DIY Phono Stage WIP

The heaters in all the valves come on and glow brightly, so I know the heater supply is ok. I've checked over the PSU and the circuit seems fine and the voltages are what you'd expect when there's no load being drawn. Both channels are dead so I think you're probably right that it's something common to both channels which in my mind suggests either a fault in the B+ supply at the amplifier end or a faulty valve. I've got a pair of old mains powered computer speakers that I assume could be used as a test amp. I take it I'd just rig them up so I could connect them to the output from each valve in the amplifier section and see if there's a signal when the power is applied?

 

yep,but you olny need 1 channel,wire the input of the test amp to a test probe and the earth to the earth of the unit under test,feed an input to the pre amp and test for a signal at varies points,id start with junction of r23,r7,r8, then r10,r16 and c5.

 

Thanks for the advice. I'll give it a go tomorrow and see if I can track down the fault.

 

Doh! I feel like a right dimwit. My father looked over the circuit for me this evening and immeadiately spotted that I'd got R3 and R9 round the wrong way i.e R9 junctioning off R3 and not vice versa. So the amplifier stage is up and running at long last .

 

Wow it's looking good ... any later pics?

Where did you get the transformer from? I've always fancied having a go myself (building a tube/valve amp) - but I've never known where to get the mains transformer from.

How much did the components cost you in total?

So many questions lol ...


Steve

 

Steve, the transformer was sourced from here www.bluebellaudio.com. They also stock a wide variety of items associated with valve amplifiers.

In terms of cost, the transformer was £55 including shipping; the MDF for the case was £4; the valves and valve bases were free (courtesy of my fathers stock of old military/industrial valves salvaged from a variety of sources); the aluminium was also free courtesy of the college where my father works; the core electronics cost £20 including shipping from the USA. With additional components i.e the switch, IEC socket, umbilical connector, wire costing around £10-20 and £20 for the paint/laquer for the cases. So in total it's cost in the region of £120 to put it all together.

I'll post some more pictures of the finished units once I've touched up some of the paint and given the laquer it's final polish and finished up the last little bits of the PSU circuit such as wiring up the obligitory blue LED in the power switch .

 

I'm surprised it's so cheap ... by far the most expensive part seems to be the transformer.

I'll be very interested in seeing the final pictures ... and yes, the LED has to be blue.


Steve

 

Steve, my estimates of the costs might be a little out as I bought stuff from a variety of sources over the space of a month or two and don't have the receipts to hand. I obviously saved around £20-30 by already having the valves; plus about £10-15 by having the ceramic valve bases and also by building my own MDF cases I saved on paying around £20 for a manufactured steel/aluminium enclosure.

 

Still, very impressive all the same. Looks like you've done an excellent job - and I'm sure you'll benefit in terms of not only nice sound, but also the satisfaction that you built the wee beasty yourself

Steve

 

Well it's running as I type this. Can't really comment on the sound yet as the valves and bits and pieces are no doubt warming up and settling down. But I'm really quite impressed that I've managed to build something that actually works! The most complicated DIY I'd done before this was some interconnects/data cables.

Next up it's time to start rolling those tubes, it's currently got Tung-Sol 6SN7GTs and a Sylvania 6SL7GT in it, but I've got RCA and Raytheon 6SN7GT's to experiment with and I've still to fix up the blue LED (forgot to get a resistor to step down the voltage). Oh, and once I've stopped being amazed that I've got it to work I'll take some photos .

 

well done bloke,
Hope you have many hours pleasure trying new parts as i did

 

Cheers, so far I've found that out of the old valves my father salvaged only the RCA and the Raytheon 6SL7GT's and 6SN7GT's are actually still functional, the remaining valves either didn't work at all or only one channel was working. The Tung-Sol's that I had running in it were very muted compared to the Raytheon's that replaced them today suggesting that they were on their last legs/defective. I finally found some suitable resistors in my father's stockpile of various electronic componenets so I've got the blue power LED working. But in typical fashion the batteries in the digital camera ran out of charge as I was taking the first picture to 'show off'. Hopefully I'll have some photos to add to the thread tomorrow.

Just out of interest Penance, what tweaks/alterations did you find to make the most noticeable difference to your Cornet? Not that I'm in anyway dissatisfied with the current performance of mine, just looking for ideas for future tweakery.

 

Pics pics pics! Now! Dammit!

Any plans to making a valve pre/power amp?


Steve

 

Main components that have a noticable effect are caps and resistors in the signal path. I have settled with TRT Dynamicaps and will be trying Kiwame resistors soon.

 

Steve, as promised here are some pictures of the phono stage in action. I had to increase the value of the resistor on the power LED this morning as I discovered last night that it could happily illuminate my room sufficiently that I could read LP sleeves with all the other lights out .

Penance, thanks for the suggestions on what to look at tweaking when I get the tinkering bug.

As for other DIY projects, I am now quite tempted to try and build an integrated valve amp or a pre/power combo.

 

Brilliant ! well done that man, I'm impressed. It's great when something you have been slaving over for hours actually sparks into life and works.

Mind you don't burn your retina's out with that bright blue LED

 

Sid and Coke, thanks. That's the LED with only 10 mA powering it! It was originally on 20 mA and illuminated the whole room. All I've got to do now is double check the earth on the inputs as I've got a very low amount of hum, probably just a slightly loose connection.

 

Nyagh!!! <drooooooooooooooooooooooooooooooooooool>

That really is pretty damn impressive. I have a pretty good understanding of electronics - and have built all sorts of stuff, but I'd feel well out of depth with valve gear (perhaps experience has taught me that 300+V is bad, very bad!). You've also done a fantastic job with the cases - I know that anything I attempted would turn out pretty poor in comparision. I wouldn't even know where to start with cutting holes in aluminium etc ... and even if I had the tools I'd probably screw it up.

The slight hum might be a poor earth - or you might have some hum coming from the power supply. Do you have a scope? If so - hook it up and see how much ripple you're getting after it's been rectified and supposedly smoothed ...

And yes, that blue LED looks pretty bright even now! Kinda nice though ...


Steve (still impressed)

 

Steve, thanks for the comments. The LED has now been dropped to 7mA so it's not quite so bright, almost on a par with the brightness of the LED's on the RA-02. In terms of building the cases etc. I was lucky that my father has a lot of DIY wood working/metal working experience and the tools to go with it so I was able to get him to help me. As I say I've never built anything like this before, but I was very conscious of the possible dangers of high voltages and currents, so I double checked everything I did and had my father check for me. But to be honest if you can solder and have a grasp of electronics I can't see you going too far wrong, unless you do something blatantly stupid. If your keen to try building a valve amp try checking out www.worldaudiodesign.com who sell a range of really nice looking kits that come with everything you need and comprehensive instructions/schematics, I think a few people on here have built them and been impressed.

I sourced the hum to a possibly dodgy earth connection on the input RCAs, to be honest though it wasn't actually noticeable unless you had your ear against the speaker cones with nothing running and the volume ramped up so it might actually have been the normal 'hum' of the valves and electronics in the circuit. Regardless the stage is sounding pretty damn good to my ears, and I'm pretty sure I can pick out a few more details and the sound is richer compared to the RA-02's inbuilt phono stage. All I need to do now is give the cases their final cut/polish to bring the laquer to a nice shine and buy some more vinyl .