pre-amps - Something's got me started!

[andrew Ivimey]

who needs 'em eh!

I have had a WAD pre2 for years. It is smashing and it just works. But recently with the Amity experiments (not over yet, oh no!) the idea that a preamp is a good idea was bangingly obvious from the start (or use a third stage in the amplifier already).

Driving Bugle, for example, is so much better with a preamp because ... impedance issues from the CDplayer / bigger voltage swing- whatever!

I needed a preamp that was very low impedance output, the finest sound quality and with an power output greater than a kick from a mule.

Choices then I see as, pushpull transformer coupled like Amity etc amps. SE transformer coupled like WAD, or all those other deisgns from grounded grid to cathode follower this and that. A valve, any valve just pick one. why not a 2A3 with an input step-up transformer - Miller caps - ooops, maybe.

I will try SE transformer aiming at more power and lower impedance. I will try low anode resistance triodes (e.g.12B4A) the classic 26 and 76 in any config I can copy/dream up, melt and so on because tehy are classic lovely sounding valves. I can see me getting round to a more Amity like designs but phew, onandonandonandon.

ramblings - is it worth it?

I acquired a couple of homemade, not quite working preamps for parts, a few weeks ago and thought I'd mayaswell just try and get them working, first. Imagine a 6SN7, first stge input, second cathode follower; pedestrian but solid. The other preamp was this but with paralleled 6SN7s. And one has a valve PSU and one an SS PSU. It was easy to get them up & running and I found what you'd expect. - LOUD and paralleled has more welly. Parallelled is also a hint of softness and neither will probably drive Amity properly, but we'll see.

So, has anyone any ideas, short cuts, experience of the finest preamp in the world to share. It is easy to get LOUD and loud is attractive but I need the best!

[Cressy Snr]

Hey Andrew

How about a 46 as a preamp valve?

Steve

[andrew Ivimey]

oops, missed that one out.

I have plans for 46 and 47 for amps (PP & SE and transformers) but playing with that second grid could do 'interesting' things to the sound, couldn't it!

You were going to play with 46, I believe - whither next!?

And no-one mention 10Y, please. It is supposed to be 'to die for' but they are getting to be nearly as rare as hen's teeth, like VT52s!

[Cressy Snr]

oops, missed that one out.

You were going to play with 46, I believe - whither next!?

I was all set to turn the first version of the PX25 amp into a three stage amp with a 46 as driver and 6a6s as a gain stage but thought better of it and went down the direct-coupled/DRD route.

Steve

[Cressy Snr]

Hi Andrew

Here's a very silly line stage I knocked up on the back of an envelope today.

It's perfect if you've got a couple of miles of cable to drive Gain around 1.5.

With a bit more HT. It could be cranked up to 80mA without it even breaking sweat



Steve

[andrew Ivimey]

Hello Steve, thanks for getting back to me.

I believe this sort of thing gives impedance all over the place as we go through the audio spectrum even though we start off with lovely low impedance because the valve has anode resistance about as low as you can get.

But I need gain too, lots of it! I think I will end up cooking WAD2 output transformers with something powerful.

By the way, does anyone out there know the anode resistance of the PT15; not for pre-amp you understand but right now in SE I am using 3.5k and it sounds a bit harsh. 5K and 10K trafos are waiting in the wings. I expect them to sound better! And as they are hammonds I can try ultra-linear; more bangs for the buck!?

[pre65]

By the way, does anyone out there know the anode resistance of the PT15;

The Marconi data sheet says 7,600 ohms !!

http://www.mif.pg.gda.pl/homepages/fran ... p/PT15.pdf

[andrew Ivimey]

Hm, okay, thanks ...7.5k rather high then and a nominal 3.5k trafo is then rather low.

I'll go for the 10K Hammond (- it was not bad with GM70 and much better with 211).