Flickering tubes on my AR M100! Help!

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I don't know why, but lately the output tubes (6550 Sylvania's) are flickering in tandem...
I don't fully understand the full circuit design, but I'll describe the symptom first, then the design topology.
There are 2 pairs of 6550's for my AR M100 (mono amp) work in a balanced-output fashion. Lately, the pairs are flickering (pulsating bluish light) one pair after another - often occurs during the initial cycle-on and sometimes well after continuous playback - accompanying slight hum and a couple of loud thumps to the loudspeaker. Onetime, the playback was interrupted by loud hum and silence intermittently and when I look at the tubes, one pair's entire metal structures were glowing red!!
The design, based on what I can understand from the schematics: Amplication - Signal to Input amp/Phase Inverter to Cross Coupler to Driver Amp to Output Tubes. (Well, I may be missing something in between.)
The Power section - AC to Rectifier to Cap to 630V to Clamp Circuits (+433V for the small signal tubes described above and +330V for the Screen) The rest of the circuitry looks like feedback to me.
Anybody has a clue as to what is happening to my amps???
This only happens to one mono amp. I tried swapping tubes between the 2 mono's. The same thing happens.
Thanks for any advice you can give me!

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Topic - Flickering tubes on my AR M100! Help! - DC 18:29:20 12/21/00 (9)

Re: Flickering tubes on my AR M100! Help! - PaulRS 08:55:46 12/24/00 (0)

Re: Flickering tubes on my AR M100! Help! - steven oda 19:57:48 12/21/00 (7)

Re: Flickering tubes on my AR M100! Help! - Story 20:04:35 12/21/00 (6)

Re: Flickering tubes on my AR M100! Help! - DC 21:34:47 12/21/00 (5)

Check the 7119 driver valve - it may be worn out and cause bias fluctuation (nt) - mb-de 00:54:44 12/22/00 (4)

Re: Check the 7119 driver valve - it may be worn out and cause bias fluctuation (nt) - Beam Tube 11:46:16 12/22/00 (0)

Re: Check the 7119 driver valve - it may be worn out and cause bias fluctuation (nt) - DC 02:02:25 12/22/00 (2)

VHF oscillation, maybe? (nt) - mb-de 08:11:42 12/22/00 (1)

Re: VHF oscillation, maybe? (nt) - DC 02:14:31 12/23/00 (0)

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