6C33 design comments?

[nickds1]

Hi,

What do people think about this design:

Ernst Rößler's 6C33 monoblock

It's used in this design to great visual effect: Impressive (but a bit loud for me!)

Being a monoglot Brit, I used Google Translate to render the German into rather amusing, but quite useful, English.

Is this a reasonable design to play with?

Cheers

[pedroskova]

I can't comment on the schematic, but for a PP 6C33 with 6v heaters, you'll need somewhere around 13 amps per channel to light them up ...which is great for winter time. Your heater transformers will be bigger than your B+ transformers.

[jane]

It looks like the time constants in the RC couplings are tight clustered (220nF*270k and 470n*180k). Watch out for phase shift inside the NFB loop!

Jan E Veiset

[SY]

A couple things jump out at me, one minor, one potentially fatal.

Minor: the phase splitter is not a particularly great one. I don't know why the designer chose one that's inherently unbalanced, has unequal time constants on each side, and has asymmetrical distortion side to side.

Fatal: The use of fixed bias for output tubes with a very high transconductance is a near-guarantee of output stage runaway and failure at some point.

[tubetvr]

Fatal: The use of fixed bias for output tubes with a very high transconductance is a near-guarantee of output stage runaway and failure at some point.

Yes, that is a risk but I don't agree that it is "a near-guarantee of output stage runaway and failure at some point" it has not been a problem for me neither in my SET with 6C33C nor with any of my OTLs. I know that it can be a problem with tubes like 6336 or 6AS7 as I have had that experience amy years ago when experimenting with OTLs.

Regards Hans

[SY]

I've got a couple of dead 6528s from my life-lesson.

[Miles Prower]

What do people think about this design:

Short answer:

Elaboration:

The phase splitter is terrible. This sort of circuit may be acceptable for the simplest projects where fidelity isn't a prime concern. Otherwise, it is unacceptable due to its inherent imbalance, and especially the unbalanced harmonic distortion which is endemic to this entire family of phase splitters. It also is not good that the inevitably required AC balance pot is connected to the 300Vdc rail. These "pots" aren't known for their insulation. A shocking experience awaits here An LTP or cathodyne would have been the superior choices here.

This splitter stage is connected to two, separate voltage amps. Even though these are two triodes in one bottle, this does not guarantee balance between sections, even if perfectly matched out of the box, they will not stay that way over time and useage. Another source of AC imbalance and unnecessary distortion. A differential should have been used here. Possibly, inadequate grid drive for the finals.

RC coupling into fixed biased finals: combined with gNFB guarantees wicked clip behaviour. A transient overshoot will turn on the gk parasitic diode and increase negative grid bias on the finals. That excessive bias will persist until it leaks off, and while it does, the finals operate in a less linear manner. Needless to say, this does not help the sonics at all.

Fixed bias on 6C33Cs -- a big no-no, and definitely not recommended by the manufacturer. These triodes with the uber low R(p)s and high g(m)s are as delicate as germanium power transistors when it comes to DC stability. You either cathode bias, or use a DC servo, or you'll have to watch 'em like a hawk so's to be able to cut power when (not if) they go red plate.

It looks like all the RC couplings cluster their phase shifts around roughly the same turn-over frequencies, a big invitation to instability. Also, looks like the gNFB is excessive here.

There are better designs out there. I'd pass (no pun intended) on this one.

[tubetvr]

Fixed bias on 6C33Cs -- a big no-no

you'll have to watch 'em like a hawk so's to be able to cut power when (not if) they go red plate.

How many 6C33Cs have you destroyed in this way? You must have a lot of experience of this tube as you are so sure about what will happen in any design that uses it with fixed bias.

Regards Hans

[JoshK]

So that you actually gets some idea of what circuit you should build, not just how bad this one is, someone previously recommended that I try the crystal palace topology from Morgan Jones. I took a long look at it and it seems like it would work well. Just need to modify the output stage for the different tube accordingly.

[nickds1]

Originally posted by JoshK
So that you actually gets some idea of what circuit you should build, not just how bad this one is, someone previously recommended that I try the crystal palace topology from Morgan Jones. I took a long look at it and it seems like it would work well. Just need to modify the output stage for the different tube accordingly.

I've been stashing up 13E1s because one day I want to build the Crystal Palace (the floor in my listening room has recently been re-enforced and I've new three-phase feeders into our plant room...) I'll have a look at MJ again tonight.

Problem I have is that this'll only be my second amp (previous was a Bevois Valley), and I just love the look of 6C33Cs. The only tube I like more is the 13E1 (which is built like a tank). 6C33s are CHEAP and I would like to build a nice amp with them too...

I'm still trying to work out how to test them on a Hickok 6000A or 539C.