6C33C SE-OTL

[Ray P]

As some of you will be aware, I like my flea-power OTL amplifiers, currently in the form of my Transcendent 300B SE-OTL amps. I have been thinking about a project to rebuild the Transcendents but in John Broskie's latest TCJ a 1watt 6C33C OTL design caught my eye (about 20% down the page);

http://tubecad.com/2018/04/blog0419.htm

It just so happens that I have one of Broskie's Aikido boards and 6C33C aren't too expensive so I'm thinking it might be good to do something different.

Any comments/observations before I get too carried away?

[pre65]

Might be worth PM ing ACPEN of this forum. David has experience of OTLs with the 6c33s and it's smaller brother.

[SimonC]

I'm a fan of the 6c33, but its a tricky beast to drive.

I've never tried them with an aikido front end, been on my list to do for a long while now. I messed around with a few variations on a c3g driven stage, and that just wasn't up to the job. You need something that can swing both voltage and current. JB's aikido variants that are effectively a gain stage + cathode follower would be my starting point these days.

Don't under estimate the power supply, they might not need mega-voltages to make them sing, but they do need (by tube standards) massive amounts of current. This tends to make the power supplies 1) very heavy and 2) very hot. They are great tubes for keeping your workshop warm in winter with

The other watch out is valve sockets, these oxidise, lose spring pressure and are generally a pita to keep working. If you can find some 'aftermarket' ones at a sensible price I'd get them. It will save you from hours of frustration trying to keep the original Russian ones working from one day to the next.

6c33 OTL - Bring it on, I'd love to hear what these tubes could do OTL

Simon C

[andrew Ivimey]

That about sums up my experience too.

Nevertheless - Onwards!

[Ray P]

Thanks chaps, that's helpful and encouraging; I'm going to start collecting up bits and pieces with a view to breadboarding something.

Simon, thanks for the steer on the sockets, I've seen similar comments elsewhere. These look to be a good solution (at a price);

http://www.partsconnexion.com/CONNEX-73297.html

I've also found a current sink kit that might fit the bill for the 500mA CCS requirement on the K&K Audio site (see 'Two Terminal Cathode Current Sink') and avoid using paralleled LM317 devices;

http://www.kandkaudio.com/other-kits/

I'm going to mail JB (hope I get a reply as he's notorious for ignoring mails) to see if he has more information than was published in his article, maybe a power supply. If I need to, working out a power supply from scratch will be new to me and quite a challenge; even basic things like how much current would be required so I may need a bit of help.

One thing in my favour with something like this is using 15ohm Lowther units.

[ed]

I was looking at this Ray, reminding myself that it's cropped up before...

like before I can't help thinking this is just a valve variation on the single ended mosfets that I was playing with sometime back...I kept coming back to the remarks that people were making about the heaving great caps in the signal path. Personally I don't think I ever noticed any problems but there were lots of mutterings...

I'm probably missing something obvious, as usual.

[Ray P]

Ed, I guess you're not wrong about the concept...

As for mutterings, I think we should just ignore them because its the result that matters. My 300B SE-OTL amps that were liked at the last Owston meet have have a similar population of caps.

[andrew Ivimey]

I'm happy to contribute a pair of these brutes. I'm sure there is a couple in the shed...somewhere.

[ed]

I'm sure there are..

swidt snigger

[Ray P]

I'm happy to contribute a pair of these brutes. I'm sure there is a couple in the shed...somewhere.

Would be glad to take up the offer if you can find them once you've moved the couple on.

[Ali Tait]

Ed, I guess you're not wrong about the concept...

As for mutterings, I think we should just ignore them because its the result that matters. My 300B SE-OTL amps that were liked at the last Owston meet have have a similar population of caps.

Certainly were, by me at least. The quality of the treble is the best I’ve heard I think.

[andrew Ivimey]

...which is pretty weird when all I got out of the 6s33 SE amp I brought to Witham one time was totally splendid up to, well, less than 1kHz anyway and didn't pursue the matter. What I did do though was to buy some appropriate switched mode power supplies that could supply 12 volts at 'n' amps where n=as many amps as possible without the valve sockets cracking up in the furnace. Ah well, another unfinished project.

[pre65]

...which is pretty weird when all I got out of the 6s33 SE amp I brought to Witham one time was totally splendid up to, well, less than 1kHz anyway and didn't pursue the matter. What I did do though was to buy some appropriate switched mode power supplies that could supply 12 volts at 'n' amps where n=as many amps as possible without the valve sockets cracking up in the furnace. Ah well, another unfinished project.

I seem to remember some wag saying it was a very good bass amp.

[andrew Ivimey]

might have been good for a sub woofer then; what a dog!

[simon]

I do remember that amp pretty clearly Andrew (that's really quite good for me now considering how many years ago it was!) and it really did play tunes in the bass. I'm about to play with £30 of class D ebay subwoofer amp, but can't help thinking about your amp...