Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Topic started by: Entropyman on December 16, 2018, 03:12:45 pm

Title: Foreplay wiring diagram
Post by: Entropyman on December 16, 2018, 03:12:45 pm
Several years ago I purchased a Foreplay used, but did not get a wiring diagram with it.  Now that I have retired I am starting to get back into tube audio and DIY, so I was trying to figure out what I really have.  Near as I can tell it has been modded somewhat, and has the "Anticipation" upgrade, but I don't know much more than that. Pretty scary looking under the hood, and I would like to clean a lot of it up, but don't have a good place to start..... Is there a wiring diagram for the base foreplay available (or preferably the original build instruction with test voltages, etc.  located somewhere?  I searched but did not find anything.

Phill B
Title: Re: Foreplay wiring diagram
Post by: Paul Joppa on December 16, 2018, 05:33:21 pm
That's going to be a challenge!

The Foreplay came out in 1998. I see in the picture that there is a wire to pin 9 of only one of the 9-pin tube sockets; this indicates it has the oldest power transformer, with 12-v filament power. (There were two later revisions, the last of which came out in 2005.) As far as I know, there are no electronic versions of the original manual. The audio circuit is a voltage amplifier (pins 1-2-3) direct coupled to a cathode follower (pins 6-7-8). It was very popular as a base for experimentation - below are the changes I see at first look:

I see the "Sweet Whispers" stepped level control, which was an upgrade. My spreadsheet to calculate the resistor values is dated 2003; I don't have a manual.

I also see that you have the "Anticipation" upgrade - the four circuit boards that act as plate and cathode loads for the voltage amplifier and cathode follower sections respectively.

The large yellow Auricaps and the adjacent black V-caps are replacements for the original output capacitors (I see that the V-caps are disconnected at one end.)

The selector switch (center front) apparently uses only two of the three positions.


As I recall, the voltage amp (VA) had around 50v on the plate (pin 3) and 1.5v on the cathode (pin 1), with the grid (pin 2) at ground. The cathode followers must have had 50v at the grid (pin 7, same as pin 3), a couple volts more at the cathode (pin 8), and the power supply voltage of 160v or so at the plate (pin 6). That's not a complete list, but if you are within 15% of those voltages the circuit is probably working correctly.
Title: Re: Foreplay wiring diagram
Post by: Alonzo on December 16, 2018, 06:49:17 pm
Salvage the Auricaps and VCaps and buy a Quickie, you get a cheap into to DHT and a good preamp for around 100$.  Lots of forum advice on upgrades and changes to get your feet wet until you decide on the next level kit.  And from owing them both, the Quickie is as good or better than the Foreplay, IMO YMMV.
Title: Re: Foreplay wiring diagram
Post by: Entropyman on December 18, 2018, 04:34:31 am
Thanks guys.  I may just try to make my own wiring diagram for it.  Some of the workmanship is a bit sketchy, so at a minimum I'll pull it apart and rework the wiring....I have a Welborne Labs Cato pre amp kit as well, which apparently is going to be challenge #2....They've gone ghost, and I cannot find anything on line about the build. Hopefully I have all the parts.....
Title: Re: Foreplay wiring diagram
Post by: Entropyman on January 25, 2019, 09:26:07 am
I have been digging into this unit a little more, and while learning quite a bit, cannot figure out why the tube pinouts are not symmetric.  B9 has both 0.1mF cap and a 68 K resistor to ground ( Tab 13/14 ).  Could anyone provide insight on why?  All the other pins appear to be the same on both tubes A and B. 
Title: Re: Foreplay wiring diagram
Post by: Paul Birkeland on January 25, 2019, 09:45:45 am
The heater feed is a 6.3V AC winding that feeds pins 4/5 and 9 of each 9 pin socket.  The caps and resistors in question bias up the heater winding a bit and are not part of the audio circuit.  Since there is only one heater winding, there's no need for more than one set of these parts.
Title: Re: Foreplay wiring diagram
Post by: Entropyman on January 27, 2019, 07:05:28 am
Thanks for the reply, but that doesn't appear to be the case.  This is a Foreplay I, so it gets the 12.6V, but I did locate the wiring diagram for the anticipation upgrade that shows the layout.  Pins 4/5 of tube V1 and V2 are connected, but only pin 9 of V2 goes out to ground through the cap and resistor.  I apologize if these seem like stupid questions, but part of this exercise is for me to learn about the circuits....
Title: Re: Foreplay wiring diagram
Post by: Paul Birkeland on January 27, 2019, 07:22:42 am
Yes, the FP-1 had the 12V winding, the FP-2 had the 6V winding. 

To heat a 12AU7 with a 12V winding, the 12V winding connects to pins 4 and 5 and nothing connects to pin 9.
To heat a 12AU7 with a 6V winding, pins 4 and 5 are connected together, then the 6V winding connects to pin 9 and pins 4/5.

To bias up the heater supply, a resistive voltage divider is connected between B+ and ground, with the junction of those resistors connecting to the heater circuit (anywhere in the heater circuit, it could even be pin 9 in the 12V setup).

The heater winding on the power transformer is just a long piece of wire wound onto a plastic bobbin.  The heaters in the tubes themselves are also a very low impedance path.  All of this is common between both of the 12AU7 tubes, so it is redundant to have these parts for both of the tubes.