Problem with my Bogner. Correct diagnosis (tubes)?

I picked up a Bogner Shiva and I went through the following:

- I fired the amp up and all was well for about 5-10 minutes of playing then a very noticeable hum/hiss began. This isn't my first tube amp by any means, so I know there has to be at least something wrong. The sound was very shrill and steady. It was also on both channels, therefore I believed it to be a microphonic tube in either V1 or the PI.

- I tapped on every tube with a pencil eraser while the amp was on, however I couldn't really hear a noticeable tapping sound coming from the speakers.

- I let the amp cool overnight and took the tubes out today. One of the 12AX7s (an ARS Chinese) has a very noticeable jingling when it is gently moved around. I believe that this tube is my huckleberry.

- I moved around all the tubes and put the believed culprit in V4 to see if there was a difference. I started up the amp and played through it really quick and the sound is completely muffled; the channel and master volumes needed to be very high to even get a level lower than what could be considered bedroom volume. This to me confirms the idea of a shot preamp tube or tubes.

- Summary

• All tubes light up
• There was a very shrill and steady hiss/hum
• I checked the tubes and one 12AX7 had a noticeable rattling
• I moved the tubes around and the amp is now incredibly muffled (makes me believe this is a preamp problem)

I am going to order some new tubes from Doug's Tubes, but prior to doing so, I wanted to see if anyone had any other ideas about what could be going wrong with the amp.

 

Alright, well I replaced all the tubes, but I am still experiencing the main issue, but I think I may know part of the reason for it.

I got the new tubes yesterday (Doug's Tubes is fast!) and rotated the preamp tubes throughout the amp and found that I had two bad preamp tubes in there. I also replaced the powertubes and everything was going well.

I played for about 40 minutes with no problems. This was with the back panel off the amp.

Today I reinstalled the back panel and after playing for about 20-30 minutes, that ultra annoying high pitched squeal came back.

This is leading me to believe the amp is getting too hot and this squeal is a result of that. I have a Trafficgreen2+ Shiva (new version with the variable boost and bright switches) which lacks the fan that prior Shivas had. I am wondering if the high plate voltages of the EL-34s in the Shiva could be overheating the amp and causing this annoying squeal. I have owned quite a few amps and it is easy to tell that this isn't just natural tube hiss or white noise. It definitely doesn't belong.

I have swapped all my cables and taken everything out of the chain, but the noise is still there.

Here are some things about the squeal:
- Only starts after about 20-30 minutes
- Goes from no squeal one minute, to a faint squeal which then increases in volume rapidly
- The squeal cuts through playing
- The squeal exists when the guitar volume is off
- Happens both when there is something in the loop and when there is nothing in the loop
- When the amp is switched to standby, you cannot hear it through the speakers of course, but you can hear a squeal coming from somewhere

Taking this to a tech is my last resort at the moment. I really cannot afford the fees right now and I don't want the amp to be sitting in a shop for a month or longer.