I present u a budget reference power amp
THE 845AA kit price 2499 euro Build 3999 euro
To cut a long story short , I am tired of making amps that weigh 30Kgs and my favourite tube is the 845 . Even the GM70 is not as good and neither is the 813 or 811 and even 300B doesnt have all the qualities of the 845 , the 2A3 comes close but has no power ......
The problem with 845 SE amplifiers is that u need a very high voltage , lots of power , and even the driver stage has extreme requirments from a designer point of view.
So how to solve all these requirments , I allready build an outerlimit , a Captain 845 , and then the commercial version the Enterprise which is still available in kit or build.
But I wanted something lighter , but just as good.
Since today evrything is supposed to be greener = more efficient , meaning mass produced switched powersupplies are the norm today and this is for the greater good of mankind. tube amps are not exactly enviromentally sound, except in the winter where the extra heat provided is not lost :-) Really guys in the winter I appreciate these amps most. In the summer I listen mostly to my 300B SE or my Boully 88 ( a KT88PP amp ) when i need power.
Lets look at the requirments for a SE 845 amp.
U need 100watts of plate power , 35 watts of filament power + driver filaments 10 watts + driver power approx 30 watts and a negative bias as well. Powerwise a good 200 Watts to be on the safe side including losses.
But u need 1200 Volts high voltage , and 450Volts driver voltage at least.......well filtered as u are in SE mode so any powersupply noise or ripple will just drip into your speakers.
The electronic powersupply has a few very interesting features that are far more easy to include then when using transformers at line frequency ( 50 or 60Hz ) .
You can much more easily filter it since u are at a very high frequency , the transformers are reduced to a fraction of the size. and u can also electronically make start up delay integrated into the board and also have short circuit protection and current limiting as well which is a real security benefit. If u ever witness a 1200 volt short u will understand this is not just some luxury but a real bonus.
All transformers are handwound , since switched powersupplies are not very RF friendly some shielding is necessary but once shielded i would say amazingly quiet :-) Its a great product , and I think once my stock of power transformers is gone I'll never ever go the power transformer route again. Except for pre amps :-)
This powersupply I wont share any schematics , It is sold build only as it takes great expertise to also adjust it. (a hint is voltages are set by varying magnetic fileds in transformers and the leakage inductance is used to regulate these)
Picture of the power supply , the prototype the finished prototype and the schematic of the amp.
A word on the schematic :
Since low cost was an issue the chosen tubes are cheaper , the PC86 and PL36 are great sounding tubes used properly , the PL36 especially is actually a 2A3 in triode mode and its choke loaded and direct coupled to the 845 so u can drive it easily in class A2 , the LL9202/85mA output transformer has enough inductance for great low end and compared to its bigger brother the LL1688 and LL1691 is just limited in its outputpower, but I measured 25 watts of output power so I think its a good compromise for its price.
The sound is spot on of what u can expect of a top of the range 845 style amp, and I do state many commercial 845 amps barely sound better or as good as a standard 300B amp, which is again proof of how hard it is to make a good 845 amp.
Enjoy the pictures and schematics , ALL PARTS ARE AVAILABLE just email me or call .
This build amp is also for sale
And the first prototype
Some curves of the tubes PC86
And the PL36 triode connected in pdf file