Poor mans 845PP by Benny & Stephan & David
This project is finished but supplied without chassis ( the pictured chassis
is Davids design and the cabling is really crazy )
as you all have noticed this project developed following the SE 845 Outer Limit and follows some of the footsteps and philosophy so we can get a state of the art design keeping a close budget in mind .David owned a Jadis PP EL34 amp and got exicted by the Outerlimit , since he didn't have the budget for that and wanted the power and sound of an 845 tube something had to be done , I was working on my Outerlimit monoblok 845pp and suggested we use that as a base for his amp , the 2A3 drivers are substituted for 6BL7 , the high voltage was lowered for the 845 as this offers lots of practical advantages , cheaper powersupply and easier to drive the outputtubes .many things omitted like current sources , vapor mercury rectifier tubes , paper in oil caps ect .but we did use an interstage for driving the outputstage ., and a long tail phase splitter as the input stage with a negative voltage for optimum performance .
Many top quality parts everywhere where needed .
As usual first simple discussion of basic philosophy .
1 What are we aiming to get ?
2 How much power we want ?
3 How high end should this be ?
4 Aesthetic design criteria ?
5 Budget ?
Basically those are common questions , as usual the answers make the difference because many contradictions can co-exist in one sentence or life or a project .
The answers .
1 A beautiful and powerfull amplifier that has killer looks and is compact as well .
2 Well enough of that whimpy SE stuff , 845 is the choice ,especially in PP from pure class A to class B you can vary output power from 50watts to 200watts .
3 Well offcourse as good as possible given the budget , it should be excellent, PP has several advantages that SE doesn't have and the interstage design eliminates coupling capacitors .
4 Small !!! if that is possible ? , lots of chrome look
5 As cheap as possible , while keeping a very high standard , Our minimum standard is quite high , why bother making mediocre stuff , why spend all the hours ?
The circuit V1.0
As described , input stage is a classic log tail phase splitter , since we
benefit from having high voltages we can load the tubes optimally , then we
are capacitor coupled to the 2nd stage which is a mini PP amplifier , it dumps
its power into the interstage , which is used to drive the 845pp stage , this
eliminates the need for capacitors , too many coupling caps seem to reduce the
soundstage , an interstage here and there solves the problem . the 6BL7 is a
very rugged tube that accepts a max working voltage of 550vdc .
It is also enough powerfull to drive the 845 in positive grid bias territory .
Again negative bias is the way to go , and this for all stages , I just can't understand why people bother with autobias , it is acceptable in guitar amplifiers but in high end ?
diode rectified , capacitor filtered then choke filtered for each channel and
again capaitor filtered , the high voltage is triac controlled for a simple
switch on delay .
then we use the 2x 35volt windings into a voltage doubler to obtain the 200volts of negative bias zener regulated that serves also for the long tail ( tube sees a current source this way from its cathode to gnd ) and a transistor bias adjustment for each tube .
Final note :
The chassis looks great but David chose the layout in function of optical beauty
, and not in logic of cabling , so this is a real mess , but Stephane cabled
most of it and I take my hat off for his patience , I wouldn't want to go hunting
for a screw in that amp !!!!!
greetings to all ,
Benny & Stephane & David ( very happy owner of a fantastic amp )
copyright webdesign 2002 aquablue Benny Glass