NFB normally converts any current source into a voltage source.

This depends upon the open loop impedance, amount of feedback, etc.

Even if the Ro of the open loop amp is infinite,
series voltage NFB converts it into a voltage source.
It doesn't take much NFB to get a DF of 10.



The snag is that my impression is that some valve designs with output
transformers only use a modest amount of feedback. As a result they can
have output impedance of the order of 0.5 Ohms. (e.g. the amp Iain
mentioned in another thread recently). This level of output impedance
allows for quite audible changes in the response with loudspeakers whose
input impedance varies with frequency.

Rubbish, imho.

99% of ppl would remain totally unaware if a 1 ohm resistor was placed betwen
their speakers
and a low Ro amp.

Regarding SS reliablity, all I know is that there are many failures,
and mainly due to shorted speaker leads.

Patrick Turner.

"Jim Lesurf" wrote in message


FWIW My recollection is that when we had valve TVs years ago, that
calls to the repair man were more common. Mainly to replace valves.
Indeed, we used to have nearby high street shops that did this. Such
things vanished later on as with SS units many shops contracted any
repair work back to the makers or a central agent as it wasn't worth
their while to keep a repairman employed any more just for the shop.

When all we had were valve-based electronics in the US, many
consumer-oriented stores including hardware stores and drug stores had tube
checkers. Repair shops for home electronics proliferated to the point where
just about every commercial strip had at least one. Electronics stores were
dominated by shelf after shelf of replacment valves.

Today, shops that repair consumer electronics are regional businesses. In
1960 there were three within a mile of my current residence. Today the
nearest one is about 5 miles away.

"Patrick Turner" wrote in message

Jim Lesurf wrote:

In article , Patrick Turner
wrote:

[snip]

A known tube amp hater pointed you in the terrible direction of one
of the the worst designs ever to be seen at

http://members.aol.com/aria3/output.htm

This type of audio amp uses tubes to be directly connected to the
load, and it is called an OTL, or output transformerless amplifier.
They are notoriously unreliable. But some sound excellent while
working.

I assume you are referring to Trevor above.

Nope, never assume anything while I am around; it was arny who gave
the references.

Agreed. However, I bumped into them following posts made by supposed
tube-lovers on URA.

I don't have direct experience of the valve amps he quoted, but my
(perhaps unreliable) recollection is
that they have had good reviews in magazines from reviewers that
like/prefer valve amps.

OTL tube amps have appalling load matches to the tubes.

Only when people go cheap on the tube count. It does take an apalling number
of tubes to adequately drive a reasonable-worst-case speaker that dips just
below 4 ohms.

It'd be like using a couple of bjts to drive 0.2 ohms.

This cab work, its just a matter of picking your implementation.


Tube amps with output transformers are able to achieve a better
match between the load and the tubes so the matching of tubes is
relatively unimportant, and the reliablity is far better, so that
5,000 hrs can be expected from an output tube such as a 6550.

It is probably correct that the power efficiency tends to be better
using transformers.

Far better.

Not an absolute rule, but one that is well-rooted in practicality.

However I'd avoid using 'match' here as speakers tend to be
designed assuming the power amp is a low-impedance voltage source.

NFB normally converts any current source into a voltage source.

There are few if any perfect voltage or current sources in the real world.
Furthermore, there are such things as amplifiers that use NFB to produce a
more ideal current source.

If you listen 2 hrs per day, 365 days per year, you get 6.8 yrs out
of an output tube. Whilst some solid state gear might last that long
without a service, a lot do not,

No idea what percentage of what commercial designs you call "a lot".
Perhaps you could specify?

It's partially a matter of how much degradation you are willing to tolerate.
For the inveterate tube, egregious amounts of distortion are apparently
considered to be a feature, not a bug. For those of us who were into sonic
accuracy, it was kind of depressing to see our amps with EL-34 output pairs
lose 10-20% of their clean output capability within 6 months.

Enough to keep me busy repairing the darn things.

It's a business! ;-)

And I am only one of maybe 50 repair blokes in a town of 300,000 ppl.

Says something good about the actual reliability of modern electronic
components.

FWIW most of the amps I use are 20+ years old, and from personal
experience, and working in the field in the past I'd say most of the
SS amps I know about survive quite well for well over 6.8 years with
no necessity for any service. Don't have any reason to assume that
the designs I knew were unusual. Hence I would not have said "a lot"
as you do above. I might have said "a few", but I don't know what
percentage, designs, etc, you have evidence on.

A well-built SS amp will run continuously for a decade or two. You don't
wanna know how may tubes it would take to keep a tube amp within 90% of spec
for the same period of time.

That said, I assume that you'd regard this as irrelevant as the OP
was asking for a valve amp recommendation, not a comparison of
opinions on reliability and long term service costs: valve versus
transistor. :-)

and the cost of a fix for a high end SS amp may well be more than
the cost of re-tubing in say 6 years.

The cost of buying a good new SS amp can be less than the cost of re-tubing.

SS amps go phut as often as a TV.

Balderdash, not that I see a lot of dead TVs, either.

Mine has had 3 failures since 1982.

Bragging or complaining?

My TV is up-to-date enough that it probably couldn't have existed in 1982.

Its been parked for 6 years since then, and since the last Olympics
when it went bung, and I decided TV was a bad influence anyway....

The less TV the better, agreed.

The 1975 SS AM/FM receiver with 30 watt amps in it has had two
services before I parked it in a cupboard.

Bragging or complaining?

So I have spent my hundreds fixing modern "reliable" appliances....

I've been through a lot more electric coffee pots and vacuum cleaners than
amps.

If you listen 2 hrs per day, 365 days per year, you get 6.8 yrs out
of an output tube. Whilst some solid state gear might last that long
without a service, a lot do not,

No idea what percentage of what commercial designs you call "a lot".
Perhaps you could specify?

It's partially a matter of how much degradation you are willing to tolerate.
For the inveterate tube, egregious amounts of distortion are apparently
considered to be a feature, not a bug. For those of us who were into sonic
accuracy, it was kind of depressing to see our amps with EL-34 output pairs
lose 10-20% of their clean output capability within 6 months.

What an outright lie this guy tells!.

I have an amp with a pair of Sylvania EL34 that are at least 35 years old,
and it gets used every other day in my workshop for testing client's speakers,
and where I need a
low impedance source for testing crossovers.

The emission is down about 15% over new tubes but the amp still measures 0.15%
at 25 watts.

I have measured it several times since i built it 6 years ago using the second
hand tubes
I aquired cheaply, and which are expendable if they fail.
None of the claimed degradation
in the cleanliness of the power output has occured.
Many other amps that I have built and sold and serviced year later for
occasional nopisy pots, or to allow some upgrade have shown no audible or
measurable
reduction of power or increase in thd.



Enough to keep me busy repairing the darn things.

It's a business! ;-)

And I am only one of maybe 50 repair blokes in a town of 300,000 ppl.

Says something good about the actual reliability of modern electronic
components.

There shouldn't be a single failure of an SS amp in my town in any given year.

But maybe thousands of failures occur.

There is no point in arguing with Arny.

Patrick Turner.

325.
Fill the pie with stew, place top crust and with a fork, seal the crusts together
then poke holes in top.
Return to oven and bake for 30 minutes, or until pie crust is golden brown.



Sudden Infant Death Soup

SIDS: delicious in winter, comparable to old fashioned Beef and Vegetable Soup.
Its free, you can sell the crib, baby clothes, toys, stroller... and so easy to
procure if such a lucky find is at hand (just pick him up from the crib and
he?s good to go)!

SIDS victim, cleaned
½ cup cooking oil
Carrots
onions
broccoli
whole cabbage
fresh green beans
potato
turnip
celery
tomato
½ stick butter
1 cup cooked pasta (macaroni, shells, etc.)

Remove as much meat as possible, cube, and brown in hot oil.
Add a little water, season, then add the carcass.
Simmer for half an hour keeping the stock thick.
Remove the carcass and add the vegetables slowly to the stock,
so that it remains boiling the whole time.
Cover the pot and simmer till vegetables are tender
(2 hours approximately).
Continue seasoning to taste.
Before serving, add butter and pasta,
serve piping with hot bread and butter.



Offspring Rolls

Similar to Vietnamese style fried rolls, they have lots of meat
(of course this can consist of chicken, beef, pork, or shrimp).
Who can resist this classic appetizer; or light lunch served with
a fresh salad? Versatility is probably this recipe?s greatest virtue,
as one can use the best part of a prime, rare, yearling, or the
morticians occasional horror: a small miracle stopped short by a

celery, green onions, and parsley.
Place roast on top with fat side up.
Place uncovered in 500° oven for 20 minutes, reduce oven to 325°.
Bake till medium rare (150°) and let roast rest.
Pour stock over onions and drippings, carve the meat and
place the slices in the au jus.



Bisque à l?Enfant

Honor the memory of Grandma with this dish by utilizing her good
silver soup tureen and her great grandchildren (crawfish, crab or
lobster will work just as well, however this dish is classically
made with crawfish).

Stuffed infant heads, stuffed crawfish heads, stuffed crab or lobster shells;
make patties if shell or head is not available
(such as with packaged crawfish, crab, or headless baby).
Flour
oil
onions
bell peppers
garlic salt, pepper, etc.
3 cups chicken stock
2 sticks butter
3 tablespoons oil

First stuff the heads, or make the patties (see index)
then fry or bake.
Set aside to drain on paper towels.
Make a roux with butter, oil and flour,
brown vegetables in the roux, then add chicken stock and
allow to simmer for 20 minutes.
Add the patties or stuffed heads, and some loose crawfish,
lobster, long piglet, or what have you.
Cook on low for 15 minutes, then allow it to set for at leas

2 tablespoons each:
salt
black, white and cayenne peppers
celery salt
garlic powder
parsley flakes
brown sugar
1 teaspoon sage
2 onions
6 cloves garlic
bunch green onions, chopped

Cut the children?s butts and the beef roast into pieces
that will fit in the grinder.
Run the meat through using a 3/16 grinding plate.
Add garlic, onions and seasoning then mix well.
Add just enough water for a smooth consistency, then mix again.
Form the sausage mixture into patties or stuff into natural casings.



Stillborn Stew

By definition, this meat cannot be had altogether fresh,
but have the lifeless unfortunate available immediately after delivery,
or use high quality beef or pork roasts (it is cheaper and better to
cut up a whole roast than to buy stew meat).

1 stillbirth, de-boned and cubed
¼ cup vegetable oil
2 large onions
bell pepper
celery
garlic
½ cup red wine
3 Irish potatoes
2 large carrots

This is a simple classic stew that makes natural gravy,
thus it does not have to be thickened.
Brown the meat quickly in very hot oil, remove and set aside.
Brown the onions, celery, pepper and garlic.
De-glaze with wine, return meat to the pan and