Hi All,
After a couple of months I have finally managed to complete my Bottlehead Crack!
Started with the basic model - all was good and enjoyed but since I already had the speedball update I installed that - this is when the "fun" began!
Since then I have added:
1) 2* chokes to replace the power capacitors
2) 2* Panasonic capacitors for the output (with "helper" capacitors)
3) Cree Schottky Diodes for the rectifier
then the problems started - noise etc. Turned out to be a very bad volume control (replaced with an alps) and some "not so great" soldering. Also managed to damage one of the LEDs on the first valve and had to replace it. (Thanks to Paul and support for their help in tracking down these issues!)
Then started tube rolling (bad idea
)
4) Added switch to allow for 12au7 and 6gc7 tubes (changes the heater wiring)
6) Added switch to allow for standard tubes and E80CC - increases the speedball stage one resistor to provide more power requried for this tube
Finally got bored having to go over to the amp to change the volume control
7) added a remote controled volume.(not sure if anyone else has done this but couldn't find anything on it) (AliExpress)
remote board also had on/off control so added a 5v relay to the power input to allow for remote power control
9) of course for the above I had to add a 220v to 9v transformer (AC) - voltage from the main transformer was too high
10) created a gaussian shield around the input cable with copper tape and grounded it to reduce noise.
11) added dual diodes to earth terminal - again to reduce noise.
12) Piano black super reflective finish for the case (took a lot of 2k laquer & elbow grease!)
Anyway long and he short of it is that my super clean Crack is now a rats nest of cables. had dreadful problems with noise so have desoldered and resoldered many joints so now some of the cables don't look so great unfortunately but finally last night I got it almost 100% quiet - still a really low humm when nothing is connected but I can't hear this when music is playing so have decided to live with it. (think it has to do with the volume control board but I really like this feature). I have tried to make sure any power cables (AC & DC) cross paths with audio cables at 90 degrees to reduce cross talk and as said have add a shield ot the input cable. I also found that grounding the controller side of the volume board reduced the hum significantly.
On to Tube rolling:
Manged to get a few tubes cheap and one not so cheap (but not crazy $80 for a Tung-Sol 5998!) Favourite by a mile is the 5998 paired with a Tungsram E80CC.
All done, now to actually use the amp with my Sennheiser 660HD. Also have a pair of Koss E950 electrostatic headphones - the crack makes a great "pre-amplifier" though while the sound is fantastic, the headphones do feel very light when on (almost like a cheap toy!)!
Issues:
1) Any thoughts on the slight hum that is left would be appreciated!
2) takes a long time to warm up (30 secs)
3) right side kicks in, then 5 secs later left side - not sure why this is but seems to happen regardless of the tubes
4) since changing the volume to remote one I have to put it at 30% for a comfortable listening level, with the original/alsp it was 60% - all are 100k but thinking that maybe I as sent a linear rather than logarithmic attenuator!
W.
