I've got a copy of a Radford series three amplifier
STA15,STA25 circuits and I was after the groups opinion on
this amplifier design with it's EF86 input stage and the 6U8
as the phase splitter circuit. I'm toying with the idea of a
version using 6BW6's (6V6) in the output stage.

Mark Harriss

"Mark Harriss" wrote in message
...

I've got a copy of a Radford series three amplifier
STA15,STA25 circuits and I was after the groups opinion on
this amplifier design with it's EF86 input stage and the 6U8
as the phase splitter circuit. I'm toying with the idea of a
version using 6BW6's (6V6) in the output stage.

Mark Harriss


Mark, I recall the brand name years ago in the UK - highly regarded.
At that time I could not afford that sort of equipment, so I built
Mullard 5-10's instead! I don't recall the original circuits, so
others will have to comment on your 6V6 O/P tube idea but I thought
the original Radfords were a bit more powerful than two 6V6's would
be, e.g two KT66's (guessed.)

Cheers,

Roger
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada

Anti-spam... Reply to: SurfNews"at"sprint"dot"ca (but written
correctly)

Engineer wrote:

Mark, I recall the brand name years ago in the UK - highly regarded.
At that time I could not afford that sort of equipment, so I built
Mullard 5-10's instead! I don't recall the original circuits, so
others will have to comment on your 6V6 O/P tube idea but I thought
the original Radfords were a bit more powerful than two 6V6's would
be, e.g two KT66's (guessed.)

Cheers,

Roger

Hi Roger, AFAIK there was the mono series e.g. MA15 and the
stereo series STA15 using KT77/EL34's all the way up to a STA100 using
KT88's. They were a bit more powerful than the specified output I think the
15 series put out 20 or 25 watts.

The main bit I'm interested in is the phase splitter circuit using a
pentode for one half and a triode for the other half. Quoting from the notes
"The series 3 circuit uses a 6U8 triode pentode; the low impedance triode
being used in the grounded grid section and the pentode in the input
section. Reflected capacitance is ineffective in the grounded grid section
and significantly improved in the pentode section, enabling a bandwidth
considerably greater than that of the conventional double triode longtailed
pair. According to other articles I have the phase splitter goes from 20Hz
to 200 KHz , while the output transformer goes past 150KHz before shunt
resonance cuts in

The Radford STA25 uses 26dB of negative feedback and has
0.9% distortion at 20Hz and 20KHz which dips down to 0.08% from 200 to
2KHz.

Hi Mark.

Build it. It's a very good amplifier indeed. You won't be disappointed.
You can obtain the proper Radford spec transformers from Woodside
Electronics, in Wales. They also might have a pcb.

Good luck.

Iain Churches



"Mark Harriss" wrote in message
...

I've got a copy of a Radford series three amplifier
STA15,STA25 circuits and I was after the groups opinion on
this amplifier design with it's EF86 input stage and the 6U8
as the phase splitter circuit. I'm toying with the idea of a
version using 6BW6's (6V6) in the output stage.

Mark Harriss