SVT-VR lost power

[The Dude]

......It might arc because of that burnt resistor... I hope so.

Much more likely the other way around! The arcing caused the burnt resistor. How would that resistor being open (removing plate voltage from the tube) cause the tube to arc?

[boroman]

OK

Now I know everything.

On the tube board there was 5 Ohm / 3W resistor opened, 220 ohm screen resistor opened.
I replaced those and put Shuguang tubes back.

Guess what, those set turned out to be completely unmatched (I bought them new, seller said its matched, they do have the same measuring number stickers too). They differ between 8mA -> 28mA (!) on the same half of the amp! I've tried to set them about the same per trio but it was quite impossible to have close range. They are just random 6550s! They do bias fine and balance fine and look good on scope but putts around 180 clean watts overall which for me, by ear was more like my V4B range of clean sound... I put Sovtek set from my SVT-CL and viola, amp has way more clean headroom. So those tubes are traash...

PS: I even tried to put them in SVT-CL (that unmatched Shuguang set) - they can;t be biased in that amp! No green lights (they are either none, red, or red with green together), look: https://www.dropbox.com/s/w2ck8wssg0...%2057.mov?dl=0

[nevetslab]

So much for the credibility of the seller of those 'matched' tubes. You'd have to have discrete biasing per tube in the output stage to use those! Ashdown BTA-400 did that with their 8 power tube output stage. I generally only encounter that wide of a current mismatch after years of use. Bummer!

[boroman]

So much for the credibility of the seller of those 'matched' tubes. You'd have to have discrete biasing per tube in the output stage to use those! Ashdown BTA-400 did that with their 8 power tube output stage. I generally only encounter that wide of a current mismatch after years of use. Bummer!

This is the China seller on ebay that sells Shuguang tubes... never mind. The mismatched tubes will always be mismatched. Even if you have potentiometer to each one, and set it to the same working point, they will not work the same way

[SimTech]

2 cents here. Tube testers do use lower voltages, usually around 150v since most any tube can operate with that. Also tube testers usually ground the grid, put a resistance on the cathode to apply the proper bias, then apply voltage to the plate and then measure the throughput current. They don't test for microphonics or in this case high voltage arcing. Also if you have a tube with a weak grid, remember that the GRID ISN'T TESTED by most tube testers even my TV-7B. I've been bit by microphonics numerous times on my TV-7. Sure it has a jack to connect a radio antenna to so you can listen as you tap the tube but I've never done that with mine.