Combining different Bottlehead bits in a common chassis

[epistaxsis]

Over on this thread on the foreplay3 forums http://www.bottlehead.com/smf/index.php/topic,1064.msg9135.html#msg9135 where I was asking about what to do with the PT-3 transformer and its impact on the EF86s of the eros I now feel that this subject is better off handled here.

Attached is a pdf of the ultimate (i.e. includes surround sound and power amp phase splitters) layout of what I want to do.

Unlike the previous diagram i have made it sort of scale and added plenty of text to the bits on the drawing.

So to ask my questions:

1) Can the PT-3 be shielded in some way to stop it interfering with the EF86s?

2) Can I run the eros / fp3s in the stereo section without coupling caps?

I look forward to the responses

[Grainger49]

That layout looks like you might have enough room under the hood for the 4 circuit boards in the Eros.  There are 3 in the Extended FP III, which uses the 12AU7 you show.  You may need to refine it when you have the PCBs in hand.

1) I can't help you there.

2) You must have an output (coupling) cap on the Eros and the FP III circuits.  There is no interstage cap in either.

 

[Doc B.]

1) Can the PT-3 be shielded in some way to stop it interfering with the EF86s?

There is no substantiation that this is even a problem yet, so there is really no answer. You have to establish a layout, build it, determine if there is a problem and then work out a solution if there is indeed a problem.

The short answer to the general question of how to deal with hum pickup in high gain tubes that comes from magnetic coupling is that distance is the simplest and most effective solution. Also one can determine the direction of lowest radiated magnetic field from the transformer and use that to one's advantage. Magnetic shielding is a very tricky thing to do right because you might need multiple layers and different materials depending upon the strength of the field you are trying to divert (magnetic fields are not "shielded" so much as diverted through magnetically permeable materials)

[Paul Joppa]

>> 1) Can the PT-3 be shielded in some way to stop it interfering with the EF86s?

Probably, but it will become a research project. First step is to determine if that is necessary - nothing but trial and error will work reliably there! For shielding close to the transformer where the fields are large calls for soft iron, and fairly thick - maybe 80-100 mils? A strap around the core (covering the exposed edges) has been suggested. A high-mu shield can would be the next step. Hard to source!

Some alternatives are remote power supplies, PT-5 (Eros) power supplies for the Foreplays, or doubling up - a single PT-3 can power two Extended Foreplay III circuits, so you could get away with only the furthest two PT-3s. It's not clear what the phase splitter is, or what power it needs - that would be the closest transformer after the Eros' own unit.

>> 2) Can I run the eros / fp3s in the stereo section without coupling caps?

No, but the Eros output cap can be smaller since you know the load impedance and are probably well shielded. Could go as low as 0.1uF I think. That makes a better cap more affordable!

[howardnair]

epis--i am fairly new at all this 3 years or so thats it-i thought of combining the eros and foreplay which i believe has been done--i backed off for several reasons-the magnetic shielding for one -2] it was more work to combine them than not- and 3rd]  bottlehead foreplay and eros are designed as separates and they work wonderfully as such-- doc and paul spent alot of time designing them as separates-and combining the two together is one thing but what you are proposing i think is asking for a lot of problems with maybe no solution

[howardnair]

epis--go to the gallery and look at wardsweb old pics--the stacked units idea may help with your consolidation problem--granted everything would not be in one chassis but 2 or 3 chassis- two chassis with a 3rd built on top or any configuration would at least give you one structure-with some creative implementation !!!!!!!!!

[epistaxsis]

Thankyou for all the replies.

So the magnetic interference issue is a suck it and see.

That is OK with me :-)

I thought I would be on the road to nowhere with the coupling caps (apart from the eros) - but it was worth a shot...

As you can see there is a new version of the sketch attached.

The big news is that I have separated the power supply section of the amplifiers from the audio section!

This, in theory, gives me what I want to do with regards to the audio circuits.

However I am well aware of the fact that I'll have fun and games with regards to wiring the heater supplies

The required 365mm (14") distance would effectively be a second shelf on the wall racks which feature 25mm (1") of slate too.

So it is doable if I use very good screened cable.

Mounting the PSUs elsewhere has triggered another old idea of mine for this pre-amp (it has been in my virtual design studio (my head!) for over a year now) - using 6V batteries for the heater supply!

What would happen if I used a 6V supply?

How critical is that 0.3V for the heaters?

Would using a 12V battery and wiring in series be a better idea?

I look forward to your responses.

P.S. the phase splitter part of the amp is the first stage of a push-pull power amplifier (I know you're all probably going to tell me SE is much better but I do need the grunt from 845 valves for the midbass!)

The idea being that we now have a pseudo balanced connection between the two units making the effect of the long interconnects between pre and power amp decreased (I wire like the US i.e. short speaker cables - not the set ups we tend to have here in the UK).

THe reason it is all blank on the sketch is that I am arguing with a mate over valves and circuits etc. (I want PP PX25s for treble and PP 845s for midbass - he is trying to say PSE is better...)

[Grainger49]

Let's see, you show 11 12.6V tubes and 5 6.3V tubes.  I don't know what is in the phase splitter.  You could take that odd 6.3V tube and put a series resistor on it to simulate another heater.  Then run it all with a battery.  You might end up with a car battery, maybe a good sized motorcycle battery for the proper amp hour rating.  (It's a joke, they don't draw many amps total).

There are a lot of guys who are not running SET you won't get arguments from us.