I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to build it
"from zero", just for the pleasure of doing it. Ideally a vintage model.

What would be the model or schematic recomended by the experts of this
group.

Please don`t recommend e-Bay.

SW

On 26 Feb 2005 23:20:59 -0300, Sothern Winds wrote:

I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to build it
"from zero", just for the pleasure of doing it. Ideally a vintage model.

What would be the model or schematic recomended by the experts of this
group.

Your posting name must be something interesting, rather than a
simple misspelling. Care to share?

Classic amplifiers that might interest you would include the
Marantz 8 and the Dynaco Stereo 70. Neither is appropriate today,
although the choice of a push-pull pair of EL34's would be a
great choice for homebrew.

The really interesting and important part of tube amplifiers is
the output transformers, so any real design starts there. So, let's
flip the question back to you; have you chosen an output transformer?

And, Good Fortune with your project,

Chris Hornbeck

Sothern Winds wrote:

I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to build it
"from zero", just for the pleasure of doing it. Ideally a vintage model.

What would be the model or schematic recomended by the experts of this
group.

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

Patrick Turner.



Please don`t recommend e-Bay.

SW

On Sun, 27 Feb 2005 15:00:19 +1100, Patrick Turner
wrote:

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

Pretty elegant design, IMO. One could argue that the coupling
caps to the output stage are overlarge, but dynamic performance
including both within and without clipping *balanced together*
likely justifies it.

A generic design for the OP would need to include many of its
important elements, including the Zobel and the phase-lead in the
loop, but might need to include some trim to the feed-foreward
compensation (between the two bottles). Care to comment on the
particularity of the .047uF/1M and 330pF/220k to a particular
OPT?. It looks, at first glance like the feedback network's
group delay will dominate, but you'll know fersure.

Thanks,

Chris Hornbeck

Sothern Winds wrote:

I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to build it
"from zero", just for the pleasure of doing it. Ideally a vintage model.

How much experience do you have in building vacuum tube
electronics (tube amps or tube radios)? Several pitfalls
can trap a newbie trying to build an otherwise excellent design
with excellent parts. Physical layout of the parts is one
area that can bite you. Ground loops is another. Kits
and rebuilding vintage amps take care of these for you.
And knowing how to handle high voltage circuits is
necessary. If you know these things then great. Otherwise
start with something easier, say a 1 watt single ended
amp.

Chris,

Thanks for you answer. The name of my posting comes only from the damn
feeling I have, that when posting to news is better to keep anonymous.
Actually I can`t explain exactly where the name comes from ..... Sorry
......

About your affirmation "Neither is appropriate today", I`d like to know
more. From my stand point, frequency response, noise and distortion from a
Dyna ST70 are very good for an audio amp and that probably 99 out of 100
people could not find a difference in the sound of it when comparing with
an amplifier with a more modern design.

I,m not trying to mean that is not worth to build a newer design, specially
if cost is similar.

This is a controversial matter among audiophiles, that I think has been
discussed by many people in the past. Any opinions?

Best regards,


Chris Hornbeck wrote in
:

On 26 Feb 2005 23:20:59 -0300, Sothern Winds wrote:

I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to
build it "from zero", just for the pleasure of doing it. Ideally a
vintage model.

What would be the model or schematic recomended by the experts of this
group.

Your posting name must be something interesting, rather than a
simple misspelling. Care to share?

Classic amplifiers that might interest you would include the
Marantz 8 and the Dynaco Stereo 70. Neither is appropriate today,
although the choice of a push-pull pair of EL34's would be a
great choice for homebrew.

The really interesting and important part of tube amplifiers is
the output transformers, so any real design starts there. So, let's
flip the question back to you; have you chosen an output transformer?

And, Good Fortune with your project,

Chris Hornbeck

Patrick,


Thanks from the pointer. I didn`t knew about your (I suppose) web site.
Is really interesting and very complete.

I`m really considering to build a 5050.


SW


Patrick Turner wrote in
:



Sothern Winds wrote:

I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to
build it "from zero", just for the pleasure of doing it. Ideally a
vintage model.

What would be the model or schematic recomended by the experts of
this group.

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

Patrick Turner.



Please don`t recommend e-Bay.

SW

About your affirmation "Neither is appropriate today", I`d like to know
more. From my stand point, frequency response, noise and distortion from a
Dyna ST70 are very good for an audio amp and that probably 99 out of 100
people could not find a difference in the sound of it when comparing with
an amplifier with a more modern design.

" Neither is appropriate today" is, unfortunately, a classic
responce you get when mentioning the ST70. I service vintage and modern
tube hifi and guitar amps and the only thing I can say about a lot of
newer amps is that they did address the problem of marginal current
capacity and heat in the power transformer. Some modern output
transformers are a cut above, but the price! I could buy a good used car
for what a pair costs. Anyway, that said, there is nothing wrong with
the design or sound of the Dynaco ST70. I own one, and have serviced a
ton of Dynaco amps and in AB tests they hold their own against most of
the modern sacred cows. In fact, the ST70 sounds better then many modern
amps. So don't you believe anyone who tries to put down these amps.
Classic amps made by Scott, Dynaco, Fisher, Marantz, and McIntosh will,
if properly restored, hold their own against pretty much anything made
today, and I have heard some pretty expencive amps. Are there amps that
sound a little better? Yes there are, but the price of these amps is
astronomical. Also, you can easily upgrade the capacitors, resistors and
power supply of an ST70 and end up with an amazingly detailed sounding
amp. So building one from scratch will allow you to use this classic
design while also allowing you to upgrade certain parts like the power
transformer and to use higher quality coupling capacitors and resistors
to end up with a real performer.


I,m not trying to mean that is not worth to build a newer design, specially
if cost is similar.

This is a controversial matter among audiophiles, that I think has been
discussed by many people in the past. Any opinion

You have that right. and what I just posted will most probably stir
up a stink that will see this thread reach 100 posts extolling the
viortues of this amp or that amp and poo-pooing the venerable little
ST70, so be prepared for a hurricane!
Bill B.

"Sothern Winds" wrote

I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to
build it
"from zero", just for the pleasure of doing it. Ideally a vintage
model.

What would be the model or schematic recomended by the experts of
this
group.

Consider getting a book with a few well-documented designs in it,
together with general explanation of how valve stuff works and how
to choose components and layout.

Some have been recommended here many times. I used Van der Veen's
"Modern High End Valve Amplifiers" and Morgan Jones' "Valve
Amplifiers", but if you want classic designs other books may be more
suitable. Morgan Jones has a new book out but I don't know if it's
any good. Anyone seen it?

The biggest difference between modern and classic is in the power
supply. Do you want to use valve rectifiers?

If you have modern lowZ speakers and want a simple design with
easily-available output transformers, you will need a design with
overall negative feedback. You will probably end up with a pair of
EL34 or 6L6 per channel in ultralinear mode. Mullard, Leak and
Williamson spring to mind.

I don't go along with the "starter amp" concept. Think like your
making the amp you are going to use and be happy with. Then you can
justify the expense of decent output transformers, which is the key
to success.

cheers, Ian

"Sothern Winds" wrote in message

I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to
build it "from zero", just for the pleasure of doing it. Ideally a
vintage model.

What would be the model or schematic recomended by the experts of this
group.

Please don`t recommend e-Bay.

How about a recommendation that you check out rec.audio.tubes?

Chris Hornbeck wrote:

On Sun, 27 Feb 2005 15:00:19 +1100, Patrick Turner
wrote:

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

Pretty elegant design, IMO. One could argue that the coupling
caps to the output stage are overlarge, but dynamic performance
including both within and without clipping *balanced together*
likely justifies it.

The time constants are large in the couplings.
One could perhaps use 0.1 uF, and still get LF stability,
depending on the OPT inductance.

There is an LF step filter between the power amp input tube anode
and the input to the LTP, so that open loop gain and phase shift is
reduced
in the critical band from 1 Hz to 20Hz, to make recovery after overload
and LF stability excellent with NFB applied.





A generic design for the OP would need to include many of its
important elements, including the Zobel and the phase-lead in the
loop, but might need to include some trim to the feed-foreward
compensation (between the two bottles). Care to comment on the
particularity of the .047uF/1M and 330pF/220k to a particular
OPT?.

These networks have to be worked out for optimal
critical damping and will vary with the OPT chosen for the job.

One of the issues when DIYing an audio amp is that
any given schematic will not give the same stablity margins
with different OPTs since the shunt C, leakage inductance and primary
inductance
all may be different.
Many magazine articles of the 1950s showed splendid samples of DIY
amps using Partridge or Savage or some other hi-end OPT,
and it was possible to apply 26 dB of NFB without a problem.
Ppl then went to buy an OPT from a far cheaper supplier,
and the OPT was POS, and wondered why the amp oscillated badly,
and that 26 dB of FB was quite impossible, and that it was impossible to
critcally
damp the circuit like I have done in mine without reducing the
closed loop bw to 30 Hz to 15 kHz.
I get 10 Hz to 65 kHz, and pure capacitor loads loads can be connected
without
causing oscillations, loads do not have to be connected to prevent
LF oscillations.
All amps should be unconditionally stable.

It looks, at first glance like the feedback network's
group delay will dominate, but you'll know fersure.

The open loop response is about 20Hz to above 20kHz with all the step
networks in place,
and the FB levels this out.
The OPT I use is my own with less than 4 mH of LL, bags of Lp, and a
small amount of Cshunt.

I just spent a day restoring a Leak TL12 mono amp and
when set for 9 ohms output load, it oscillated at LF without load
connected,
and any value of cap between 1 and 0.1 uF caused it to oscillate
violently.
LL is far too high in these amps.
The use of a pentode input tube allows the miller effect of the LTP
to cause a pole at too low an F.
After changing the load match to suit 4 ohms, and a
few other mods, it behaved a little better, but the Leak OPT is a POS.

Patrick Turner.






Thanks,

Chris Hornbeck

Sowthern Winds wrote:

Patrick,

Thanks from the pointer. I didn`t knew about your (I suppose) web site.
Is really interesting and very complete.

I`m really considering to build a 5050.

I don't do kits for ppl, and you need some basic
abilities with using tools, plus a decent tool kit,
and somewhere to work it all.

You could build a stand alone line stage pre-amp first,
then make the power amp later if you feel comfortable with your skill
levels.

If you have never built much of anything before, a tube amp
is a **BIG** challenge, and there are voltages that can send you north.

Patrick Turner.



SW

Patrick Turner wrote in
:



Sothern Winds wrote:

I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to
build it "from zero", just for the pleasure of doing it. Ideally a
vintage model.

What would be the model or schematic recomended by the experts of
this group.

http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

Patrick Turner.



Please don`t recommend e-Bay.

SW

Sothern Winds wrote:
I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to build it
"from zero", just for the pleasure of doing it. Ideally a vintage model.

What would be the model or schematic recomended by the experts of this
group.

Please don`t recommend e-Bay.

SW
Have a look at the Ella at www.diyhifisupply.com. I built one and it
blows away amps costing 10 times as much.

Wayne,

I went to the www.diyhifisupply.com- Specs and price are very impressive.

But after reading some comments I found at their support forum
(http://www.audioasylum.com/forums/DIYHiFi/bbs.html) I get concerned about
if their claims are true (see the "ELLA New schematics scary experience").

You said you built one. How was your experience?

Regards,

SW




Wayne McDermott wrote in
:

Sothern Winds wrote:
I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to
build it "from zero", just for the pleasure of doing it. Ideally a
vintage model.

What would be the model or schematic recomended by the experts of
this group.

Please don`t recommend e-Bay.

SW
Have a look at the Ella at www.diyhifisupply.com. I built one and it
blows away amps costing 10 times as much.

On Mon, 28 Feb 2005 11:28:06 +1100, Patrick Turner
wrote:

The time constants are large in the couplings.
One could perhaps use 0.1 uF, and still get LF stability,
depending on the OPT inductance.

In the particular case of low frequency stability and
overload recovery, I tend to fall into the DTN Williamson
camp and favor (what's nowdays called) a single dominant
pole compensation. It's not necessarily optimum for
extended response, but it is optimum for overload
recovery, and, as the old saying goes "If it don't sound
good loud, it don't sound good.".


These networks have to be worked out for optimal
critical damping and will vary with the OPT chosen for the job.

One of the issues when DIYing an audio amp is that
any given schematic will not give the same stablity margins
with different OPTs since the shunt C, leakage inductance and primary
inductance all may be different.

And in the case of high frequency stability, neither of us
lean toward the op-amp model of a single dominant pole. A
good, finished design that includes loop feedback *will
neccesarily* require some cut-and-try both in the forward
and the feedback paths. As your circuit elegantly shows.

Note 1: If conventional solid-state amplifiers were paid the same
level of attention to these same issues, they'd be happier
campers.

Note 2: Maybe a good topic for a coming-soon-to-a-newsgroup-
near-you thread might be "Optimizing the open loop and closed
loop performance of fed-back amplifiers", concentrating on
measurement strategies.

Note 3: The mid-1990's craze for homebrew SET's, and, importantly,
zero loop feedback, may be a reflection of how *wrong* this issue
is typically treated among us builders.

Thanks,

Chris Hornbeck
"Anarchy only works among those who can control themselves." -ha

Chris Hornbeck wrote:

On Mon, 28 Feb 2005 11:28:06 +1100, Patrick Turner
wrote:

The time constants are large in the couplings.
One could perhaps use 0.1 uF, and still get LF stability,
depending on the OPT inductance.

In the particular case of low frequency stability and
overload recovery, I tend to fall into the DTN Williamson
camp and favor (what's nowdays called) a single dominant
pole compensation. It's not necessarily optimum for
extended response, but it is optimum for overload
recovery, and, as the old saying goes "If it don't sound
good loud, it don't sound good.".

Williamson's amp had more RC couplings than the Mullard 520 et all.
Therefore W needed at least 100H in the OPT even at low levels
to get a low enough pole between the load and Ra of the triodes in
parallel.
Therefore he use 4,400 P turns on a core of 32 mm tongue x 44 m stack.

Bean counters universally hated Williamson because he said whatcha have
to do, and nobody wanted to do it.

GE knew that nobody took W seriously, so they even provided notes in the
back of a book in 1957 called '17 amplifier circuits from 5 watts to 1,100
watts',
so that ppl would be able to use OPTs with only 20H of minimal P
inductance,
and still get good bass, **and stability** with a lot of NFB used.

Many Williamson amps oscillate badly since the OPT ppl have tried to use
isn't
anywhere near what W said it had to be.



These networks have to be worked out for optimal
critical damping and will vary with the OPT chosen for the job.

One of the issues when DIYing an audio amp is that
any given schematic will not give the same stablity margins
with different OPTs since the shunt C, leakage inductance and primary
inductance all may be different.

And in the case of high frequency stability, neither of us
lean toward the op-amp model of a single dominant pole. A
good, finished design that includes loop feedback *will
neccesarily* require some cut-and-try both in the forward
and the feedback paths. As your circuit elegantly shows.

Its impossible to define the nature of the OPT shunt C and LL until
the OPT is in the amp.
The aim of a good amp dsign with tubes is to have the open loop
BW without the zobel compo networks of phase tweakers
as the same as closed loop BW with all the tweakers in place.

In my 300 watt amps I got 300 kHz of OPT BW due to
the 5P x 6S interleaving, and the fact that since RL is only 1k a-a,
and the core so large, only 1,100 primary turns are needed
for good bass, and the Lp and LL is mainly determined by the Np squared,
so LL is below 1 mH.

But I cannot allow the amp to have a final BW with NFB of 300 kHz.

That would only work if the load was an R.

I like to get 50 kHz of BW even with 2 uF and no other load,
although one of course cannot get full mid frequency voltage
of around 45 vrms across a 2 uF cap at 50 kHz.

But at 4.5 volts of output, its easy.

In fact at low levels any value of cap between 0.1 and 10 uF shouldn't
cause
more than 3 dB of peaking in the sine wave response anywhere.

If the peaking is above 20 khz, well and good, as it should bw with up to
5 uF, but with
10 uF the peak is below 20 khz.

The tweaking of the open loop phase and gain reduction above 20 kHz is a
must
even if the open loop BW is 300 kHz.
That's because there is still leakage L and some HF poles from the miller
effect,
and so a wide band amp with OPT will simply oscillate
at a higher F than one with more LL and lower miller poles.

The trick is to use a **minimum** of phase tweaking to control both the HF
gain and
HF phase shifts caused by miller, so that a BW of 65 kHz can still be
obtained
with a pure R load, yet no pure C load will cause more than 1.5 dB of
peaking in the
sine wave response at any F below 20 khz.

Usually ESL have some R in series with their dominant C component/s
and some R in parallel with all that, and then the amp
set up the way I do it will have a flat response into any real world load,

and act better than many SS amps I have tested.

The compo C across the global FB resistor can also be minimised
by placing a small L, maybe only a couple of uH
in the speaker return wire in the amp and taking the
low value R used in the cathode of V1 to a point where
negative current FB is generated as F rises above 20 khz.
The current in the L tweaks the phase 90 degrees, and so "fits"
the delays in the rest of the amp, and the CFB allows the Ro of the amp
to become higher above 20 khz where low Ro is a pest.

Yesterday I added such an L into a leak TL12, a truly HORRIBLY
unstable amp compared to mine, and was still able to get 50kHz of BW
into 8 ohms, and LF stability without a load and HF stability with any
sort
of pure capacitance, and a far better ability to drive a speaker such as a
Quad ESL
which is like 15 ohms strapped with 2uF and 1.6 ohms in series.
I was able to not have to use any phase tweaker zobel at V1 to
control the miller C between the EF36 and following 6SN7 LTP.

The generation of some CFB at HF is actually more effective
with low value reactive loads at HF than trying to rely
solely on caps across the global voltage NFB R.
But zobel across the output, usually 10 ohms plus say
0.22 uF are a necessity with amps with poor stability margins or no margin
at all.


Note 1: If conventional solid-state amplifiers were paid the same
level of attention to these same issues, they'd be happier
campers.

Many SS amps have passive zobels with LR and CR after their outputs
which really is back at the emitters of the output stage,
also where the NFB is sent back to the diff pair.

The average SS amp couldn't have the 60 dB of global NFB used
if the L component of the LR zobel was included,
especially without the R strapped across the L.
The tube amp has the L in there where we like it or not,
and it is the leakage L.
if the impedance ration of the opt is say 6k to 8 ohms,
or 750:1, as it is in an old Leak set to 4 ohms,
and the LL is 50 mH like it is with a Leak at the primary, then
we have a massive 66 uH in series with the output.
and in a tube amp its impossible to have *effective* global FB
unless we take it from the output.
Since the LL is so high, only a limited amount of NFB can be used.

But if LL was say 4 uH then the pole caused by LL and the load is
at a far higher F, and much more FB can be used.

4 to 10 uH is commonly used at the output of SS amps
with an R of between 4 and 8 ohms, and then there is a following
series RC and perhaps a series RC from the output to 0V.

The first RC acts to have the output emitters see an R load at above say
100 khz, and thus the output device gain is limited and stability assured.

the rest of the L and C and R hanging off the SS outputs make sure
C loads of any value never cause the amp to actually see the low impedance
of a C
type of load.
Square wave tests with SS amps can show serious over shoots
with cap or RC loads, and most of this over shoot is merely a
function of the passive filter components of the "outpboard" network.
A CRO on the output emitters usually show a good square wave
The square wave at the output of the diffamp, ie, the early error signal
should show a smooth curved line of the horizontals and thus show
that the amp is ever so easily compensating for the droop in the open loop

response regardless of load or oscillations in the load current.

Good tube amps should show minimal oscillations in their
error signals with a square wave.


Note 2: Maybe a good topic for a coming-soon-to-a-newsgroup-
near-you thread might be "Optimizing the open loop and closed
loop performance of fed-back amplifiers", concentrating on
measurement strategies.

The theme has been done to death.
A search of rat archives under my name and 'NFB' and 'critical damping'
should give you at least a book or two to read.




Note 3: The mid-1990's craze for homebrew SET's, and, importantly,
zero loop feedback, may be a reflection of how *wrong* this issue
is typically treated among us builders.

Not really.

There **is NFB in triodes**.

There are at least 2 ways to get to an amp with
low thd of less than 1% at any full power, and with Ro = less than 1 ohm.

One is to start with a Horror like a Leak amp, and place the
original 26 dB of NFb around the circuit and that'll give
Ro less than 1 ohm and thd a lot less than 1 %.

If we take the FB away, and set the Leak amp up for 9 ohms, we
will find that Ro = 12 ohms, very high for a triode amp but it is due to
the effective
winding resistance and transformed plate resistance being equal to
the sum of 2.7 and 9.3 ohms respectively.
the "9 ohm" load match gives the PP KT66
a load of 2.9 k RLa-a plus the 850 ohms of winding R a-a.

Without NFB, the Leak would be an attrocious performer.

BUT, if we then said OK, we will use a quad of
KT66 in triode, and then use a low winding loss transformer
that has 5% total losses, so that Rw at the output was only 0.4 ohms,
and then we will have an impedance ratio of 2,666:1,
then the Ra-a of 1.6k of the 4 x KT66 will ap[pear at the output as 0.6
ohms.

The total Ro will be 1.0 ohms.

The load seen by the 4 tubes with 8 ohms connected will be 2,666 x 8 =
21,333 ohms
With Ea of 450v, we should get a peak swing down to
say 100v, so Va-a = 500 vrms so Po = 11.7 watts,
and it will definitely be all class A, and remain class A even if
4 ohms is connected, and po will increase lots.

The thd of KT66 in triode with such loads is around the 1% at 12 watts,
but then even unloaded, at 500 vrms a-a, the thd won't be much less;
we have reached the residual level of thd at high V swings.

But the thd will decline with Vo, so that at 2 watts, which gives 93 dB
SPL
into todays insensitive speakers of about 90 dB/W/M,
the thd should be arond 0.4 %, and mainly only 3H.

Tubes such as 300B should do a little better.

So we have out 12 watts at 1% and 1 ohm, and the onlt NFB we have allowed
is the
NFB put there by the God of Triodes when he said "Let There Be Triodes!",
in about 1903.

The drive amp driving the output stage has to be designed carefully
but getting 70 vrms grid to grid at 0.1% thd without NFB is easy.

With similar logic, 4 parallel KT66 in SE will produce
low Ro, but have maybe 3% thd at 12 watts, but its mainly 2H
instead of 3H, which will be nearly absent, since the 3H appears
in output class A stages because of the imbalances of the difference
between Gm
with PP class A tubes turning on or off.
Few devices have a very pure symmetry of turn off/turn on Gm with respect
to output voltages.

The well set up triode amp with no loop FB can sound utterly delightful,
and it is hard to build a bad sounding one in either PP or SE
providing the basic rules are followed, thd below 1% at
normal levels used, Ro less than 1 ohm, and enough damn power
to give yourself a ceiling.

A quad of KT88 or KT90 in triode are rather better than KT66, 807, 6L6 or
EL34 imho.

Patrick Turner.

Sowthern Winds wrote:
Wayne,

I went to the www.diyhifisupply.com- Specs and price are very impressive.

But after reading some comments I found at their support forum
(http://www.audioasylum.com/forums/DIYHiFi/bbs.html) I get concerned about
if their claims are true (see the "ELLA New schematics scary experience").

You said you built one. How was your experience?

Regards,

SW




Wayne McDermott wrote in
:


Sothern Winds wrote:

I`d like to build a hi-fi tube amplifier, 35 W/channel. I want to
build it "from zero", just for the pleasure of doing it. Ideally a
vintage model.

What would be the model or schematic recomended by the experts of
this group.

Please don`t recommend e-Bay.

SW

Have a look at the Ella at www.diyhifisupply.com. I built one and it
blows away amps costing 10 times as much.


Hi SW,

I had some issues during construction with inaccuracies in the
instructions, as well as some incorrect pots being supplied but they
were very quickly resolved.

At first I was disappointed, I found the amp to be "flat" and lacking
midrange. This was due to

1. I had not given it enough time to burn in (50 hours music is
recommended) and

2. I was running it without a preamp. Though the Ella can drive a
CD/ipod source directly it was working too hard to fill out. When I
inserted a preamp (my Rotel RA-985BX with the power amp switched out)
the performance was utterly jaw dropping. Everyone who listens to it
admits it is a sound they have never heard before. It truly does live up
to all of the superlatives the audio press use, and I haven't tried any
of the upgrades available yet.

It is also great fun to build and very simple. I wholeheartedly
recommend you try one !

Kind Regards,

Wayne McDermott

Some have been recommended here many times. I used Van der Veen's
"Modern High End Valve Amplifiers" and Morgan Jones' "Valve
Amplifiers", but if you want classic designs other books may be more
suitable. Morgan Jones has a new book out but I don't know if it's
any good. Anyone seen it?

I have the 2nd edition. An excellent book though with the caveat that I
was trained as an electronics technician so that the theory he covers in
3 pages is well known to me. However if you do not have a reasonable
grounding in electronics it would be a difficult book, and, more
importantly, you are dealing with some potentially fatal voltages and
currents. Make sure you have good ELCB protection.

Wayne McDermott

There are several old books with detailed chassis drawings and wiring
layouts of amps and preamps and a few are available-from OCSL and other
sources- as reprints. You can copy them exactly with good results.
Projects which put the power supply on a separate chassis are more
idiotproof and lend themselves to later improvement-you will want a DC
filament supply with slow warm-up and a delayed, impulse-limited B+
supply eventually.

The stock Dyna ST70 has a lot of faults but if you can get someone to
sell you one cheap you get a chassis, two output transformers that are
pretty good (if they are good), and a power transformer which is of
good quality but undersized. It can be used to drive one channel nicely
or a bigger one can be fitted or an outboard power supply can be built.

The stock circuit is poor, uses a hard to find tube, and is built on a
PC board that hasn't aged well at all typically. They do make a fine
target if you own a T/C Contender with a .410 barrel or a small
smoothbore muzzleloader. Remove the 7199s and give them to a Hammond
organ buff or a Black church with one. The board shatters beautifully
with any good .410 skeet load.

However, Dynas have become semi-valuable now and so starting from
scratch is more likely. You can fab a chassis or have one bent up by
any sheetmetal shop, custom radiator shop, or by any aircraft facility.
(The latter will quote a freakish price but since GA A&Ps start at ten
bucks an hour getting one of the guys to bend one up after hours might
be reasonable.) Found items like cookware, automatic transmission oil
pans, truck radiator end tanks, and even scrapped Lycoming engine
crankcase halves are possibilities. Good sources for output
transformers are Sowter in England, Lars Lundahl in Sweden, and
several Oriental makes which can all be found on the Internet. If you
want a U.S. unit, some high end vendors have been known to be cool
about selling transformers and others total pricks. Never hurts to ask.
Cary used to have a nice back door business.

As much as I hate regen radios, building a simple one such as in the
old Alfred Morgan kids books might be a good project if you have no
experience whatever. Large stocks of AA5 tube radio kits were available
in some of the catalogs advertised in electronic journals (sic) like
Nuts and Volts as recently as 2000, so there may be some out there.

Many vintage amps are cramped, awkward sons of bitches. You can always
part them out for the iron though, especially Scotts and Fishers,
however they are worth much more as complete units. Sell them on ebay
and buy new stuff.

This is until you blow up one of those output transformers. Start
simple.