Digital guy comes to tube land. I tried my first amp and like the
sound.

If anyone is interested: http://wosch.teratronik.com/amp/amp.html

Maybe you RATs can point me to all those big blunders I made...

I shamelessly posted a few questions onto that webpage and would
be happy for a few hints and tricks.

Keep glowing...
--
Olav Wölfelschneider

Hi RATs!

Welcome! You did the most important thing right: Got it working and listened

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

Hi Olav,

Maybe you RATs can point me to all those big
blunders I made...

Hmmm, let's see ...

- you feed the screen (g2) from an OPT tab more
positive (nearer to B+) than the plate. This
is very unusual. Is this a drawing error or
intentional? If intentional, what's the objective
behind that?

- the cathode resistors (R3, R7) of the ECC83 systems
seem to be extremely high (probably since you wanted
to reduce the gain of both stages). What plate current
do both ECC83 systems draw (each)?

At least for the gain/driver stages there probably are
better alternatives than using high gain (mu=100) sections
and then "destroying" the gain by such extreme plate/cathode
resistor ratios. At least on first sight I see no point
in using two such stages after another when a single
one with a less mu type triode section would do instead.

Or am I missing something?

Tom

--
If in doubt, mumble.

Tom Schlangen wrote:
Hi Olav,


Maybe you RATs can point me to all those big
blunders I made...


Hmmm, let's see ...

- you feed the screen (g2) from an OPT tab more
positive (nearer to B+) than the plate. This
is very unusual. Is this a drawing error or
intentional? If intentional, what's the objective
behind that?

This is usually called "Ultra-Linear" (UL), although originally UL
referred to a specific tap ratio (43% if I remember correctly). It
applies local feedback to the screen grid, and basically acts as a
topology halfway between normal pentode operation, and triode operation
(you could view triode operation as UL with 100% tap).

- the cathode resistors (R3, R7) of the ECC83 systems
seem to be extremely high (probably since you wanted
to reduce the gain of both stages). What plate current
do both ECC83 systems draw (each)?

At least for the gain/driver stages there probably are
better alternatives than using high gain (mu=100) sections
and then "destroying" the gain by such extreme plate/cathode
resistor ratios. At least on first sight I see no point
in using two such stages after another when a single
one with a less mu type triode section would do instead.

There may well be lower total distortion using this approach, since each
stage will have a lot of local feedback to linearize the response.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

Hi Fred,


This is usually called "Ultra-Linear" (UL),
although originally UL referred to a specific
tap ratio (43% if I remember correctly).

Oh, Fred I, would simply call it pentode mode;
there's not a little bit UL in this circuit ;-)

Tom

--
The first rule of magick is simple: Don't waste your time
waving your hands and hoping, when a rock or club will do.

Hi Fred,

At least for the gain/driver stages there probably are
better alternatives than using high gain (mu=100) sections
and then "destroying" the gain by such extreme plate/cathode
resistor ratios. At least on first sight I see no point
in using two such stages after another when a single
one with a less mu type triode section would do instead.

There may well be lower total distortion using this approach,
since each stage will have a lot of local feedback to linearize
the response.

This may be, but:

A certain ammount of K2 distortion even may be desired
to cancel out the distortion of the power stage. And
probably the EL504 (a TV sweep circuit power tube)
generates quite some distortion in pentode (not UL)
mode.

Without having listened to such a design of course,
just from viewing the schematics, I still think that
the gain of roughly 4 x 4 = 16 of both stages in that
topology more easily and straight forward could be
obtained from other circuits with less parts (e.g
coupling caps) with more "fitting" tubes, than with
two capacitor coupled grounded cathode stages using
a high mu double triode like 12AX7/ECC83.

I mean, a gain of 16 really *screams* for other tubes
than a 12AX7/ECC83, and for a single stage with
a medium mu triode (even without a cathode bypass
cap).

At least, this is a rather unconventional way of
doing things, so maybe we both miss the point - lets
see what the author had in mind if he cares to elaborate
on this circuit and why he used this topology and
what its benefits are.

Tom

--
Life: All in all a bad game, but graphics are really good.

Olav Wölfelschneider wrote:


Digital guy comes to tube land. I tried my first amp and like the
sound.

If anyone is interested: http://wosch.teratronik.com/amp/amp.html

You may want to add in parallel a cap across R12, the 1K ultralinear
tap resistor. G2 needs a low impedance source to operate in ultralinear
mode properly. Not sure if 1K is low enough or not. G2 will draw some
plate current (which is varying according to what the control grid is doing)
and this varying current will (in a nonlinear fashion) vary the voltage
on g2
in addition to what the ultralinear tap and quiescent g2 current is doing.

"Tom Schlangen" wrote in message
...

This is usually called "Ultra-Linear" (UL),
although originally UL referred to a specific
tap ratio (43% if I remember correctly).

Oh, Fred I, would simply call it pentode mode;
there's not a little bit UL in this circuit ;-)

Hi Tom

I've seen something like this done locally, selecting the taps on a similar
cheap PA output transformer to make a P/P ultralinear tube transformer (a
use for which it was _never_ intended).

If you look closely at the schematic, the screen is fed at some part of the
total primary impedance, but whether it's the usual ~43% is beyond my
knowledge, or maths ability (I suspect it's a bit higher).

It is, however, a basic UL circuit .....

Cheers

David

Hi Olav,

regarding the question on your homepage about
NFB and oscillation:

Obviously you got positive feedback - so simply
ground the other end of the OPT secondary winding
and get the feedback voltage from the (now) not
grounded end.

Tom

--
The first rule of magick is simple: Don't waste your time
waving your hands and hoping, when a rock or club will do.

Hi Olav,

You give me way too much credit. I still have much theory
to learn.

Hey I am a beginner in tube DIY myself. Just don't be
confused with all these fancy concepts out there to
consider, but stay with common sense and stone-age
rules like "best amplifyer is a wire with gain".

I think most of tube circuit benefits (including
good sound) are comming from the sheer simplicity
of very few active elements in simple circuits.

Think about why e.g a 6SN7 is considered to be
the most linear active device in audio technology
invented and known so far in universe :-)

Where there is no fuss, you don't have to cope
with correcting it. Keep it simple, understand
it and learn from it.

At least these directives worked for me :-)

Anyway the driver stage was once from a historic
pre-amp design where there was a tone-control
network between the two stages.
Of course, this ate away a lot of the gain, so
they took two stages.

Ah, now this makes sense.

I built the tone network, didn't like it and
removed it. Now it seems I have too much gain
on my hands.

So you have at least two options to improve on your
circuit now:

- Use NFB to reduce gain by intention to gain more
over-all linearity and less distortion instead
of gain (just try it, but don't misuse it apply
too much to cure other design faults. 6-9 dB
should be enough for a "common" two stage SE
design using "common" valves and circuits. EL504
probably will need more than that to be tamed
together with the OPT you use

- go for less gain (which means go for the gain you
really need) without NFB, using less parts, and
so on.

You have to try out to find the way you prefer
sonically and technologically wise. As a beginner
myself with only few amps built so far I prefer
to just keep it simple, so I have a chance to really
understand what is going on.

I'm thinking about trying the SRPP stage from
http://www.ndh.net/home/kboehm/kt88.htm instead.

Ah, Mr. Boehm, from the same very little provider
I have my own home at. The circuit shown at Mr.
Boehms site is quite generic, but of course there
is absolutely nothing wrong with that. Try it,
measure it (as he suggest, with diffent tube
types) and understand how it works! Be
aware that you probably have to lift heaters so
you don't exceed max. heater/cathode of the
tube you intend to use.

But, you should consider wether a SRPP circuit
suits your intention _besides_ being know to be
okay within certain design rules and margins
in the first place.

In my first project:

http://www.ndh.net/home/schlangen/ro...ecl82pset.html

I had an ECL82 triode section to spare per channel,
so I could have used a SRPP circuit instead of the
simple grounded cathode stage I used in the end.

Or I could have built a grounded cathode stage with
a cathode follower (but what for? no hard load
to drive ...) or a cascode or a constant current
draw pair, or what else one could do with two
similar triode sections.

I simply stayed with the single triode section
because it worked nicely and I did understand its
operation at that moment.

Not need for "fancy" stuff like SRPP there in this
circuit at that moment, which I wasn't able to
understand at that time anyway.

Some months later and after trying other things
(like SRPP, cascodes, or simple paralleling of
triode sections) on the bench, I designed and
built the second amp:

http://www.ndh.net/home/schlangen/ro...t/el34set.html

and although being eager to apply all what I have
learned from the first amp and all experiments
thereafter, I stayed with simplicity and just
paralleled two triode driver sections because
this approach just does and _delivered_ what was
needed. And the resulting amp simply sounds gorgous.

This doesn't keep me from experimenting - just the
opposite is true. But for things intended not
for experimenting but for daily use I just keep
it simple, reliable and proven, and concentrate
on the really important "other" issues, like
electrical safety, and so on.

using an ECC83?

As said already, ECC83 is a very high amplification
factor tube. ECC83 definitely is very suitable for
its intended purpose, but that is just the point ...
if you don't need this high gain, there are better
(e.g more linear) candidates.

As for the tube selection... since I'm just beginning
my stock of tubes is limited. I've got several
EL504 (which got me started with tubes)
a few low grade ECC83 from EI/Jugoslavia and some
russians.

I wouldn't worry about brands at the moment. Just
primarily go for a correct electrical design, and
after that start tube brand rolling and after that
just use what you like yourself most sonically wise
(in a certain circuit).

Then think about why this tube of the same type
sounds other than that other tube in the same
circuit and draw your conclusions, and use what
you like, ignoring what other folks say.

At that moment you probably are already one step
above those folks who say that Telefunken, Amperex
buggle boys or whatelse is fashionable at the moment
is "the best brand" without knowing what they are
talking about. Just remember that a tube circuit
not only consists of a tube, less than a certain
tube *band*.

So, just find out yourself about certain brands
and circuits and so on, but before that built a
solid base of knowledge and understanding
to derive your opinion from.

I tried something with two of the russians, but
they where microphonic like hell... (6S2P & 6SH49P-DR)

So you probably snipped your fingernail against
some of them and heared a more ore less loud click
out of your speakers compared to other brands.

Okay, but the question is, if this really matters:

Do you snip your fingers against the tubes in
the amp you are listening *music* with? Do you
have 100dB efficiency horns that shake the
ECC83 input tubes when driven by your EL504
output stage so hard that there are audible
interferences?

Do you have a very low signal level stage (e.g
phono inputs) with these tubes, were microphonics
of input tube really is an issue?

It is good that you consider such things, but
don't let them deflect you from what *really*
matters for the intended purpose you design the
actual circuit for.

Right now the amp is disassembled, as I am working
on a chassis.

That is the hard and no-fun stuff, if one doesn't
have appropiate tools and doesn't want to fall
back to generic off-the-shelf chassis.

Maybe my homepage can give you some hints for
chassis construction; there are (short)
sub-chapters with pictures how these
chassis are made.

Welcome to the wonderful world of tube!

Tom

--
MS-DOS is the worst text adventure game I have ever played:
Poor vocabulary, weak parser and boring storyline.

Hi Olav,

I wrote:

- go for less gain (which means go for the gain you
really need) without NFB, using less parts, and
so on.

E.g by dropping one of the two gain stages and
optimize the remaining one :-)

Tom

--
Anyone who considers protocol unimportant has
never dealt with a cat. - R. Heinlein