Hello, I have a Fender Vibrolux Reverb Custom in which I removed the circuit board and replaced it with a eyelet board just like they did back in the day. I didnt have any problem for some time and I started to get a squel at times and buzzing and then disappear. I did notice that it followed from the normal channel to the vibrato channel and then I removed the Phase inverter tube and replaced it with another tube. It was gone. I had a old RCA nos tube in the PI socket. thanks for your video......
"I recapped the amp, I changed out the carbon comp resistors, the tubes are new, hopelessness starts to set in..." 100% accurate. The fifth step for me (prior to watching this video) was deducing that by performing these updates on a 50 year old amp, it was now more powerful and thus out of spec compared to the original design. Thanks for the help!
Had this happen on a Collins KWM2A, on 6AZ8 that acted as 3rd transmit audio amp and second stage receiver IF. When I turned down the MIC gain, it was a high pitched squeal in receive mode. Yes, it had a cathode follower. In my case, I had just replaced the tubes. re-capped and cleaned the tube pins. I got rid of my squeal by swapping out another 6AZ8, however, I'm sure the initial cause has to do with re-capping, or perhaps just a bad tube. If it returns I'll put a stopper resistor on the grid. Also, I found a great way to locate a squealing tube is to simply place your hand near it, fingers around it (your fingers make a great inductive tuning capacitor) and it should change frequency as you move your fingers around the outside glass. Thanks for the advice.
Very helpful to understand squealing. Now, I have yet to start to work on the amp that brought me here, It's a 4 input copy of a circa 1970 Hiwatt. Both channels oscillate depending on the settings, partially affected by the EQ and presence control but mostly by the volume, past a certain volume with a certain EQ it'll squeal. The preamp is board mounted so there's not that much you can do wiring wise, I have replaced a couple out of spec resistors and caps with a subsequent reduction in noise but it hasn't affected the squealing. There's a couple shielded cables in the first part of the preamp which I replaced and it improved. V1 has 2,2k cathode bias resistors on both triodes as does the first triode of V2 (second triode is a cathode follower), only V1a has a cathode bypass cap (0.047uf), second one and V2a just have the resistor. Any tips on where to start? I don't have an oscilloscope and still don't really understand where the squealing comes from exactly so I don't know if the volume having an effect on it is an important factor.
Is a 10 microfarads sufficiently large to stop the squealing? That’s what I have for a cathode bypass capacitor in the gain stage of my amp following the tone stack. But I have squealing. I have a printed pcb and trying to get a grid stopper resistor will require surgery I would like to avoid, especially if it didn’t work
It will not stop the squeal. A cathode bypass cap is designed to stop the feedback current through the cathode resistor. It shunts AC to ground and maintains the bias voltage. Squeal is a parasitic oscillation and a grid stopper is required.
@@deepblueharp yes, I just paralleled a 30 microfarad capacitor to the 10 microfarad cathode bypass capacitor on the 2nd gain stage after the tonestack. It didn’t help at all. I will try a 10k grid stopping resistor next. I get the high pitch squealing on the clean channel - but get the humming like in your video on the soak channel. So will have to do something there as well
I have a similar issue on my VHT Special 6, it helped a bit to move the output wires away from the preamp but it still sounds terrible. I have changed all the caps (amp is 13 years old) but no luck, any further ideas? ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-13HJgjXa1HU.html
