06-11-2013, 08:55 PM
A long time ago I made a test speaker/bench amplifier, see:
http://golbornevintageradio.co.uk/forum/...hp?tid=692
This was quite quickly replaced by a filleted Roberts R600: http://golbornevintageradio.co.uk/forum/...hp?tid=833
I still used it to play MP3 players/DAB radio through though.
A few months ago I bought one of the repro Philips Super-Inductance sets and said, in jest, that it'd sound better with an EL84 in it. For all that I was joking it did get me thinking about building a valve amplifier.
Most designs seem to use expensive driver valves like ECC82/3s or EF86s. The obvious thing would be to use an IC for this but I couldn't find a circuit. Oddly there are plenty around with valves driving IC outputs which strikes me as utterly pointless as if there is such a thing as 'Valve Sound' it'll be coming from the output stage.
Eventually I came up with this which is the output circuit from a VHF61 driven by a µA741 circuit from the Burr-Brown Handbook. A 741 may be SS but does have a certain vintage quality.
I know that the 1000µF capacitor on the 741 and the 47kΩ grid-stopper are both larger than needed but it worked fine.
I still felt I'd copped out a bit so have recently been playing around using an EF80 and finally settled on with this:-
As can be seen, I included negative feedback but after assembling it properly found that it wasn't needed.
I also tried using a 6CH6 as an output valve. In the lash-up version this was as good with no NFB as the EL84 was with it but on the finished version the EL84 is definitely better so that's what I've stuck with.
I have received appreciated help from Alan but it's failings and limitations are mine own.
Research and Development
Final Version
- Joe
http://golbornevintageradio.co.uk/forum/...hp?tid=692
This was quite quickly replaced by a filleted Roberts R600: http://golbornevintageradio.co.uk/forum/...hp?tid=833
I still used it to play MP3 players/DAB radio through though.
A few months ago I bought one of the repro Philips Super-Inductance sets and said, in jest, that it'd sound better with an EL84 in it. For all that I was joking it did get me thinking about building a valve amplifier.
Most designs seem to use expensive driver valves like ECC82/3s or EF86s. The obvious thing would be to use an IC for this but I couldn't find a circuit. Oddly there are plenty around with valves driving IC outputs which strikes me as utterly pointless as if there is such a thing as 'Valve Sound' it'll be coming from the output stage.
Eventually I came up with this which is the output circuit from a VHF61 driven by a µA741 circuit from the Burr-Brown Handbook. A 741 may be SS but does have a certain vintage quality.
I know that the 1000µF capacitor on the 741 and the 47kΩ grid-stopper are both larger than needed but it worked fine.
I still felt I'd copped out a bit so have recently been playing around using an EF80 and finally settled on with this:-
As can be seen, I included negative feedback but after assembling it properly found that it wasn't needed.
I also tried using a 6CH6 as an output valve. In the lash-up version this was as good with no NFB as the EL84 was with it but on the finished version the EL84 is definitely better so that's what I've stuck with.
I have received appreciated help from Alan but it's failings and limitations are mine own.
Research and Development
Final Version
- Joe