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A valve line pre amp for the DIY fishy

Thanks David. I do have a load of large green 18ohm wirewound, about 5W I think so I'll try them first. Later on it'll be interesting to see if regulated DC for the heaters sounds any different!
 
Great accessible little project. Thanks very much for posting. Maybe it can become the valve alternative to the Pink Fish/Piglets Dad B4?

I fancy a go at this as a first foray into Valve builds as well. Had a good look at the design and mugged up on some valve theory with Morgan Jones's book.

A couple or three newbie questions if I may please?

The ECC88 is a double triode. I've read it can be used in parallel. Are we doing that in this design or is it one half for each channel? There are two valve bases at the top of the BOM, so perhaps we are leaving one half of each ECC88 unused?

I have a 12v transfomer I can use for the heater but will need to drop the voltage to 6.3v. Will a LM317 based circuit be OK for current at 1.5 Amps max?

Can I make the higher wattage resistors up from 0.5W metal film in series or are wirewound better for other reasons?

Thanks for the inspiration.

John

Thanks John. Ok if it was two triodes in parallel I would have shown that on the schematic:) Many of the parts on the BOM come in minimum order quantities.. there will probably be 5 listed for some parts where only two are required. If in doubt schematic wins over BOM;)

You can use any method of obtaining the 6.3V.

You can indeed make up the resistor values from series or parallel combinations of lower Wattage resistors. Careful of the Voltage ratings of some smaller resistors! Series will help here. As I mentioned earlier don't use a wirewounds for the anode resistors.

Something not on the schematic that occurred to me is that pin 9 should go to ground. It's a screen between the valve halves.
 
The story so far:- Needed a solder flux 'fix'!
Just junk box bits and make it fit as you go along.

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A few more build guidelines. C7 (47uF) and everything to the right of it should be close to the mains transformer (the two 22R resistors can more conveniently be fitted to the valve base. Not critical) and should have its own ground but not to chassis.
Take the negative output from the rectifier to the negative of C7 and then from C7 not the rectifier take a wire from here to the main star ground, which does connect/bolt to the chassis. ALL OTHER GROUNDS GO TO THIS STAR GROUND INDIVIDUALLY. Do not even use the chassis to ground the phono sockets.
Keep the wiring from TX to rectifier and from rectifier to smoothing capacitor (C7) short and direct to minimise noise.
The 6.3V heater wiring from TX to the valve base should be made from a tightly twisted pair, as in Snowman Al's example, and preferably kept close to chassis. Try to keep this away from other wiring etc.
Remember that electrons have no concept of neat! Attempting to, for example, bundle all the wiring together in a nice neat wiring loom would almost certainly mean bad mains hum!
The experienced constructor will know where they may or may not take a liberty or two and get away with it... but if you're not in that group then follow what I've said and, if in doubt, think short, direct wiring rather than making it three times as long so it can be bent in nice neat looking right angles etc. This doesn't mean wiring should be so short that it is taut and pressing on other parts of course...
 
Yes it is Alive!
Lash up number 1 working ...

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Running off my bench supply at the moment and sounds very 'weighty'.
With 210 volts on the top of the 30k resistors we have about 56 / 57 volts on the anodes and 1.05 volts on the cathodes. (So roughly 4.5ma per side.)
Very slight hum, to be expected with a lash up. A bit longer listening then a tidy up.
 
That's great! You should have 250V on the top of the 30K resistor, 66V on the anodes and 1.3V on the cathodes, within usual tolerances allowing for valves of different age or make etc. It was optimised for these voltages so if you can get the HT up to 250V on the top of the 30K's that would be good.

Also the design as posted has huge isolation between channels etc by having the two separate 47uF decoupling caps each fed by its own 15K resistor.
In spite of being a very simple circuit the schematic as posted was carefully optimised...

There should be 340V on the top of C7. This is with a mains TX secondary of 250V. If using a slightly different mains TX with say 240 or 270V secondary then the 15K resistors R13, R14 should be changed to make the voltage on the top of the 30K resistors 250V.
 
I looked a little more tonight - the TX I have is already in an enclosure with switches, so I’m going to use an umbilical and a smaller enclosure for the valve and stuff I think;

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Bits I’m missing ordered - I went for higher uF caps (220uf), so should be good to go in a week or so.
 
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One suggestion- The addition of a 1M ohm resistor from the output terminal to ground would eliminate the build up of any DC offset on the output capacitor and keep a grid leak path from the grid of the triode to ground should the wiper of the volume pot fail open circuit or become noisy.
 


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