Marshall TSL60 bias problem

[Andrex88]

hello everyone I would like to ask the opinion of you fans on a guitar amp a marshall TSL60, I have an unstable bias problem, practically if I set it to hot at 38mv after a while it starts to rise or fall with a variation of 6-8mv at volume 0, I replaced the resistors R12-R13-R24-R25-R76-R2, C12, C69 the 22k bias potentiometer and the Zener diode ZD3 all that concerned the bias regulation circuit. But nothing seems to have changed actually all the replacement parts I measured with the tester and they gave the same measurements as the new parts. The thing if I then start playing at medium-low volume oscillates 39mv 42mv, if instead I start playing at medium-high volume the mvs rise until the tester no longer scores you can see that it goes from 39 to 50 80 120mv up to 200mv, then the tester goes off the scale and doesn't score anymore and although with the guitar I'm far from the amp when it's about to end the sustain effect starts whistling which is an impressive thing, as I stop the strings it stops whistling and the tester starts to score again and returns to 38mv but it is never stable. Attention the oscillations occur only when I start playing the guitar repeatedly, even if you leave the volume of the amp at full throttle and the guitar volume at zero there is no oscillation it remains at 38mv and then oscillates of those 6-8mv. I can't figure out what triggers this problem as long as it is. I am an enthusiast but also a beginner in electronics so have pity, I am also trying to understand what correlation there is in raising the G3 grid value with the guitar signal input in the sense why as soon as I start playing with medium-high volume G3 rises, shouldn't it stay on about 38mv? While with medium-low volume all this does not show up except with very slight fluctuations. Thank you very much

[Jon Snell]

Check the DC voltage across ZD3. Does it go down in value (less than 50volts?) If it does replace the three capacitors; C61, C62 & C69.
Check for dark colouration on the PCBoard where the HT line and Bias line runs.

[g1]

Idle current measurement is for no-signal condition only.
Once you play you have to unplug the guitar and wait a while for it to stabilize so you can get a reading again.
And if you are playing with the meter connected, you can have strange things happen like whistling. Or the whistling could be something else unrelated, like microphonic tube or reverb issues.

[Andrex88]

the ht line and bias in the pcb seem apparently perfect, capacitor C69 has already been replaced together with C12, I will try to replace C61 and C62, check the voltage on ZD3. So if I understand it is normal that when the guitar signal comes in and the volume is high in the amplifier, it will increase the grid value every time the guitar sends the signal.

[J M Fahey]

Of course.

[g1]

Yes, any signal or even noise on the grid will cause the cathode current to change.
If you have no signal connected, you could remove the PI tube to eliminate any noise getting through to the grid. Also any fluctuations of the line voltage will cause fluctuations of the cathode current.

[The Dude]

As already said, bias is an idle current setting- no signal applied. If bias remains constant and correct bias is obtained in that state, you don't have a bias problem. When you say you have "oscillation", is there actually a howling "feedbackish" sound, or are you just observing the guitar signal on a scope? If you are playing high gain and loud, some feedback is normal at the end of notes. I'm not sure there is a problem with the amp.

[Andrex88]

I checked the voltage across the ZD3 and it gives me fixed 50v, no I don't have an oscilloscope, I'm trying to learn electronics because I have recently become passionate about amplifiers, I usually do this procedure to adjust the bias, even if I leave the guitar connected I keep it with the volume at zero, as well as the volume of the amplifier at zero I warm the amplifier well and then I proceed to adjust the potentiometer, I leave it for half an hour at minimum and then I double check, I attach a video that I did a little while ago. Obviously the whistle is heard when I use the distorted channel with gain and volume at max while the master volume is just above half..


https://www.youtube.com/watch?v=MagJ...channel=andreb

[Jazz P Bass]

You really need to understand the purpose of biasing an output section.
In the broadest sense most push-pull outputs are in Class B when biased 'cold'.
This will cause the crossover point to have a notch. (sounds bad)
In the o scope method: By raising the bias current until the notch disappears, you bring the amp into Class AB. (sounds lovely)
That is all there is to it.
Now, enter the Web of too much information, tube values all over the place, endless twiddling & tweaking........

[Andrex88]

it would be interesting to try with an oscilloscope, maybe I could get one, would you have some advice for me on which model to buy. I would really like to learn.

[Andrex88]

I would also like to show you another video, where you can hear a background hum, I have a fender stratocaster dave murray signature with HHH humbucker Seymour Duncan, practically this happens, both in the clean channel and in the distorted channel you can hear this hum with the guitar disconnected , in the clean at medium-high volume, in the distorted from medium-low volume upwards, on the other hand by connecting the guitar and using the clean channel the hum of the bottom remains unchanged, while in the distorted channel the hum doubles. The guitar cable and the guitar trying them on a fender amp is perfect. And another problem that I forgot to mention, I thought it could be the transformer that anyway even at zero volume feels like a vibration in the frame and then affects the circuit.

https://youtu.be/h3MSmOZx4-U

[Jon Snell]

I can hear mains hum from a high gain pre amp stage.
As the pot fixings (nuts) earth to points on the board, I would fit all of the nuts first to ensure good grounding of the pots then listen again.

[Andrex88]

i understand the nuts of the potentiometers, however it is a problem that was also fully assembled, i was wondering if it could come from the heating circuit. Or from the diode bridge.

[Jon Snell]

If it were the diode bridge then it would be 100HZ not 50HZ.
Check R14/15 grounding the heaters.

[Helmholtz]

If a diode (connection) in a rectifier bridge goes open, the result is half-wave rectification and 50Hz (60Hz) ripple.

I wonder if the noises stay the same with the chassis further away from the speakers.
At such close distance between tubes and speakers I'd expect all sorts of unwanted couplings: mechanical, magnetic and electrical.