an extra 1/2 tube on ch1, and the gain/tone mod's Leo suggested.
    Have you tried mixing the channels at the input of the third gain stage as suggested in posts 4 and 9?

    The gain of the two channels would then be the same as Channel 2 (as required) with no extra gain and noise to deal with. It only needs one extra R an C. I've marked up the schematic below.

    Click image for larger version. 

Name:	deluxe_reverb_aa763_mod.gif 
Views:	98 
Size:	94.1 KB 
ID:	50880

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    Thanks everyone, reading . . .

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    Here‘s a link to a typical 80s style channel cascading mod for SF Fender amps, but you get only one „gainier“ channel.
    https://www.premierguitar.com/articl...a-super-reverb
    Have fun
    Zouto

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    Last edited by Zouto; 10-24-2018 at 01:39 PM.

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    Quote Originally Posted by Dave H View Post
    Have you tried mixing the channels at the input of the third gain stage as suggested in posts 4 and 9?

    The gain of the two channels would then be the same as Channel 2 (as required) with no extra gain and noise to deal with. It only needs one extra R an C. I've marked up the schematic below.

    Click image for larger version. 

Name:	deluxe_reverb_aa763_mod.gif 
Views:	98 
Size:	94.1 KB 
ID:	50880
    Oh very cool idea, thanks, will try that as well. Finally got a space cleared out in the basement, and some old kitchen cabinets and a piece of old kitchen counter for a workbench. Now no one complains about solder burning odor I can tinker more!!

    I pulled the parts out from the mostly failed attempt to use both sides of the spare 12AX7 that would have been the tremolo tube. Will try all 3 suggestions. Dang this is fun stuff, thanks everyone!

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    Quote Originally Posted by Justin Thomas View Post
    Or, you can mod the amp to habe the third gain stage shared between both channels, sp you dont HAVE to add a stage. Also, I think that would put the channels in phase, which is another bonus!

    Justin
    yes, one could replace one or more of the 100k plate resistors on the first two stages with between 150k to 470k (depending on whim), or reduce the cathode resistance (to get hotter bias and more tube current through the load)

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    Also, keep in mind that the gain of the reverb (vibrato) channel will be a lot higher if the amp is built without the vibrato channel because the 50k load of the vibrato intensity pot isn't in place.

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    Awesome, thanks Tubeswell, thanks Chuck!

    Trial 1:
    Well, stole some time that I should have used to re-install the dishwasher, and other house repairs, took the failed attempt components out, switched the bias cap and resistor and plate resistor to the same values as in the preamp (on this 1/2 of 12ax7 that would have been tremolo). 820R resistor, 25v 25uf cap for cathode, and 100k plate. I had a trim pot + resistor installed from the failed attempt, left that in.

    With this configuration, output of the normal channel goes through a voltage divider that has a trim pot, into 1/2 a 12ax7 biased as above, then through another voltage divider 220k/100k into the mixing resistor.

    By poking around through other schematics, some calculations that are probably wrong, and some dumb luck. OK lots of dumb luck: sounds much better than before. Normal channel is a little more 'gainy' (just a touch) and should be in phase with the reverb channel. Even trim pot at max, volume isn't wildly out of spec compared to the reverb channel. Sound quality is pretty good. Noise, hum, about what they were before I started the hack fest.

    No sparks, no smoke, which is nice.

    Next trial, some fiddling with the mixing cap+resistor (dry to wet) on the reverb channel, as Leo suggested.

    Will also try the "both channels into extra reverb gain stage" suggestion after that.

    General question for you all: when you do these kinds of tone "tuning" experiments, its hard to do a side by side, for me anyway, it takes up to a week before I can get time to make the changes, then I forget what the exact tone was like. Do you record trials then listen to side by side? Recording is a whole other ball of wax.

    This poor amp chassis has been sitting on a stand on my desk for many, many months. Nice to get it making sound again! Can't thank you all enough for answering my many, often stupid questions.

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    Quote Originally Posted by Chuck H View Post
    Also, keep in mind that the gain of the reverb (vibrato) channel will be a lot higher if the amp is built without the vibrato channel because the 50k load of the vibrato intensity pot isn't in place.
    Chuck, thanks that would explain a lot. I could not figure out, on this poor hacked amp, why the gain was so different between the two inputs (normal vs vib). I looked for ages to find a wrong component, resistor, bad tube. This is what I asked for, I just didn't know it.

    Never liked tube vibrato, and after reading about the bias bumping going on, liked it less. My brother is a real guitarist, brought over his strymon box. Its got fantastic tremolo, a bunch of variations, and won't (that I know of) beat the OT bias.

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    Quote Originally Posted by tubeswell View Post
    yes, one could replace one or more of the 100k plate resistors on the first two stages with between 150k to 470k (depending on whim), or reduce the cathode resistance (to get hotter bias and more tube current through the load)
    Thanks Tubeswell, i would like to try that, to get just a bit more gain on both channels. I got lost in the calculations the last time, back to the books and see if I can get it right. Wil try 150KR, since I have them, and it seems like not a huge change.

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    Quote Originally Posted by mikepukmel View Post
    Wil try 150KR, since I have them, and it seems like not a huge change.
    As you go higher with the anode resistor, the load line flattens out a bit, giving the stage more output swing for a given input swing. Not a lot, not like a 470k will magically create 470k/100k = 4.7 times more gain! A few dB is all. For the high-gainers, that few dB more above the noise floor helps. For 150k from 100k, you might not notice it, especially if the stage is loaded down (haven't looked at the schem for your DLX). But go for it! Not a huge change, as you suppose, but will be a tweak in the right direction.

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    Quote Originally Posted by mikepukmel View Post
    Chuck, thanks that would explain a lot. I could not figure out, on this poor hacked amp, why the gain was so different between the two inputs (normal vs vib). I looked for ages to find a wrong component, resistor, bad tube. This is what I asked for, I just didn't know it.

    Never liked tube vibrato, and after reading about the bias bumping going on, liked it less. My brother is a real guitarist, brought over his strymon box. Its got fantastic tremolo, a bunch of variations, and won't (that I know of) beat the OT bias.
    Disconnecting the vibrato circuit at the pot is part of several "SRV mods" for Fender type circuits to add gain and improve fidelity at either end of the spectrum. IIRC it's reported that Stevie had the vibrato disconnected at the pot in (ironically) his famous Vibroverb. Rendering it just a "-verb" I guess

    EDIT: He also had the "normal" channel tube pulled. This heats up the bias on the remaining vibrato channel by leaving only one tube running through the previously shared cathode circuit. It also raises the gain and changes feel a bit. But then it's also reported that he used a 5751 input stage tube, which would then decrease gain a bit. Anyway... Offered for your consideration. Certainly it was a very good sounding amp.

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    not like a 470k will magically create 470k/100k = 4.7 times more gain!
    Don't forget the internal plate resistance (maybe 60k for an ECC83 with bypassed cathode resistor) which acts in parallel to the plate load (referring to the current-source tube model) and prevents much increase of gain with higher plate resistors. No sense to go higher than 220k.

    img013.pdf

    The theoretical (non-achievable) maximum no-load gain is given by (around 100 for a 12AX7).

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    Quote Originally Posted by eschertron View Post
    As you go higher with the anode resistor, the load line flattens out a bit, giving the stage more output swing for a given input swing. Not a lot, not like a 470k will magically create 470k/100k = 4.7 times more gain! A few dB is all. For the high-gainers, that few dB more above the noise floor helps. For 150k from 100k, you might not notice it, especially if the stage is loaded down (haven't looked at the schem for your DLX). But go for it! Not a huge change, as you suppose, but will be a tweak in the right direction.
    Thanks, will try that! As Im learning slowly, Im finding that smaller changes spending a little time to get a feeling for what they do, are better for me to sort out, than going hog wild.

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    Quote Originally Posted by Chuck H View Post
    Disconnecting the vibrato circuit at the pot is part of several "SRV mods" for Fender type circuits to add gain and improve fidelity at either end of the spectrum. IIRC it's reported that Stevie had the vibrato disconnected at the pot in (ironically) his famous Vibroverb. Rendering it just a "-verb" I guess

    EDIT: He also had the "normal" channel tube pulled. This heats up the bias on the remaining vibrato channel by leaving only one tube running through the previously shared cathode circuit. It also raises the gain and changes feel a bit. But then it's also reported that he used a 5751 input stage tube, which would then decrease gain a bit. Anyway... Offered for your consideration. Certainly it was a very good sounding amp.
    Excellent, I can't wait to pop that V1 tube out and sounds *just* like Stevie! Heh. Ok seriously, very cool info, thanks. Oh my poor neighbors, the experiments Im going to try!

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    Quote Originally Posted by Helmholtz View Post
    Don't forget the internal plate resistance (maybe 60k for an ECC83 with bypassed cathode resistor) which acts in parallel to the plate load (referring to the current-source tube model) and prevents much increase of gain with higher plate resistors. No sense to go higher than 220k.

    img013.pdf

    The theoretical (non-achievable) maximum no-load gain is given by (around 100 for a 12AX7).
    Awesome, thanks for the info and chart! 220k is a lot higher than the stock 100k (to me anyway, 220% seems like a lot) with changes that order, would you measure up everything, voltages and re-calculate the bias? How far can you go without risk of toasting the tube?

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    I'll draw up the schematic, and post it. Based on how it sounds and smells I don't think I did anything extremely stupid.

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    Quote Originally Posted by mikepukmel View Post
    Thanks, will try that! As Im learning slowly, Im finding that smaller changes spending a little time to get a feeling for what they do, are better for me to sort out, than going hog wild.
    I find myself making a list of changes, putting them in, and then trying to figure out which one *didn't* do what I expected

    Quote Originally Posted by mikepukmel View Post
    I'll draw up the schematic, and post it. Based on how it sounds and smells I don't think I did anything extremely stupid.
    The anode resistor makes a wee difference in the dynamic behavior of the stage. A little gain, a little change to the clipping characteristics, etc. Nothing drastic. Like the difference between Hungarian paprika and Tex-Mex paprika on your deviled eggs. No way changing the anode resistor would make any part of the amp "melty". Well, maybe if one put in 150 Ohm instead of 150k!

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    Thanks E! Little tweaking is great. I learned the hard way, that a fender type thing isn't going to sound like a 100W marshall, and well we don't want it to

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    For some reason, back when, I had put an extra dropping resistor off the choke + cap to feed the reverb driver.

    THis change, I just realized I used the wrong cathode resistor. I had a 1.5K on the bench but soldered in a 820R. Think I doubled or halved something. Dang.

    Click image for larger version. 

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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    Excellent, I can't wait to pop that V1 tube out and sounds *just* like Stevie!
    But then you'll also need 2 JBL E130-8s. At least that's what he used in 1986 when I had a chance to take a glimpse at his speakers and equipment from very close during a show in a German TV studio. He had two '64 Vibroverbs stacked with only the top one powered and connected to both 15" speakers.
    Rene Martinez also told us that he used RCA 6L6s.

    I liked his sound better than his later multi-amp setup with EV speakers.

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    Last edited by Helmholtz; 08-22-2019 at 03:04 PM.
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    with changes that order, would you measure up everything, voltages and re-calculate the bias? How far can you go without risk of toasting the tube?
    You can't go wrong using one of the examples from the chart that suits your B+.

    Only increasing the plate resistor will increase gain but not plate current or tube dissipation. But it will cause off-center biasing resulting in reduced clean headroom and changed clipping behaviour. To avoid this the cathode resistor should adjusted as well.
    The chart takes care of this.

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    Last edited by Helmholtz; 08-22-2019 at 04:42 PM.
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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    You can't go wrong using one of the examples from the chart that suits your B+.

    Only increasing the plate resistor will increase gain but not plate current or tube dissipation. But it will cause off-center biasing resulting in reduced clean headroom and changed clipping behaviour. To avoid this the cathode resistor should adjusted as well.
    The chart takes care of this.
    This is really important. It's easy enough to just bump the plate resistor value for gain. A lot of "mods" do this. I never liked the sound of replacing stock 100k resistors with higher value resistors. Then when I got beyond rote modifications and began to learn more about design I discovered that it was the bias change that I didn't like and not the gain or impedance changes. I've since graduated to where I do sometimes use higher value plate loads than the ubiquitous 100k and with no tonal detriment when biased properly.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    Cool, thanks. Ive read about some amps purposely reducing clean headroom to get 'unbalanced' clipping. it has a particular kind of sound? Until then, I always thought clipping meant both top and bottom of the wave.

    If you use a higher plate load, then re-bias, that means changing out the cathode resistor for some other values as well?

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    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by mikepukmel View Post
    If you use a higher plate load, then re-bias, that means changing out the cathode resistor for some other values as well?
    Yes. You can use the load line to calculate, or just do what I do. Use values previously demonstrated as good sounding in classic designs. There are examples by many major manufacturers, but the most obvious would be the AC30 using 220k loads and a shared 1.5k cathode resistor for two triodes. Similar to Fender BF amps using 100k loads and a shared 820 ohm cathode resistor. So for a single stage that doesn't share the cathode circuit you'd need a 3k resistor. Not a standard value. 2.7k or 3.3k would work fine. With 2.7k seemingly being the preferred choice with a 220k plate load among designs I've seen. These are also the values used by Fender on some models.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    3K is a standard value for 5% parts as is 750. Fender used 10% parts for the most part.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
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