Vibro champ cutting out at high gain

[LaszloS]

hello brothers,

I have a vibro champ that i built frim a kit.  I did two mods on it  a switch on the negative feedback and the tonestack bypass. I have been playing it on ten usuallly with the tone stack bypassed and there is something odd happening.  at full gain with a pedal driving it in front it starts to cut out.  sometimes it will cut out a little and come back and sometimes it cuts out altogether except you can hear the guitar signal faintly  but much quieter. 

this has happened a few times and it went back to normal.

i have tried switching the powertubes (right now it has a JJ 6v6S) but once it goes into this wierd attenuation mode I switch the powertubes and it doesn't do anything then, as if magically, it goes back after a while.

I will check the voltages when I get a chance but I wanted to know if anyone has any ideas to what this could be.

cheerz LZ

[plexi50]

Sure you dont have a ground that is not soldered solid? Input jacks or cathode cap?

[jjasilli]

It seems that you need to tame signal strength:

1. NFB:   http://geofex.com/ > tube amp debug page > vol drops out : note last comment on parasitic oscillation.  Sufficient NFB may help @ strong signal levels.

2.  Blocking distotrtion:  http://aikenamps.com/ > Tech Info > Advanced > What Is "Blocking" Distortion?  The tonestack attenates signal which was amplified by the 1st preamp stage.  So there may be too much signal strength hitting the input of the tone revovery stage, and or the power tube; made worse by the booster pedal.  This could causae blocking distortion.

3. Grid Leak values.  Temporarily, you might try using jumper wires to place a (shunt) resistor, say 470K or 1M, in parallel with the grid leak resistor to: a) the tone recorery stage; then b) the power tube.  The grid leak resistor for a) may be the vol pot.  

4.  Remove one or both cathode bypass caps in the preamp stages.

[LaszloS]

thanks for the links, i think maybe it has something to do with the tremolo.  the volume is basically cut in half when this happens. and the tremolo no longer works.

maybe somebody could weigh in on this, if it was a parasitic oscillation above hearing range would the volume be attenuated like that and cause the tremolo to not work?

the geofx site seems to indicate if it was P.O. it would be a dead cut.

the tremolo voltage seems to be oscilating between 140Vand 180V on the pin which is connected to the 6V6

[HotBluePlates]

Did the amp work before the mods?

[jjasilli]

Did the amp work before the mods?

Or did you incorporate the mods into the build?

the tremolo voltage seems to be oscillating . . . on the pin which is connected to the 6V6
VibroChamp trem is fed into the cathode pin of the tone recovery preamp stage, not into the power tube.  Maybe you should post a schematic of the amp as actually built & modded.

[Tiny_Daddy]

Could be "blocking distortion" caused by a too-large or missing resistor on a control grid, or maybe you need a grid-blocking resistor somewhere - schematic needed.

[LaszloS]

Ok something happened which probably shouldn't of happened but the amp is back to normal now.  while taking the voltages off the 6v6 on pin 3 the multi meter pin sliiped causing it to short out against the heater wire. there was a blue spark and full volume was restored.  this problem confuses me because once the amp gets quite it stays quite at all volumes and input levels (until now).  I have tried swapping all tubes with no effect.

for the layout i followed the triode layout http://site.triodestore.com/AA764VIBROCHAMP140-18027120V.pdf which to my eyes apears to be the stock vibro champ setup.  the mods are just a switch between the bass pot to ground to bypass (jumping the 18k resistor) the tone stack and a switch on the negative feedback (between the speaker out and the 2.7 k resistor

[jjasilli]

. . .the amp is back to normal now. . .  Not clear: is the amp functioning properly now?

[LaszloS]

Yes the volume is bacK to full levels.  i don't understand what that could've done. 

the only thing I can infer from it is that the problem somehow isn't the power tube, because swapping them didn't change it.  it was only when the output transformer lead attached to the powertube got jumped to ground (off the pin of the 6V6 to the heater) that the volume went back to full.

[jjasilli]

It is possible that there was a stray wire strand or bit of solder that was causing a partial, intermittent short, which worsened when the lugs warmed up.  And that the pyrotechnics    melted it away.

[rzenc]

Yes the volume is bacK to full levels.  i don't understand what that could've done. 

the only thing I can infer from it is that the problem somehow isn't the power tube, because swapping them didn't change it.  it was only when the output transformer lead attached to the powertube got jumped to ground (off the pin of the 6V6 to the heater) that the volume went back to full.

A colder solder joint - probably pin 3 plate - could have been "reflowed" due to current flowing and heating taking place. I once shorted accidently pin 3 and pin 2 on a 100W plexi build and it instantly melted the jumper wire insulation which connected plates on V7 and V8 tube sockets and also heater wiring. Mains fuse popped. Not the kind of thing that I would recommend for reflowing cold solder joints  .

With Respect

Rzenc

[LaszloS]

huh, 

A colder solder joint - probably pin 3 plate - could have been "reflowed" due to current flowing and heating taking place. I once shorted accidently pin 3 and pin 2 on a 100W plexi build and it instantly melted the jumper wire insulation which connected plates on V7 and V8 tube sockets and also heater wiring. Mains fuse popped.

so now thats done  should hav no more problems?

this vibro champ seems to run pretty hot with the stock set up. I have 370 on the plate (pin3) and 22 on the cathode (pin 8) (470R cathode resistor with a 25uf/25v cap) 

that seems to be 48mA if the resistor is acurate i think.

could running it to hot cause it to do this?

Again confusing cause I had already tried reflowing all those pins before the "pyrotechnics"

lz