Oscillation Mystery

[Trout]

OK, I must be the most unlucky guy when it comes to trying to build 2 matching anythings.

A few weeks back I built a fender champ clone kinda amp. With a couple tweaks I ended up with an amazing sounding rendition of the 1960ish champ AA764.

So, my younger brother falls in love with this amp. Well ok, I'll help him out and build one.
I start the project by of course punching and drilling and grinding out an exact copy on another 6 X 10 Hammond chassis.
I happened to also have a second matching PT & OT.
4 hours later, All the hardware is in place, I start by wiring in my usual Power,Fliaments,PT & OT. and slowly get to the wiring in of the Resistors and Caps.
No problems encountered, Actually was sort of fun seeing as I had 1 amp done for referance.
Every part used is exactly the same as in the first amp with the exception of a tone cap, Mine has an old sprague black beauty, His has an orange drop. (.047)

So, after about 6 hours of carefully wiring and soldering in each and every part. Its Time to make noise!!

Welp, I get plenty of power, I guess a solid 4.5W. I get Plenty of great tone response from the guitar and Everything looks and sounds very sweet indeed. >>>>>UNTIL<<<<

I raise the volume to full, The treble to full, And I Cut Off? Like Flipping a switch!!
Also, unlike the first version, I get a severe squeel/ Oscillation while the guitar is not plugged in.

If I reduce the treble control, I can run at full volume, Or if I lower the volume I can run full treble. But I Can Not run both at maximum.

I have heard this type of squeel once before when I had reversed the OPT primary leads & also once when I had not grounded the speaker jack.
But in this case, Those are (according to color code) Correct and well grounded.
I have rechecked all joints,grounds, Swapped out tubes, Yada Yada, Nothing helps. (So Far)

Any ideas would be helpful, I am going to try to beat this thing tonite seeing as I have a few free hours .
Trout

[Trout]

OK,

I found the input Squeel Issue, The Input jack was not switching/making good contact to short mode with cord removed.
Whew that was an easy fix.
Now If I can get it to not reach cut-off when the pots are set at maximum I will be in great shape.

I popped a 5881 in to see if there was any direct change in the symptom Vs the 6V6gt's I have tried. " No Change"

I am pretty sure its pre-amp related, But I still dont see anything that could be the direct cause unless its bad pots? They are new from parts express. All the electrolytics are from them and everything was purchased together.

The only possible variance I can see would be the resistors used seeing as they are all NOS ohmites Lil Devil carbons. I have built several other amps with these and not had any issues thus far.

Right now as it stands, The cut-off is very sharp, Like flipping a switch at around 95% setting on the tone or volume controls. The
Schematic Is correct to both my working and non-working units.
Trout

[tubelab.com]

When the amp is operated in the "cutoff" condition can the signal be traced to find the point of loss? Have you tried another 12AX7? If I had to make a wild guess, I would look for an open condition at one of the grids. A bad pot could cause this if the ground connection went open. Try temporarily bridging a 100K resistor from each grid to ground to see if this helps.

Another possibility is a strong oscillation (above the audio range) that causes the cutoff. Check to see if there is any voltage that changes from the working to the cutoff conditions.

[Trout]

Originally posted by tubelab.com
When the amp is operated in the "cutoff" condition can the signal be traced to find the point of loss? Have you tried another 12AX7? If I had to make a wild guess, I would look for an open condition at one of the grids. A bad pot could cause this if the ground connection went open. Try temporarily bridging a 100K resistor from each grid to ground to see if this helps.

Another possibility is a strong oscillation (above the audio range) that causes the cutoff. Check to see if there is any voltage that changes from the working to the cutoff conditions.

First Off,,
WELCOME BACK!
I Hope some normality is coming back to you and your family!

I Put a 12AU7 in, and I can not get it to cut off and output was conciderably lower. I put in a 12AT7, Pooof, Same cut-off condition. 3 Different brand 12AX7's Same.
I left my meter at my brothers so I will have to pull voltages off tomorrow afternoon.
My piece of junk simpson is out of Battery again so I can't check much as far as ohms.

So looks like tonites a wash.
Gene

[Joel]

This problem is described in Weber's book. It's definitely an oscillation. Things to address would be circuit leads, and whether you have adequate decoupling between stages. Reducing the output impedance of the power supply may help too.

Joel

[Trout]

Originally posted by Joel
This problem is described in Weber's book. It's definitely an oscillation. Things to address would be circuit leads, and whether you have adequate decoupling between stages. Reducing the output impedance of the power supply may help too.

Joel

I am inclined to fully agree that theres an oscillation issue, The part I am not understanding is simply this. I have 2 units I personally built Both using the same PT, OPT,Pots Caps,and Chassis layout. Amp "A" sings sweetly with no issues or apparent oscillation, Amp "B" clearly has an issue.
This is a really simple amp and I am sure the units are true to the original schematic. I do not understand how fender could have produced (guessing) thousands of these amps with a 50% error possibility?

The decoupling is set to exactly as shown on schematic.
Could you expand a bit on lowering the power supply impedance?
The leads(signal) are very short & shielded.
gene

BTW, Today I replaced EVERY cap in the amp with different brand NEW caps, I replaced and routed differently the signal leads on the pots,
Other than sockets and transformers & chassis I litterly rebuilt this thing only to arrive at the same point.

[anatech]

Hi Gene,
At least you are consistant!

Sometimes the position of the parts is all it takes to cause oscillation. It could be the design is borderline stable, and with the other transformer, it's not stable. Try bypassing the plate resistor in the preamp stages one at a time with a small capacitor. Maybe 50 pF to start. That should introduce a high frequency cutoff.

-Chris

[Trout]

Originally posted by anatech
Hi Gene,
At least you are consistant!

Sometimes the position of the parts is all it takes to cause oscillation. It could be the design is borderline stable, and with the other transformer, it's not stable. Try bypassing the plate resistor in the preamp stages one at a time with a small capacitor. Maybe 50 pF to start. That should introduce a high frequency cutoff.

-Chris

Oh My Gosh Yes, I should have started designing oscillators!! Or Hey Wait, I know, Valve dog whistles!! Yeah Thats It, Come to think of it, No wonder our pooche hides at the other end of the house when he see's me headed for the garage!

I wonder<ponders> a 5640, a few calculater batteries,a el-cheapo opt made of fine wire wrapped around a old iron barn nail, a piezeo speaker,and the body of a marks-a-lot magic marker.



I'll try the 50pf's tonite, Got my VM back a bit ago so I'll pull a few referance voltages also.
Gene

[anatech]

Hiya Gene,
LOL.

I design real good oscillators sometimes. Wanna start a club?

Seriously, I hope you don't think I was poking at your design abilities. They were not real Fender transformers were they? If not, other things can happen with the same schematic. Just a possibility.

-Chris

[Trout]

Originally posted by anatech
Hiya Gene,
LOL.

I design real good oscillators sometimes. Wanna start a club?

Seriously, I hope you don't think I was poking at your design abilities. They were not real Fender transformers were they? If not, other things can happen with the same schematic. Just a possibility.

-Chris

Hey,, I am a very light hearted guy! Honestly even if they dont always work, Its still a lot of fun trying to solve the mystery & learning more and more about this great hobby!

Nope, not old fender transformers. They came from some old Devry Technical Intutute class project kits.
I think years ago, before sillycone, & Before Compfusers they had radio and TV repair classes. You know, Earn Big money in the rewarding technical field ads! haha
Anyway, I aquired several of the original class project power supplies (built) I have built several low power amps with 6AQ5's, EL84's and 6V6gt's with these. of course results have varied.
Gene