Full volume after 2 minutes

Are you saying that no matter where the gain and volume controls are set the amp produces full output? What if all controls are set to zero? Is this with an instrument plugged in?

 

Once the situation occurs, what happens if you pull preamp tubes? Start with V1 and pull in order with the phase inverter being last. Does the situation change with the removal of any 12A-7 tube? I am going to think that you have an osicillation is occurring and taking the output to full roaring output. Have you put that scope at different points in the circuit to see where things are going wild?

 

The first place I would look is here;
as the LDRs are the common denominator here.
Check/replace C37

 

Yes. With the amplifier switched off and no latent high voltage on the smoothing capacitors, measure the resistance of the LDRs. They should be very high resistance and when the amplifier is powered up change value when each LDR is selected. To prove the LDRs, link the resistor parts out temporarily with a 10k resistor bridges across the LDR side. Hopefully if my diagnosis is right, there will be full volume straight away. If LDR1 is linked out the volume will be reduced. Just think of them as variable resistors, the more light, the lower the resistance.
Your layout is not to be trusted as at first glance the power switches are drawn wrong!

 

Imho, the amp needs to be on a tech’s bench so that voltages could be observed during the time of low volume versus the time of normal operation. If you are that tech, put it on the bench. How old is the amp? SLOs have been around long enough for the electrolytics...like the one Jon Snell points to....are weak and in need of replacement.

 

So Jon your diagnosis is the switching supply voltage is causing the amp to "switch" as it starts to function after two minutes? Or is something else being dragged down?

Trying to learn.

 

What we must do first is diagnose what is actually happening.
By measuring the off load resistances, they should be very high to reduce the level of signal.
Now, when we put current through the LED part of the LDR, the resistance should go down, allowing for more signal or if current is instant, acting a little like an electronic switch. A Light Dependant Resistor.
Once we know they change value as current is passing through the LED sections, we know they are working as expected.
So, if the resistance changes over say 2 minutes when the amplifier is working and first switched on, they are either all faulty or there is an issue with the drive to the LEDs. Until that point is reached we are clutching at straws.
I hope that explains it for you.

I just read what I typed through and amazed myself as I am dyslexic and it even makes sense to me!

 

Has it ever worked properly, or has it always had this two minute lag?

 

Transpose the lower pair of wires, both power and standby to the centre pins. or link bottom to centre.

 

So, a build that has worked for a ‘long time’. Jon Snell’s path if investigation is interesting. As for the switches, they must be wired differently from your drawing shown in post #6, or the amp never would have powered up, would it?
Have you gone through the test procedure Jon Snell outlines for those diodes? if that investigation does not point to the problem
and since you say the amp has worked properly ‘for a long time’, one wonders what a ‘long time’ is? When did you build this amp? if the diodes are good, one might suspect electrolytic caps??

 

So, this is the LDR that is connected to the control grid of the second triode in V2, correct?