: : Last year it happened to me twice that the output tubes
: : in Model Ts (two different amplifiers) went into a run-
: : away mode ,6550s glowing like oranges in a fruitbasket.
: : In both cases 2 tubes on one side of the primary, the
: : bias voltage on that side would decrease to about half
: : its normal value. So I suspect some kind of grid current
: : When I checked the tubes on a tester afterwards they
: : were done for (shorts). What causes this ? Ageing of
: : tubes ? Both the Model T and the Sentura II use an ultra-
: : linear output transformer ,which means that the plate and
: : screengrid get the same voltage ,500+ in case of the T ,
: : where the maximum allowable voltage on the screengrid of
: : a 6550 is 400 volts. Could that be the cause of the tube
: : malfunctioning? Who has tried something with EL 34s ,
: : 7581s or 7027 ? All those fall within the limits of the
: : design. I will do so myself soon ,clean up the sockets
: : etc.
: : I recently got a Sentura II ,now the third tube is a 6AN8
: : right ,not a 7199. Is that a difference between a I and II
: : Thanks Tos
: : Volts for a 6550.
: :
: :
: "Normal" aging of tubes won't cause this... but as tubes get used, they might encounter a few bumps or hard knocks in their lifetime... and this could be a cause.
: A similar thing happened to me. I got 4 new matched 6550's (2 pairs) for my Model T. Popped the old tubes out, popped the new ones in, turned it on to let it warm up and stablize before I checked the bias settings. Everything seemed fine at first, but then the hum level through the speakers jumped in volume. It was easy to see two of the 6550's glowing bright red, so I quickly shut off the power.
: After it cooled down... I swapped the tubes, reversing the side of the primary the tubes occupied. Turned it on, but watching carefully now, and the same thing happened... but on the opposite side of the primary... so the problem was following the tubes.
: Using only one pair, one tube on each side, I was able to isolate it down to the particular tube. I returned it and it's matching pair (Antique Electronics) and got a replacement matched set. Popped them in and all was fine with the world.
: They checked the offending tube on their tube tester and at first it tested fine... but after sitting for a few minutes, the bias voltage dropped quickly. Apparently, the heat and expansion was causing some internal connection to take place.
: I haven't experimented with replacement of the output tubes with something besides 6550's. I guess I'm happy with them. If you do try... it would be interesting to hear your results.
: Most every tube amp I have exceeds the design specs on the output tubes.
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Hi all: This is the first time I have posted here. I am a collector of sunn gear and I'm really happy to see this page. I have a couple of Model T's in my collection and I've tried both 6L6's and KT88's in place of the 6550's. The KT88's were the clear winners. They have a big low end that stays tight
with out sacrificing the mids or highs. 6550's sound sterol in comparison. As for the orange tube problem, this is caused by poorly matched tubes.
