Tim Mellow OTL Amplifier Build

[Ray P]

As I mentioned on the 6C33C OTL Thread, I've ordered a set of PCBs for a build of Tim Mellow's 6C33C OTL amplifier and as that generated a little interest I thought I would start a fresh topic straight away.

Here's a link to an article on the 6C33C that includes the schematic for a variant of the amplifier using Russian miniature valves as well as links to the original Tim Mellow article (published in AudioExpress). The page also has some other useful/interesting links relating to the amplifier and 6C33Cs.

http://www.vk6fh.com/vk6fh/triodeamp2013.htm

I'm not blazing a trail with this build, if you search Audio Talk (or do a more general Google search) for 'Mellow OTL' you'll find other projects to build this amplifier.

Why build it? I've had excellent results with some flea-power SE-OTL projects so I thought it was time to try something with a bit more grunt and one that eliminates the output cap that is used with the SE-OTLs, I have some 6C33Cs with suitable ceramic sockets (along with some other bits and pieces) and the Mellow design seems to be well regarded, so why not?

Anyway, the PCBs I've ordered are courtesy of Ares Audio. Whilst retaining the core of the Mellow design they've developed it a little, simplifying the power supply and designing an additional module to 'manage' the amplifier with microprocessor controlled start-up and protection.

The Ares Audio webpage for the amplifier project is here;

http://www.aresaudio.com/otl-power-amplifier.html

and the PCBs are available here;

http://www.aresaudio.com/kit-e-pcb.html

The site is in Italian but Google Translate will take care of it and when I mailed them the reply was in good English.

I've ordered PCBs to be able to build the amplifier as two monoblocks.

More to follow in due course.

[RhythMick]

Watching

[Max N]

Also watching
Keep the builds coming Ray, you're on a roll!

[rowuk]

Are the 6C33C valves reliable enough for OTL use? I have heard them die with a bang...

[Ray P]

From a thread on DIY Audio;

6C33s will hold together OK if you precondition them for abour 72 hours. If you don't do that, you will loose twice as many out of the box!

To precondition any tube, the plate, grid and cathode are tied to the filaments and the tube lit up (no B+ obviously) for whatever time it seems to need. 72 hours seems to do it for the 6C33.

Anyway, there seem to be plenty of fine-sounding and reliable 6C33C amplifiers out there.

[Dave the bass]

To precondition any tube, the plate, grid and cathode are tied to the filaments and the tube lit up (no B+ obviously) for whatever time it seems to need. 72 hours seems to do it for the 6C33.

I've not heard of that process before, whats the thinking behind doing that Ray?

Interested of Dartford.

[Nick]

Slight guess, but with tubes like the 100th that have no seperate getter, you have to do something similar theough in that case you need anode current by running it as a diode as well to get the anode up to rad heat as the metal of the anode is designed to absorb any gasses in the tube fefor hitting it with its normal HT voltage which can be 2.7kv or so.

I am guessing with these its about driving gasses out of the anode and getting the getter to absorb the released gasses so the vacuum is good.

[Dave the bass]

Aah. Yes, ta. You've jogged a brain cell, that process
was chatted about at Owston a longtime ago.

It was the tying of the Anode, Grid and Cathode together that I couldn't quite understand. Like why that would be done or necessary in the quote Ray linked.

[Nick]

I can only assume its to prevent any contact potential from developing on those elements. But I don;t know for certain.

[Ray P]

I've not heard of that process before, whats the thinking behind doing that Ray?

Dave, I posted that in response to the comment questioning the reliability of 6C33Cs in OTL amp. It was originally posted by the guy behind Atmasphere OTL amps so he has a modicum of experience with OTLs and 6C33Cs so I just took it at face value for now - I ought to follow it up at some point to understand the process but I imagine Nick won't be wrong.

[Ray P]

And in other news...

The chap from Ares Audio sent me mechanical drawings of the PCBs so I've started to think about setting out some metalwork - the main priority being to give the 6C33Cs plenty of ventilation and to space them away from everything else.

Also, I've just had notification that the PCBs have been shipped.