Laney Pro Tube Screeching noise

AndyYPablo

Member
2018-03-16 12:58 pm
Hi All,

I'm having continuing issues with my Laney AOR Pro Tube 30 combo amp.

When switched on, as soon as the tubes begin to glow there is a sharply rising screeching noise. This noise is affected by the master volume.

The noise does not occur when the power tubes are both removed, and is quieter when only one power tube is plugged in.

No input noise can be heard through the speaker, only the screech.

Please see here for a demonstration with both tubes

Does the noise ceasing when power tubes are removed suggest any possible areas i need to look into for repairs? Or do I simply need to replace both tubes?
 
If the master volume control turns the screeching down then the fault is before the volume control. If the presence control controls the sound then it could be the phase splitter with an unbalanced anode load. Remove the phase splitter ECC83, if the noise stops it is either that stage or before it. Difficult to guess without working on it.
 

Rod Coleman

Member
Paid Member
2004-07-25 6:55 pm
UK
Preamp stages with instability & squealing is common in modern amps that use cheap grades of electrolytic caps in the preamp's local decoupling capacitors (22µF 500V for example). The Illinois Capacitor TTI series that Fender use cause this problem all the time. The capacitors degrade badly, if the amp has not been used for a long time, and that often provokes the instability.

But it could also be any of the preamp valves, so if you like to fix things by substitution, you can start there.

Get the schematic, if you have to debug it further than that. The decoupling caps connect from the local anode supply to "ground".
 

AndyYPablo

Member
2018-03-16 12:58 pm
Have your speaker leads been swapped? Try them the other way.

If not measure the dc resistance of the primary side of your output transformer.
Probably about 150ohms plate to CT with a transformer that size. Should be fairly even both sides.

Of course power should be off.

Hi, the issues continues with the speaker cables swapped. I am struggling to identify which wires are plate and CT, and unsure I could reach those connections without clipping some shorter wires that are keeping the pcb from coming away from the enclosure.

JonSnell Electronic said:
If the master volume control turns the screeching down then the fault is before the volume control. If the presence control controls the sound then it could be the phase splitter with an unbalanced anode load. Remove the phase splitter ECC83, if the noise stops it is either that stage or before it. Difficult to guess without working on it.

I have made a short video to show how each pot affects the screeching. It seems to be affected the same way any output would normally be, aside from the fact that the noise cannot be turned down completely by the master volume. This was filmed with the master volume set at 0. I will look at the schematic and try to figure out how to remove the phase splitter.

Rod Coleman said:
Preamp stages with instability & squealing is common in modern amps that use cheap grades of electrolytic caps in the preamp's local decoupling capacitors (22µF 500V for example). The Illinois Capacitor TTI series that Fender use cause this problem all the time. The capacitors degrade badly, if the amp has not been used for a long time, and that often provokes the instability.

Caps is likely the first thing I will look into replacing as it's the cheapest, and upon closer inspection, mine look like this. This is the only visible issue to my untrained eye. Whatever the issue turns out to be, these can't be helping.


All wild guesses.
You need to scope the amp to be certain.

or, as many backpanels state:"refer servicing to qualified personnel".

Emphasis on *qualified*

Very true, however there are some very good places to start troubleshooting highlighted. This will be stripped down for parts and sold if i'm unsuccessful, as the cost of professional repair is too high, and I will likely recoup the cost of buying it as I found the cab contains a valuable speaker when I popped it open.

Thank you all for taking the time to reply, I'm going to try some of the things suggested here and see if I can't get it back to gigging condition.