Hi all,

I'm an electronic tech, I worked a litle with tube, more with transistors.
I'm not an enginner but I do play arround with circuits

I would like to desing from different schematics an OTL Class A amplifier
arround 50 watts, I have a few questions...

About feedback, Were do we draw the line ? Do we need feedback in our
tube amp ? How do you calculate it ? Is it better to have some or not ?

Using Class A, Do the tube worn-out faster ?

What do you think about the 6c33 tube ? Could it make an nice HI-Fi system
?

Mario

"Mario Tardif" ha scritto nel messaggio
. ..

Hi all,

I'm an electronic tech, I worked a litle with tube, more with transistors.
I'm not an enginner but I do play arround with circuits

I would like to desing from different schematics an OTL Class A amplifier
arround 50 watts,

WOW, that's BIG!

I have a few questions...

About feedback, Were do we draw the line ? Do we need feedback in our
tube amp ?

For an OTL, definitely yes, or You'll need dozens of power tubes to get to
the output impedance required.

How do you calculate it ?

See above, as much as needed according to FB eqs. to get Zout down to say 1
Ohm

Is it better to have some or not ?

Using Class A, Do the tube worn-out faster ?

No, unless You push them too close to the power dissipation limits. Keep
some 10% lower and You'll have no problems.


What do you think about the 6c33 tube ?

Great tube. I used it in a SE class A unit, but with transformer coupling
(900 Ohms). It is one of the obvious choices for OTLs together with 6S41,
6S19 and 6AS7/6080. A second way is using TV tetrodes ie. EL509,
nevertheless the 6S33S is the apparent first choice.

Could it make an nice HI-Fi system

If You manage to design a good circuit (and OTLs are a real mess) it surely
will.

?

Mario

Mario Tardif wrote:

Hi all,

I'm an electronic tech, I worked a litle with tube, more with transistors.
I'm not an enginner but I do play arround with circuits

I would like to desing from different schematics an OTL Class A amplifier
arround 50 watts, I have a few questions...

About feedback, Were do we draw the line ? Do we need feedback in our
tube amp ? How do you calculate it ? Is it better to have some or not ?

If you want 50 watts of class A with tubes in an OTL amp, you will need an
enormous
number of tubes because the plate resistance of tubes is very high
compared to the speaker load.

Nearly all OTL amps are class AB1 or AB2, with very little class A power.
They mostly use a lot of applied NFB, 40 dB isn't unusual.

If you want 50 watts in class A with tubes, forget OTL
unless you want an extremely inefficient amplifier
compared to what you could get with an OPT,
which will allow you to match the load properly and avoid using
all the necessary NFB if any.

Even with an OPT, to get 40 watts from tubes in class A means that plate
dissipation
will have to be about 110 watts at idle if you use tubes like KT88/6550,
and perhaps a lot more if you use transmiting triodes.

So a quad of KT88/6550 can make 40 watts of class A in UL with an OPT easily,
and need only 16 dB of NFB to get a good DF and low thd.



Using Class A, Do the tube worn-out faster ?

Yes, unless you run the tubes at only 2/3 the rated maximum continuous
dissipation.
So a 6550 has Pda = 42 watts, but it lasts ok if run at 28 watts.
4 will give 112 watts of Pd, and if efficiency maximum in UL = 40%,
expect a maximum of around 45 watts of class A.


What do you think about the 6c33 tube ? Could it make an nice HI-Fi system
?

Its a very nice tube for hi-fi.

It is used for OTL, in the circlotron circuit,
and you'd need 4 of these tubes to get reasonable AB performance
with a lot of NFB since there is very little class A power.

One guy I know abandoned OTL using 6C33 after blowing up a tube.
He uses a pair in parallel SE with an OPT and gets better sound he says.

Tubes are low current swing, high voltage swing devices, and they prefer an
OPT to deliver power properly; the OPT is like a gearbox that converts the
power from an engine with high revs and low torque to the rear
wheels....VVVRRRMMM!!....
and away we go, ( with an OPT ).

Patrick Turner.




Mario