Here we are ten years after the birth of the SE84A and this amp is more than still around but doing well.  After getting into designing of tube amps I soon came to realize some of the less well known features of this amplifier.  So I thought I would start a thread on some of the technical circuit aspects of the amp that I feel account for why it works so well.  Steve if he feels inclined can jump in here as well.  I am going to number them so as to make responses easier for others.

1)  The output transformers  This one feature accounts for so much of the amplifiers fidelity, it is an enigma as to why they have been criticized so much.  Of course you can obtain better OPT's, but at a cost of more than the entire amplifier it really doesn't make a lot of sense.  What most of the critics are against are the very things that make these stock OPT's work so well.  You have to look at the symbiotic nature of the entire amplifier.  The higher impedance of the OPT's primary accomplishes some very note worthy things, greater power supply ripple rejection, less of a load on the tube which increases the gain of the circuit slightly, the power supply can then be made ultra fast which increases transient current delivery making for a fast airy sound.  Then if you use low impedance speakers (with a benine impedance plot) you can lower distortion and the combination is killer good at ultra low cost.  Coupled with this is a very gradual sloping frequency response on the bottom end that makes the best use of the available 2 watts.  The synergy of this circuit design with this tube is nothing short of genius, if you listened to 50 different combinations of operating points with different OPT's i am sure that blind listening tests would put you with this combination every time.  That is of course if you like the open airy detailed high resolution sound that is the base of the imaging character of the SE84 series of amplifiers.

2)  Metal film resisters  Steve usually uses metal film resisters, these sound pretty neutral and allow a lot of detail through, especially in the grid stopper position.   Some other types of resistors have more noise, so these are a good inexpensive resistor with a big bang for the buck.  Non-inductive wire-wounds like the ones that can be found in the PS are also very good.  Carbon comps should be used carefully, as they are very noisy, but sometimes you need to use them with high gain circuits.  But if you use metal film resistors you need to use better quality caps.

3) Metalised film caps Here is another good choice in the SE84 amp, they more transparent than than electrolytics, and some other types of caps,  the original SE84's used Solen fast caps, but it looks like above that Steve is also using GE Metalised caps which are also good.  One upgrade many have tried is to replace the cap off of the first stages plate with a teflon V-cap, which takes the amp to a whole new level after breakin for a very effortless sound.


Many have tried to copy the Decware SE84 amp, but none have really done it successfully, as the synergy of the above is part of what I feel makes it work so well.  There are of course some upgrades you can make to the amp, but the basic design is what makes it work so well.


TG

Good points Guys!

Remember though there were about thirty amps that were not "A" before revision A.

I've had three Decware amps, one before revision A that became a revision A, a Select and my current pair of E34L Monoblocks.  Incredible value and years of enjoyment.

The Revision that was A was a change to allow SV83s and 6N1P