strange problem with hot rod deville

[phsyconoodler]

I have a hot rod deville amp that has a very srange issue.It splats when played like one of the speakers is shot.
 I have tried different tubes,bias,checked every solder joint in the amp,replaced the drive-line coupling caps,all filter caps and even subbed in a good output transformer.All to no avail.And of course tried different speakers.
  It makes no difference what I do with this amp it seems.I have a good solid bias voltage,the preamp and output voltages are all within spec.
  Here's what was done on the amp previously:
 1) different mid cap,treble cap and bass cap
 2) bright caps on the volume and drive controls
 3) different slope resistor

 The amp worked just fine for a couple of months but now it's doing the splat-thing and driving me crazy.

 Any ideas where to look from here would be appreciated! When you get mad at something it's hard to think straight,so my mind is drawing a blank right now....

[Willabe]

Sounds like you've done about everything you can do to find what's wrong. Small out side chance it's a bad tube socket?

Other than that, my moneys on a bad PCB trace or pad. Does it splat when the amp is hit hard with a heavy pick attack/accent or does it do it all the time?


               Brad        

[John]

Have you tried Hoffman's listening amp?

[phsyconoodler]

It splats on hard pick attack.Sounds ok when strumming lightly. Checked all the tube sockets...

 I may replace the components that I modded originally and see what happens......anything at this point

[Willabe]

It splats on hard pick attack.Sounds ok when strumming lightly.

I was thinking that excess vibration was shaking a PCB trace/pad open?

Maybe try using a separate speaker cab with a long speaker cord to isolate the amp head from any vibration? Although that could just prove that a component has a bad lead where it connects to the body of the part.

Johns idea is a good 1 to isolate where it is happing in the amp. Narrows done where to look.



              Brad        

[stratele52]

IMO this could mean when the amp need more ( lots of ) current when more signal ( you hit with hard pick attack ) circuits can't deliver it and your sound "splat" ;

Check for defective solder , PCB trace half broken.

Chance if you plug Voltmeter on power supply (or other voltage rail ) , you'll see voltage drop if you hit hard the strings same time. If meter is fast enough.

Be carefull ; guitar is grounded and amp have lethal hi voltage/ current

[phsyconoodler]

I have a separate speaker cabinet that I tried;no luck.
  I replaced all the components of my mods;no luck.
As I bias the tubes hotter it gets less splaty but it's still there.Different tubes are better/worse and still splaty.
 I tend to agree about the current demand idea,but it's not rearing it's ugly head as to where the issue lies right now.
:(

[Jack1962]

I have had these amps in with this problem and similar ones as well , a easy solution is to use a 12AT7 in the PI socket and to use a lower gain tube in V2( if my memory serves me correctly lol ) but that is just a treatment of the symptom not a solution to the problem.

[LooseChange]

Have you checked the plate resistors on the PI? Check the voltages o that tube.

[phsyconoodler]

I'm leaning that direction now.Thanks for the lucid response Loose old buddy!

[phsyconoodler]

Loosechange I love you man!(not like that though!)
 One bad plate resistor will ruin your whole day.It measured 25k out of the circuit.should have been 100k.I replaced both the 82k and the 100k and the amp is happy again!

 I must have skipped checking the voltage cause of the crappy location.

Lesson learned: don't skip a step thinking it can't be that!!!!!

[LooseChange]

Thanks for the love!
I get at least 2 of these in a month. I've got these suckers down.

[nandrewjackson]

Bad pot?

My homemade preamp was doing the same thing.  I had a 500K volume pot interstage between 1st and second triodes.  IT wasn't completely bad, but when a hard attack would go through it, it would splat out.  Low strumming wouldn't affect it.

New pot, problem gone.

[stratele52]

Have you checked the plate resistors on the PI? Check the voltages o that tube.

Congrat , We often forget  those PI resistor 

[Slimtim]

congrats on the fix,good job LC.

[phsyconoodler]

I get tons of Hot Rod Fender amps in for repairs and mods but this is the first one that killed a PI plate resistor.

 

[LooseChange]

Wow... first one!. I've seen a bunch like that. Now you'll get one a week. :)

[stratele52]

Wow... first one!. I've seen a bunch like that. Now you'll get one a week. :)

Same as me.