Updated: 20-Jan-03
Doede's Audio Tweaks...

Tweak of the week:
Decoupling of the Output-Transformer directly to the 300B Cathode in a SE amplifier

How did this tweak see daylight?? I already noticed in some designs, I found  here and there surfing around on the Internet, that the Output Transformer was DIRECTLY decoupled by a small capacitor  to the Cathode of the End Tube. If you think a second about it, this makes a lot of sense, as the "Voltage Source" which is "embedded" in the Tube is connected directly between de Anode and the Cathode, with the Rp as the output impedance. Therefor you will have the most DIRECT signal path to the "load", which is the Output Transformer, when this load is connected between Anode and Cathode. This will not be possible, as we need some biasing of course.... In practice (you can see that also very nicely in the circuit below) the Output Transformer is connected with one end to the Anode, so what is the PATH now to the Cathode?? Well just follow the signal from the other Transformer connection..... Through the decoupling capacitor of the Power Supply, than through the Rk decoupling capacitor and than the de-humm potentiometer. So the Idea is now, to Shortcut this circuit  completely to BOTH Cathode connections. Just have a look at the RED modifications below, simple eh???

Why the low value of the Capacitors???? Well, as the other capacitors are already there, there is no effect at all at the frequency response. The only effect will be the shorter path. My explanation is that this will work any way from middle frequencies upwards with best result for high frequencies, so 2 uF will certainly do. Now we can pick easily the nice, fast, transparent Auricaps.....

Click Picture for High Resolution Image

Click for High Resolution Image


OK, nice technical explanation, BUT, how does it SOUND ????

Well, you better get your welding iron hot !!! This appeared to be a very valuable tweak !!!

To make it kind of scientifically "correct" I used the proven method of my "A-B-Switches" which are standard built in my amp for purposes like these. So I was able to make a very quick A-B comparison.....

Started listening to some pop, jazz and classical music and than switched over to the Tweak Position. Immediately the effect was there !!! No need to go up and down and do careful comparison. As soon as the switch is in the other position, the sound stage is gaining extra detail. The sound stage is becoming clearer and some what larger, but the GREAT STEP is without any doubt the big gain on micro detail. The recognition, pin point was definitively a step forward

I rate this as a major tweak, the 5 points kind of thing, I found the step bigger than, for example, replacing the 27 tube for a 227 (Meshplate). Actually, the sound improvement looks similar to the effect you get when you put meshplate tubes in the amp (triode and rectifier) in stead of the plate versions. Only that this tweaks result is clearly more evident !

Keep in mind: for this really small effort (4 Capacitors of 10 $ each and 30 minutes work), this is a truly great tweak..... 

Happy Listening and Building !!!!

Doede Douma


IMPORTANT: The information provided on this page is intended as guide for DIY activities and therefore free to copy and or publish. If any one wishes to use any of the information from my WEB site, please make sure to refer and footnote to my URL Link as source! Doede Douma