The Doddington Special

What people are working on at the moment
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andrew Ivimey
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#1 The Doddington Special

Post by andrew Ivimey »

Did anyone try this little ECL82SE amp?

I've got all the bits, though almost none are quite to spec! (typical) and I have started marking out and drilling, but I wondered has anyone else done this remarkable looking little number? could have my time and precious blood!
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Nick
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#2

Post by Nick »

Have you a link to the circuit?
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#3

Post by andrew Ivimey »

er.... it must have been on the WD BB

consider 180vHT@43ma. Triode has 300mH as anode choke, there is a cathode resistor and cap. 1mfd coupling cap into pentode section which uses (only) 150mH as gride choke, there is cathode cap and resistor. Output trafo is 3k. 10henry choke from HT to anode choke on the triode.

And Bob's yeruncle... we shall see what we shall see.
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#4

Post by andrew Ivimey »

ooops for mH read H. those Hammond 156s are used all over the place.
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Dave the bass
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#5

Post by Dave the bass »

BOBA was an ECL82SE Andrew, that was choke loaded then parrafed into 2 tortoise TX's.

I think it was one of Nicks designs, it certainly got me going in the right direction.

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#6

Post by Andrew »

Here it is....
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#7

Post by simon »

That's the one I've been intending to build for a while but haven't had the chance. Might of known that you'd beat me to it Andrew :wink:. Very interested to see what you think.
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#8

Post by Dave the bass »

Whats the purpose of them thar 2 big chokes in series on the triode stage for?

Thats a bit different innit? Even the local Valve-Village-idiot (me) knows there's normally resistors in that position :D

EDIT...and theres anuvver hanging off the grid of the Pentode. Who's on commission from Hammond :lol:

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#9

Post by Andrew »

That thars a choke loaded anode on them thar triode.

Thus presenting a near 'infinite', and much more linear, load to the triode than say a resistor and allows thar triode to swing twice the B+ to boot.

There was a thread on the old BB where they talked about "coke loaded adodes", a typo I hope as I suspect that would be something completely different.

cheers,

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#10

Post by Dave the bass »

Aha, thanks Andrew. That must be a 'spensif way of building a 1watt or so SE amp! Bleeding heavy too I'd imagine.

Re- "coke loaded adodes"... isn't that what faded 70's rock stars used to suffer from?

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#11

Post by Andrew »

Dave the bass wrote:Aha, thanks Andrew. That must be a 'spensif way of building a 1watt or so SE amp! Bleeding heavy too I'd imagine.

Re- "coke loaded adodes"... isn't that what faded 70's rock stars used to suffer from?

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coke or choke - spensif either way I should imagine..... :shock:
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#12

Post by Nick »

Thus presenting a near 'infinite', and much more linear, load to the triode than say a resistor
Just to be real picky like for t'lad (Dave),

The choke load isn't in any way more linear than the resistor, a resistor is about as linear as it comes, but what Andrew is talking about, i sthat te choke provides a very high AC load to the triode, and a low DC load, so the DC operating point is the anode at B+, but the AC load line is just about a horizontal line, which is a more linear way to operate a triode.

Its the same sort of thing as a CCS load, but in that case it requires a higher B+ as the CCS "consumes" voltage as well.

The choke load stops becoming perfect when the frequency of the signal means the reactance of the choke falls, so the line stops being horizontal. So its important that the inductance is such that this doesn't happen at any frequency we are interested in.

Its also why its important that the cathode resistor is bipassed, not haing it bipassed increases the anode resistance, so the frequency at which the choke stops being a infinate load becomes higher.

The grid choke is doing a similar thing. Its high impedance to AC works like a large grid resistor would in that position, but its low DC resistance, allows any build up of DC from grid current to find a easy path to ground, instead of having to go through the normall large grid resistor.

I have always been a little unsure of the logic of that last part, any grid current is going to be the result of signal peaks. If its from the signal, isn't it going to be AC at the same frequency as the signal, so no more able to pass through the choke than the signal?

Though I can see it working for occasional non periodic excusions into grid current.
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#13

Post by andrew Ivimey »

so, thats told you our DTB, and I spose its told me too. Its all drilled and a few of the important things added, like grommets. I have vaguely suitable chokes from Hongkers and only a pair of the right Hammonds, the PSU is hugely overspcc'd but we'll see when it comes to the choke (I'm going choke loaded but solid state) and then I need the TIME !!!

The output trafos are double C core AEs because that'sm all I've got. I believe our Doddington used cheap and cheerfuls from Ask Jan.


hm... could be a right mess!

But for witham, I'd like to introduce PushPullSEX amp- its okay!
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#14

Post by pre65 »

Hi-a Push Pull Sex amp sounds rather rude ! :shock:
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#15

Post by andrew Ivimey »

Yeah well you all had your chance at Witham a while ago with SEX amp.

For 1 watt it really is splendid and yes I'm biased but 6DN7 is better for me than 6EM7. The push pull jobbie manages nearly 4 watts per ch. It has very unlikely feedback and yet it works really well.

There is a PSE version which I haven't gottene round to.

Like a butterfly, I spose, I flit around all over the place and don't listen to the music.

Don't get me chuntering on about speakers.
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