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6C33C/6H6P amp - which one?

I consider an amp for my electrostats and decided to go for something with 6C33C tube. So I have some questions:
- the esl's are Martin Logan Scenario (hybrid with 20cm bass, impedance varies between 6 in the mid and 3 ohm at 20 kHz,
89 dB/w)
- would I have to use PP or would SE with this tube be ok,
how many 6C33C tubes would I need in SE configuration and how many in PP ( I think about PP and two pairs per channel but
maybe SE would make the deal? )
- if I should go for PP then which mode - ultralinear or triode..?
- what amount of feedback should I consider..?
- how good damping factor and control of the amplifier can I achieve...? (I thought about transformer with higher resistance so
that there's higher damping factor, what value would be ok?)
- is 6H6P a good idea for line stage and for driver(is it really
better sonically than typical 6992...?)


Martin Logans are not efficient enough for SE unlesss you want to use tubes that are not very good for audio. PP is your best bet. I have some stats that while not in the same league and are not hybrids, I use a PP 7868 or an 6L6 family power tube. They sound great. They are combined with a woofer using a passive cross and a different amp and it sounds great. It is funny to me that the center channel of my home theatre sounds better than most systems. If you can bi amp so much the better. The ES need a lot of power and it could be that the woofer does too. An active cross and an amp for each could really improve your sound. An active cross and your favorite tube amps are the best, quickest and possibly cheapest way to get these to sound like you want,
6C33C : thermal (in)stability

if you toy around with the 6C33C (you are holding the tiger by his tail, you know that?), you have tackle the thermal instability of that tube, too. It has 500mA of quiescent current. It heats up and changes its transconductance. And smallest supply voltage changes affect the effective bias, too, due to its terribly high transconductance.

Strong advice, build an active bias voltage regulator with a timeconstant of, say 1Hz, it must be a PI-regulator with zero regulating deviation. The reg has to keep the quiescent current constant by changing the bias voltage.

pieter t

Disabled Account
2009-04-12 11:13 am
6C33 SET 15 Watt Class A amplifer
i think you can modify this one

There is a lot of voltage gain in this design (two 6SN7 stages cascaded).
Then quite some gNFB.
I would load the 6C33 at it's cathode in stead of anode (same output transformer), and skip the feedback. Now you need the voltage gain, and the output stage will be lower impedance (local FB).
Some time ago I heard an excellent 6C33 amp with this topology :)
I know the 6c33 termal instability, but i can find example of anodic current regulator:
I gathered some real life experience lately about 6C33C as I made the OTL on them. I checked and did brake-in for about 20 pcs of NOS tubes (mfg 1967-1991), learning the tolerances issue and behaviour of the tubes. I got only 2 pcs out of them appearing bad (inner shorts), both made in 1960's, which I take as a very good result.

This thermal instability issue is over epmhasized. It appears only when running those tubes near absolute maximum allowed power, near anode glowing. And this is not the way how to use the 6C33C (and other same kind of thick anode tubes, 6C18C and 6C41C for example)!

If you are driving 6C33C at up to 35-40W continious on anode, nothing bad happens and tubes are very stable with fixed bias. Yes, there is the "startup gap" as Romythecat explaines, but it's not a problem, it's one of charecteristics of the 6C33C. If you keep above in mind, you do not need auto biasing.
The 6C33C can take a lot of current in impulses, but do not heat it continuosly at the edge of maximum, leave the headroom. When the anode gets red it bends and this means the end for the tube.

Actually I fell into those 6C33C tubes, my OTL sounds just so much better than all OT amps I had ever made, I left all thoughts about OT anymore at once. And of course - fixed bias gives the best sound.
I suggest to try 6C33C, but try as OTL. It's a very much different but pleasant world in Tubeland.


2002-08-15 10:16 pm
I suggest PP with simple CCS's bias and Toroidal outputs. I have a strong feeling that this will sound every bit as good as OTL but be about an order of magnitude more efficient.

EI's have intrinsic limitations which Toroidals avoid.

Just my tupence worth.

i see this


i can use 6h6p for ecc99 ?
why 65h anodic inductor?? i havent space for another iron core...


2002-08-15 10:16 pm
The huge airgap will create all sorts of losses in the core. This is bad enough in modest current designs (<50mA), but we are talking about 500mA here. High frequency roll off and sloppy bass will be almost inevitable. Worst still - it will cost an arm and a leg to get that sloppy performance.

Notice that the pair who built a 15W SET with these monsters boasted of a soft sound - read as heavily rolled off and compromised sound. There is a huge difference between rolled off, and a unfatiguing sound. If an amp is fatiguing it has an intrinsic stability problem and rolling off the top end is a rather brutal method for solving that.

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The huge airgap will create all sorts of losses in the core. This is bad enough in modest current designs (<50mA), but we are talking about 500mA here. High frequency roll off and sloppy bass will be almost inevitable. Worst still - it will cost an arm and a leg to get that sloppy performance.


500mA? You are kidding :(
SE 6C33's run on 150 to maybe 250mA; a good operating point for instance is 200V / 200mA (40 watts of dissipation).
Your 500mA would not allow more than 120V at the anode not to surpass maximum anode dissipation; please check specifications first before posting nonsense.
The 6C33 is a low Rp tube. Despite the need for the air gap a good quality opt for the 6C33 easily outperforms a 300B in the bandwidth department (high frequency and bass). This statement is out of experience, not based on forum rumours :rolleyes:


2002-08-15 10:16 pm
Even at 200mA an SE output is going to be rolled off- all SE outputs are by design.
PP is the best way to use these to get the best performance. I have not used a 6C33C but have built 6080 PP amps which work on very similar lines. PP using these types of tubes will best a 300B SE amp - and I have that from someone who put their 300B amp into retirement after building a PP 6080 amp (for the cost of 1x300B valve).