Hot Rod Deville OD channel sounds like a machine gun

[punkykatt]

http://el34world.com/charts/Schematics/files/fender/Fender_hotrod_deville.pdf

Hey Guys,  anyone have any experience with The Fender Hot Rod Deville OD channel making machine gun noise??  Clean channel works good. Any help is much appreciated.
Punky

[punkykatt]

Found two bad 22uf/500v filter caps in the PS. 

[shooter]

glad you found it, but I'm confused why the clean channel worked, not enough to loose sleep though 

[punkykatt]

Yeah, that puzzles me why too.  It was C35 & C36 filtering the preamp.  The amp did hum a bit on the clean channel. When you played, you did not hear the hum.
While checking the resistors , diodes and transistors on the PCB, I happened to notice  some crud on the + side of those two caps. I then put my newly purchased Blue ESR meter to use and sure enough got  no reading at all on those caps. I  temporarily clipped in some new 22/500`s over those and BINGO no more machine gun in the OD channel and no hum in the clean.   That  meter saved me mucho time, I love that ESR meter. Great investment for sure.
Punky

[Willabe]

Found two bad 22uf/500v filter caps in the PS.

What brand? 

[punkykatt]

Illinois

[PRR]

> Found two bad 22uf/500v filter caps

IMO, this scheme begs for subsonic oscillation, from thub-thub to RAT-TAT.

It has way too many high gain stages on one filter cap.

In OD mode a 4th stage is added. Evidently it all worked for Fender with good caps.

Perhaps with dud caps at X and maybe Y, any unbalance in the 5th stage caused the oscillation.

[punkykatt]

Thanks PRR for sharing your view.  Hopefully  this experience will help  other techs that get one of these amps in for the same problem. I could have spent hours and hours on this amp. 

[frus]

Sorry to hijack the thread, but does anybody know why Vibro Kings have like 4-5 gain stages on the same node (including PI)?


http://www.thevintagesound.com/ffg/schem/vibro_king_schem.gif


A friend of mine got low frequency oscillation past half on the volume knob, I added one more power node for the (first two stages I think) and it was gone. But why the design in the first place?

[sluckey]

But why the design in the first place?

Cheap. Sloppy engineering. Perhaps Mr. Zinky as a designer/accountant? Who really knows. Fender also ran 6 triodes from a single node in the very successful 50 year old AB763 amps.

[frus]

Yes, but that's still only 2 gain stages in a row from the same node (as far as I can see)


Vibro King is a point to point amp (in the mid 90's), and for instance I noticed all the care they've taken to try to star ground it (a LOT of ground wires going around), so they weren't actually cutting the corners as usual (and those amps were expensive). But then decided to go for "whole preamp from one node" idea. I thought I was missing something and there actually was some reason for this

[PRR]

> 4-5 gain stages on the same node

Three in cascade; since V2A has some PSRR and (lame) local decoupling, and the long-tail driver is (nominally) balanced. (I have been bitten by the same scheme when a resistor drifted 50%, so I'm not in love with it.) Also note the added bass-cut between V4A and VOL. (I do wonder what C5 is for.) And C34+C35 makes a fairly large filter. (But then why not throw 5K between and take two taps?)

It was the 1990s. Music was in a funny place, drugs were changing, and Fender was trying to be bold even at the risk of mis-steps. Little things like wise design were secondary.

We also appear to have 460V on a "330V" EL84. (HTH can a reverb pan stand that much drive?