811a SET amp.

What people are working on at the moment
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Paul Barker
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#211

Post by Paul Barker »

Excellent you remembered protecting with a zenner, I would have blown up the first pair without you😂
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#212

Post by Paul Barker »

Looks like the 12bz7 also 300v Max, so I maybe didn't need to worry so much about that. I'll still consume the excess across a resistor for safety.

I'm in the process of ordering the lm234 and associated resistors. Decided to take the option to make it temperature stable with a diode which acts in opposite way as temperature changes, requiriing a slightly more complexed resistor formula. A 56 ohm for the lm234 and a 560 ohm for the diode, according to the formula should give 2.5 mA set at any temperature. Not expensive to add one diode and one resistor to provide this feature, so may as well.

The original grid cap which Phil pointed out was drawn unconventionally is what makes the original triode ccs function at AC. Now the main roll of the triode is to provide the headroom for the AC swing, which the lm234 cannot. But it seems harmless to allow the triode to also act in ccs mode.

So I'm going to sketch the plan out here and invite criticism, in case I have missed something.
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#213

Post by Paul Barker »

Here is the temperature stable CCS calculator

The formula is =((0.0677/B4)+((0.0677+0.65)/(10*B4)))*1000

B4 is the R1 value R2 is 10 times R1 the above formula provides the set current expressed in mA.

The circuit below shows the position of R1 and R2.

For the time being I have left out the voltage divider network with which to bias the whole circuit positive or negative. As previously mentioned I want to try it as it is first. It does however occur to me that Stephies operating point of +9v would easily be achieved with a PP9 battery on the input. That is to say Battery plus on grid of v1 negative through 1 meg resistor to ground. Phono in centre connector then stands at +9v so pre-amp DC isolation requirement. an inline cap on this board would solve it, but in most situations would represent a duplication cap, so as we are talking here to scratch builders, let them know the science and apply as it best fits their needs. If you don't know the science don't attempt it.

sorry forgot to write on the two level tracking Zenner diodes together are 1N5378B 100v 5W.


Image
Last edited by Paul Barker on Tue Jun 02, 2015 6:00 am, edited 1 time in total.
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#214

Post by IslandPink »

I must admit I'm struggling to follow the details here, but it's a very interesting thread and I'm keen to hear the amp(s) when it/they get built !
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andrew Ivimey
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#215

Post by andrew Ivimey »

Me too & ditto for a listen. Twould be nice for someone to knock up a simple 811 amp then as a comparison.

Our Ed did a splendid 572 amp for an Eggborough do. I loved it.
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#216

Post by Paul Barker »

The detail is that the circuit ACF2 is what Nick found and used to drive the 833a the last time he and I did a collaboration at Owsten.

Nick set the bias with a resistive divider network on the input, but on the negative rail to ground as for the 833a we always needed negative bias.

Apart from the level shifting nature of the circuit, which facilitates the above feature. It provides a high input impedance and low output impedance (2 or 3 ohms) and the output is "virtually an undistorted replica of the input".

I can't recall the transient positive drive we observed on the day but the power of the amp was not limited by the driver.

The McDonals article states that with the 5687 it can drive up to 100mA of positive grid current with 3 ohms output impedance. Frequency response more or less flat up to 0.5Mhz -3db to 10Mhz and beyond.

Nick was sold on it before trying it and it showed itself very capable at Owsten.

Nick did use the 6bx7 but the 5687 capable of 100ma positive drive would suffice for the 833a. though I had intended to use the 6bx7 it was a simple mistake when I mounted the sockets, and I decided to just run with what I had done.

The bias setting flexibility is an enormous benefit to so many of the amplifiers we make.

But this is not a voltage amplifier, it is an Augment Cathode Follower from beginning to end.

But because of the high input impedance it doesn't require more than voltage amplification in front of it. Though folk will always wish to make preamps loaded with niche valves and components, and what ever sweet sounds suchlike presents to the input of this circuit , it shall convert it to a low impedance high current drive with little alteration across a wider range of frequencies than erstwhile we have enjoyed.
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#217

Post by IslandPink »

It does seem very good for the job. I was annoyed with myself I didn't get over to hear the Eimac 100TH amp Nick built, which used that driver .
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#218

Post by Paul Barker »

I didn't realise Nick used it for that valve.

There is something a little mysterious about it, I would like to see snap shots of the various dc voltages across all the valves. It doesn't add up the statement MacDonald makes about the 300v rating of the valves not exceeded. This is why I put the extra resistor on the tail of the already long tail, to spare the blushes of the 6sl7.
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#219

Post by jack »

I too am really enjoying this - tonight's reading is the ACF article - loads of loverly maths :)

My maths has been rusty recently, but #2 son who is doing Civil Eng. at uni was having a bit of bother so I had to re-learn a lot of the stuff I did years ago - hairy maths, but was really quite good fun (in a masochistic sort of way)

Following what you're up to to Paul with great interest...
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#220

Post by Paul Barker »

Well it's Nick's baby I am just picking it up.
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#221

Post by Paul Barker »

OK I have now grown into the Google Docs changes and can once more figure out how to allow you guys with thelinkto edit the spreadsheet. Please don't change anything but the value of R1. Experiment with which value gives the current set which you desire. R2 is automatically changed for you. Obviously it is always 10 X R1.

If I were any good at maths I could write it so that you enter desired set current and it provides you with the resistor values. But I am not that bright. So you have to try resistor values until you arrive at the solution you want. But at least I have saved you from trawling through a very boring document to find the maths.
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#222

Post by Nick »

IslandPink wrote:It does seem very good for the job. I was annoyed with myself I didn't get over to hear the Eimac 100TH amp Nick built, which used that driver .
I didn;t use the ACF to drive the 100th that time, I had not found it then, That amp used a pair of E130L for each chan, one as a cathode follower the other a active load for the cathode follower.
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#223

Post by Paul Barker »

Do you have the 100TL's still Nick? Want to bring them to next meeting and I'll make my output stage adaptable.

I expect they are gone though!
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#224

Post by IslandPink »

Ah, I got the drivers mixed up - thanks for reminding me, Nick.
The ones Nick had/has were 100TH, Paul , not 100TL . They are about £35 as singles from time to time on EBay .
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#225

Post by Paul Barker »

Yes I just realised my mistake swell. They were TH.

I'm not really wanting to increase the size of my collection :shock:

Though I recognise they may sound better than anything else I own.
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