OTL Asylum OTL, Output Transformerless Amplifier User Group.

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ OTL Asylum ]

---

[ Alert Moderator ]

---

MA240SE

Posted by Lew (A) on April 10, 2006 at 08:39:04

In Reply to: 6C33C-B & Novacron revisited posted by Legendre on April 5, 2006 at 16:50:26:

According to Ralph, I bought the last pair of MA240 chassis' that were available. This was about 7 or 8 years ago. Ralph sent them to me along with all the parts for a pair of monoblocks in two huge and very heavy cartons. (The MA2 transformers weigh a ton each, or so it seems.) For those not in the know, the MA240 was basically an MA2 in which the output tubes were 6C33Cs (3 pairs per chassis), instead of 6AS7s. Thus it utilized the massive MA2 transformers which give 400V for HV. Ralph provided a schematic whereby the amps could be constructed with three dual-section driver tubes per chassis. Thus my amps are "special" in that each output tube is driven by its own driver section, thereby making it possible to set the bias for each output tube individually. I obtained a set of matched 6C33Cs from Victor Khomenko of BAT. Perhaps the matching of the tubes and the capacity to bias each tube separately account for the fact that I am still on my original set of tubes with no failures, after so many years of use. The amps were originally built for me by a local friend, but after that I started to study tube circuit design with the aid of books, etc. In the ensuing years I have added the following upgrades, many of which are now also standard on factory products: (1) Pi filtering in power supply; also bias supply and supplies to input stage vs driver stage are separately isolated via LC filters. (2) Hexfreds of course. (3) Paul Speltz' LV power supply mod, using CRCRC filtering, with very low value Rs. (4) Separate tube-based CCSs for each driver section; this required installation of three additional dual-section triodes (ECC99s), a tight fit. (5) Changed 12AT7s in input stage for 6900s (a fancy beefed up milspec version of the 5687). (6) Use a cascode-type CCS for input stage; this gives very high Z which enhances CCS function. (7) All teflon coupling caps. (8) All Vishay, Caddock, or Mills resistors in signal path. (9) Solid core thin gauge silver wiring in signal path. (10) Probably more that I cannot remember, but I can think of more things to do in future; e.g., add regulation to HV supplies, expt with low amount of NFB to reduce output Z. I kind of wish there was room for one more pair of 6C33Cs per chassis, but it ain't gonna happen.

I still use AC filament supplies, because these amps are dead silent, and if noise is not a problem, AC sounds best.

I am very happy with these amps.

This post is made possible by the generous support of people like you and our sponsors:

VH Audio

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---