Re-building Audio Mirror amps
for 6C33 triodes
on a lot of web search, the 6C33 tube has a lot
of aficionados and the discussions on how to use the
tubes best are endless. Dos and don'ts by the numbers and
people are much divided when it comes to how these tubes
should be treated. The 6C33 tubes are often used by OTL
builders, but also PP designs seem popular. For SET
usually only one tube is used, where PSE versions seem
So, I mailed Vladimir at Audio Mirror to hear if he would provide the schematics for the 6C33 amp and not only did I get a prompt reply, but also the schematics for an improved version where fixed bias is replaced my an N-channel power mosfet circuit, that is: Auto bias.
If you read the tread here: http://www.goodsoundclub.com/Forums/ShowPost.aspx?postID=6140#6140, there are a number of things here that shouldn't be done, not least auto bias, but I decided to take my chances with Vladimir's suggestions.
Having done the mods according to Vladimir's recommendation I can say that this is the most transparent amp I've ever had running my speakers. Next, as Vladimir says, the bass grip and dynamics are phenomenal. Bass transients have a punch not even the JungSon's 80 class A watts can do much better.
The thing about this
amplifier is heat and hum. The latter I'm sure I'll
overcome; a minor residual hum that isn't audible from
the listening seat, but noticeable when standing next to
the speaker. 95 dB speakers are not forgiving.
Fixed bias version
17-04-2011: To make a long, long story short, the input stage of this design proved seriously unstable and after frying a lot of mosfets, the direct coupled input stage was abandoned in return for the original design including fixed bias. This version may not have the magic touch of the auto bias, but something I can live with. Thanks to Morten for lending me his 6C33 amps for copying.
setting up this amp - although taking quite a few hours - was expected
to be an easy task. Not so! Having plans for a second 6C33 amp, I
purchased two Lundahl LL1693 OPTs. These can be wired for 300 ohms
primary impedance and are to my knowledge the only off-the-shelf OPTs
available for this application. Obviously other manufacturers can wind
OPTs for two 6C33 tubes if asked.
To the left Jacmusic wiring. Notice the +-marking (encircled in red) has changed compared to Lundahl specs despite the wire numbering being the same. At the bottom the wiring I used leaving the two primary windings in parallel as should be. There are two options here, letting each tube see its own 600 ohms primary - or connecting the two primaries in parallel (300 ohms) and running wires from a central point to tube anodes. I don't think it makes any difference.
So, time to power up the amps again - and sweet music came through the speakers! Finally! The Long and Winding Road...
First of all this set-up
has very low hum and trying to get even lower I supplied shielding to
the filament supply wires. Minor improvement. The 100 ohms resistors
from filament wires to ground is something that has to be tried. I found
the shown ground resistors to produce least hum and I have to put the
ear to the pulp to hear hum from my 95 dB DTQWT speakers, so no further
work in this area for the time being. Maybe a double choke some day
These Lundahl OPTs indeed seem very good. With regard to transparency I don't think there's any difference from the auto bias version. Vlad's comments on sonic differences may be just a tiny bit exaggerated and from pics provided it appears his amps were fitted with Sonicaps, which to my ears are seriously bad and impairs mid and treble performance. With the best coupling caps available, I think the differences are small. The auto bias version may excel in bass performance and a certain holographic magic to the midrange, but for the time being I can live happily with these amps, truly the best I have had running my DTQWTs, OBL-11s, QUATTROs and even the PRELUDEs, the latter really not at thunderous levels.