Pretty much everything in the preamp is standard tube amplifier construction. However, you do need to follow some standard tube layout and construction procedures. Keep the power and signal wires separated. If they need to cross, do so at right angles. The same is true for resistors and capacitors. Keep signal bearing components away from the power supply filter capacitors and especially transformers. I prefer to place power supply components in one section of a chassis and the signal portion in a separate area at least 2 inches away from the power supply parts. For more design and construction tips, see my Design and Construction of Vacuum Tube Amplifier tips and suggestions. Suggestions for a Tube Amplifier Wiring Color Code are also available.
The tube amplifier power transformer specified in the plans (Edcor XPWR083) is an inexpensive ($15 US, August 2010) PCB mounting type rated, 200 V at 40 mA and 8 V at 1.5 A. It is unshielded and if possible should be either enclosed in a metal enclosure (Photograph 3) or at least separated from the remainder of the circuitry with a shield. There is no reason other transformers can't be used; this one has the right ratings and and is inexpensive.
ForeWatt Type II Preamplifier with remote control circuitry
The heater supply is regulated DC. The heaters must not be connected to the same ground as the signal, B+ or chassis. They need to float or tube failure may occur. The only connection of the heater circuit to the rest of the circuitry is a "bias" tap. This is developed from the main B+ supply and is about 1/3 the B+. No current flows through this connection (perhaps a tiny bit of leakage). It only establishes a reference for the heaters so that the difference between the heaters and any of the cathodes is less than 100 volts. One quirk of SRPP and other "totem pole" circuit configurations is that one of the cathodes is elevated quite a bit above ground. In a SRPP the cathode on the upper triode has a voltage potential of roughly 1/2 the applied B+. So with 215-225 V on the anode the cathode is over 100 V. During start up this value is actually 1/2 the full B+ (of about 275 V). This value exceeds the tube rating for heater to cathode voltage and will result in tube failure. Perhaps this will not occur immediately, but it will as the tube ages. I have had this happen. Not a good thing. It usually results a huge spike in the output that could be catastrophic further down the audio chain.
Use shielded wire from the inputs to the selector switch and to and from the volume control. As there in no provision for channel balance (IMHO no well designed system should need one) use either a quality variable resistor (I use Alps Blues) or a stepped volume control / attenuator.
ForeWatt Type I Preamplifier - Interior View
One area that is always of concern is what to do with the metal chassis if you use one. Most inexperienced builders tie all the grounds to the metal chassis. This is nearly always going to result in ground loops. Depending on your electrical codes (which vary by country) you should connect the chassis (and actually any exposed metal parts like transformers) to the AC mains ground. In the US this is the third wire (supposed to be green). The signal and B+ grounds are connected to the chassis through a type X2 rated capacitor and a resistor in parallel with the capacitor. The capacitor is a special rated type designed just for this application (one brand is RIFA) and will range in value from about 0.1 to 0.33 uF. The resistor is typically a common carbon one of 100-150 ohms and rated at 1/2 Watt. A related mistake made by novice builders is to directly connect the input and output jacks to the chassis. The ground portions of the jacks should go to the "floating" signal ground in the circuit. Either of two types of ground system can be used for the signal and B+. I have used both "star" grounding and "buss" grounding in my projects. Either can work satisfactorily. In this project a modified star ground system is used. One star grounding point was used for each channel with a single connection to the B+ negative.
I used a type a LM317 variable voltage regulator in the heater supply as I am familiar with their operation (and have lots of them), but a fixed voltage regulator like a 7806 can be used as well.
The B+ is filtered through a total of 4 stages. Another thing I don't do is use chokes. I have nothing against them; I just don't need them to get the performance I require. The final filter stage to each tube is isolated from the alternate channel and each uses a polyester capacitor. I have found that using poly (polyester or polypropylene) caps in the final stage seems to improve the overall quality of sound. It is possible that an electrolytic with a suitably sized bypass cap would work well also.
The one component (other than the vacuum tube) that has the most effect on the sonic signature of the preamplifier is the output capacitor. The choice of brand, size and type are significant in this component. I recommend no less than 0.47 uF if you are feeding a high impedance load (50 k-ohms or more and short cables). Values as high as 10 uF are suitable for lower impedance (5 k) loads. I tried several types and sizes and finally settled with a 1 uF Russian K40Y9 paper-in-oil (PIO) capacitor in one version and a 1 uF Jantzen Standard Z capacitor in the other preamp. I suggest you use whatever brand and type you like as it will color the sound a bit. Some types I tried were Auricaps (nice clean sound with a little less bottom end), Jantzen (good overall performance, but not as detailed as the K40Y9), WIMA MKS (OK but not as nice sounding as the K40Y9), a cheap unnamed one (to protect the innocent) - it was horrible, bass OK, everything else edgy and not really listenable). So pick something you like.
The remote control portion of the second type of preamp is powered separately by a small 12 volt SMPS. Any kind of 12V power source will work fine provided it can deliver over 600 mA. It must be a type that is isolated from the AC line as the inputs and control circuitry are attached to the negative side of the 12 volts. It cannot use the same power source as the tube heaters as they have a positive bias on them to protect the heaters and cathodes from failure as noted earlier. See the schematics for the components I used. The instructions provided with the remote control module are a bit unclear so be sure to watch the interconnections. I misconnected them several times apparently without damage. Another thing that was confusing was that the module automatically selected input 2 on start up. This is not really a problem; just connect the component you use most to that input.
I used relays and a switch to activate the bypass function. The relays can operate off the same power supply as the remote control module. Just be sure there is sufficient current available. I used an underpowered one at first and had the relays chatter when the motorized volume control was operating. Very bad for the speakers. The MV-02 motorized remote volume control module is approximately $35US (August 2010). Remember that there can be significant shipping costs on items from Hong Kong so check first. It comes complete with the hand held remote control. It is a collection of assemblies that interconnect with ribbon cables. The instructions are a bit vague so read them carefully. Out of a total of five MV-02 motorized remote volume control that were ordered, one failed to operate properly. Despite the one DOA module, they are still a great buy. It the MV-02 comes with an Alps Black 100k stereo motor volume control. This potentiometer alone is worth the price. You can design a "passive" preamplifier around the module itself. Essentially, the bypass mode in the Type II schematic is just that. The indicator LEDs are not included, so you will need to provide your own. A series resistor of about 2 k is required for use with them. Since the module uses a common signal and power ground it could not be powered from the existing preamp power supply. I used a Mean Well PM-05-12 SMPS module to power it. This SMPS takes 100-240 VAC and converts it to 12 VDC. Importantly, it has 1000 volts of isolation of the inputs to the outputs. I attach it main power supply just after the power switch.
