# Help me with oscillations please!



## Guiary (Oct 10, 2007)

Alright I'm running out of Ideas!

I have a Peavey VTM120 which I've begun modifying and everything has been going well, loving the sound etc. But I'm suffering from some nasty oscillations with my pregain set anywhere above 4.5. I mean it isn't so bad when when I'm facing away from the amp but just when I turn and face it...

The purpose of my mods were to increase the gain, tightness, bass response, increase the mids and make them more usable, and remove the bright cap.

The mods I've done are all found throughout the internet as JCM800 mods and since the amps are very similar build wise that was my reference point.

OK my questions:

Would adding a bypass/filter cap on the plate resistors help with that nasty ass ear piercing feedback?

also I've tried increasing R4 (located on the preamp board) to 4.7K which I was refered to in order to decrease oscillations, but it hasn't helped too much. Should I increase the value even higher?

Thanks,

Gary


----------



## Hamm Guitars (Jan 12, 2007)

Are we talking oscillations or feedback?

Can you get the amp to oscillate with no input and moving the controls?


----------



## Guiary (Oct 10, 2007)

Hamm Guitars said:


> Are we talking oscillations or feedback?
> 
> Can you get the amp to oscillate with no input and moving the controls?


At a volume of say 6ish with no input and volume of 1 or so gain at about 4.5.

I've just decreased the gain by replacing R5 with a 5.6K resistor which has is helping out a lot but still there.


----------



## Guiary (Oct 10, 2007)

Alright it's a little more stable now. I've decreased the amount of gain going to V1 by sticking in a 5.6K resistor in R5 and bridged a .022uf Cap across it. While keeping R4 at 3.9.... still have to try 4.7K.

Now I can get it stable with a volume of about 3.5 and gain from 0-8 (didn't try more cause... will no reason.) but as soon as the volume hits 4.... Not stable anymore.

Until I score some caps for bridging the plate resistors I'm working with the bare minimums!

Just a little update for those 17 people who've read this thread.


----------



## Wild Bill (May 3, 2006)

Guiary said:


> Alright I'm running out of Ideas!
> 
> The mods I've done are all found throughout the internet as JCM800 mods and since the amps are very similar build wise that was my reference point.
> 
> Gary


I guess you've been finding out that the electrons may not agree with you about "very similar"!:smile:

Looks to me like you already had some extra stages of gain, compared to a JC800. This is compounding your problem.

You have two things to worry about when you start making these kind of mods to an existing design. 

The first is that the more gain stages you have the easier it is to get oscillations. Oscillations are usually caused by stray capacitance between wires or pcb traces. Signal gets coupled back to a previous stage, in phase. The amount of signal of course increases as you change parts values to increase the gain of stages, making a stable layout become an unstable one.

The second is that with modern printed circuit board amps your layout is kinda cast in stone. It's kinda tough to move a copper trace around to find the stable spot.

Tweaking gain is best done in small doses. With a JCM 800 one of the common methods of increasing gain is to lower that 10K cathode resistor feeding the cathode follower to the tone stack. I never lower that to less than 6K8. Guys have brought me their amp after they or someone else has hacked a mod and often I find that resistor has been changed to something ridiculous, like 3K3 or even 1K5. The stage almost HAS to oscillate!

My advice is to put everything back to stock. Then leave the 1st triode values alone. That stage is the most critical and easy to get into trouble. You MIGHT get away with increasing R1 to 220K for more gain. Maybe not, again depending on how Peavey ran those circuit board traces.

With the following stages make smaller changes. The idea is to get a bit more gain out of each stage rather than bags of gain out of one or two. When you add all the gain up you end up with the same amount but the stages are much more stable. That's what Peavey did with the 5150. A better overdriven sound and a more stable layout/circuit.

Do NOT change the cathode resistors on the cathode followers! Those are R10 and R14. A cathode follower has no gain. Changing the resistors just screws things up.

Good Luck!

:food-smiley-004:


----------

