Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: vampwizzard on April 23, 2019, 08:08:37 am

Title: NFB circuit 5F6A vs JTM45
Post by: vampwizzard on April 23, 2019, 08:08:37 am

Folks,

Been gone for awhile caught up in work/performing music/coaching wrestling but back now. A guy in Nashville saw and heard that matchless clone I built and asked me to commission an amp with Fender cleans and Marshall distortion... made him go play a JTM45 and got him sold on that.

Just got the parts to convert my 5E3 build which wound up using 6L6's to a 5F6A (nearly identical circuit to the JTM45) and was looking for advice on the NFB circuit. Rob Robinette's 3-way switch https://robrobinette.com/Generic_Tube_Amp_Mods.htm#3-Way_Negative_Feedback_Switch (https://robrobinette.com/Generic_Tube_Amp_Mods.htm#3-Way_Negative_Feedback_Switch) gives advice for balancing the NFB based on what secondary tap you pull the NFB from. Since my OT is from the hammond AO-29/8Ohms, should I be looking to size my NFB resistors right along with the chart? 56K for the 5F6A and 20K for the JTM45?

Thank you all! Looking forward to this one.

Title: Re: NFB circuit 5F6A vs JTM45
Post by: sluckey on April 23, 2019, 08:48:36 am

Rob's info is a good starting point. If you like the results then call it done. Otherwise, experiment with different values. Many people even use a pot to vary the amount of NFB. There's really not a right or wrong here.

Title: Re: NFB circuit 5F6A vs JTM45
Post by: vampwizzard on April 23, 2019, 09:19:50 am

Thanks, Sluckey. He intends to take this thing on the road so Im going to try and bulletproof it the best I can. May bring a pot for test purposes and dial it in before swapping in a fixed value. The other request was to mimic channel switching because he doesnt like pedals.. so I plan on just doing the channel jumpering/cascading preamps via the footswitch. Lest I have to demonstrate a guitar volume control.  :icon_biggrin:

Title: Re: NFB circuit 5F6A vs JTM45
Post by: MakerDP on April 29, 2019, 10:34:01 am

Look at how the AX84.com October amp implements its variable NFB control... I LOVE this control in my builds. With that control the October can be 5F6A-ish or JTM-ish with the twist of that knob... and anywhere in between. (Also, flip a switch and now it's Plexi!)

Title: Re: NFB circuit 5F6A vs JTM45
Post by: vampwizzard on April 29, 2019, 08:48:09 pm

I took a look and that is certainly one hell of an amp. Ill probably try the pot on the next iteration.