12E1 SE Amplifier

[Paul Barker]

Indeed so. I used to just muck about with amps, now I am mucking about with speakers catridges etc priorities are adjusting.

[The Shadow]

Here's a nice simple linestage I knocked up to go with the power amp.

I was just using what I had available so I could play around with a very simple circuit. The intention is to use a 5687 or 12B4A later on.

400V HT is possibly bit overkill for such a little valve but it allowed a flatter loadline and a higher value plate load which I think is supposed to give less distortion. It swings plenty of volts too. Anyway 400V was what I had going spare on the power supply, so it was that or nowt.

It sounds surprisingly good and gives the system a fuller sound than the cathode-follower 6J5 preamp.

A 5687 or 12B4A will have a lower Zout but the curves don't look as nice. Anyway we'll see what gives when they arrive as they say.

Steve

[The Shadow]

Here are the 12E14 power amp and ecc88 linestages in their new finishes.
Apologies for the quality of the photos but Ant broke my digital camera.

Back spray-painted clear polycarbonate sheeting is a great material to work with, is of course non-magnetic and extremely tough. It can also be planed like wood if you have a sharp enough plane iron.

The first areas in which I saw this or a similar material used in the DIY context were James's open baffles and Andrew I's blue LED festooned GM70 experiment. I then picked up on it again on the Tubelab homepage.
The rear paint spraying idea was picked up from 1960s radio tuning dial glasses, old Meridian CD transports etc.

Now then; if I really make a serious attempt to show some self-discipline; this set-up might last more than a couple of months; after all the circuits have now been whittled down so much that they cannot be simplified or messed with any more.

Steve

[Shane]

Any chance of a schematic for the line-stage power supply?

Thanks,

Shane.

[The Shadow]

I'll see if I can dig it out Shane.

TBH the HT is far too high at 400 odd volts for a single ECC88 which means that the 47K (48 actually) anode load resistors had to be made up from four 12K 10W wirewounds to cope with the dissipation caused by the massive voltage drop needed. The PSU was intended for 6SN7/6J5s which unlike ECC88s do need that level of HT to perform at their best.

Steve.

[Shane]

Thanks Steve. What would you recommend for B+ if you were starting from scratch?

[The Shadow]

Originally Posted by Shane

Thanks Steve. What would you recommend for B+ if you were starting from scratch?

Morgan Jones used an HT of 285V is some of his designs in "Valve amplifiers"

I reckon around 200 - 280V would be about right for a single valve. The ECC88 works fine at low voltages. One of Jones' rules of thumb is that the cost of a power supply is proportional to the square of its HT voltage. So the lower the HT you can get away with the cheaper it'll be.

Steve

[Shane]

As you might have guessed, I'm thinking of cribbing your design for my first scratch-build project. I have a transformer which gives me 375-0-375 and a Mullard EZ80, so I just need to work out what goes in between. All suggestions gratefully received!
Initially, it will be used drive the power amp of my A&R A60, but hey, we've all got to start somewhere...

[The Shadow]

Originally Posted by Shane

As you might have guessed, I'm thinking of cribbing your design for my first scratch-build project.

ooooh steady on Shane, I wouldn't do that

The 375-0-375 transformer with a cap input supply and SS rectification will put out around 500VDC with no load which again is too high for an ECC88, which will put next to no load on such a beast of a transformer. A choke input supply would put out less than 375V (about 340 I think which is again still a bit high) and you need to be drawing a certain minimum current for the choke to operate properly otherwise you will end up back at 500 odd volts or possibly even more pretty darned sharpish if you are not careful.

The Hammond 369BX 150-0-150 @ 75ma transformer would be a far more appropriate purchase at approx £39 from Phillip at Bluebell Audio. He will also be able to give you plenty of advice. In fact you will have trouble trying to get him to shut up once he gets going

He was incredibly helpful to me when I started building my own amps. Giving him a call would be an excellent idea. He'll talk you through the whole process if you buy your bits off him.

Steve

[The Shadow]

Here is the final schematic.

The anode loads were changed to 22k and the cathode resistors to 820R which has given an improvement in smoothness and clarity from the linestage. We now have 267 volts at the anodes which is less of a voltage drop than before and is not throwing as much HT away as heat.

That makes me feel a little easier.