One morning while having coffee, switched on my tube amp to warm up the tubes and play some classics, but one power tube goes bright, smoke appears inside then goes off
a few moments later the silvery coating on top became ghostly white...R.I.P 6AR5... :'( :'( :'( pull out the tubes and upon close examination, a crack is revealed at the base...
perhaps now is the time these bottles should take over.
after some metal working to fit the new tubesockets...
As can be seen the 12AV6 driver is dwarfed by the 6F6 clear tops.
Following changes were made:
1. 400R cathode resistor were replaced by 150R.
2. 10K feedback resistor connected to OPT secondary replaced by 250K, this time connected directly to plate of 6F6, in series with 1uF/350V motor starting caps & directly connected to cathode of 12AV6 driver tube.
3. Aside from power switch, i installed a B+ switch so that tubes are fully warmed up before supplying power to the tubes, to prolong the service life of the tubes.
First listening impressions: at power on there was an audible hum even at 4 ft away from the drivers. This is bad, steering away from the schematics, I have to do it my way, gathering up the knowledge i learned from the web site recomended by sir Tony also in this thread, just backtrack a few post( thanks again sir!
) I put a 1K res in the screen grid and shunted by 100uF/450V Nichicon caps to ground(the original schematics connects the screen grid directly to B+).
Second listening impressions: Finally the hum is barely audible with my ears sticking closely to the cone, great!!, listened to Maria Rita Mariano's "Dos Gardenias" I listened to this before but this time its kind of a euphoric, her vocals others may put it "to die for". This amp has taken me several levels up in tube audio!
Btw i am now using PHILLIPS AD50600 5" highly efficient drivers, so the 1Watt/ch output from 6F6 sounded even more powerful than my room warmer the JLH 10W/ch class A SS amp!
Bass- gone is the slight "boominess" from the former 6AR5, taut and having that punch.
Highs- Natural, airy, totally absence of sibilance.(I am using tweeter from my onkyo bs, the pHillips driver fits snugly in the Onkyo enclosure)
When I played Jim Brickman's "By Heart" sung by Ann Cochran, I noticed that my wife listened seriously, she later told me she was "a little bit carried away emotionally" though I know it is more than a bit.
My efforts now rewarded, I have no further plans of tweaking. It may not look glamorous on the outside, but the performance is really something!! I have yet to make that wooden plinth on the sides(my carpentry skills is very poor! but with determination it can be accomplished
)
Also I would like to give credit to
AT&S audio transformer by sir Edrel Sison, running at 43mA, they stay cool even up to one hour operation, they warmed up only due to heat conduction from power transformer because of close proximity, they have proven their worth sonic & physical.
my daughter ejoying her cd of Celine Dion
