Some type of nasty oscillation? '69 Laney LA60BL, mp3 attached (8 seconds)

[Gtr0]

Hello MEF. I have a '69 Laney LA60BL that seems to have attracted some type of... oscillation? I am not experienced enough to say. I can scope it out, but I won't have the time for a few days, so I just wanted to know if anyone may have had experience with this before and maybe knew what it is right away... you never know.

At first it only appeared as a sound on palm muting... as if to appear as something was overloading. Then I noticed while recording the attached clip that when I hold an low chord (in this case a standard low D power chord) it will come out... you'll hear it quite plainly at 6 seconds into the clip... before that it only appears on the tail of notes... I suppose it takes a second or so to generate?

If you have any clues, that would be great, it could save me a lot of time. If not, thanks anyway!!! I will figure it out eventually.

[Jon Snell]

Have you tried a different known good loudspeaker? If not the speaker, crossover distortion comes instantly to my mind as it does not sound to me like oscillations.

[mikeydee77]

Plus one on the speaker thought...

[Gtr0]

Thank you kindly!

I am using a Suhr Reactive Load and IR unit. It presents a more proper Impedance Curve (based on 4x12 with G12M celestions) than many other loads on the market. I use it with a huge array of different amps including Marshall 1959's (which in my experience is the ultimate test for any load or attenuator). And so the first thing I thought was that I was overloading the Suhr reactive load, but it wasn't the case (there is a trim knob and red led when it is being overloaded).

This behavior just started one day... I do not play the amp very much but I do make sure to fire it up for an hour or so at least every three to four weeks... so one day I turned it on and this was the result. I turned it off and revisited it a week later to find it wasn't a one-time anomaly. The amp is all original except I replaced the tubes and all electrolytics with F and T before turning it on the first time in early 2020 when I bought it. It ran and sounded rather glorious until this "tearing fart" sound started to appear.

oh yea, I also installed a bias circuit when I bought it.

In other words, I haven't made any changes to the amp, or been inside of it, in almost 2 years... so this is something that just happened, more or less, out of the blue. I had run it several times through this reactive load previously.

[Jon Snell]

What is it like on a loudspeaker.
The load you are using may be the issue but you say you installed a 'bias circuit'. Have you checked the bias ... could be crossover distortion and a simple cure.

Rule; Always remove aditional stuff that could affect the outcome first to clarify where the 'fault' may be and check your work. Just because it is relatively new, doesn't mean it is still OK.

[Gtr0]

I will have to recheck the bias. Last time I did (about a year ago) it was good... but I do not remember all of the stats... and then I was using a TAD Bias Master. This thing puts more than 570 VDC (if I remember correctly) on the EL34 B+... this is with the 250VDC setting on the power transformer.

Two years ago with lots of help (including you, @Jon Snell) I was able to get the bias circuit adjustable. https://music-electronics-forum.com/...60bl-pleeeease

As for a loud speaker... yes, I know this is absolutely best to cut as many variables out as possible... I'll have to talk to my neighbors... here in Belgium, too many homes (including mine) are attached to the neighbors... a sort of "row house" situation. I'm not a fan.

I pulled the head out of the chassis... took a look to see if I could find anything obvious... which I could not. I plugged it in, still made that tearing sound... then I took off the 12ax7 cover on v1 and the sound went away... I went on to play for about 30 minutes and the sound came back once in that time... scratching my head for sure. I do know the sockets are all very old and there is a lot of wiggle room with all tubes. Perhaps a session of re-tensioning the pins is in order.

Another concern of mine is the grounding scheme this amp has going on. Before playing it, when I first bought it, all grounds in the amp (including the mains power!) were grounded to the control pots bus wire. Then I guess it assumed a connection to the chassis from the pots. One of the first things I did was drill two holes, one at the mains connection for a dedicated ground connection and the other at the input jack to ground everything else. But currently the speaker jacks and power tube cathode grounding is still through the pot bus wire and to ground via the input jack grounding I added to the chassis.

Should I make a dedicated, third ground lug location for the speaker jacks and cathode/bias circuit? Picture attached with some added graphics to show the connections..


Thanks!

[Jon Snell]

If taking off the valve can stopped the sounds, could be a dry/bad chassis connection as looking at the fuse holder fixing nut, that is corroded from dampness over time.
If the bias voltage starts to float, due to poor grounding, that would make the crossover noise that you are experiencing.