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Ian Thompson-Bell Ian Thompson-Bell is offline
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Default Which tube expert can do custom works?

Iain Churches wrote:
"John Byrns" wrote in message
...


The BBC seems to have used "low level mixing" as late as the 1950s,
necessitating only one microphone amplifier for typically 4 microphones.


There are some interesting BBC pics of Type A equipment at:

http://www.btinternet.com/~roger.bec...pea/typea1.htm


Iain




RDH4 cites a 1946 BBC article about the design of this desk. I managed
to get my local library to get me a copy. Fascinating.

Cheers

Ian
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John Byrns John Byrns is offline
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Default Which tube expert can do custom works?

In article ,
"Iain Churches" wrote:

"John Byrns" wrote in message
...


The BBC seems to have used "low level mixing" as late as the 1950s,
necessitating only one microphone amplifier for typically 4 microphones.


There are some interesting BBC pics of Type A equipment at:

http://www.btinternet.com/~roger.bec...pea/typea1.htm


Interesting, I have a BBC "Training Manual"/Book from 1955 that has a
couple of pages that describe the Type A equipment along with a block
diagram of the mixers components. More to the point of this thread the
book also includes an approximate schematic diagram of the AMC/2 "A" or
microphone amplifier mentioned on the web site in connection with the
Type A equipment. It is interesting in that the AMC/2 amplifier uses a
single pentode tube/valve with shunt feedback, probably not a topology
that would find great favor with this group.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
  #363   Report Post  
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Ian Thompson-Bell Ian Thompson-Bell is offline
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Posts: 493
Default Which tube expert can do custom works?

John Byrns wrote:
In article ,
"Iain Churches" wrote:

"John Byrns" wrote in message
...


The BBC seems to have used "low level mixing" as late as the 1950s,
necessitating only one microphone amplifier for typically 4 microphones.


There are some interesting BBC pics of Type A equipment at:

http://www.btinternet.com/~roger.bec...pea/typea1.htm


Interesting, I have a BBC "Training Manual"/Book from 1955 that has a
couple of pages that describe the Type A equipment along with a block
diagram of the mixers components. More to the point of this thread the
book also includes an approximate schematic diagram of the AMC/2 "A" or
microphone amplifier mentioned on the web site in connection with the
Type A equipment. It is interesting in that the AMC/2 amplifier uses a
single pentode tube/valve with shunt feedback, probably not a topology
that would find great favor with this group.



Indeed. ISTR it uses a good old 6AU6 tube - I think the circuit is given
also in RDH4. I heard the choice of tube (which was apparently common in
UK TV receivers of the day, was so that an OB crew could easily get
replacements at the local radio repair shop.

Does your training manual also show the means of mixing? - the article I
have only has a block diagram that omits many of the details.

Cheers

Ian
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John Byrns John Byrns is offline
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Default Which tube expert can do custom works?

In article ,
Ian Thompson-Bell wrote:

John Byrns wrote:
In article ,
"Iain Churches" wrote:

"John Byrns" wrote in message
...

The BBC seems to have used "low level mixing" as late as the 1950s,
necessitating only one microphone amplifier for typically 4 microphones.

There are some interesting BBC pics of Type A equipment at:

http://www.btinternet.com/~roger.bec...pea/typea1.htm


Interesting, I have a BBC "Training Manual"/Book from 1955 that has a
couple of pages that describe the Type A equipment along with a block
diagram of the mixers components. More to the point of this thread the
book also includes an approximate schematic diagram of the AMC/2 "A" or
microphone amplifier mentioned on the web site in connection with the
Type A equipment. It is interesting in that the AMC/2 amplifier uses a
single pentode tube/valve with shunt feedback, probably not a topology
that would find great favor with this group.


Indeed. ISTR it uses a good old 6AU6 tube - I think the circuit is given
also in RDH4. I heard the choice of tube (which was apparently common in
UK TV receivers of the day, was so that an OB crew could easily get
replacements at the local radio repair shop.


I found some BBC amplifiers in the RDH4 that use 6AU6s, but I assume
these were a later design than the AMC/2 which both the web site Iain
cited and the manual/book say were used in the Type A equipment. The
Type A equipment appears to have been a range of studio consoles, not OB
equipment, so being able to buy replacement tubes at a radio repair shop
wouldn't have been very relevant. Both the web site and the book/manual
say that the AMC/2 used a single type AC/SP3A pentode. I couldn't find
any data on this tube on the usual web sites, I am curious what the
transconductance was to see how they got a gain of 50 dB from the single
tube. It would also be interesting to know what the primary impedance
of the output transformer used in the AMC/2 was. The gain was
switchable between 50 dB, 40 dB, and 30 dB, reading between the lines I
assume no feedback was used at the 50 dB setting.

Does your training manual also show the means of mixing? - the article I
have only has a block diagram that omits many of the details.


It only offers a block diagram, perhaps the same one you have. The
block diagram shows the mixing being done with two "mixing pads", one
for the main output and one for the echo send. I assume these "mixing
pads" are simple 600 Ohm resistive mixing networks.

I am going to have to write to the webmaster of the web page Iain cited,
the book/manual I have appears to answer the question asked on the web
page relating to the mystery of what happened to the Type A Marks III &
IV, as well as giving a different spin on what the Mark I was.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/
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