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Thread: Gain stage bypass - query

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    Question Gain stage bypass - query

    Hi Guys,

    I am looking for some advice, so I (hopefully) only have to do this once.

    The attached circuit shows a 4 stage preamp that uses 3 stages for Clean and adds the 2nd stage (V2B) for Drive.
    The circuit is shown in Drive mode and when switched to Clean, Relay 1A enables the Volume Pot and Relay 1B enables a full Bypass of the (Drive) 2nd stage.

    Would you guys advise grounding the 2nd stage (V2B) output, as shown, to remove any possibility of signal or noise coming from V2B entering the grid of V1B; or will the Clean Bypass be sufficient to ensure uncorrupted Clean sound?
    Modifying the PCB traces and installing wiring to enable Relay 2 to ground V2B output is a PITA job, but if it is advisable I will do it, rather than have the Clean sound compromised.

    All advice, opinions, comments, gratefully received.
    Thanks, in advance.

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    Old Timer J M Fahey's Avatar
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    As shown, "Clean Bypass" bypasses nothing, since Drive extra triode always receives signal and itīs output is always connected to next stage through 1M resistor, so you NEED to ground its output or at least have Ry1b select either/or Clean/Boost output.

    As is, it only enables/disables Clean but Dirty is always enables (unless you add the second relay set of contacts that is)

    EDIT: to see it better, draw the Clean path with green pencil, Boost with a red one, and think which path gets interrupted (or not) when activating relay.

    Also, since itīs a new design , and such always require troubleshooting and fine tuning, I suggest you first build it on an eyeletted board or similar, with extra "spare" eyelets all over the place, which allow you to mod things at will.

    Only when you are 100% happy with your results you commit to a PCB.

    Just as an example, google "Bob Pease lab bench" .
    He was a mythical designer ... look at the cabling nightmare his breadboards and whole bench eventually became ... for good reason

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    Last edited by J M Fahey; 09-11-2019 at 04:34 PM.
    Juan Manuel Fahey

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    Thanks for your response and advice Juan.

    This amp is not a new build and is already on a PCB.
    It is a Fender Blues Deluxe; I took on the job, to repair some issues and do some mods to the Drive mode, against my better judgement, to help the guy out.
    I've worked on a few of these Fender PCB amps, with their flexi boards and flimsy traces.
    They are a PITA to work on and nothing you do, saves them from future failures; except for the one where I replaced the board with a turret board and digital reverb. This is the last one I will do.

    After your suggestions and those of another Technician; I have revised my approach to the switching, as per the attachment.

    Bob Pease was quite an exceptional man; very interesting reading about him and I will revisit.

    Cheers, Noel

    Click image for larger version. 

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    Supporting Member TomCarlos's Avatar
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    Juan... you mentioned Bob Pease..... I had the pleasure to meet and work with Bob during my time at National Semiconductor (many years ago). I can vouch for the photos that are online. His desk was an ABSOLUTE MESS. However, he knew the exact location of every piece of paper that he ever collected. The lab area was also a mess. Bob was intelligent, a great engineer, personable, and a please to work with.

    Yes, before going to PCB, build a test board. That is what we did before committing product ideas to the Integrated Circuit build. Back in the day, we called them Bread Boards!!

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    Supporting Member Jazz P Bass's Avatar
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    Tom.
    Happy to hear that you actually knew Bob Pease.
    I loved that shot of him ditching a computer off of the roof!

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    Quote Originally Posted by NoelW View Post
    Thanks for your response and advice Juan.

    This amp is not a new build and is already on a PCB.
    It is a Fender Blues Deluxe; I took on the job, to repair some issues and do some mods to the Drive mode, against my better judgement, to help the guy out.
    I've worked on a few of these Fender PCB amps, with their flexi boards and flimsy traces.
    They are a PITA to work on and nothing you do, saves them from future failures; except for the one where I replaced the board with a turret board and digital reverb. This is the last one I will do.

    After your suggestions and those of another Technician; I have revised my approach to the switching, as per the attachment.

    Bob Pease was quite an exceptional man; very interesting reading about him and I will revisit.

    Cheers, Noel

    Click image for larger version. 

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    I, for one, hope Fender never stops making their Blues/Hotrod amps.
    Some brilliant member here said ‘The Fender Hotrod series is to amp techs what Microsoft Windows is to IT guys, a reliable and consistent form of income’
    (To paraphrase)

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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by NoelW View Post
    Thanks for your response and advice Juan.

    This amp is not a new build and is already on a PCB.
    It is a Fender Blues Deluxe; I took on the job, to repair some issues and do some mods to the Drive mode, against my better judgement, to help the guy out.
    I've worked on a few of these Fender PCB amps, with their flexi boards and flimsy traces.
    They are a PITA to work on and nothing you do, saves them from future failures; except for the one where I replaced the board with a turret board and digital reverb. This is the last one I will do.

    After your suggestions and those of another Technician; I have revised my approach to the switching, as per the attachment.

    Bob Pease was quite an exceptional man; very interesting reading about him and I will revisit.

    Cheers, Noel

    Click image for larger version. 

Name:	Blues Deluxe. DRIVE-CLEAN SWITCHING-revised.jpg 
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    I don't like this option either. I'm not sure what the 2M pot function is. If it's it's just a fixed trimmer and not user adjustable this may be ok. Otherwise when it's at full CCW rotation it will bleed that 1n cap to ground variably when in clean mode.

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    Quote Originally Posted by SoulFetish View Post
    I, for one, hope Fender never stops making their Blues/Hotrod amps.
    Some brilliant member here said ‘The Fender Hotrod series is to amp techs what Microsoft Windows is to IT guys, a reliable and consistent form of income’
    (To paraphrase)
    I hope they don't stop making them either so all you techs, who like the business they bring, can keep making a living from them.
    I find they consume more time to fix than the average guitarist around here wants to pay for.
    Maybe your clients are more understanding.

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    Quote Originally Posted by Chuck H View Post
    I don't like this option either. I'm not sure what the 2M pot function is. If it's it's just a fixed trimmer and not user adjustable this may be ok. Otherwise when it's at full CCW rotation it will bleed that 1n cap to ground variably when in clean mode.
    Chuck,
    The 2M pot is a multi-turn trimpot and it is mounted internally.
    It is there to adjust the maximum Drive signal level to the owner's satisfaction, soundwise; then it will most likely be replaced by a fixed resistor.
    There is the 250k Drive pot between the 2M (or whatever it ends up as) and ground as well.

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    Quote Originally Posted by Chuck H View Post
    I don't like this option either.
    Chuck,

    What don't you like and can you offer some advice?
    I am always grateful for suggestions or corrections from knowledgeable Techs.

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    Bent Member Chuck H's Avatar
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    Do it like this. One six pin relay (actually 8 pin with the actuator voltage). I changed the 510k resistor to 1M so that I could add a 1M to the other side of the relay switch for a total 500k (instead of 510k) in drive mode. Same for the 1M added to the clean pot. I hope this doesn't complicate what's possible on the board or any of your other limitations.

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    Last edited by Chuck H; 09-13-2019 at 02:44 AM.
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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by NoelW View Post
    This amp is not a new build and is already on a PCB.
    It is a Fender Blues Deluxe
    ;
    Ouch!!!
    What you did now is better, you are actually switching a triode stage in/out the circuit, but there is a problem which will drive you crazy: Blues Deluxe/Deville/etc. series are NOT two channel amps but SINGLE channel ones, with all gain stages already there, and they attenuate some, ground pots or leave them floating, etc., as needed to clean it up.

    Simpler (for them) to build or layout but since "everything is in the signal path, always" design is always a compromise, distortion is not too good and clean, although acceptable, is not Blackface clean, too much mud in the path.

    You are trying to *really* bypass some stages but it will drive you crazy because itīs not built that way.

    Personally, I would "tweak" Distortion but nothing too deep, too much effort for little improvement.

    If it were a personal build, Iīd plain tear the whole PCB off and build, say, a Soldano or something there, even one of the simplified ones.
    Or check what Laney does in the VC series:
    Click image for larger version. 

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    I took on the job, to repair some issues and do some mods to the Drive mode, against my better judgement, to help the guy out.
    I've worked on a few of these Fender PCB amps, with their flexi boards and flimsy traces.
    They are a PITA to work on and nothing you do, saves them from future failures; except for the one where I replaced the board with a turret board and digital reverb. This is the last one I will do.
    Famous last words

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    Juan Manuel Fahey

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    Quote Originally Posted by Chuck H View Post
    Do it like this. One six pin relay. I changed the 510k resistor to 1M so that I could add a 1M to the other side of the relay switch for a total 500k (instead of 510k) in drive mode. Same for the 1M added to the clean pot. I hope this doesn't complicate what's possible on the board or any of your other limitations.

    Chuck,

    That looks like a good variation to the cct.. The 1Ms will work well if I can implement the changes.
    The problem is; changing the Volume control from rheostat to potentiometer is not easy because of the board limitations. This why I used the 680-560//510 divider to get somewhere in the ballpark of necessary attenuation.
    These resistors and some of the other caps and resistors and the trimpots are mounted on a separate aux. turret board so they can be changed, relatively easily, to alter signal level, as needed. There is a trimpot for V2B Rk adjustment also.
    Applying the grounding mute to the Drive Input or Output is going to be difficult, due to the closeness of the traces to each other and not much trace to attach a wire to. I don't want wires falling off the board.

    Do you think a mute is necessary now that the 3rd stage input is only connected exclusively to stage 1 Clean or stage 2 Drive?
    If a mute is really necessary to eliminate any signal bleed-through, I will try and find a way to do it and make the wires secure.
    I favour an Input mute, but it is more difficult to do than the Output mute because of the board layout.

    Thanks for the help.

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    Quote Originally Posted by J M Fahey View Post
    Ouch!!!
    What you did now is better, you are actually switching a triode stage in/out the circuit, but there is a problem which will drive you crazy: Blues Deluxe/Deville/etc. series are NOT two channel amps but SINGLE channel ones, with all gain stages already there, and they attenuate some, ground pots or leave them floating, etc., as needed to clean it up.

    Simpler (for them) to build or layout but since "everything is in the signal path, always" design is always a compromise, distortion is not too good and clean, although acceptable, is not Blackface clean, too much mud in the path.

    You are trying to *really* bypass some stages but it will drive you crazy because itīs not built that way.

    Personally, I would "tweak" Distortion but nothing too deep, too much effort for little improvement.

    If it were a personal build, Iīd plain tear the whole PCB off and build, say, a Soldano or something there, even one of the simplified ones.
    Or check what Laney does in the VC series:
    Click image for larger version. 

Name:	Laney01.png 
Views:	19 
Size:	10.4 KB 
ID:	55190


    Famous last words
    Juan,

    I agree with everything you said and this is why I did just that to one of these amps, a few years ago.
    I gutted the chassis except for the trannies and other useful bits and made a complete new turret board with 2 separate channels (sharing Stage 1 and the FX loop driver), with a digital Reverb; this amp is still being gigged without problems.
    Unfortunately, I couldn't talk this bloke into doing that and I'm now past the point of making anything out of the job, but I want to give him an amp that works well, regardless.
    Pride in my work prevents me from deliberately doing a crap job, even if it costs me.

    "Famous last words" indeed!! (I can only hope that I don't weaken and take on another)

    I have a VC30 in my workshop now, so I will investigate further.

    Chuck H made some good suggestions also.

    Thanks to you all, for making your experience available on this informative forum.

    Cheers, Noel

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by NoelW View Post
    Chuck,

    That looks like a good variation to the cct.. The 1Ms will work well if I can implement the changes.
    The problem is; changing the Volume control from rheostat to potentiometer is not easy because of the board limitations. This why I used the 680-560//510 divider to get somewhere in the ballpark of necessary attenuation.
    These resistors and some of the other caps and resistors and the trimpots are mounted on a separate aux. turret board so they can be changed, relatively easily, to alter signal level, as needed. There is a trimpot for V2B Rk adjustment also.
    Applying the grounding mute to the Drive Input or Output is going to be difficult, due to the closeness of the traces to each other and not much trace to attach a wire to. I don't want wires falling off the board.

    Do you think a mute is necessary now that the 3rd stage input is only connected exclusively to stage 1 Clean or stage 2 Drive?
    If a mute is really necessary to eliminate any signal bleed-through, I will try and find a way to do it and make the wires secure.
    I favour an Input mute, but it is more difficult to do than the Output mute because of the board layout.

    Thanks for the help.
    There are pro's and con's to muting input or output. Since you only have one additional gain stage I think muting the output might be favorable. One reason is that the power supply for that amp (unless it's modified) operates like phase stages. That can cause problems with any output bleed on the power supply that isn't adequately referenced to 0V (like a filter cap that has a little age on it). This can cause instability. If you would like to mute the input that can work too. As long as the variable attenuation resistor for the drive "channel" is greater than about 220k you could relay ground the grid of the drive stage AHEAD of that resistance rather than relay grounding the output behind the 1M series resistor. Such a series resistance at the input of an additional stage isn't always available (see later info) so grounding at the output is somewhat standard. On that note...

    You may want to re think your attenuation strategies. 1M and 2M attenuations for high gain circuits create a SHIT TON of white noise (HISSSSS!!!!) Lowering series resistance in high gain circuits can only help. This typically requires some out of the box thinking and creative alternatives, but it can pay off.

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    Quote Originally Posted by Chuck H View Post
    There are pro's and con's to muting input or output. Since you only have one additional gain stage I think muting the output might be favorable. One reason is that the power supply for that amp (unless it's modified) operates like phase stages. That can cause problems with any output bleed on the power supply that isn't adequately referenced to 0V (like a filter cap that has a little age on it). This can cause instability. If you would like to mute the input that can work too. As long as the variable attenuation resistor for the drive "channel" is greater than about 220k you could relay ground the grid of the drive stage AHEAD of that resistance rather than relay grounding the output behind the 1M series resistor. Such a series resistance at the input of an additional stage isn't always available (see later info) so grounding at the output is somewhat standard. On that note...

    You may want to re think your attenuation strategies. 1M and 2M attenuations for high gain circuits create a SHIT TON of white noise (HISSSSS!!!!) Lowering series resistance in high gain circuits can only help. This typically requires some out of the box thinking and creative alternatives, but it can pay off.
    Is grounding the Drive output going to have much effect now that the output will be isolated from the 3rd stage grid, when in Clean mode and vice versa?
    Unless there is a risk of capacitive or inductive coupling causing bleed-through of signal/noise to the 3rd stage. ??
    I have installed the dropping resistor (33k) and the decoupling cap (33uF) to separate the added V2B gain stage from the others to help avoid bleed-through; is this not enough, you think??

    I have had concerns about the HISS also and will change values to suit if there is any. The aux. board makes this easy to do.
    I can drop a resistor to ground from the 2M trimpot-Drive pot junction to create a divider and then fiddle the values to suit. This another reason for using the trimpot; I can use it to get a divider ratio and then alter the resistor values to avoid HISS.

    My main problem is working within the restrictions of the congested, already populated, poorly made PCB of a 1 channel amp that is pretending to be a 2 channel amp.
    I probably should have resisted the owners request to have good overdrive onboard and told him to get a good pedal and use the Clean "Channel".
    Too late now, so I will have to make it work.

    Other Techs have had success with overdrive mods to these amps, apparently.
    Steve Dallman, on the Fender Forum (FDP) has done some, using just the Drive stage "Bypass" method and his clients have been happy with the results.
    I was concerned about the Bypass not being sufficient, which lead me to ask you blokes for your advice; and as you and Juan have pointed out; the "Bypass" is not really a proper bypass and will likely cause issues that I don't want.

    So on we go.

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    Bent Member Chuck H's Avatar
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    It's not just the V2B drive triode that can be problematic. Notice that the V2A triode is in phase with the input triode (V1A) and fed from the same power supply filter node. This has been a pitfall for this amp model in general. I do think that the branched dropping resistor and cap are suitable for isolating the drive input triode. In fact I think 33k is going to produce lower than desirable voltage, but maybe not.?. Still... Best design would be to operate V1 from one filter node and V2 from another. Since you already created a branched node I would think this is possible. And I would probably just run them in series with a lower resistance than 33k, but you may already have a contrary guru plan that you're trying to follow. Same goes for the channel switching. The method I demonstrated is standard practice. And for good reason. Many have tried to implement channel switching with a simple SPDT relay and failed. SOMETHING always seems to cause bleed in this scenario. Inductive, capacitive or via power supply coupling. If you look at successful commercial designs (which you should have already) you'll see that they ALL ground the drive channel somewhere when it's not in use. You were ready to implement a third relay switch if necessary. Why, now that I've shown you how to do it with two do you want to use only one?

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Chuck H View Post
    Same goes for the channel switching. The method I demonstrated is standard practice. And for good reason. Many have tried to implement channel switching with a simple SPDT relay and failed. SOMETHING always seems to cause bleed in this scenario. Inductive, capacitive or via power supply coupling. If you look at successful commercial designs (which you should have already) you'll see that they ALL ground the drive channel somewhere when it's not in use. You were ready to implement a third relay switch if necessary.
    We are trying to paddle up the waterfall here, we have everything against: VERY high gain , very high impedance, buzzy/aggressive signal (distortion) full of harmonics, constricted space, etc.
    All conspire to make buzz jump channels, contaminating the clean signal.

    Common complaint is "I can still hear distorted signal in the background".

    So you must not only switch between clean and dirty *output* signal, you must also internally mute dirty channel so
    distortion isnīt even generated.

    Have a look at the cheesy/cheap "splatty" Marshall we are following in another current thread:



    Here we have two separate, *parallel* preamps, which are footswitch selectable.

    Not just a single triode gain stage in/out of a preamp but two fully separate ones, notice both get signal from the straight from the input jack, and share nothing.

    Instead of an SPDT relay selecting between clean and dirty channels they permanently join them through 220k mixing resistors and alternatively ground one preamp output or the other ... in principle the same thing, but, of course, cheaper and simpler.

    So "Left" transistor grounds Clean volume wiper to mute it, and "Right" one grounds , not Boost channel Volume but to make it stronger, straight grounds V2a plate and yet it clearly was not enough, so they added a third "contact/switch" to also ground Boost channel Gain pot wiper using the "Center" transistor.

    So they are grounding/switching OFF two points on the dirt channel.

    So if you have an unused relay section, do something similar.

    You might add a transistor switch as a mute similar to the Marshall amp but then you will have to create and add some control line to activate it, not worth it on a single job, so in this case it might be more bpractical to addn a relay gluedn to thye chassis, legs up, and wire it as needed.

    BUT, even with output grounded, some dirty splatter

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    Juan Manuel Fahey

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    Hi Chuck & Juan,

    I hear you both, loud and clear and I am going to implement your advice, which I always agreed with.
    It was just the degree of difficulty and suspect reliability of the traces that lead me to looking for a simpler way to do this.
    But life and amp repairs wasn't meant to be easy, but the right way is the best way.

    I have spent some time completing most of the other work and I think that adding another relay is the best method to mute the Input and Output of the Drive stage, when in Clean mode.
    The closeness and fragility of the traces to the existing relays, concerns me.
    I have the components for the added Drive stage mounted on the aux. board, which will enable easy connection to the mute grounding points.

    All I have to do is figure out how to wire in the new relay, to the existing circuit, so that the panel and foot switches will operate it as well.
    My knowledge of SS electronics isn't great and I am concerned whether the 16V +- power supply can handle another relay. There would then be; two ON and one OFF in one mode and one ON and two OFF in the other.
    I have mounted 15W aluminium body resistors externally to a heat sink to replace R85 & R86, which are renowned for burning the PCB.
    Blues Deluxe Schematic attached.

    I am also going to decouple all the stages from the power supply, to further aid the separation of the gain stages; as per Chuck's advice.
    I am thinking that individual dropping resistors and decoupling caps, coming off the "X" Supply node will be simpler to arrange than doing a series drop and cap arrangement, due to the standard layout.
    Maybe 10k resistors and 10uF or 33uF caps, both of which I have in small radial sizes, which will make mounting easier.
    (Chuck, I only fitted the 33k resistor, so I could easily parallel others to find the best voltage drop, if it was too much. With a measured 330V available, at "X", 33k would have dropped about 30V)

    I have also changed the voltage divider 2M resistor at the Drive Input to a 680k with a 220k from the Drive pot top to ground.
    It is a bit difficult to change the Volume pot from rheostat to potentiometer, so that will have to stay.

    What do you both think of those changes, now.

    I really appreciate the advice and help from you both and I don't mind being told that I'm wrong or misguided, when I am.

    Cheers, Noel
    Attached Files Attached Files

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    Last edited by NoelW; 09-14-2019 at 07:24 AM.

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