Power Tube Failure

@jea48 

The VT50 is a power amp, the OP’s questions regard his 6SE line stage. You are aware of the difference between a preamp and a power amp?

ARC VT50 Schematic 

"https://www.arcdb.ws/Database/VT50/ARC_VT50_schematic_and_parts_list.pdf

ARC VT50 Setup Instructions With Complete Retube

https://www.arcdb.ws/model/VT50"

11) If test equipment is available verify channel-to-channel balance is < 1/4 dB. If not, power down, swap around 6922s. Turn on. Recheck steps 4-8. Recheck channel Bias.

"And there is why it can take hours. When the 6922 tubes are moved into another socket the bias procedure starts all over again for the tubes moved to a different tube socket.

That is actual hands on experience talking. Been there done that...

Time is money... That’s why if you buy 6922, (small signal), tubes from ARC for a VT50, VT100, VT100 MKII or a VT200 you will pay dearly for the tubes to be tightly, closely, matched. 

No I haven’t bought tubes from ARC. I have had the best luck buying 6922 tubes for the VT50 from the Tube Store. Tell them up front what the tubes are going in."

I looked at the circuit and Q1, 2, 3 are part of AR’s proprietary method of using the two 6922s as phase inverters to provide a symmetrical voltage swing to the output tubes. You are not biasing them but ensuring they deliver the proper voltages. You bias the output tubes using R29 and want to see 50MV DC across each channels test resistor. Though you may find the process arduous no such procedure is required for a line stage. 

the 12 in 12AX7 denotes it’s maximum heater voltage though it can be successfully used in a circuit at 6.3 volts.

@faustuss E88CC (6922 / 6DJ8), not E83CC. 12V is definitely no-bueno on the former. I goofed because I have a preamp with an internal switch between 6DJ8 / 12DJ8. The Seimens E88CC glowed like the sun lol. Tough tubes. 

Tubes that have not been powered up for an extended period of time will often make sporadic noises since all the various elements that make up the components inside the tube have a tendency to move around in the vacuum on an atomic level and are quickly scavenged up by gettering.

This particular set was brand new Russian KT88 Mullard, noisy (intermittent, not steady) even after biasing and settling for 20 minutes. Definitely not good. A tube crapped out soon as I put signal through. They still test great, strangely (mA, gm). New power tubes should be generally quiet, apart from maybe an isolated soft pop burning off some impurity. 

And I get that vintage tube circuits are durable and stable, even over decades. I’ve had some vintage Eicos & Heathkits and love them. I was simply chuffed that instead of popping a sacrificial fuse I get a red LED indicator on the bad tube slot and no fuse needing replacement (still has the main fuse as final safeguard).

@mulveling 

Thanks for the information.  I checked-out the the MaxiPreamp II & MaxiMatcher that you own.  The metrics seem just what I am looking for, and the pair seem very appropriate who has as many tubes running has your system requires.

Today the price-point (~$2k) seems a bit high for my TWO -12AX7 PhonoPre and ONE 6550-PreAmp.  Still, I have back-up tubes that need to be tested, so I may pull the trigger at some point.

Very best,

@mulveling @jea48 @faustuss - thanks for keeping it lively!

... you have made this conversation one of the most strings that I have read on AG in sometime :-)

faustuss Regarding the statement "your 6SE it is employed employed as a voltage regulator...", does this imply that the Ref-6 SE's 6550 will likely LAST LONGER (more slowly degrade) than a a similar tubes that are used for direct power output?

Background info: even though the Ref-6 SE's 6550 (Sovteck) is not a power tube, it reaches 180+F, thus it seems to be very inefficient (causing it to degrade?).  Consequently, would you recommend that this voltage-regulator 6550 be occasionally tested like 6550s which are used as power tubes? 

Best

errata;   one of the most INTERESTING strings...