A somewhat historic occasion - Foreplay III retires

[Paul Birkeland]

Hi:

At the risk of getting ahead of ourselves, I have a question about the 300B preamp.  I know that EML mesh-plate 300Bs are generally (or is it it specifically?) NOT recommended for Paramount amps.  Putting aside the cost issue, would it be correct to assume that they would work just fine in the 300B preamp?  Thanks.

Gerry 

This is a misconception that seems to come up quite often.  The issue with EML tubes in the Paramounts was with the original directly coupled 2A3 circuit; this is what the soft start circuit addressed and solved.  There was never such an issue with the 300B's, though the de-rated EML mesh 300B might not be the best idea. 

[Thoburn]

OK. First let me say that I am less than a novice when it comes to circuit design. But, I'm curious why the new Foreplay IV uses 300B tubes. Why not 2A3 or even 45.  I ask this because I have the impression (from reading other reports, not personal experience) that the 45 sounds 'better' than 2A3 which sounds 'better' than the B300. Is there a simple explanation for this? One that even I would understand?

[Paul Joppa]

The 45 was dropped because the output impedance (same as plate impedance) is 2.5 times higher than a 300B. We did a lot of experimenting with different designs to get the source impedance down and the available current up, and have so far concluded that both make an important difference.

As a preamp, a very quiet filament power supply is needed, and it's both difficult and inefficient to regulate 2.5 amps at 2.5 volts. That discouraged the use of 2A3s. 5 volts at 1.2 amps is, in important ways, four times easier. (This is a problem that I will continue to work on ...)  Meanwhile, and pending resolution of the last details of the circuit, I think it can easily be made to work with 6A3s, or (with a socket change) 6B4s.

With bigger changes, a few others may be workable - we haven't yet explored the limits of the design.

[Doc B.]

I suppose the simplest explanation is that 300Bs actually sound better than 2A3s or 45s.

[Gerry E.]

This is a misconception that seems to come up quite often.  The issue with EML tubes in the Paramounts was with the original directly coupled 2A3 circuit; this is what the soft start circuit addressed and solved.  There was never such an issue with the 300B's, though the de-rated EML mesh 300B might not be the best idea. 

What added to the confusion was that I had consulted with George from Tubes USA.  He's the major EML seller in the U.S..  He specifically recommended that I go with the standard solid-plate EML 300Bs for my Paramounts.  Maybe this was just personal preference.  My plan now is to get a pair of EML mesh-plates and try them and the solid-plates in both the new preamp and Paramounts to see which way sounds better.

Gerry         

[Albert B. Broman]

I would like to reask the question from xcortes.  Could this be easily adapted to a headphone amp?  Is this in any way related to the semi-retirement of the Smack headphone amp?

[Doc B.]

Well, I guess you can turn anything into something it wasn't originally intended for.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fwww.collectspace.com%2Freview%2Fspaceshuttlecafe01.jpg&hash=32c3423ba01bde58225ec377b42038120f0c9142)

Output impedance is around 500 ohms. You would need to add output transformers. When you do that the gain drops a bunch. and you lose the advantage of the OTL line level circuit. A better choice for a 300B headphone amp would be Paramounts. They are pretty amazing with AKG K1000s.

We are discussing re-releasing the Smack concept in a somewhat different form.

[Randy Taylor]

Been a couple of months since the Foreplay III was retired and the talk of the new Foreplay IV, just wondering if the Foreplay IV will be out soon. I have a old '02 Foreplay and was hoping to upgrade this winter to a Foreplay III when it was retired, but now have to look into the Foreplay IV when it comes out, i just have not heard anything on it in a while.

[Jim R.]

Randy,

The new preamp will be called the Bee Pre and there is now a separate board for it -- located just below quickie.

HTH,

Jim