New user, first build, almost works!

[Trapland]

Hello everyone. I'm new to the forum, I've read a lot here and I finally decided to jump in. I'm a player, small time collector and electronics hobbyist. (Im a radio amateur, Extra Class. My nerdiness knows no limits)

I built my first ever guitar amp. I started on April fools day and lit her up four days later. It worked! I had no step by step, just followed the schematic and layout for a Weber Natalie kit. It's basically a JMP 4 holer without the low sensitivity inputs, an added PPIMV and running four 6v6s.

[IMG:936:699]http://i463.photobucket.com/albums/qq35 ... 6b4238.jpg[/img]
[IMG786]http://i463.photobucket.com/albums/qq35 ... b92a68.jpg[/img]

I'm liking it, but I have . a minor problem that I hope you can help with. ?!?! Please!

I think these things are all related, bt I'm not sure so I will list them.

The presence control oscillates when cranked up. It's more sensitive at higher loudness levels. Moving leads around improves it, but doesn't fix it.

Although not necessarily a problem, my ppimv is not shielded. The layout did not call for it, and I don't know if its causing problems.

The treble control is HOSED UP! It works fine either on zero or ten, but anywhere in the middle and loudness goes down and a low level distorted sound is there behind the regular sound. IT'S HORRIBLE! This ONE thing is driving me crazy! I've spent about 5 days trying to figure it out.

Anyone have any ideas on the treble thing? Could it be caused by feedback oscillations? As long as the treble is full up or down and the presence is less than full,mother amp sounds great! It seems weird that the treble would work full up if its an oscillation issue. The schematic and layout are here, in case anyone wants to look....maybe its a layout mistake?

https://taweber.powweb.com/store/natalie_schem.jpg
https://taweber.powweb.com/store/natalie_layout.jpg

Anyway, hi everyone!

[xtian]

The Presence circuit is part of the negative feedback (NFB) loop. Howling feedback is a common issue, easily fixed by swapping your output primaries (usually blue and brown, confirmed in the schematic).

This may solve your treble control issue. But if it does not, check the tone stack wiring again. Then check it again. Then post closeups.

Cheers, and welcome, and congrats on your build!

[strelok]

Moving the leads around suggests the issue with howling might be due to the long red lead coming off the presence knob. I would suggest running it under the filter cap board and shortening it so its no longer than it needs to be. Keeping the leads short helps prevent feedback loops from occurring between the various stages.

[Trapland]

Thanks for the welcome.

I don't think I have a reversed OT issue. I can play cranked with no issues. It's not until I both crank the master AND THE PRESENCE that I have problems.

I have spent several days triple checking the tone stack. I can't find anything that's not like the layout. However, I haven't checked the layout to make sure its correct to a marshall style schematic.

I'm really hoping someone will say " oh ya! Treble messed up in the middle of its range. I had tht happen and I did yada yada yada"

I may shield up the ppimv but I really want to fix this treble thing first. Unless that IS the problem.

[Trapland]

Moving the leads around suggests the issue with howling might be due to the long red lead coming off the presence knob. I would suggest running it under the filter cap board and shortening it so its no longer than it needs to be. Keeping the leads short helps prevent feedback loops from occurring between the various stages.

The red lead is hanging above the board as a test. It was below the filter and it was bad. Getting it well above and away helped the oscillations a lot. I'll figure out how to finalize it after I gt the treble issue solved.

[M Fowler]

Twist your red pairs of wire on MV together and same for yellow pairs on MV to reduce noise. Shielded wire is normally used but twisting should help.

Can't tell from photo is third lug on mid pot is going to ground.

[strelok]

I may be completely off the mark here, and I'm certainly no expert, but after looking at the schematic I can't tell how in the heck the presence knob does anything other than vary the overall level of negative feedback coming into the phase inverter. Maybe someone else could chime in on this, seems unlikely that there would be that sort of mistake on their schematics so I feel like I'm probably just missing something. It is almost 2am here.

[VacuumVoodoo]

Amputate C25. Any change?

[Trapland]

I may be completely off the mark here, and I'm certainly no expert, but after looking at the schematic I can't tell how in the heck the presence knob does anything other than vary the overall level of negative feedback coming into the phase inverter. Maybe someone else could chime in on this, seems unlikely that there would be that sort of mistake on their schematics so I feel like I'm probably just missing something. It is almost 2am here.

I wondered this too. When I couldn't figure out the treble thing, I started comparing the circuit parts to marshall. It very much like a JMP except that darn presence circuit. I would normally think there would be some cap shunting high frequencies to ground. In this circuit, everything is series and the control pot is only a variable resistor in series before squirting the neg feedback in. I can't understand how it affects frequency. I sure cant hear anything. Maybe i have to change it to a more traditional JMP style presence control.

VacuumVoodoo: I haven't clipped the cap yet, what would you expect to happen? I'm trying to learn here.

As a side note, I completely disconnected the presence and thus the negative feedback. It did not fix the treble problem at all.

[Stevem]

Having all of you preamp tubes so close to the bottom of the board and the pots maybe a source of the oscillations?
Checking the install of all your coupling and bypass caps so that the out bound lead is feeding the signal on down stream might be the fix also.

[VacuumVoodoo]

VacuumVoodoo: I haven't clipped the cap yet, what would you expect to happen? I'm trying to learn here.
As a side note, I completely disconnected the presence and thus the negative feedback. It did not fix the treble problem at all.

C25 shorts any AC feedback signal straight to Mother Earth thus leaving only DC and ultra low frequency feedback in circuit. Which is meaningless in this application.

[xtian]

I completely disconnected the presence and thus the negative feedback. It did not fix the treble problem at all.

But did it solve your oscillation problem?

[xtian]

Having all of you preamp tubes so close to the bottom of the board and the pots maybe a source of the oscillations?

I don't think so. I've build three 50-watt plexis (1987 circuit) using this 6xM chassis with tubes in the locations you see. No problems. I like this chassis a lot.

[Trapland]

I completely disconnected the presence and thus the negative feedback. It did not fix the treble problem at all.

But did it solve your oscillation problem?

Yes. Rerouting or disconnecting the presence wires fixed the feedback problem.

I still have the treble problem. The treble control workers normally when full up or down. In the middle of its range the sound is quieter with an even quieter distortion paralleling tha normal sound.

[Teleguy61]

I have built a Natalie kit.
I don't recall any issues with oscillation on startup. I would suggest that you disconnect the treble pot and measure it with your DVM to be sure it is functional. Pots do fail, and are sometimes faulty. Worth a check.
Just my two cents, but, between the presence circuit and the PPIMV, it was tough to get it to open up and speak.
It is an excellent mod platform though, and mine is currently a D-Lite, with built in D-Lator, mid cap selector, OD entry selector,and a full/half power switch.

I've been looking at Rocket threads recently though, and it may be about to become a 6V6 Rocket.
It could happen!