Nicks Aikido 300b amp

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Nick
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#1 Nicks Aikido 300b amp

Post by Nick »

Ok, I have been playing with the driver stage of the 300b amp I have been working on, and found something out that might be interesting to the folks using aikidos.

First, a quick run down of the amp, I will post later about why various bits are as they are, but for the moment it consists of a monoblock series regulated supply. 230v torroide isolation TX, to voltage doubler using uf4007 diodes, and 100uf electrolytic caps. There are LCR traps tuned to 120kHz (which I measured the resonant frequency of the torroide to be) before the diodes in series, and one after the caps in parallel with the supply, the traps are intended to remove the reverse current ringing from using SS diodes. I can’t tell how well its working as I don’t have a skope with a timebase that will let me look close enough at the right place. That then splits into teo 15H chokes, followrd by 100uf caps. Then two sets of series regulatots, using 6080’s as the series tube, and the pentode from a ecl82 as the control tube, and the voltage reg is a pair of 0D3’s. Basically it’s the circuit on Steve Benche’s pages about regulators.

The 300b driver stage is an aikido with both stages using 6n6p. And before that there is an ecc40 to provide voltage gain. The aikido and ecc40 are using a B+ of 350v, and then via a mundorf cap to the 300b grid. The 300b is running with a 1k cathode resistor, and a B+ of 460v, @ 70ma.

So, that’s almost as it was when Paul heard it at Steve’s a few weeks ago. But I have been playing today at seeing if I could reduce the amount of 2nd harmonic the driver stage was producing, 3rd and above is very low, but there is a bit more 2nd than I wanted, leading to a bit too rich a sound for my liking. I expected the cause of the distortion to be the ecc40, but it turns out that that’s producing bugger all in the way of distortion, the source of the problem was the first stage of the aikido.

I had the 6n6p running at about 17ma each, using a 330R resistor in the cathode of all three valves (with cathode resistors). That seems to work well for the second stage and it drives the 300b well, but what I found was that by reducing the current in the first stage, far below where the curves for the valve would make you want to do so, I found that I could reduce the level of 2nd harmonic by about 8dB. I have gone from using a 330R resistor in the cathode to a 2k8, and gone from 17ma to 2.8ma, and I think the amp sounds much better like that.

If there are people reading this unsure what I mean by the aikido then this is the general shape of the circuit. Though that image uses different valves to mine it gives the general idea.


http://www.tubecad.com/2004/AikidoAmp5.gif
Andrew
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#2

Post by Andrew »

Hi Nick,

That's an interesting find. You are running the 6N6P very 'cool'. Did you measyure it at say 4-5mA or at any otehr points of the way down.

Have you thought of trying another dual triode and see what that does, say compare the distortion to one that's text book operating point is 2-2mA.

I've recently come to the conclusion that my Aikido is a bit under voltage at 250v and I'm thinking of sticking a cap iin before it to get up to about 300v.

cheers,

-- Andrew
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Nick
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#3

Post by Nick »

Did you measyure it at say 4-5mA or at any otehr points of the way down
Yep, I kept increasing the cathode resistor 150R at a time, the 2nd harmonic went down about 1dB per 150R.

I wonder if whats going on is by increasing the bias on the first valve, it gives more headroom for the signal from the ecc40.
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#4

Post by Andrew »

Hi Nick,

In John Broskie's blog when he goes on about ECC83 -ECC82 Aikido to drive the 300B he says something like he started with the 910R cathode resistor but found 2k to sound better. Now this is contrary to Mr B's ethos of running you valves hard. So, I reckon you could be onto something there.

OK that gives me two things to try bigger cathode resistor, I'm using 910R, and upping the voltage.

cheers,

-- Andrew
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#5

Post by IslandPink »

That's a very interesting result Nick. Pretty non-intuitive .

Mark
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#6

Post by Nick »

I am going to have to start pulling this apart. I have finally admitted to myself, that somewhere in the process, this amp stopped sounding as magic as it could. Somewhere about two montsh ago, it lost something special. It still sounds good, but it used to sound much better than good. I don't think its the supply, its sounded good with that supply, its something I have changed in the driver stage.

So I will have to remember what I did, and work backwards. I guess this is when taking notes would have helped :-).
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#7

Post by Andrew »

Be interesting to find out what it was.

cheers,

-- Andrew
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Nick
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#8

Post by Nick »

Ok, first thing that seems to have helped return the magic. remove the DC heater supply from the DRIVER stage. I wonder whats going on there, but the dynamics and micro-detail has returned. I think that was partly what was missing.

I will do some more listening and tweeking and see how it goes.
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#9

Post by Andrew »

Nick wrote: I wonder whats going on there.....
Me too, why would that make a difference on an IDH valve, could it be crap/hash from the reg getting in? I wonder if ferrites on the heater pins might help?

Did you get chance to take a look at those articles on 317s I sent? There's stuff in there about how they're output is not a clean as you might expect.

-- Andrew
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#10

Post by shaun »

hi nick

i think i got the same sort of sound when i dc regulated the fillaments on the my 76 front end. all there but a bit uninvolving and sort of grey. when i swopped in some current source supplies the sound seemed to get it's soul back. i know it should not make a difference but to my ears i did.

worth a try?

take care

s

nice place by the way :D
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Nick
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#11

Post by Nick »

Oh, no, I forgot about them. I will have a read.

Yes, it was my thought that hash from the regs was involved. I will swap back after a few days and see if its just my ears playing games.
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#12

Post by richardcooper2k »

any developments since last post nick ?
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#13

Post by Nick »

Well, I am currently using a 6em7 like valve as the first two stages, driving a 1:1 interstage into the 300b grid, I think thats a better sound to me. I still haven't got back to the sound I had before though, so maybe I was in a better frame of mind when listening then. Hard to say.
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#14

Post by richardcooper2k »

sounds like you've tweaked the power supply and then started again with a new amp for it !
can you say how it sounds different to with the aikido,etc ?
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#15

Post by Nick »

Well, the most obvious change, is the soundstage is set slightly further back behind the speakers, and there is a greater sense of reality. Nothing obvious in terms of sound, more high's, less lows that sort of thing. Much more in the way it fools the head into accepting it as a slice of reality.

Not a big change, but I think I am at the place, where very small changes become important.
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