Gain stage bypass - query

[NoelW]

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.

[J M Fahey]

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

[NoelW]

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

[TomCarlos]

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!!

[Jazz P Bass]

Tom.
Happy to hear that you actually knew Bob Pease.
I loved that shot of him ditching a computer off of the roof!

[SoulFetish]

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

[ATTACH=CONFIG]55182[/ATTACH]

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)

[Chuck H]

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

[ATTACH=CONFIG]55182[/ATTACH]

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.

[NoelW]

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.

[NoelW]

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.

[NoelW]

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.

[Chuck H]

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.

[J M Fahey]

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:

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

[NoelW]

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.

[NoelW]

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:
[ATTACH=CONFIG]55190[/ATTACH]


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

[Chuck H]

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.