OTL amps, my pick for the tube used.
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its been out of production for about 20 years. The tooling was destroyed with the plant I’ve asked Mike Matthews for his views on this, from what I understood his factory in Russia has all the dies and tooling, and sell them on New Sensor and the sockets at reasonable prices much cheaper if you join and login. https://shop.ehx.com/item/6c33/tubes-vacuum-sovtek/ https://shop.ehx.com/item/7big/ |
@bobheinatz, here is what I can say at the moment. The design is past the prototype stage and a few pilot amps will be built for listening tests.These will be monoblocks with moderate power and so far have been tested on a pair of Quad 63 with great results. I am going to have the prototypes in my system shortly on my Quad 57s. Ideally these will work quite well with 16 and 8 ohm box speakers and can include an external autoformer if you have speakers with lower or more difficult loads. I can say while Roger studied the H3aa circuit as part of his research, this OTL will have more in common with the earlier Futterman designs which were simpler and in the process Roger was able to further reduce the parts count in half. A Pentode/Triode switch will also be available. IIRC Roger mentioned the Futterman amplifiers have 7 adjustments with two pages of instructions to set up the amp. Roger simplified this by installing a meter with a 6 position switch so you can view the current in each tube and easily see if one is becoming weak or running too hot. There is also just one bias adjustment as the tubes are supplied computer matched. The only other adjustment is the bias for the driver stage and that is done on the same meter. |
The 633C initially became popular for its unusual shape and very low cost. Even in small quantities they could be had for $13 including socket. Very unusual to sell a tube with a socket. They actually recommenced replacing the socket with each tube renewal! Now I see M Mathews is offering them for $142 so this is either new production or NOS. I was told that there were warehouses everywhere with this tube stacked to the ceiling. Don't worry, never will run out, I was told. This tube is very close to our 6336 made only by Tungsol and equals 3 x 6AS7 in parallel. It has other features and I recommend take the time to read the 3 paragraph description. The number is strangely similar. https://frank.pocnet.net/sheets/127/6/6336A.pdf I recently worked on a VK-75 amp which uses 4 of the 6C33. When turning the amp over to work in it the tubes keept falling out of their sockets. Replacing the sockets in most 6C33 amps I have seen is not always easy... but it should be. On to the tube I use, the 26DQ5 has all the attributes one can desire in a OTL output tube. Those are
When you get right down to it Sweep Tubes are the only choice for making OTL amplifiers. |
