In article , Don Pearce
wrote:
On Sun, 29 Jul 2007 14:36:37 +0100, Jim Lesurf
wrote:
[snip]
FWIW The most recent addition has included circuit diagrams of some
early Radford valve amps. These aren't yet mentioned on the main Audio
Misc page as I plan, first, to add more Radford material. But the
Radford circuits can be found directly via.
http://www.audiomisc.co.uk/Radford/c.../circuits.html
JIm - interesting circuits, but they look like it took a bit of fettling
to get them flat and stable. Do you know the history of the pole/zero
combination across the first anode load (ok, i can see that Mullard felt
it necessary in the first place)? Also C7/R13 in the first Radford
circuit. Was it all there to make a circuit that was able to handle
overall feedback without honking?
Some more of the details should be clear when I put a version of the recent
HFN article up on the site, and add some more info.
The magazine article only appeared this month, and I like to give about 6
months between magazine publication and web publication. FWIW The current
'circuits' page was done early as a reference for the HFN article as the
don't want circuit diagrams in the magazine these days. warning="plug" If
anyone is interested, the article is in the issue on sale now, cover-dated
September. /warning
The difficulty is as you'd expect. Radford and Bailey engaged in what
Armstrong always called "continuous development". i.e. Radford was an
incurable fiddle-to-see-if-it-can-be-improved type. So the circuit might
change without the model number showing this, or with any announcement. :-)
Alas, this process often seems to have gone unrecorded. As a result, the
details of the chosen circuit values changed irregularly with time.
In fact, the Mk1 diagram on the page is based on the circuit given by JLH
for the original MA/STA, but I suspect there are other versions which
differ in detail. Also, the JLH version seems to have one or two errors
IIRC.
I don't have a copy of an 'official' Radford diagram for the Mk1 MA/STA
which I can identify as such.
The Mk 2 changed during the four months when HFN ran the original series on
it! Have various versions of the Mk2.
Do you mean C7/R13 of the Mk2 as shown (diagram nearest bottom of the
page)?
In effect, IIUC the simple answer to your last question is "yes" - although
I don't know the full history of this. And am handicapped by having a
relatively limited understanding of valve designs.
According to the info I have, the Mk2 with the triode-pentode, etc, has a
much better power bandwidth, etc, than the Mk1. I understand this was a
result of a combination of Radford's drive and transformer skills, and
Bailey's work on the gain behaviours of the active stages. So I presently
assume that Bailey was behind the RC tweaks to flatten/extend the
gain/phase response. This in addition to Bailey being the one who came up
with the triode-pentode splitter as being a key change. The references
certainly indicate this quite clearly.
Above said, I am still collecting info on this, so if anyone has relevant
documents (or was there!) I'd welcome input. :-) I have various other
circuit diagrams and manuals, etc. But a snag is that many are not dated or
clearly identified, so have to be put in context by a sort of 'forensic
archeology' approach where I have to deduce in what order, and when,
changes were made. Bit like 'Time Team'... :-) The snag is that there may
be changes I have no info about at present, despite having a fair bit of
paperwork.
Some of the later designs like the 'renaissance' one are easier, but aren't
really Radford in the same sense. Some quite interesting designs, though.
Some changes are fairly obvious and easy to link to a given Mk/series, but
others are not.
All being well, I hope to flesh this out, and add circuits, etc, for the
later Radford designs, once the relevant 'round tuit' appears on my desk.
;-
FWIW, though, I will be doing something similar with the Sugden class-A
amps, and will probably do that first as the material I have is better
ordered. Plus the HFN article that started that appeared some months ago,
now. ;-
An advantage here is that the people at Sugden were able to give me some
help wrt the circuit evolutions for the A21, etc, as they still had the
original documents and knew enough of the details. They have been very
helpful. Alas, I have had no 'inside' contacts for Radford... But if I am
lucky, the initial page(s) may prompt something in time.
One of the purposes of setting up on the new server is that it will give
more space for things like diagrams. So I should eventually be able to
include a fair range of info. Only problem is that there are still only 24
hours in a day... and I probably drowze though more of them than I used to.
;-
Slainte,
Jim
--
Electronics
http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc
http://www.audiomisc.co.uk/index.html
Armstrong Audio
http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc and Armstrong websites, plus Radford circuits.
Linear Mode
