Orange TH30C output volume drop

[alxkol3x]

Hello, dear users of this forum!

The gear:
Orange TH30C, no mods in amp section (custom speaker installed), all tubes (PA and preamp) were changed ~6 months ago, no previous issues

The problem:
While playing through the Dirty channel (2 out of 4 power amp tubes are switched off) on Full output, I noticed a volume drop. After 3 or 4 minutes, the volume incresed back to normal, and the, suddenly, dropped again and didn't return to normal.
After switching to Clean channel, I noticed that switching between Half and Full mode doesn't change the output volume of the amp. To be sure, I disengaged all my pedals and plugged directly into the amp Input -- no changes. On the Dirty channel, however, switching between Full and Half settings was working, and the volume was changing, but the output level was still lower than before. The characteristics of the amp sound (tone, distortion, response to the differnt output levels of different instruments) is unchanged.

What has been done to solve the problem:
(1) Since changing between 2 and 4 PA tubes was leading to the volume change, it was concluded that problem (most likely) is not due to faulty tubes in PA section.
(2) Since switching between Full and Half mode was working on Dirty channel, I supposed that problem can be due to a faulty preamp tube in postion №1 (Clean channel stage 1&2, see attached media). After changing the tube to a new one, the problem persisted.
(3) After inspecting the preamp tubes while the amp was working, I found that the preamp tube in position №5 (PI tube, see attached media) was glowing brighter than the other ones. After changing it to a working one, the problem still persisted.
(4) Fuse was checked, and it's not blown.

So, what can it be? I know that Half mode limits the plate voltage of EL84 tubes, but why does it affect only the Clean channel? I understand that lower output volume in general (which can also be observed in my case on both channels) can be due to faulty tubes, but it doesn't explan why switching works on Dirty channel.

Thank you for your time and help!

[AlNewman]

First, unless you're comfortable, have appropriate tools, and know proper safety procedures, you shouldn't be working on an amp besides basically changing tubes.

Normally the first thing to do with an intermittent problem is clean all the pots, jacks, connections, switches, etc.  Quite often that is the cause of the problem.  Tighten all screws, pots, etc.  What's labelled TX connectors on the schematic are often loose and make a bad connection.

Then a lot can be determined by voltages.  There is test points on your amp to test the cathode voltage in full and half mode.  It looks like the relay for switching the power tubes also switches the impedance of the OT.  Unfortunately I don't see any other voltages on the schematic, but there are test points, and if you record the voltages and post them here, somebody should be able to confirm if they make sense. 

If voltages look fine, then chopsticking the amp will sometimes reveal the problem area, which could be a bad solder joint, ribbon cable, connector, etc.

At that point, you may need to trace the signal to find where the problem starts.  I don't recommend doing ANY of the above tests at full volume.  In fact nearly every time you record DC voltages, it should be done at idle.

Here's a schematic, seems like it should be close to what you have.

https://el34world.com/charts/Schematics/files/Orange/Orange_TH-30.pdf