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A valve line pre amp for the DIY fishy

Arkless Electronics

Arkless Electronics

Trade: Amp design and repairs.
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Cheap valve, and what, gain of 4x or so fixed by shunt feedback, plus simple cathode bias. Hard to go wrong :)

PS I like the call-out of UF4007 diodes - dirty cheap, and very effective/quiet in any low-current supply.
 
I'm tempted, as I've been looking for valve project to learn the ropes on. Regarding the transformer - is there something off the shelf you had in mind or can we just add a few turns to a 1:1 isolating toroid?
 
It's just a generic 250V and 6.3V small mains transformer. This will be found to be the most easily available TX for valve use and various ones are available. You could of course wind the extra turns on to a toroidal isolating TX yes.
 
I`m quite tempted - it`s 44 years since I last built a bit of valve kit and I probably have all the bits somewhere - trouble is I don`t actually need it...
 
I also designed an output mute circuit for it which grounds the output until the valve has warmed up but in the form above the circuit is so simple that the mute circuit doubles the complexity! I didn't want to put off the novice with having to also build a piece of SS circuitry on veroboard for the delay circuit, which a novice could find more challenging than the actual amp... It also of course requires rectifying and smoothing the heater supply in order to power the SS circuitry...
 
Obviously experienced constructors will be able to go ahead with nothing more than the schematic, but if there is real interest I'll post a BOM for it with order codes etc and find a suitable TX people can order for it.

If wanting more than the one input then a source selector switch will be needed before the vol control.

I put the same design up on aos, with BOM, but no one built one and its all been deleted since so I thought I'd offer it up here and see what interest there was:)
 
I'll probably give it a go, thanks Jez - specs of a suitable TX would help :)

Cheers, Richard
 
Dowser said:
I'll probably give it a go, thanks Jez - specs of a suitable TX would help :)

Cheers, Richard
Click to expand...

0 - 250V @ >40mA and 0 - 6.3V @ >0.5A secondary, primary to suit your countries mains supply (still in 220V land these days Richard?). A centre tapped 6.3V winding is no problem and means you can get rid of R16 and 17 and connect the centre tap to ground.
 
Between the bypassed cathode and the feedback loop, it should have decently low output impedance even without a follower.
 
Yank said:
Between the bypassed cathode and the feedback loop, it should have decently low output impedance even without a follower.
Click to expand...

;) IIRC about 120 Ohms ish. THD about 0.02%, FR about 5Hz - >100KHz. These are rough figures from memory of when I designed it and did measurements on the prototype.
 
Very good of you to post it up Jez, hopefully it might encourage a few people to dip a toe into thermionic waters.
 
Thanks Barry. It's ideal for a first build as it's as simple as it gets...

I found some old notes and it's about 450R output impedance, so still well low enough, THD 0.013% @ 5V RMS output so <0.01% at normal levels.
 
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Arkless Electronics said:
0 - 250V @ >40mA and 0 - 6.3V @ >0.5A secondary, primary to suit your countries mains supply (still in 220V land these days Richard?). A centre tapped 6.3V winding is no problem and means you can get rid of R16 and 17 and connect the centre tap to ground.
Click to expand...

Thanks - yes, in Swissieland still - think I may even have a spare TX, need to check.

Thanks, Richard
 
Is it possible to reduce the overall gain to ~unity? I'm using a Leak ST 20 at the moment and the gain structure with most pre-amps is not ideal.
 
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