Gain stage bypass - query

[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!

[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!!

[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

[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]

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.