Greetings,
My AMC CVT 2100 has issues since I installed a set of Shuguang KT88.
The ZD501 and ZD502 keep shorting and blowing the mains fuse. The 270 ohms resistor is still intact.
I assume the zener diodes provide a kind of regulation but I am wondering if this feature is necessary with an ultralinear OT. I always thought that a safety resistor of 100 ohms was adequate.
The SM suggests 55mv across each 2 ohms resistor which is only 27.5 ma cathode current per tube; am I missing something?
Your help is appreciated.
Cheers
My AMC CVT 2100 has issues since I installed a set of Shuguang KT88.
The ZD501 and ZD502 keep shorting and blowing the mains fuse. The 270 ohms resistor is still intact.
I assume the zener diodes provide a kind of regulation but I am wondering if this feature is necessary with an ultralinear OT. I always thought that a safety resistor of 100 ohms was adequate.
The SM suggests 55mv across each 2 ohms resistor which is only 27.5 ma cathode current per tube; am I missing something?
Your help is appreciated.
Cheers
No experience with this specific amp, but considering the rated 80+W output from each pair of KT88, it'll run on a pretty high voltage. At least 500V probably.
Ok for a good quality KT88, but already out of spec for a good 6550a (max 450V in UL).
Same for the 100k grid leak resistors: ok for KT88, too high for 6550a (max 50k for fixed bias).
What is inside the Chinese KT88 that you have? Who knows. The early ones were pretty bad. Redplating under 25W, poor grid alignment (high g2 current), thermal runaway. Later there were pretty decent ones too.
Important to keep the current low, so less likely to overheat and run away.
If the 12V zeners feeding the g2 are blowing, that means lots of current. Badly aligned g2 grids, oscillation or both.
Do you have the old tubes to try of those hold up better?
Ok for a good quality KT88, but already out of spec for a good 6550a (max 450V in UL).
Same for the 100k grid leak resistors: ok for KT88, too high for 6550a (max 50k for fixed bias).
What is inside the Chinese KT88 that you have? Who knows. The early ones were pretty bad. Redplating under 25W, poor grid alignment (high g2 current), thermal runaway. Later there were pretty decent ones too.
Important to keep the current low, so less likely to overheat and run away.
If the 12V zeners feeding the g2 are blowing, that means lots of current. Badly aligned g2 grids, oscillation or both.
Do you have the old tubes to try of those hold up better?
The old tubes are Svetlana 6550C, they needed to be replaced after many years of daily use. I have put them back after changing the diodes and the mains fuse, no bang, no smoke. BTW plate voltage is 717V.
No way I would run an EL34 with 700+V on the screen grid. They draw a lot of screen current, more than beam tubes like the 6550/KT88 😱.
Uprated KT versions like the KT90 (600V max) or KT120/etc (650V max) might survive?
Uprated KT versions like the KT90 (600V max) or KT120/etc (650V max) might survive?
According to some tubes vendors, there are Sino KT88 not recommended to operate horizontally.
The KT88 are less than 35mm apart with a fan underneath; still tight in such confined space.
With the right voltage there are many other candidates like the KT77, the 7581A etc.
I'll use a set of 6550A and change the grid leak resistors as suggested by Parafeed813.
I'll keep you posted.
The KT88 are less than 35mm apart with a fan underneath; still tight in such confined space.
With the right voltage there are many other candidates like the KT77, the 7581A etc.
I'll use a set of 6550A and change the grid leak resistors as suggested by Parafeed813.
I'll keep you posted.