6C33 Vacuum Tube

[Gerry E.]

Doc, PJ:

I'm curious if you ever considered the using the 6C33 tube in one of your amp kits?  The obvious advantage is that you can get about double the power of a 300B in a single-ended design.  OTOH, maybe the required voltage/current is too dangerous for a kit?

The reason I'm asking is because I recently came across a graph that extols the virtues of this tube (see attachment).  Maybe that graph is an oversimplification and there's more to it than that.  Thanks!

Gerry

   

 

[Paul Joppa]

Did you notice the expected lifetime of 750 hours?  :^)

I know PB built one some years ago, maybe he will post his experiences.

I've looked at it several times but never been happy with it. The linearity is fairly poor, making it hard to get the expected power without resorting to feedback. And the data sheet implies that longer life can be obtained by running it at 30 watts maximum dissipation. Reportedly, there are few sockets available, and most of them can't really take the 6 amperes of filament current - the socket burns out faster than the tube. Gain is low, like a 6080, so the driver needs more voltage than the power tube, which complicates the power supply.

In the end, we offer the Kaiju which can be bridged into a 16-watt monoblock. If you've followed the recent club meetings you may have seen the experimental 50-watt SEP amp using four 6550s per channel. No idea yet whether that will become an actual product, but it does show that we are always exploring new ideas.

 

[Paul Birkeland]

(I hit "post" right as PJ posted, but I'll leave the overlap anyway)

I've built a 6C33 SET amp, so I can offer some commentary.

First and foremost, a 6C33 is not a directly heated tube, and therefore suffers a little bit in terms of linearity when compared to a 2A3 or 300B.  In my own project, I found that a little bit of global feedback helped straighten that out a lot, but that imparts its own issues that you have to be prepared to deal with.

Another strike against the 6C33 is the heater current required to make the tube operate.  I had to be very, very careful when selecting tube sockets for the 6C33, as many of them would not support the heater current requirement.  Each 6C33 needs about 7A of heater current, and even my 6.3V/20A dedicated filament transformer groans a little bit when I turn the amplifier on! 

On the good side, the 6C33 can be operated at pretty safe voltage levels and doesn't need a particularly insane amount of high voltage current.  It is also pretty easy to make a nice 600 Ohm/20W gapped output transformer, but the size of the requisite parallel feed capacitor makes series feed a more sensible choice.

Ultimately, since the 6C33 isn't directly heated and needs feedback anyway, the more common approach is to run a 6550 or KT-88 at near maximum operating conditions for roughly the same power.  The same could ultimately be done for a 300B with reduced tube lifetime expectations.

As far as 6C33 lifetime goes, I have found them to last much, much longer than 750 hours (I'm still on my first pair, and the amp is almost 10 years old, though it doesn't get a lot of use now).

-PB

 

[RayP]

Since I am listening to my 6C33C amp right now, I thought I might comment.

It's not an easy tube to work with. The socket and heat issues are significant. You might like to read more about it at Romy the Cat's site.
http://www.goodsoundclub.com/Forums/ShowPost.aspx?postID=6140

Without being an expert like Paul and PB, I suspect it would be challenging to develop a kit using this tube. Aside from the significant heat issues, a typical parafeed Bottlehead circuit would need a large and presumably expensive plate choke to handle the current.

I have also found that the tubes I have been getting from Russia and Eastern Europe have been less than reliable. Usually one out of two is noisy on arrival, even with cleaning treatment.

Still, I like the amp and enjoy listening to it a lot. If you want to know more about it, read http://audioratbag.blogspot.com/2015/01/cat-vomit-special-6e5p-6c33c-parafeed.html

Like most of my amps, it is designed to solve another problem or investigate another design technique rather than produce the highest quality sound, so I am not recommending that you follow my example.

ray

 

[Doc B]

You might like to read more about it at Romy the Cat's site.

I'll will post a different opinion here. Roman Bessnow is a troll. He has long since been banned from my forums.

 

[Paul Birkeland]

Without being an expert like Paul and PB, I suspect it would be challenging to develop a kit using this tube. Aside from the significant heat issues, a typical parafeed Bottlehead circuit would need a large and presumably expensive plate choke to handle the current.

Still, I like the amp and enjoy listening to it a lot. If you want to know more about it, read http://audioratbag.blogspot.com/2015/01/cat-vomit-special-6e5p-6c33c-parafeed.html

The plate choke actually isn't too bad.  If we assume that you're using a 600 Ohm parallel feed transformer, than a 5-10H choke will get the job done.  The One Electron PRC-1 is 5.5H at 230mA for $85, not too bad.  The Magnequest BAPC-300 will also get the job done (what I ended up using) with 12H of inductance up to 300mA.  With the low impedance of the 6C33, using a power supply choke as a plate choke is potentially an option, so a Hammond 193Q is also worth considering.  Having a plate choke will let you use a meager power supply voltage that isn't much more than 300V (assuming also that you go with fixed bias, which I would also recommend).


The real issue is that for my amp, the parallel feed capacitor should be roughly 40uF, which is a little crazy.  I would strongly suggest sweeping the amp you linked to with the 4uF capacitor, then with a 100uF capacitor.  I suspect 4uF is way too small.

 

[RayP]

Interesting. I might try the plate choke sometime.

I'm not too worried by the parafeed cap being too small since I bi-amp and cross over about 80 hz.

ray

 

[4krow]

  I can only say this much for sure. I look at some of the mystical/wild tubes offered out there, and wonder which would be best to use for audio. There is no doubt that quite a few of them would be worth the effort, and yet, all that there is to consider boggles my mind. Not enough time/knowledge, too many tubes.

 

[Gerry E.]

As the OP of this thread, I wanted to mention that I got to hear a single-ended 6C33 amp in my system just a couple of days ago.  It was an expensive Tube Guru unit.  It uses four 5U4-G (or Russian equiv.) rectifiers!!!???  The extra power was nice but I'm not sure the sound was as smooth as my Paramount 300Bs.

In the end I passed on it, mainly because it didn't have 4-ohm taps (neither do my Paramounts with upgraded MQ iron).  I need that for my new speaker (woofer) configuration, something I wrote about in another thread.  Instead, I picked up a pair of old Electraprint DRD 300B amps from a local acquaintance.  He was virtually giving them away and not only do they have 4-ohm taps, they have 2-ohm as well. 

It may be crazy but who knows - I could bi-amp with the DRD amps on the woofers and the Paramounts on the mid-range/tweeters.

Gerry           

 

[Paul Birkeland]

It uses four 5U4-G (or Russian equiv.) rectifiers!!!??? 

I wonder if it was setup for a tube rectified bridge.  IMO, with such vicious current demands, I stuck with the UF4007 for my amps.

 

[Gerry E.]

I wonder if it was setup for a tube rectified bridge.  IMO, with such vicious current demands, I stuck with the UF4007 for my amps.

Here's a quote from the builder (his emphasis): "I made a FULL TUBE RECTIFIERING CIRCUIT there is not any DIODE!!  NOBODY Makes the same On the Planet!"

Gerry

 

[Doc B]

Here's a quote from the builder (his emphasis): "I made a FULL TUBE RECTIFIERING CIRCUIT there is not any DIODE!!  NOBODY Makes the same On the Planet!"

Gerry

Ah, caps and exclamation points. It must be true.

 

[xcortes]

An alternative truth maybe?

 

[mcandmar]

"I made a FULL TUBE RECTIFIERING CIRCUIT there is not any DIODE!!  NOBODY Makes the same On the Planet!"

How can somebody have such outdated ideals yet write like a millennial. The mind boggles.

 

[2wo]

But I read it on the internet...John

 

[Paul Birkeland]

You can't get DC without at least one diode...

 

[RayP]

Since it appears that the amp is made in Hungary, I think we can give them a bit of leeway with their use of English.

By the way, here is a partial review.
https://www.stereophile.com/content/another-green-world

However $10k does seem to be a bit excessive even though the casework looks beautiful.

Incidentally, take a look at their GU-81 amp with the yellow coverings for the B+ which presumably is well over 1000 volts. Definitely too scary for me to even contemplate building.

http://www.tubeguru.eu/poweramps/

Another example of the GU-81 -
https://arstechnica.co.uk/gadgets/2016/03/icon-audio-mb-81-russian-valve-amp-200w/

Somehow I don't think there will be a Bottlehead headphone amp using these tubes in the near future.

ray