Hoffman Blues Junior- Treble Pot turning Volume down at 9 with noise

[Lucenauer]

Hi everybody, new to the forum here. I have succesfully finished the Hoffman Blues Junior conversion, with some elements of the original Blues Junior. I added the reverb on a separate PCB, which I can turn on with the footswich, and I´m very happy with it. However there is a strange effect that involves the volume and treble controls. When I turn the treble pot up (around 9 o´clock) volume drops instantly, and sound is terrible, choking and crackling. At high Volume settings the effect appears even sooner, and Master control doesn´t have any effect on it so I assume that the problem is at the preamp. The Medium and bass tone controls work just fine. I tried different values for C5 (220 pF, 330 pF) and replaced C2 and C8 (0,022uF), but this artifact still occurs. Have anybody experienced something similar? Cheers

[Latole]

Welcome to the forum.

To help you, we need to see the schematic as build.

[Lucenauer]

I just used the Hoffman version, but to add the reverb I made it like in the original creamboard layout. In that schematic, reverb circuit runs in parallel to R18 (430k). Besides that, schematic is basically Hoffman's, although I tweaked bass cap for a 0,1uF.

https://el34world.com/schematics.htm#Hoffman_Blues_Junior_Conversion

[Latole]

Am I to understand that you don't have the tele schematic you're building, that you haven't drawn a schematic of what you want to do?

I wouldn't be able to build an amp without errors if I didn't make a schematic of what I wanted to do first.

[pdf64]

I guess that the amp is unstable, oscillating at certain settings.
To help you fix that, you need to provide its schematic, layout and build photos to the thread.
Handraw stuff if necessary.
Images can be uploaded to imgbb and their bbcode links posted.

[Latole]

  "    When I turn the treble pot up (around 9 o´clock) volume drops instantly,  "

Your treble pot may have a wire or a capacitor to the ground.

[Lucenauer]

Thank you all for your replies. The layout of the preamp is exactly the one that Latole posted. Only difference is a 0,1 uF value for bass cap. I am positive that the problem is at that part, as there isn't any influence of Master or Reverb controls, neither by turning Bass nad middle tone knobs.
I checked if there's any ground/capacitor connected from treble pot lug to ground, but it's not the case.
I posted the project on a spanish guitar forum, maybe you can open the pictures from links below.
If they don't help I could make some more. As this is my first valve amp project it's maybe not as neat and clean as it should be, I know. Hope it's not crucial on this matter

https://www.guitarristas.info/index.php?controller=forum&action=view_attachment&attachment_id=192656

https://www.guitarristas.info/index.php?controller=forum&action=view_attachment&attachment_id=192655

[Latole]

Thanks for the photos, it's very interesting to see a new project.

It's a good start, but could you make sure that all the welds are solid, there are some that look dubious to me, without wishing to criticize you.

[Willabe]

There's an eyelet above the 2nd preamp tube, it's in the middle of the board. Has 3 R's an 1 radial e-cap.

That eyelet doesn't look soldered.

[Latole]

There's an eyelet above the 2nd preamp tube, it's in the middle of the board. Has 3 R's an 1 radial e-cap.

That eyelet doesn't look soldered.

IMO , there are few unsoldered or "safely "soldered.

[Latole]

I checked if there's any ground/capacitor connected from treble pot lug to ground, but it's not the case.

There are something you did not see.

[Lucenauer]

That eyelet is safely soldered, picture may  be confusing. I will upload some better pictures

[Latole]

If you think your solder joints are solid, use long-nose pliers and shake the wires on the terminals, which should resist.

  It would have been desirable for you to learn how to solder properly before embarking on this project.

This what I can see and they have many more :

[SEL49]

Your board layout looks nothing like Hoffman's layout.

[Latole]

Good point SEL49

[Lucenauer]

I'll check solder joints then. Schematic is Hoffman's, layout was redisigned into the board and materials I could afford where I live. Anyway, thanks and cheers to you all

[SEL49]

Hoffman's schematic has been proven over and over. We know it works. But your layout has not been proven. We can't tell if you made any mistakes drawing your layout or actually wiring errors, wrong component values, etc.

Posting your layout drawing would be very helpful. Your pics are not detailed enough for us to figure out if your as-built circuit matches the schematic.

[Lucenauer]

Quick update on this matter. The problem is solved. Input and preamp cable shielding is very important. As I´m using valve sockets pcb, I installed the grid stopper 10k resistors (R2, R7, R15) on the main pcb. At first I used unshielded cable from the resistors on the main pcb to valve socket pcb. When I changed to shielded, the breaking point was higher, still not gone. To get rid of the oscillation I had to increase R15 value. I tried 22k,33k and 47k, and it got slightly better with every change, but only with 68k was it perfect. No oscillations, no decrease of the treble tone range that I miss. It still shines a lot in certain settings, to a point I wouldn´t ever use, at least with a Stratocaster plugged in. I hope this can be useful to somebody. Cheers

[tubeswell]

I just used the Hoffman version, but to add the reverb I made it like in the original creamboard layout. In that schematic, reverb circuit runs in parallel to R18 (430k). Besides that, schematic is basically Hoffman's, although I tweaked bass cap for a 0,1uF.

https://el34world.com/schematics.htm#Hoffman_Blues_Junior_Conversion

You say it's a Hoffman version, but that's not a Hoffman board layout, and Doug's BJ has no reverb

[Lucenauer]

In my first post I said that I was doing the Hoffman conversion of my blues Junior using some original Blues Junior elements like the reverb. I wasn't pretending to replicate the very same Hoffman project, but to have a solid circuit I could trust, using previous parts and with some other limitations. Therefore, I had to redesign the Hoffman project into the available turretboard (smaller than the original one), and I used the upgraded valve socket board that I had installed in the amp previously. It's pretty obvious that there are differences with the Hoffman version, but the scheme at the preamp was the same (besides the reverb wet/dry part). My fault was not to post the layout (lazyness, not to much freetime, you name it), and I give that to you all. Sorry for that, but the amp WORKS. And after the last post it works like a charm, very stable, no oscillations, silent. No soldering issues, even if in the poor pictures I posted (again, my fault) some sharp eyes detected many of them.  Considering that the Hoffman version is a version of a preexisting circuit, please take mine as a version of Hoffman's, an awful one if you like. But a working one. I just posted to find out if there are some other people making the same approach I did, and I really hope this can help somebody with the same trouble. If you don't consider it appropiate or even worth of being in the forum, feel free to report it to moderators. I think that the main purpose of Hoffman's is to make  a great sounding but fragile amp usable and it certainly helped me a lot to achieve it. It may not be orthodox, I'm not an engineer but a musician that has made dozens of DIY projects, and I've been learning with every single one. So did I with this amp, and now I will enjoy playing through it. Salud