Wednesday, February 8, 2012

Tube Taster Linestage

Some of you who look at the Bottlehead internet Forum might remember a post by Doc B back in January of 2001 extolling the virtues of Dalwhinnie single malt whiskey. It provoked a flood of messages where the boozers amongst us extolled the virtues of Lagluvin, Laphroaig, Balvenie etc. My contribution follows:

A couple of years back, spouse gave me a twelve pack of sample size bottles. It was very instructive to go through them and see what I actually liked. My favorites were Dalwhinnie and Lagluvin.
What I need is a twelve pack of various tubes to taste. Now that is an idea. Designing a generic input tube tester should be possible for 7, 9 and octal.
The idea lingered, longer than the Lagluvin, and I decided to design and build a Tube Taster. Something where I could try out these various input tubes such as 6N1P, 5965, 76, 6SN7 etc and get an idea of the sonic flavor in a standard setup. Like many of you, I have a stash of tubes that I was going to do something with one day, a couple of 76’s, 6N1P, 5965, 5867, 12AU7, 6080 and so on. I decided that I would just take a standard 12” x 12” piece of aluminum, put in a bunch of 9 pin and octal sockets plus a couple of five pin sockets for the 76’s and see what could be done. I also wanted to do it inexpensively and simply.

I really enjoyed John Day’s Valve article on the Paraline with the output transformers. Looking at the schematic, I realized that I could use the guts of it and replicate it a number of times using switches. At this point, it probably would help to see a schematic.

Everything inside the box is repeated six times. For example, there are six R3 resistors, six R4 resistors, six R2 resistors and six C1 capacitors per channel. However, there is only one R1 resistor and one C2 capacitor and one output line transformer per channel. The key to it all is the two pole, six position switch for each channel.

As you can see, I use a two pole, six position switch for each channel. One pole is used to connect to the input resistor, R1. The other pole is connected to the capacitor C2 connecting to the output transformer. Since there are six positions, I can accommodate a maximum of six different tube types. Let’s look at wiring tube 1. Connect position 1 to the plate side of resistor R3 that connects to the plate of tube 1. Connect position 7 to the grid of tube 1.

It does not matter about the position of the switch, for each tube, current will flow from B+ thru each R3 to each tube thru the appropriate R4 to ground. The grid resistor for each tube ensures that the bias is set correctly for each tube. Since the topology is parafeed and no DC current is passing thru C2 or the output transformer, each tube operating point is stable and predictable irrespective of switch position. In fact it would be possible to listen to a 76 through the left channel and a 6N1P though the right channel.

If we desire to actually use tube 1 and the switch is positioned to contact positions 1 and 7, then the signal will flow R1 - pole A – position 7 – grid 1 – plate tube 1 – position 1 – pole B – C2.

If we desire to actually use tube 4 and the switch is positioned to contact positions 4 and 10, then the signal will flow R1 - pole A – position 10 – grid 4 – plate tube 4 – position 4 – pole B – C2.

I finished up with two five-pin sockets for the 76’s, three ninepin sockets and two octals.  To make things simple, I tried to use a standard operating point of 200 volts and 10 ma since that point is pretty easy to pick out on most plate curve graphs. Incidentally, the 76s run at 6ma.

The six position switches are break-before-make Lorlin 10WA155 from Mouser at the time of build $2.57 each. They are a nylon design rated to 1000 volts. The design calls for 200 volts to pass thru the switch, so to reduce the risk of electrocution, take care in the quality and design of the switches you use in this position. In addition, use non-metallic knobs. There are a lot of wires in this linestage and color-coding is useful if not essential. It also helps to number the tubes 1 thru 6 to avoid confusion, particularly when you use non-duals such as 76s.

The output transformers are Edcor WSM10k/600 at a whopping $8.24 each (at the time, currently $10.23). They arrived nicely packed between two pieces of plywood and appear to be built to a higher standard than the line transformers I get from Radio Shack, and the core was wrapped in a fetching shade of yellow tape. On their website, they use M6 for core laminations. These transformers are wonderful for playing with. No doubt the more expensive transformers are better, but you can go a long way with these at very little cost.

The power transformer is a simple global use dual primary 115-230, dual secondary 115-230 43va unit. Typically, these cost less than $20. Mine actually came from Allied, but Mouser and Digikey also sell them. The 43va rating is a bit low if you plan to run all of the tubes at once. I usually run with half the sockets empty. You might consider lashing out and buying the 80va model for a few extra bucks. I simply had a 43va unit on hand, left over from another incomplete (failed) project. It’s pretty easy to get 300 – 320 volts with the usual ultrafast diodes and a simple CRC or CLC smoothing circuit. There is nothing special here, though I did some of the Foreplay mods to the power supply. I’ll leave them up to you.

What is special in the power supply is the turn on circuit that I filched from an old Glass Audio article that credits an even older document. I sometimes wonder if there is really anything new in this field. It requires that the 6 volt transformer be separate from the high voltage transformer, but they are cheap. Simply turn on either one of the DPDT switches and the heater voltage is applied. Wait whatever period you think is necessary for the heaters to get warmed up and then turn on the other switch and the high voltage is applied. It does not matter which of the switches you turn on first. Turn off is just as simple. Turn off either one, which turns off the high voltage, and then turn off the other to remove the 6 volts from the heaters. I use this arrangement on my power amps as well. I used a Radio Shack 12.6VAC CT and wired 3 tubes to one 6.3VAC half and the remaining tubes to the other 6.3VAC half. The DPDT switches from Radio Shack work fine.
Be aware that this arrangement does not work if there is a short power outage. Where I live we get a lot of outages and when they occur I simply turn off the switches if I can before the power returns. If the power is out for a short time I figure the filaments are still hot.

When changing from one tube type to another on the preamp, I first remove the high voltage first from the amp, and then remove the high voltage from the preamp. I then turn the two rotary Lorlin switches to the different input tube and then turn on the high voltage for the preamp and then the high voltage for the power amp. It takes but a few seconds with the heaters still in operation the whole time. Turning the Lorin rotary switches without following this routine results in a pop at the loudspeakers.

So the big question is whether you can actually hear differences between the various tubes. The answer is yes, which is not surprising. When I first started to get music to appear at the loudspeakers I quickly shuffled between the various tubes and immediately noticed differences. Gain levels vary, of course, so I needed to adjust the volume to do real comparisons. Actually, I found it works best to resist the temptation to flick around between the tube types. Certain tubes seemed to suit certain music and I seemed to be consistent in my tastes. If I put this in whisky terms, if it’s really cold, wet and miserable outside, I prefer the peaty Islay types like Lagluvin and Laphroag. Otherwise if it’s just plain wet, then Dalwhinnie works for me. If it’s dry, then I prefer Ezra Brooks bourbon. If it’s after dinner, then Pierre Ferrand cognac is the tipple of choice. Variation according to your needs is wonderful.

You will notice I have not said which tubes I prefer. The reason is simple, I don’t want to influence your taste. Build it yourself and then you will know what you yourself really prefer.

Construction. The unit is designed to be turned over easily and worked on. I took the unit to a Bottlehead meeting in Elkton, Maryland, and its looks caused a sensation. Takes the whole business of fit and finish to a whole new level. Perhaps "gawdawful looking" seemed to be the most appropriate description.  Some of you may know that the old fashioned artisans used to put in a deliberate flaw in their projects to show that only God was perfect.  I decided to leave the half torn off yellow sticker from McMaster on the top aluminium plate as my deliberate flaw. Just to make sure that God noticed, I did the same thing on the acrylic bottom plate.

The Edcore transformers.

Wiring this linestage was a challenge. I used wire from IBM Type 1 cable. It's got some sort of non flammable foam insulation and I like using it. If it came from IBM in those days you could be sure it was top quality.

Sound quality. Let’s just say it sounds a whole lot better than it looks. Definitely not duct tape sound. There is something quite special about the parafeed topology, even in a preamp.

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