Stout TMB possible NFB issues

[Ambugaton]

Hey Everyone,

I am pretty new to amp building, this being only my 2nd build. I built this amp based off of the layout and some build pics on the forum but I couldn't quite understand how the NFB was supposed to be hooked up. I believe I also made some errors in the speaker jack wiring as well. I will be able to post pictures later today but just wanted to see if someone had a diagram that included the NFB.

The problem I am having is a large amount of distortion at very low levels. I know this amp doesn't have a ton of clean head room but anything above 1-2 on the volume knob starts to distort. When I push the amp up into the 4-5 volume range there seems to be an odd sound associated with the distortion... almost like a phaser type of sound (something a UFO might make). Also, only one of my speaker jacks work. Probably an easy fix though.

The amp is SUPER quiet and all my readings are spot on with what the diagram has. It still sounds great but even with a minimass I have to set the volume at a minimum to avoid excess distortion and the odd swell/phase/alien sound. I will upload pictures later but in the meantime if someone has had this issue or might have a solution... please let me know. Thanks!

[EL34]

If you are talking about my Stout, all my layouts and build info for all my stuff is on the library page here.

http://el34world.com/schematics.htm

[Ambugaton]

Yes I used the layouts that you provided. Thanks by the way! The layout lables the NFB but doesn't tell me where to connect it to the impedence selector. I just replacated what was on your build pic but because of my lack of knowledge/experience wasn't exactly 100% I was correct.

I will upload some better pictures later... these I just happened to have already.

[HotBluePlates]

The layout lables the NFB but doesn't tell me where to connect it to the impedence selector.

The layout doesn't show the output transformer directly, but the schematic does. And the Feedback labels on the schematic indicate a wire from R20 to the 8Ω tap of the OT secondary.

[sluckey]

Page 4 of the pdf shows where to connect the feedback wire.

[EL34]

It's shown in a few places on the PDF


The power supply layout shows the rotary switch and feedback and so does the schematic
http://el34world.com/Hoffman/files/Hoffman_Stout.pdf


Check the PDF file again and tell me if you see both of those locations?

[sluckey]

Also, only one of my speaker jacks work.

The jack that doesn't have the jumper between the ground lug and the switch lug will not work unless something is plugged into the other jack.

[Ambugaton]

I feel like an idiot now. This build was a few weeks ago and now I remember that I actually was concerned about how the output jacks are tied into the impedance selector. Looks like both of the jack tips connect to two locations on the selector. I think mine is just directly to the 8ohm tap.

Regardless of my ignorance is the layout/schematic has anyone had a similar issue? I get almost no clean head room with the amp. The noise associated with the distortion I will try to get a recording. Sounds similar to a phaser.

[EL34]

Have you done this yet?
http://el34world.com/Forum/index.php?topic=17701.0


It is an invaluable tool that works really well

[sluckey]

I get almost no clean head room with the amp. The noise associated with the distortion I will try to get a recording. Sounds similar to a phaser.

This may be a NFB phase issue. Swap the OT plate leads at the tube sockets. Get better? Worse?

[Ambugaton]

Have you done this yet?
http://el34world.com/Forum/index.php?topic=17701.0


It is an invaluable tool that works really well

Thanks! I did something very similar when building but wasn't sure if the second connection on the selector was the ground or not.

[Ambugaton]

I get almost no clean head room with the amp. The noise associated with the distortion I will try to get a recording. Sounds similar to a phaser.

This may be a NFB phase issue. Swap the OT plate leads at the tube sockets. Get better? Worse?

I tried this a while ago. I got major oscillation when volume was increased so I know they are correct as is.

[EL34]

Thanks! I did something very similar when building but wasn't sure if the second connection on the selector was the ground or not.

I am saying to do it now and double check everything


Logic dictates that the amp would be working of everything was correct

[Ambugaton]

I went through and double checked my build vs the layout. All is good there. I did notice that the resistor connecting the nfb to the impedance selector measures only 4.3k when installed. I've tried several resistors here and get the same results. Any thoughts?

[Ambugaton]

Here is a close up of the NFB.

[sluckey]

I did notice that the resistor connecting the nfb to the impedance selector measures only 4.3k when installed.

Perfectly normal. R19 (4.7K) is basically parallel to the 82K NFB resistor.

[Ambugaton]

I swapped the two eh's around that came with the kit and it made a difference so I think I will try a 12at7 in the pi slot.

[Ambugaton]

Well, I tried different combinations of 12ax7, 12at7, and 5157. Turns out that I liked the 5157 in the V1 spot and the EH 12ax7 in the PI. The 12at7 I tried was the mullard but was not the praised military CV4024. I didn't care for the mullard much at all. I thought putting a lower gain tube in the PI would smooth. I found that it made the amp grainy and pretty dry.