schematics for 6C33 single ended OTL?

[vincent_brient]

I would like to build a pair of 6C33 single ended OTL monoblock amplifier, no feedback. Is there an complete available schematics?
I have found http://www.super-otl-audio.com/Schematic.htm but components values are not given.

I would like to use it for an Altec 604 horn tweeter (16 ohm). Power needs to be at least 5W and outpout impedance can be as high as 50 ohms because it improves this tweeter performance. I also need at least 35dB global gain.

[tubetvr]

Hi,

To build a cathode follower OTL is probably the easiest you can do, even though the efficiency will be terrible low. It is a very straightforward job and probably the only thing you need to think about is to ensure that the driver stage give enough voltage, using a SRPP as driver is probably a good idea. A better idea is to use an active load or to build a SRPP as output stage.

Even better, (and the only really practical way) is to build a push-pull OTL working in class AB, this would give you reasonable output power, lower distortion and better efficiency, see here for one schematic http://www.tubetvr.com/otl.html

BTW, Why don't you want to use feedback?

Regards Hans

[vincent_brient]

Thank's.
I tried to use no-feedback and single ended to get the best sound quality, especially for a 105dB/W/m treeble compression driver.

[Jeb-D.]

If going single ended i'd suggest using a solid state constant current scource for the cathode load. So there wouldn't have to be as many tubes. The heaters on those 6c33c's are power hogs. it would increase the overall efficency of the amp, decrease the size and increase the performance of the amp.

A Class A push pull could be good too. Class AB are good for power and efficency but not the greatest for sound, IMO.

A class AB with feedback, well you might as well be building a consumer grade solid state amp. It may have good bass but that would probably it.

[vincent_brient]

"solid state constant current scource for the cathode load"

Do you think that the curent source technology can affect the sound? I mean: do I risk to get a transistor sound when using a solid state current source? If not yes indeed the efficiency and reliability would be improved. I also would need a big heatsink for the ~50W current source.

[tubetvr]

A class AB with feedback, well you might as well be building a consumer grade solid state amp. It may have good bass but that would probably it.

You have obviously never heard an OTL amp have you?

Regards Hans

[Jeb-D.]

The solid state constant current source will not give you a transistor sound. The transistor is not responsible for signal amplification it just supplys a constant current. It would be the same as using a tube as a CCS except you can get away with a smaller supply rail with transistor. It will allow you to extract the most out of the single ended amp. A choke will work, but a while back I was looking for one for this application and was unable to find one. It would have to have high inductance & DC current ratings in order to have good bandwidth.

Chances are you would have to parallel a couple because of the high idle current. If designed right, probably 1 transistor to supply every 2 tubes. Since alot of power BJT's are rated at aprox 120W which is twice the 6c33c's rating. The cathode follower topology does have negative feedback whithin itself, but it's much more natural compared to global feedback.

[Shoog]

The secondary of a microwave transformer works well as a choke. I have one in my powersupply which happily copes with 1/2A of current. It gets hot though because it burns 25W of heat. I have heard of people using them as plate chokes, but cannot comment on how they sound. I might try it myself with my 807SE which currently uses a TT21 tube CCS.

Shoog