[Engineer[_2_]]

On Jun 29, 7:34*pm, Patrick Turner wrote:
On Jun 26, 8:05*am, John Byrns wrote:









In article ,
*John L Stewart wrote:

John s short note accompanying his amplifier invites us to comment &
offer suggestions. I did 7 or 8 years ago & found that John was already
happy with his amplifier just the way it was then. And I guess he still
is. And John comments freely on the work of others.

I don't think I ever liked it "just the way it was", there have always been a
few details that need improving. *

First, I would like to change the input tubes to the equivalent of 12AU7s,
unfortunately I don't think there is an octal equivalent for the 12AU7, so I
would have to somehow adapt 9 pin miniature sockets to the existing chassis
holes. *I remember that 9 pin miniature sockets used to be available that would
fit the cutout for an octal socket, does anyone know where I might find a couple
of those?

Second, I would like to try connecting the output tubes as triodes. *This would
require adding an LC section to the power supply, and would unfortunately cut
the power output to something less than the half the power output available in
pentode mode.

Anyway, here is my take on some minimal changes that would result in a
substantial performance improvement.
snip interesting read for brevity.

Hmm, with 240V mains here I won't be rushing into building any amp
without a PT.

When wanting to fit a ninepin socket into a larger 28mm octal socket
hole I just bore a 19mm hole in a bit of scrap metal *and bolt it in
under the existing 28mm hole. I often have to replace ALL the 1935
tubes in some horrible old radio chassis made in 1935 and fitted with
4V heater european tubes with queer sockets. I usually strip the whole
chassis, sand it dowm and paint it with a pale greeny beige color,
Circuit is totally revised, 1950s tubes are used, and I will cut out
plates of color-bond sheet metal to overlay large areas of the chassis
with the 50mm holes for euro-tube sockets. When finished, nobody cares
about the "**** that was there" and they marvel at the sound because
I've incorperated all the best ways to build an AM radio.

Perhaps there's a parable there...

But anyway, there's no reason why the SNR for such amps can't be good
if all circuitry is referred to a 0V rail. The use of an input tranny
with a floating input coil would also help against hum. Just what
other gear was used may affect noise though, TT, preamp perhhaps.

I better get on my bike and ride out of range of the old rubbish,
boots, old bottles being hurled my way because I humbly suggested the
merits of major parts upgrading to ancient old junk.

Patrick Turner

No boots or old bottles from me... As we all know, the range of radio
"restoration" goes from "everything authentic to period" (re-stuff old
caps, keep dog-bone resistors, use DCC wire, etc.) to "rebuild the
whole thing" with modern components. I'm somewhere in the middle with
a tendency to keep as much as possible if it works, but I always
replace all electrolytics and wax caps (time bombs!), but I don't re-
stuff any boxes and I don't leave the old dead ones littering the
chassis. I replace tube detector diodes with 1N34A point contact
diodes and I add audio NFB to nearly all the radios I restore (taking
out the useless tone controls - why top cut already crappy AM?) If
the original wiring is stable, i.e. insulation cracked but not
"leaking" and I don't have to move it, then I leave it... else I use
salvaged plastic covered wire in bright colours to replace. So far,
I've found no dead large IFT's but if I did I'd replace with a NOS
small, late 1950's types (but I'd likely put these inside the old
can!) Mains wire is always replaced with new plastic stuff, even 3
pin plug and wire, if to hand.
On the contrary (perhaps the main topic of this post), I invariably
rebuild tired old P-P "Hi-Fi" audio amplifiers completely because 1)
it's fun, and 2) I can do it better than the original in most
cases.
Cheers,
Roger