Re: 211 A2 amp



"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> wrote in message
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Iain Churches wrote:

"Patrick Turner" <info@xxxxxxxxxxxxxxxxxx> wrote in message
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Iain Churches wrote:

"Nick Gorham" <nick@xxxxxxxxxxx> wrote in message
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Hi Iain,

I have added a couple of inside pictures to

http://www.lurcher.org/nick/images/211/

Hello Nick. Lovely job:-)

I have sent the pics to a couple of chaps with whom
I share a workshop.

The Tentlabs units are a nice and simple way of heating DHTs,

I asked about these, because, as I mentioned to you before
I want very much to build a good SET and have been collecting
info and ideas for a while now.

AC is cheaper, but I get a little obsessive about hearing hum, and I
have
never got hum low enough to not hear it.

I don't think that's obsessive. My own view is that the
amp should be silent from the listening position. It is not
too difficult to build a PP of say 50W which is totally silent
EAS (ear against speaker) but SET is a totally different
challenge.

Why is SET a special challnge for noise?

I was thinking about AC heaters. I have heard only a few
SET amps, but all of them, with one exception (Russian built)
had audible hum from the listenking position. Hence my interest
in Nick's amp, and his choice of Tentlab.

See the info at

http://www.tentlabs.com/Products/Tubeamp/Tubefilament/index.html


My 60W amps with a parallel pair of 845 with very little global FB
pass the EAS test easily.

At a metre, there is no indication they are switched on except
visually.


I am very interested to build a good quality SET.
I shall be grateful for advice from yourself, Nick, Andre
(and of course anyone else who actually has hands-on
experience) There will probably be lots of flames too,
but who cares. I'm still going to build it:-) I will start
a new thread whehn the time is ripe.

The power supplies are now separate from the audio amp chassis.
the PT, OPT, and the two chokes for the choke input PS for dc heating
of
cathodes are
all potted, and mechanically inaudible beyond 300mm.

Here in the EU, it is very difficult (if not impossible) to find a DC
connector approved for domestic use above 70V. Even the multi-pin
NATO connectors which were used used for high voltage DC in
broadcast are rated much lower for domestic applications.

I just use plain old octal plugs and sockets, OK for one offs.
To each amp there 3 umbilical cables lightly twisted.
Two are 10 amp rated 450Vac rated mobile crane wire, *nice* and flexible
One is an orange twin mains cable, but only for low voltage dc for
turn on and the protect circuit.

If the octal plug going into the PSU is pulled out and grabbed with a
hand
the DC flow from the amp is prevented by diodes facing the appropriate
way.

The diode option is one thing that I looked at. I decided, after
speaking with the insurance evaluator, to use a safety loop and
a relay.



Unless someone has a pet dobberman or pet lion prone to enjoying
a bit of a chew on soft plastic things, then the cables won't be
dangerous.
The animals definately will stop chewing the cable at some point in time
if the amps are
turned on.

But if the animal is a potential Cruft's champion.....

I am using +680Vdc and -600Vdc approx, and careful layout
of octal socket pins with some adjacent pins not used will prevent any
arcing.

The use of octal plugs/sockets does not meet the safety
standards, I am told by an assurance safety assessor, and
no safety inspector would agree to such a thing being used
these days for DC power.

I understand.

I don't think my set up is unsafe.
after all, HV is above 1,000V, no?

We are talking about the LVD (Low Voltage directive here)
which seems to go to 1500 V ! It's very difficult indeed to
"interpret" the directives. I think the only sure way is to take
an amp for inspection. In Scandinavia we have the FIN, SE
and N standard. If the unit pases any of these, then CE is
is assured. I am sure you have something similar in Oz. In
addition a director of the company has to sign a declaration
which is given to the owner with the manual, which states that
the unit fully meets current safety requirements. The test is very
expensive (several thousand Euros) The last time I asked
about it, they required two identical units, both of which
are tested to destruction (mains transformer current capacity,
winding isolation etc etc) I pointed out that the Sowter
transformers which I use already have a test certificate at
CE rating - but, it seems that does not make any difference.

The other problem I had was that I wanted a preliminary
inspection (someone to cast a beady eye over the amp) and
comment if there were any obvious breeches of the regulations
that could be detected visually. They were not willing to do this.

But I have found out quite a lot of info on the way - things
that are considered to be crucial. They are, as far as tube
amps are concerned, mainly to do with safety - earth bonding
etc etc. One interesting thing is that many modern amps use
crimped connectors on the mains input, AC switches, fuses
etc right up to the primary of the PT.

It is difficult to believe that a crimped connector is stronger
or more reliable than a solder joint, but it seems that the
regs call for a mechanical joint. A chap of my acquaitance
who is involved with the design of GSM stations for Nokia
told me that a "mechanical joint" is, in his opinion, not a
crimped connector at all, but a wire passed through the
eyelet on the switch tag, bent over and then soldered, so
that it hgas mechanical strength and cannot become
detatched if the joint is dry.

I have also be told (by a my less reliable source) that the
green/yellow earth wire on the mains connector must be
five cms longer than the other two wires so that if the
connector is pulled out accidentally, the earth will be
the last connection to break. Hmm! I wonder!!



Are you going to help him out?

Several long emails after the first, he might have got hum to
3dB less than it was. I can't get him to
MEASURE things properly, and INVESTIGATE, and the LEARN,
and to STUDY, and to ASK WHY, and all that basic stuff I've been doing
for years.

Maybe I get him over the line though.

He has to be willing to make some effort. manybe when he
starts to get results he will become encouraged. DC heaters
on the first stage are, I would have thought, the first requirement.

Old Leaks and Lowthers are not a good combo with regard to hum.

I can believe it:-)

He's in Italy, and I said find some old guy who knows about
Geloso amps, and he should be able to work a Leak out.

The Leak schematics are easily obtainable.
There is a guy in Oz (whose name escapes me) who runs
a Leak site.

Regards
Iain



.

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