12E1 SE Amplifier

[Andrew]

Hi Steve,

How much current is your 88 passing? Why do I ask?

One school of thought for a pre is to run them hard, keeps noise down and reduces Zout. I have tried 15mA which to me sounds cleaner than say 8mA. But then again you found this didn't suit your tastes, so could be a red herring, I have a TVC after mine so the lower Zout will be a benefit.

-- Andrew

[Greg.]

Hi Steve,

I really like the simplicity of this design and would love to have a go at a scratch build of it, but like Shane I could do with a circuit for the power supply. As you say your 400V is a compromise, for a lower voltage supply, what do you suggest other than an ear bashing from Philip Bluebell?

Best wishes,

Greg

[NickG]

How much current is your 88 passing? Why do I ask?

Andrew, can't you work it out? all the numbers you need are there :-).

Greg, I would have thought a nice little power supply could be made using one of the Danbury (ex maplin) TX's and chokes. What do you think is the ideal voltage Steve?

Maybe a little torroid and 3 pin regulator for a heater supply, and use the 6.3v winding on the mains transformer for a rectifier like EZ80. That should give you somewhere near 250v.

[The Shadow]

Hi Andrew

I'm running at 10mA ish quiescent current with about 4mA peak to peak swing about the operating point. Voltage swing is approx 155V for the full 2V output of a CD player.

Now the power supply is only going through 22K resistors I'm a bit happier with it. The plates are dissipating 2.67W each which adds up to 5.34 watts. This is just below the maximum permissible dissipation of 5.7W when both triodes are operating.

So the valve is being caned with the voltage. Rather than low volts/high current people usually run these things at, I'm running it at very high volts (for this valve) lowish current. This out of necessity given my PSU which is putting out 460V rather than the 430 it did with four 6J5s, due to the very light loading presented by the valve. It does sound very nice indeed operated like this though.

Greg I think a more conventional higher current low voltage operation for this valve would use the TXs Nick suggests. What it would sound like I am not sure as the situation I had to contend with is not really the accepted way of operating this valve.

PSU schematic I am using for this preamp is below. 20uF caps are the big blue LCR polypropylene motor run caps Maplins do.



Steve

[NickG]

I'm running at 10 ish mA quiescent current

Err, if those voltages are correct its running at about 6ma,

[The Shadow]

Originally Posted by NickG

Err, if those voltages are correct its running at about 6ma,

Hi Nick
The HT is running at 467V quite a bit higher the the 400V I have on the diagram. It needs altering

I get it to about 8 and a bit mA having looked more carefully but thenI read it off the graph I drew rather than calculating it numerically.

The voltages I measure however tally just about spot on with what the loadline I drew predicted.
As one can see I am a bit restricted as to where I can run the valve at that high an HT because of dissipation issues.
But like I said it sounds OK down there.

Steve.

[The Shadow]

It's no good you know

I'm going to have to learn to conform!

The conventional wisdom it seems, having had a search about on the Interweb is to run the ecc88 at around 90Va and 20mA.
Completely the opposite end of the curves from where I am! I'm nearly 3 times that Va and only 4 tenths the current.

So Greg
A nice low voltage transformer for the HT and a 9V toroid for a DC heater supply plus a 6.3v for an EZ81 or something like that would be far more sensible than what I am using. Or you could use the PSUII you already have with the 200VHT that runs your existing cathode follower preamp.

A bit of four core cable and a Cliff plug and you will be set.

Steve.

[The Shadow]

I am having quite a nice day tube rolling this power amplifier.
It seems to work very well with a number of valves.

Stick in a pair of Mullard EL34s and it gives that lovely creamy midrange quality the EL34 is justly famous for.

Put in a pair of Sylvania 6CA7 American "fat bottle" tetrodes and we have **** kicking stomp a-plenty without aggression or nastiness.

The new Electro-Harmonix 6Ca7 fat bottle is very close to the NOS Sylvania

The ITT 12E14 puts out a nice neutral, easy and soulful groove.

A pair of GEC KT66s sounds almost identical to the 12E14 apart from a slightly more explicit treble, not better treble, just more of it.

Finally A pair of Steve's Mazda 6P25 carbonised envelope pentodes sounds just fine and dandy but not a lot of power compared to the others.

Those choke loaded 76s seem to be doing a grand job driving a range of output tubes.
I think I have now in my own humble way discovered for myself the vital importance of a good driver stage regardless of the output valve being used.

These common tetrodes and pentodes though easy to drive in comparison to 300Bs are nevertheless quite capable of complaining loudly if the preceding circuitry is not sonically up to the task.

The PT15 amp I was playing with (not very well I might add) was nice but could have been so much better if I had had a better idea of what I was doing at the time I was messing with it.

Nick and Andrew's experiments with drivers and PSUs for the 300B are beginning to make a bit more sense.

Steve

[steve s]

steve, there a couple of more valves you could try, i have some el37 and el35
that may sound good..

and some vt75 versions of the kt66

and a pair of 830a (i think) which are a version of the kt66 .. then theres all the 6l6 makes and variants.....loads of them.... keep you busy for years..

cheers

steve

[Paul Barker]

Then there's the French directly heated version of the 807, can't remember the number. Also some metal can direct heated 6v6's again memory lapse but I have some, as does Dave Dove.

left my bloody apprentice to do final connection on Friday and got a call back from Highway Patrol later that night.