811a SET amp.

What people are working on at the moment
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Paul Barker
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#181

Post by Paul Barker »

Busy testing. The three 6bx7 are all good.

Thought I'd test some 12ax7's I would expect to be budget electro harmonix look brand new test good. They test ok.

But ridiculous new price these days. Couldn't believe it. I feel sorry for people trying to get started in valve audio now.
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pre65
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#182

Post by pre65 »

I won't be getting my 6BX7 for at least a week.

In the meantime, what are your thoughts (Paul) for the +450v / -400v power supply for the ACF2 ?
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#183

Post by Paul Barker »

Might be worth the cost of a specially wound transformer for the b+ use a bridge rectifier and ground the centre tap for balanced power supply. Then use 6as7 series pass reg on each rail to give the +450v -400v.

Sounds slightly complexed, but not really, I think more simple. You are only making one power supply. The series pass regulator is the simplest easiest to implement and cheap to make, (when you already have enough 6080's to sink a battle ship).

I would just specify the transformer HT . Use seperate filament transformers. For those I usually buy any old torroid off eBay and wind the secondary to suit if not already ideal. Normally only 5 to 10 turns per volt, depending on core size.

I would just make a FRED bridge, which would involve series FRED's equalised with resistors. You could use damper diodes, but you would have a lot more heater supplies to cope with.

We will need an active preamp to get the voltage gain required, I would rather not build that as apart of this. I think you have something you can use.

Acf2 as standard has an immense input impedance so just a high mu valve should drive it, no need to add a buffer to this buffer. The reasons for the existence of this circuit is to give an immense input impedance a low output impedance near zero losses and near perfect linearity (as much as we can achieve with valves and not glorified op amps).

I know that Nick is recommending to make this circuit with the wonderful facility to bias the whole mechanism so we can fix the bias for the output stage. Yes that would be a great facility for versatility.

But I am conscious that the prime objective is to drive the 811a. As such, manipulation of the 811a filament supply gives a slight positive bias, which is all we need. True that in an ideal world for maximum power 5 to 10 volts positive bias would be better. The 811a would still produce a decent amount of power at a few volts positive.

The elegance of the original design is worth trying as stock.

A voltage divider module with capacitor isolation could be quickly bolted on the front, almost a plug in module. Most projects like 811 801a 833a GM70 GM100 (yes I have a pair) only require negative bias. So I am not at all sure it is worth implementing positive bias to the voltage divider.

So I thought, build at ground potential first for this one valve, because it lends itself to that simplicity,
and build on it in baby steps.
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#184

Post by Paul Barker »

So for HT 700vct (350-0-350) specified for 100ma would be a minimum standard. It would give the 6080 just 30v or so to work with. But a more healthy 80v on the negative rail. It would just about work. Whereas anything greater up to 800vct the 6080's could accommodate the surplus and would operate better.

Better could be achieved, and greater current capacity would benefit giant projects like GM100.

Do you have a transformer in the right range or shall we order a pair one each? If ordering we could go middle for diddle at 750vct and maybe specify 150 mA for overkill on this project but expansion potential.
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#185

Post by Paul Barker »

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Nick
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#186

Post by Nick »

The elegance of the original design is worth trying as stock.
I wasn’t suggesting altering the original design, whatever you do unless you use a transformer coupled first stage you will have to cap couple to the buffer input, that cap will have to have a resistor following it, the ACF input impedance mandates that, so taking that resistor to a pot or directly to ground makes little difference, and certainly no difference to the ACF itself.

Even if you did use a transformer, the secondary would need to be referenced somewhere.
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#187

Post by pre65 »

What about one of these ?

http://www.antekinc.com/as-4t400-400va- ... ansformer/

It might be more use if you want to use it for an amp PSU at some time in the future.

Could one use each of the two 400v secs to do a separate pos and neg supply ?

Or is that too complicated ?
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#188

Post by Paul Barker »

There is no detail. Is it blue and green 400v on each?

You could centre tap by connecting the middle whatever that is, might be start blue in each case then first green joins second blue, or vice versa. It would have to be experimented to determine direction and ensure they add and don't buck.

You can make two seperate supplies off that yes.

Thinking about that I have two cheap 400v transformers I could use. I linked to them on here when they came up. Very cheap because they were really single phase 400v primary 55-0-55 secondary. Using two you can series connect the two 110 windings across the mains and have two sperated 400v secondaries. Very high power rating.

£25 each, nobody seemed to take them up at the time.
Link to the discussion, showing my measured output voltage.
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#189

Post by pre65 »

Yep, two separate 400v secs and two separate 6.3v secs.

4 windings in all.

So it could be 400v -0v - 400v in series or 0v-400v in parallel or two separate 0v - 400v windings.

Quite versatile. :)
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#190

Post by pre65 »

Paul Barker wrote:
Thinking about that I have two cheap 400v transformers I could use. I linked to them on here when they came up. Very cheap because they were really single phase 400v primary 55-0-55 secondary. Using two you can series connect the two 110 windings across the mains and have two sperated 400v secondaries. Very high power rating.

£25 each, nobody seemed to take them up at the time.
Link to the discussion, showing my measured output voltage.
There are some 250va and 500va of the same type on Ebay for £29.99 + £10 carriage. Probably more expensive for two of those than one Antec from USA, even including Parcelthief charges.

http://stores.ebay.co.uk/breconjess?_dm ... ransformer
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#191

Post by Paul Barker »

Enough already :shock:
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#192

Post by Paul Barker »

Just searching for components on RS

Decided to use 2 20k 7 watt resistors in series as closest I could find for the 40k.

Other resistors should be much easier to pick.

Do you want me to double the order or are you going to make your own order?

Remember I have the 100v zenner's ordered.
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#193

Post by pre65 »

Thanks for the offer Paul, but I'll sort out my own resistors etc.
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#194

Post by Paul Barker »

No probs. a pot to set the bias would cost up to £10 and you'll need two.

If you want to order it though here is an

example.. It has to be combined with a few other parts, but this is the expensive bit.
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#195

Post by Nick »

I was thinking more like this

http://www.rapidonline.com/electronic-c ... ot-68-0275

With a 330k 2W resistors on either side of it.
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