Oscillation...gondo!

[surfsup]

I was trying for months to get rid of this slight oscillation. I could describe it as "harsh treble distortion" that i hear others struggle with. The paul ruby mod was suggested and i implemented that but there was still a little left. Been annoying to say the least.

I can finally claim success in swatting this oscillation to its grave...

Here's what i did, might help someone else. The leads to my tone stack pots were standard. They ran from the board to the pots much like many photos i see. At about 6-7 on the gain knob and 10 on the guitar vol pot, the oscillation would start being audible. It would kill double stops and sustain only on certain frequencies. Contrary to "short leads" i actually Lengthened the lead from the treble cap to the pot. I ran it down to the chassis, along the chassis to the opposite side of the pot then straight up between the pots against the chassis front plate. This helped.

I noticed the high E string, B, G etc sounded much better, but now i noticed a slight issue with low strings. So i did the same with the bass lead off the TMB board cap turret to the bass pot. What a difference. Its like the amp is so smooth and hold notes longer. Double stops sound totally clear and harmonically rich rather than harsh ans distorted sounding. When doing a double stop to a string bend, it was really bad before. Now, the note is bent clearly and holds a sustain much longer. At 10 on the gain and vol on guitar cranked, she's clear and sounds better than i've ever heard. Not sure i can improve it at this point.

So if you have a slight harsh overtone, you might want to mess with your TMB leads to the pots. Actually make them longer and run them against the chassis rather than clustered together in between the pots and board.

[ampgeek]

Very helpful given that I am nearing the end of an 18'ish watt layout drawing! It's an Atomic 16 pre (TMB tone stack) into a "standard" 18 watt, cathode biased PA. VVR on PA plates and screens only. I am also including both pre and post PI MVs.

I have been reading with great interest the various threads regarding fizz/buzz distortions and have also included the Ruby mod and OT primary filtering array.

Could you share your layout and circuit designs? I would like to see where mine stands against yours to determine if I need to tweak to take advantage of your discoveries.

Mine is relatively compact so there isn't much distance between the tone stack turrets and the pots. Yep...working under the shortest distance philosophy!

Also hoping that someone will make a guess at what might be happening to make the improvement in your condition.

Many thanks!
Dave O.

[surfsup]

Here's an old pic. The green lead is shielded and runs under the board now, etc... but the white/blue leads running to the pots are what I changed. I ran the two white straight down to the chassis, across and up. There is about 1/2" separation on them as they run across the chassis.

I was reading Merlin's Preamp book and he wrote how the typical Fender TMB stack should not be put on the board because these are very sensitive leads - showing a point to point. I chopsticked in here moving those around as much as I could with no difference. I decided to trust that statement that these leads were sensitive and re-run them down and across - making them several inches longer, but against the chassis and sure enough, that helped. I'm so glad this oscillation ordeal is over! It's like I have nothing to do now...

[Reeltarded]

That is excellent! Worth an experiment here later. Thanks buddy, and good job!

[ampgeek]

Cool. Many thanks!

What is the function of the switch that the blue/white leads are running near? Wondering if that might have had something to do with the distortion you were experiencing.

Nothing to do?!?! You must be dreaming up your next build by now!!

Thanks again,
Dave O.

[Milkmansound]

When I figured out a layout that worked for a fender style tone stack, I took a lot of pictures and committed it to memory! I have chased down that oscillation myself before and it's no fun! I think congratulations are in order

[Gaz]

Yeah, I've been having wires hit the chassis off the board where I used to fly them to the pots. I kinda had this voice in my head saying, "make the leads as short as possible." I think that's a highly subjective statement, and has lead many builder's astray!

After seeing some gut shots of Randall Aiken's stuff, I decided to try moving some sensitive flying wires to along the chassis, even thought it made them longer. He does it with every lead:

http://www.amparchives.com/album/Aiken/ ... ken13.html

It surely took out a little buzz that was only abated by shielding the chassis previously.

I don't think the lengthening of the wires necessarily helped you, but rather exploiting the shielding provided by the chassis. I'd got out on a limb, and say that if the wire is more than an inch or so, running it along the chassis is always better.

[rp]

Your first way of doing it looked perfectly fine to me, I doubt I ever would have suspected those leads. I prolly would have gone nuts and shotgunned the preamp. I usually run my wires along the chassis just to get them out of the way of soldering or future work but I too figured short =s always better. This is a good post, live and learn on TAG.

BTW what's your take on those Solen film power caps?

[martin manning]

Amp building 101. Pots left to right: P, M, B, T, V2, V1. No flying leads in 1959! The tone stack caps are the two yellow Astrons and the red "domino" under the V1 pot.

[surfsup]

You must be dreaming up your next build by now!!

Yea, I do have some ideas and parts already for a few amps...!

I don't think the lengthening of the wires necessarily helped you, but rather exploiting the shielding provided by the chassis. I'd got out on a limb, and say that if the wire is more than an inch or so, running it along the chassis is always better.

I agree. I didn't mean to imply it was the length that did it. I meant despite it being longer, the chassis run helped.

BTW what's your take on those Solen film power caps?

They work great but are a bit large and require thoughtful design. I plan to use them as much as possible actually.

No flying leads in 1959!

Martin, you are spot on as usual. Unfortunately I wasn't around in 1959 so it was impossible for some of us to study that amp! and...I've only been building amps for a year and a half. So we are all learning at our own pace - and I've learned a lot from you so thanks for continually sharing. (I guess I should re-run the blue one too since it is still "flying")

[martin manning]

Learning is a continuous process for all of us- The more you know, the more you know you don't know, or something like that. Studying classics like that 5F6-A is tremendously valuable, and the schematics, layouts, and photos are very accessible on the web. It's not always obvious why something was done the way it was, but we can all get better at that with experience. Sometimes there is a good reason but sometimes not, so challenging past practice is not a bad thing either, and it makes the more experienced guys think through it.

[Lindz]

Along these lines.. I'm curious for someone like Martin or other experienced builders to comment on how Soldano gets away with all the unusual stuff he does in the SLO - straight non-twisted heater wires, flying leads from the boards, all sorts of stuff that kind of flies in the face of typical amp no-no's

I've just started an SLO build as a learning experience, and am about to run all the leads and am torn between trying to pull off his style or doing more conventional wiring with twisted heater leads, dropping signal leads to the chassis etc.

I get that the very carefully placed parallel heater leads are supposed to be less noisy but would it not be quieter still or less risky to twist?

Same with flying all the pot leads...

Anyone have experience doing both styles and want to comment on it?

His style LOOKS super clean, and I am fascinated by that although a noisy, cool looking build is not going to make me as happy as a quiet, slightly less cool looking build

[Gaz]

Along these lines.. I'm curious for someone like Martin or other experienced builders to comment on how Soldano gets away with all the unusual stuff he does in the SLO - straight non-twisted heater wires, flying leads from the boards, all sorts of stuff that kind of flies in the face of typical amp no-no's

I've just started an SLO build as a learning experience, and am about to run all the leads and am torn between trying to pull off his style or doing more conventional wiring with twisted heater leads, dropping signal leads to the chassis etc.

I get that the very carefully placed parallel heater leads are supposed to be less noisy but would it not be quieter still or less risky to twist?

Same with flying all the pot leads...

Anyone have experience doing both styles and want to comment on it?

His style LOOKS super clean, and I am fascinated by that although a noisy, cool looking build is not going to make me as happy as a quiet, slightly less cool looking build

Lots of shielded cable, short runs (works for him), parallel heater wires.

One other thing is that pre-phase inverter master volume circuits can hide noise better than non-master designs if they are run with the master low. IME, a SLO (or any other similar design) shows its ugly noise when the master gets over "5."