The KT88 Matrix Amplifier


The YL1060 amplifier sounded so interesting that I decided to build another Matrix amplifier, using commonly available tubes and parts, just to see if the sound I was getting was related to the Matrix topology, or somehow related to the exotic tubes I was using. This amp ended up producing the same kind of sound and similar characteristics as the YL1060 amplifier. This version has been built by several people. One word of caution. The 6MB8 drivers work well, but I found that the distortion characteristics of those tubes vary A LOT from part to part. Some will sound constricted at low levels; others will not. I have found that I get a yield of about 20%. That is, about one in 5 sound great; the others do not. However, since this tube can be found for 50 cents(!) on sale, buying a bunch and selecting them is not a big deal, and not expensive at all.


and the power supply...


Notice that this is actually a pretty simple amplifier (4 tubes for stereo amp) although the Matrix operation looks odd at first glance.

The KT88s are cathode biased (this is Class A remember) and driven from the pentode section of the 6MB8s. There is local feedback applied around the KT88s in order to get the output impedance down (to about 2 ohms) for reasonable damping. The screen grid of the KT88 is regulated voltage.

The 6MB8 screen grid is also regulated by a shunt regulator made from the "left over" triode section in the one 6MB8 (the other triode section is the inverter necessary for proper Matrix operation.

I used Stancor and Hammond iron for the output transformers. The sound quality is set by the characteristics of T1 much moreso than by T2.

The 0.6V sensitivity is higher than many amplifiers, but those tubes are relatively quiet, so the amplifier as a whole is quiet.

The distortion of this amp is actually very good.

Distortion is < 0.2% at 1 watt and higher order components are very low.