Cheap project competition

[Paul Barker]

Originally Posted by NickG

Or, it could be the competition is such that excuses are being prepared :-)

Err, cough cough,

I've done nothing.

[andrew ivimey]

Don’t you just know when its right?

Cheapniss Mk7 (or is it 8) has evolved, switched on and I can tell this little amp is in the right ball park.

El84s were not kind to me so I returned to familiar ground; I did a spud 6S45. It is amazing that 1 watt from one small valve can sound impressive. But hard… There may be a way with this valve to get it right, but I haven’t found it yet.

So, staying in familiar territory, I wondered just how much I could slim down my version of Bottlehead’s 6DN7 SEX amp; direct coupled and solid state rectification.

I am not convinced that solid state rectification really has to be inferior even with SE. It is not elegant – we love another valve glowing all nice and warm- but it is cheap.

So, a direct coupled 6DN7, those Hongkers c cores (£25 the pair) output transformers (are fine) with SS PSU all adds up to (if NEW components) around £70.

1watt of joy – and there is a couple of tweaks to boost the output because I can only get 340 volts HT, and I‘d like more. I’d like more current too, but the cheapest PSU metal I can find is of course Maplin for mains power and choke. Already the PSU costs only slightly less than the amplifier section.

And it sounds very good – with my fostex206s – so I am around 95dB sensitivity. Oh I dream of 98, 99, 101dB!

I ‘spect I’ll get back to putting el84 (Civilisation) to rights now I feel better about all this cheapniss business.

Rock on!

[pre65]

Hi-now i'm back from my hols i need to "sort out" the 832 PP.

i need some advice on the earthing.

i prepose to do away with the bus bar and star earth.as i have 3 sections to the amp (2 single channels and a joint semi dual mono power supply) do i earth all the earths on each section together,then connect those 3 points to the IEC earth ?


Philip

[andrew ivimey]

Hello Philip, hope your hols were good.

I have done with 832s as PSE; loud but uninspiring. Don't be put off with that. I was playing, and not very seriously.

(I have three spare 832s now, when you get yours up and running and decide you need spares.)

Don't earth back to the IEC connector. If you bother with the mains earth, that should only be grounding the chassis and have nothing to do with the electronic circuit.

Choose a midplace in your circuit and create a point that all earth leads go to. It is as simple as that. You are trying to avoid any notion of an earth loop, which makes for awful hum. I am not convinced star earths have any particular virtue except that the builder is taking care not to make a hash of earths all over the place and inevitably causing HUM.

[pre65]

Hi Andrew-yes the holiday was excellent.

we all ended up in St Jean de Luz and while my friends were trail riding in the Pyrenees for 3 days i had to occupy their wifes.
we also went to see the French martyr village at Orradour sur Glane which was a harrowing experience.one visitor had entered in the visitors book "every child should see this so it never happens again".

anyway back to the 832.are you saying just ONE earth point for the whole amp?

also should i "lift" this earth to mains earth with a resistor ?

and then connect the 3 baseplates to the IEC earth ?



Philip

[andrew ivimey]

anyway back to the 832.are you saying just ONE earth point for the whole amp? YES, alternatively, one star for each channel.

also should i "lift" this earth to mains earth with a resistor ? don't connect the electronics to mains earth, even with a resistor.

and then connect the 3 baseplates to the IEC earth ? You could do this,I wouldn't, or just ground the mains transformer to IEC earth.

[david counter]

if it helps Philip
what I do is have one earth rail for the ps and one for the signal, both of which go to a common point and then onto a chassis earth post, this is connected to the IEC,

the idea as I understand it is only to have one path to earth,

[andrew ivimey]

Whereas your chassis earth can be connected to mains earth (IEC socket of you use them), your circuit earth should not.

[andrew ivimey]

I took the 832PSE apart this afternoon, but kept the front end (SRPP) and then thought, well why not? and added an el34 at the S.End.

So easy, too easy. It sounds lovely.

The circuit is similar to Steve's and JELab's but I used what I had to hand so not quite. EL34s are louder than 5881s.

Question: why did Steve connect Grid3 to earth, rather than cathode or through a 1k resistor to anode?

If all these bits were new it would have cost about £150 - I could use chinese output transformers which would shave off £50; that would get it into the cheap category then. Perhaps then it might have a place in the 'Cheap Project Competition'.

All that faffing around!

It ain't clever but it does sound rather good!

[vitalstates]

Originally Posted by andrew ivimey

Question: why did Steve connect Grid3 to earth, rather than cathode or through a 1k resistor to anode?

If I remember correctly it was Paul Bs suggestion......it worked, but I didn't question why at the time......Perhaps Paul might pick this up and explain.

I also keep reminding myself that this is the case....just in case I decide to slip a 'normal' pentode in for comparison...it might make for interesting results........This occured to me because I am currently running on kt66s with no mods to the operating point.


Regards

Ed