2A3 pre-amp

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pre65
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#1 2A3 pre-amp

Post by pre65 »

I was impressed with Nicks 2A3 pre-amp at Owston, so I've decided to build one myself.

I think that I should have nearly everything in stock to build a near replica. The PSU transformer will be the one from my first 300b amp and I have an assortment of chokes and capacitors to choose from.

I will be using 6B4-g with DIY hi-fi filament modules and possibly 6V6 in place of EL84 (supposed to be an octal equivilent).

Just got to find something suitable to build it in (or on), I think I'm all out of MDF bits now. Perhaps a bit of kitchen worktop that won't exhibit a parabolic curve ? :wink: :lol:

Are you still going to post your circuit Nick ?
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#2

Post by IslandPink »

Go for it Phil, that's a fine piece of audio kit .
Shame you're in Suffolk, I've got a house full of 18mm and 25mm ply offcuts !
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#3

Post by Mike H »

I'd like to try something likewise, but like Phil I fink a trip to B&Q for a mdf stock replenish will be necessary .. Image

Already got a few more octal relay sockets.
 
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#4

Post by pre65 »

I've been raiding the parts box, and the old 6B4-g monoblock chassis for parts and have found the baseboard from my last preamp project, a bit of kitchen worktop. That should do for the power supply. Picture is below.

There are two valves, one is the rectifier (part of hybrid bridge) and the other is a 30 sec delay valve.

It's probably overkill for a preamp, but so what. :wink:

Image
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#5

Post by simon »

Well, it probably isn't overkill - preamps work with much lower signal levels so you might as well throw everything at it including the kitchen worktop.
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#6

Post by pre65 »

And Chris (Stratmangler) wanted something potentially lethal from me, so how could I refuse. :lol:
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#7

Post by simon »

That's more like it, too many nice boxes around here these days :P
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#8

Post by pre65 »

I've just ordered six more octal relay bases from Maplins so I can mount all the valves, and that should be it apart from a couple of resistors and carbon volume pot to order from HFC.

A very cheap build if one takes only external purchases into account. :D
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#9

Post by simon »

Congratulations Phil, you have now reached a critical mass of bits! :lol:
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#10

Post by Mike H »

K-A-B-O-O-M   Image
 
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#11

Post by Nick »

Circuit of what I took to Owston
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#12

Post by IslandPink »

You mentioned Elna Silmic on the cathode. Now, it's easy enough to get 220u up to 50V , is that what you used ? or do you have a source of 100v Silmics :D
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#13

Post by Paul Barker »

elegantly simple.
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#14

Post by Paul Barker »

Have you measured the voltage gain (A)? compared to the mu? Say into a 100k power amp?

I calculate with a fully bypassed cathode resistor, using the assumption that an EL84 load is 1meg ohm into a 100k load A of 4.1, which is very impressive.

To begin affecting the gain the load has to go as low as 25k when gain drops just under 4.

At worst case 10k anode load into 47k power amp gain is still 3.7

So someone could operate 2a3 at 125v -20v (via 800ohm cathode resistor) 25mA drop 250v across a 10k resistor with a 400v supply and autobias dissipating 6 watts across the resistor (so use 12 watt minimum) and still have a viable preamp with loads of headroom and even more simplicity, that may not sound as good but 10k loadline alone on a 2a3 is pretty excellent. Your bypass cap of 220 brings -3db below 5hz in resistor model. 100 brings 10hz



Not a little fun can be had with your choice of preamp valve. We almost have a competition.
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#15

Post by Nick »

As it stands the el84 only provides a load of 6k5 (measured, may be a old el84 though), you could make it a lot higher by using a CCS in place of the 200R resistor, but I wanted to aim for simplicity and a 6k load on a 2a3 is just fine. It measured at about a gain of 4. Nice spread of 2nd and 3rd harmonics, THD less than 0.05% at 2v output.

The cathode resistor is only seeing 35v, so a 50v part is fine in my book :-)
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