Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: isaac_teller on October 20, 2023, 07:45:24 am

Title: JTM45 One wire mod
Post by: isaac_teller on October 20, 2023, 07:45:24 am

Hello. Please correct me if I am wrong. I am building a JTM45 and I want both options-standard JTM45 and cascaded input stages. From Robrobinettes site it looks like I could make it work just by adding a SPST(on-on) switch with one side going to the bright input and the other side going to the wiper of the normal volume. Is it that easy? Seems like I might be missing something. Thank you.


https://robrobinette.com/images/Guitar/Bassman/5F6A_One_Wire_Cascade_Mod_Schematic.gif

Title: Re: JTM45 One wire mod
Post by: pdf64 on October 20, 2023, 08:27:45 am

It’s a bad idea to cascade common cathode gain stages that share cathode biasing components.
It works fine when the cathode bypass cap is new, but as it ages, a positive feedback loop develops, which will eventually cause oscillation.
Hence it’s a good idea to split the cathodes if you’re going to do this.