The Tube "Squeal"

I discuss what causes the squeal, provide a sound clip, and how to fix the squeal. I would be a good practice to place a 10K resistor on the grid of the 2nd stage preamp tube that the tone stack is connected (tone stack to volume to preamp). I also discovered that a few inches of wire length difference between the volume pot and 2nd stage preamp may start or stop the squealing. "Squealing" is an effect of parasitic oscillation that is occurs in the preamp tube.
Original Work, Copyright 2021 James D. Whiteside, II all rights reserved, For consideration only, no reproduction without prior permission.

[@rlibby404]

"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!

[@jimcastillo8950]

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......

[@kylefyock4369]

Super good explanation. Thank you for this! Straightforward, no BS.

[@ThomasACarlos]

Great video. I'm going to go back and look at the few tube amps I have and the ones I worked on to see how the V1b is constructed.

[@glenng.8254]

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.

[@mikoschannel6413]

Awesome explanation. Thanks

[@augustusvonwatts555]

This helped so much. Thank you

[@Strongholle]

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.

[@deepblueharp]

Please go to by About page for my email and let's explore this in more detail. The first thing I need is the schematic.

[@voxpathfinder15r]

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

[@deepblueharp]

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.

[@voxpathfinder15r]

@@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

[@DemonKingOFFICIAL]

Never seen a cathode that didn’t have a resistor on it between it and ground. I’m a newb tho in reality.

[@deepblueharp]

The resistor is a local feedback that lowers the gain.

[@EnmandsBand1]

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? kzbin.info/www/bejne/Z2Sre5qgjcZkfrc

[@deepblueharp]

It sounds like a hum. I will need a clean 60 second audio clip to process in my software. Please email me for details.