Using Dumble overdrive on a fender twin preamp,squealing....

[gary sanders]

OK Ive started a new toy and I wanted to use the preamp of the fender twin reverb model aa769.everything else is the schematic of the good ole 124.sounds great until i turn the over drive gain up and it squeals like crazy,really high pitched noise.the presence control affects the noise too

Heres the fender schematic

http://schematicheaven.com/fenderamps/t ... _schem.pdf

I stopped at the 220K after the .047uf after the plate of V1B and it goes to the OD switch on the dumble 124 schematic

Should I have left the 220k off or is this maybe a coupling issue or just something i have no idea of? Everything is good,voltages,blah blah specs are there where they should be,somethings just pushing the high end way way over

[mbeldyga]

Sounds like positive feedback. Swap power tubes anodes.

[ChrisM]

You shield all the necessary runs?

[guitardude57]

Don't think it is PFB.

Try moving some wires a little on your OD valve. Might have some interaction going on.

Try a different tube in that position.

Did you put snubbers on the OD valve? Grid to cathode 270uF (suit to taste) Some builds need them....some don't.
Pin 1-3 and 6-8. Solder right on valve socket with short leads. See if that helps.

[gary sanders]

All shielded and using snubbers.I had 100pf on my 124 amp so i used those here too.I have some 330pf somewhere,will look at my leads for what could cause some trouble.will check all this before i turn it on again and report thanks for the help!

[talbany]

Does it do it with the guitar unplugged?

If it's PF it would do it all the time..

Tony

[guitardude57]

yes, if the OT leads are reversed the nfb is positive and it will squeal like a bitch.... as soon as off standby I would think.

I only had one case of the plate tranny wires being backwards on a Marshall build. Flipped the standby, and WOOOOOOOOOO. Turned it off....reversed the leads and beautiful!

But it sounded like it only happened on OD.....so sounds like inter-reaction somewhere else to me.

[gary sanders]

just found when i pulled the bright on the volume control on the clean channel it did it too,and when the pre volume is turned up on clean so its not just an over drive problem.I will swap my plates and see.

[talbany]

Yeah!! Positive feedback will squeal as soon as you turn the Stby off..
Standard operating procedure is to start pulling tubes..Start with the V2 pulled in OD.. see if it still does it..If it does you know it's in the output section.. If not you know it's in the preamp...put V2 back in and pull V1 turn up the OD volume and gain if it does it you know it's in the OD circuit..Try to narrow it down that way..Also If it squeals when you crank the clean channel and Master w/ all tubes in chances are something's wired wrong there.. Good Luck..

just found when i pulled the bright on the volume control on the clean channel it did it too,and when the pre volume is turned up on clean so its not just an over drive problem.I will swap my plates and see.

Sounds like you found it oscillation..

Tony

[gary sanders]

I switched the OT wires and it made a loud rumble so I instantly killed the power.will pull v1 and 2 and see if that can tell me where it may be

[gary sanders]

The problem is in the presence wiring,disconnected it from the OT and it stopped.Now how should i mod this to get the presence control working?

[talbany]

double check the wiring, presence feed resistor 4.7K and pres pot values..make sure your tapping off the 4 ohm tap..
Generally + feedback howl's/rumbles and preamp tube issues squeal.crackle.. Generally!!

Some pics would help!!

Tony

[ER]

Is your OT/speaker grounded well?

[gary sanders]

Yes the ground on everything is good.Checked all of them.

[gary sanders]

I have managed to solve the problem.Made so many changes i'll try to list them all....I changed a few caps like the .005 in the OD to an .01.Found that on the grid of V1 I had a 6.8k instead of a 68k.Changed the first resistor to a 150k from 220k.went to 5.1k grid stoppers from 1.5k and a few other things to cut highs.I noticed some amps have a small cap parallel to the resistor in the FB loop so I used a 150p.And the last thing that finally got rid of the last bit of oscillation,I changed the guitar cable.Yes the cheap unshielded cable was a factor,when I got the jack on the other end close to the chassis it started screamin.First it was w/o a cable,then it was gradually gettin better.not sure what helped the most,probably everything together.spending 2 days searching this and other forums finding everything that could cause the problem.Learned alot.Time to ROCK.Its sounding really good.used the duncan TSC and believe the last thing to do is change the mid pot to 100k from 10k.it shows a great mid boost.then the chassis goes in the cab.building is fun but playing and loving something you built is even better!!!