An Exercise in Daftness.
Posted: Sun Nov 11, 2018 4:16 pm
The output transformerless valve amplifier has to be one of the daftest amplifying contraptions ever; woefully inefficient and more useful as a room heater than actually amplifying your music signal, yet possessed of a sound so clear, sweet and musical, that once heard, it is never forgotten.
Now I've built four myself, two of which promised a lot, but were plagued with problems; so much so that they became more trouble than they were worth and ended up in my electronics graveyard. The other two were well-proven designs by Bruce Rozenblit of Transcendent Sound, in the USA, built from kits. I built up these for Nigel, one of our Ant's customers. Both worked first time without issue. One; an OTL that uses eight 300Bs (four per channel) has been with Nigel for around 12 months. The other is a single ended OTL, using six (three per channel) EL509 colour TV line output pentodes, or "sweep tubes" as they are known to our American friends. This OTL amp is waiting to be picked up by Nigel, along with a Goldring G99 idler turntable that Ant has serviced and replinthed for him. Testing this latter OTL on my Fane retro speakers has produced some extremely fine music and I'll be sorry to see it go.
So I've had a look in my valve boxes and fished out a pair of rather large DC control valves, that are Ideal for use in an output transformerless amplifier, because they have low output resistance and high transconductance, and have drawn up a circuit for single ended operation, on the back of a fag packet:
It's a three stage job with a JJ ECC99 double triode acting as input and driver stage.
Now I've built four myself, two of which promised a lot, but were plagued with problems; so much so that they became more trouble than they were worth and ended up in my electronics graveyard. The other two were well-proven designs by Bruce Rozenblit of Transcendent Sound, in the USA, built from kits. I built up these for Nigel, one of our Ant's customers. Both worked first time without issue. One; an OTL that uses eight 300Bs (four per channel) has been with Nigel for around 12 months. The other is a single ended OTL, using six (three per channel) EL509 colour TV line output pentodes, or "sweep tubes" as they are known to our American friends. This OTL amp is waiting to be picked up by Nigel, along with a Goldring G99 idler turntable that Ant has serviced and replinthed for him. Testing this latter OTL on my Fane retro speakers has produced some extremely fine music and I'll be sorry to see it go.
So I've had a look in my valve boxes and fished out a pair of rather large DC control valves, that are Ideal for use in an output transformerless amplifier, because they have low output resistance and high transconductance, and have drawn up a circuit for single ended operation, on the back of a fag packet:
It's a three stage job with a JJ ECC99 double triode acting as input and driver stage.