I recently built a 5F11 Vibrolux copy and it sounds great...for about 2 mins. Then there is this background scratchiness or white-noise-sounding garbage and the volume drops. I switch pick-ups and the volume jumps back up but eventually degrades with this scratchiness and intermittent garbage. I have tried different guitars and all do it but differently. When there is no input signal the amp is nice and quiet. So I think it is in the preamp somewhere. I have switched out preamp tubes. Put in new 12AX7s, tried different preamp tubes (12AY7s)...all do this after a few minutes. I read all about Tweed Disease and tried to ground out the turret board (not sure I really did anything useful here). I still think this is a preamp issue but I don't know enough to pinpoint the issue. Any help would be much appreciated!
Thanks in advance!
Thanks in advance!
Take some high resolution pics of your build and post it please.
I will have to find a schematic but some of those old amps didn't come with grid stop resistors. Try adding some.......you could have stability issues.....oscillations.
I will have to find a schematic but some of those old amps didn't come with grid stop resistors. Try adding some.......you could have stability issues.....oscillations.
My first thought is a grid that is not terminated. Go down the row and touch a volt meter probe to each grid. Does any of those wake it back up for a moment?
Here is a schematic:
http://bmamps.com/Schematics/fender/vibrolux_5f11_schem.pdf
Here is a schematic:
http://bmamps.com/Schematics/fender/vibrolux_5f11_schem.pdf
When you say the grid is not terminated do you mean the cathode grid (2 and/or 7) is not grounded? Sorry, I'm a chemist in a EE's world.
Cathode and Grid are two separate things. The grids for 12AX7's are pins 2 and 7 like you stated, the cathodes are pins 3 and 8.
If you built an exact clone it is unlikely that it's something in the design, so it's probably a wiring error. Go back and inch through the schematic checking all connections. We can't really tell you anything from just a schematic, you will have to post pictures of your build. An oscillation could account for both the delay of fault and the lack of power. What do you have for test equipment? Signal generator and scope would be good.
famous mockingbird and Enzo, thanks a bunch for the input. I later realized the stupidity of my comment about the cathode and the grid. Your collective input motivated me to recheck and re-wire the grounds. I went through and just re-grounded everything paying special attention to the preamp section and ensuring I didn't have a lot of opportunities for some ground loop issue. That totally solved the issue. Now the amp is very silent and then spills awesome sauce all over my earballs when I play. Thanks so much for the advice! I love this forum!
Your killing me over here hahaha!!!!!!!!!- Status
- Not open for further replies.