3W 6C33B-C SE OTL schematics ready

[vincent_brient]

Here is a schematics of a 3W 6C33B-C SE OTL amplifier for midrange compression driver and tweeter, but the frequency range and power can be extended:

http://vincent.brient.free.fr/otl3w_eng.htm

[woody]

Nice work! I am about to build something similar but with
3 6c33c-b in stead of 2.

Realy a 220 mh air core inductor need not be all that large.
~ 13 lb of 16 AWG wire on a 4 inch core with a 2 inch winding legnth ends up being about 7 inches in diameter and 7 ohms.
This form factor could probably be inproved on.

How much currrent are you running on each of the 6c33c tubes?

Keep up the good work.

[astouffer]

Any particular reason for using an air core inductor?

[vincent_brient]

Originally posted by woody

How much currrent are you running on each of the 6c33c tubes?

I am running them at 450mA (500mA when stabilised) and the plate dissipation is 40W, which is not too high since it can handle 60W.

[vincent_brient]

Originally posted by astouffer
Any particular reason for using an air core inductor?

It is more linear that an iron core inductor. I tried using a iron core inductor and sound was not as good.

[woody]

I just fired up the output stage on my 10 watt otl. Just the output stage as the driver stage isn`t wired yet but just using the
preamp output to drive the 3 6c33c-b tubes probably gives me
~100 mw of raw power all battery powered as seen in the picture. I quess next thing is a real power supply those batterys
can be a pain!

Woody

[vincent_brient]

Originally posted by woody
I just fired up the output stage on my 10 watt otl.

Did you use the output stage shown on my schematics? Battery use shoudn't be a problem.

[woody]

Vincent, no I am trying to find 2 sets of 3 tubes that measure
about the same then use a 5 ohm resistor on each cathode to
get them matched a little closer and then use a common 35 ohm
resistor as the load. Your`s is a better aproach I am just lazy!
My comment about batterys was referring to how inconvient
it is to recharge them not to mention the weight!

[Mark A. Gulbrandsen]

Come on... you guys need to get the power output at least up to the same amount of watts those 6C33's filaments dissipate....
The 6C33 ias a great tube that can do so much more... but 3 watts output is such a waste of energy.

Mark

[Alastair E]

Perhaps Im a little strange, (More than likely!) But I have an OTL, built around two 6C33-CB valves, in Circlotron style, and using basically the same drive scheme as Allan Kimmel used with his EL519 tubes, apart from the MOSFET G2 drive on the O/P bottles he used.

The heart of Allan's original Circlotron amp employs Differential Amplifiers in each signal path of the O/P tubes. One input is the sig from Phase-split and the other, the O/P from the power-tubes...

This scheme uses 100% neg feedback only around the O/P stage employing differential amplifiers (6SL7 tubes) for each 'Half' or 'side' of the O/P stage, both fed from the phase-splitter. The 6C33C-B are themselves driven by DC coupled parralleled 7044 triodes, similar to ECC99 or 5687 types, one bottle per side from the diff-amps. Bias is set by varying the grid volts of the driver-tubes. The drivers themselves are run from a plus 200V and a Negative rail of 170V. I plan to try a couple of MOSFETS in place of the 7044 just to see the difference, sometime....

Currently, Im running 200V on each half of the circlotron (driver and 6C33 Plate-supply) and a quiescent current at only 50 mA per tube.....

Thats Only 10W Plate dissipation per tube....(Damnd things stay practically Cold!)

Should sound absolutely awfull I hear you say, and I suppose it should.....

(In fact, its possible to reduce the bias-current to only 25 mA before audible effects of distortion are evident-Tests with the 'Scope, indicate that at 35mA per tube that distortion is negligable, so I chose 50mA to be on the safe-side....)

It sounds just brill. Ive tried at currents as high as 300 mA on the O/P tubes, There is No real discernable difference in sound,-At least to my ear, apart from some slight hum due to my less than perfect Circlotron power-supplies being taxed to the limit at constant 300mA loads--Really should do something about that..........

The Diff-Amp in the drive-scheme take care of the drive and O/P impedance a treat, even at hideously Low plate-diss/ high plate-impedances!

An 'Efficient' OTL IS possible!

Full O/P power, is a little over 40W, with a wonderful Big and detailed sound, Possibly could squeeze a little more if the PSU was stiffer...

(Working as I do in a Renewable-Energy sector, I couldnt 'cope' with the horrendous power waste that the usual OTL's seem to demand--Most seem to double as room-heaters! )