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.

829B lightbulb amplifier

A few years ago, a friend I worked with mentioned to me that the old gentleman who lived across the road from him was getting rid of some tubes since he was moving to a smaller house and would I be interested in taking them. I duly finished up with three show boxes of assorted tubes. There were two peculiar tubes that looked like they came from 'My Favourite Martian' and so I did some research about them. They were 829B transmitting tubes.

I looked them up in my copy of the RCA Transmitting Tube manual and also poked around on the web. Almost immediately I found Pete Millet's web site and his 829B single ended amp. The seed was planted in my brain and eventually I decided that I had to do something with them.

Here is a link to Tom Schlangen's plate curves for the 829B wired as a triode,

I tend to prefer the parafeed version of single ended and had built an STC amp and an 807 amp using the cheap Radio Shack line transformers. I had been surprised by how good these little cheap transformers can sound provided you don't need the amp to play very low frequencies. I typically use line arrays with 12 of the famous Parts Express 99 cent 4" full range drivers (269-469) so the line transformers would be ok because the arrays don't go below 100 hz. I have been building various subwoofers over the years and eventually settled on a pair of the Bill Fitzmaurice Tuba 18 horns that I built which I really like. I use an old Aragon amp to drive them and an electronic crossover.

I also had some 6C45Pi tubes waiting to be used for something and I decided to try a two stage amp rather than the three stages of Pete's design. I didn't have any suitable plate chokes however and I was pondering whether I would lash out and finally buy some good ones when I did one of my annual trips back home to Australia. As usual, the trip gave me time to think and ponder and somewhere along the line, I remembered Steve Bench's web site where he describes different versions of single ended topology. In particular, I found his comments about the resistor loaded version (#3) intriguing. "I would like to add that I find something magical about the sound of the resulting amplifier, in spite of its disadvantages". Then I also remembered the Nelson Pass Zen amplifier where he used light bulbs as resistor loads.

So what light bulb do you use? Based on Pete's amp, I figured I would probably be passing about 80ma through the bulbs and that I would need a load of about 4k ohms. I would be dropping 4000 x .08 volts = 320 volts and dissipating 320 x .08 = 25.6 watts. That's a fair bit of heat. The light bulb idea was looking better and better.
So what size light bulb should I use. Nelson Pass points out that the resistance of a light bulb changes with the voltage applied, but I figured I would start off with the simple P = VI and V=IR equations.

100115 0.87 132 
50115 0.43 264 
25115 0.22 529 
12 1150.10 1102 
This wasn't working too well. Since I was in Australia where the household voltage is 240 volts, I tried that.

100240 0.42 576 
50240 0.21 1152 
25240 0.10 2304
122400.05 4608 
That 25 watt bulb looked like a real possibility. I could use two in series for approximately 4600 ohms and a combined power capacity of 50 watts. Maybe it would work and maybe not, so on the way to the airport I stopped off and bought four of the bayonet sockets commonly used in Australia and six 25 watt bulbs. Somehow they let me carry the bulbs on board the plane in my hand luggage and they all arrived home safely.

Unfortunately, when you do the calculations, 115 volt bulbs don't produce resistances that are as useful. Also note that UK and Aussie lightbulbs have a bayonet mount.
It's about time you saw the schematic. But first, the warning.

This amp operates at very high voltages that can kill you. Do not attempt to build this amplifier unless you have already built several other amplifiers, preferably kits so that you can learn good techniques. I would highly recommend the Bottlehead kits. The exposed pins on top of the 829B are a definite safety hazard. Do not use this tube if you have small children around, or even curious adults.

I have not included the power supply. I will leave that up to you. This power supply will need to produce approximately 600 volts so be very careful. I would recommend that you look at other power supplies and the PSUD design tool. There is always considerable debate about what makes the best power supply. I typically do better with UF4007 rectification rather than tube, and I like using motor run caps.  I'm currently messing around with low DCR power supplies in my latest creation.

Since I typically use UF4007 diodes for rectification, I also use the two DPDT switch solution that allows you to turn on the 6 volt stuff first and then to throw the other switch for the high voltage. It doesn't matter which switch you throw first and the switches only have to handle wall voltage. It works very well and the only disadvantage I know about is that it doesn't handle a power outage where the power comes back on before you can turn the amp off. That doesn't happen too often. The circuit appeared in a Glass Audio but the author in the article got it from an old manual.

When you look at the pictures of the amp, you will see that I have a board that I got out of an old IBM computer that was to be thrown away. I figured it desrved to be kept in use. I don't know whether it helps or not.
The toroid come from Plitron.

The Hammond chokes and the 30uf 600VAC motor run capacitors came from a failed project where I realized I had bitten off more than I should chew. In retrospect, I was lucky I stopped when I did.

For the heaters, I use a Radio Shack 12 volt 3 amp transformer. It gets a bit warm with use. For the 6 volt wiring, I use some IBM type 1 plenum cable (solid copper 22#, FEP, twisted pair, sheilding galore). I ground the shielding. I like using this stuff for signal wiring as well.

I had read about being very careful with the 6C45Pi tubes with grip stoppers and cutting off the unused solder lugs to prevent oscillation, so I did as well as I could. So far, I haven't had any oscillation problems.

I used the euro-style terminal block that Pete suggests for connecting the antenna style pins on the 829B tubes. I have noticed that over time that they tend to loosen a bit and need to be tightened. You need to think about how much additional insulation you need beyond the insulation of the wires carring the high voltage to the plate pins.

I wired the output transformers as autoformers. To my ears, they sound better that way. I've since built another creation where I can switch from normal configuration to autoformer configuration and back with little interruption and I still prefer the autoformer way. You may differ in your opinion, they are your ears. Radio Shack has stopped selling the line transformers, but there are plenty of other places to buy the equivalents. Be aware that these Radio Shack transformers are not supposed to do too well below 100 hz. It's not a problem for me since I use a separate subwoofer.

I realized that with this amp, there were a number of things that might not work too well, so I tried to build the amp in modules so I could replace a module if it was unsuccessful.  I also like using polycarbonate instead of aluminium. Yes, I know I should use metal, but I hate using it since I always manage to cut myself on the holes etc. I tried polycarbonate and haven't gone back. I have since started to do what Pete Millet does and use single sided copper pcb and turrets for the main circuit. That works well for me and I still use polycarbonate for the rest of the amp.

I also had an IKEA Ivar side unit kicking around and decided to use it to support the modules. The power supply is at one end and the input 6C45Pi tubes are at the other. There is a reasonably logical progression from one end to the other.

Fortunately, the amp worked immediately, though I had some failures when I transported it to some Bottlehead meets in Maryland. I am gradually improving in my soldering abilities, but I'm still not too good at it.
I really like this amp and many others who have heard it at the Bottlehead meets have liked it as well. It certainly gets your attention though some have mentioned to me that their wives would never allow it in their living room. I'm not particularly good at describing sounds and would never make an audio reviewer, but Steve Bench's 'magical' description works for me.

Incidently, when the music gets loud, the light bulbs flicker in time to the music. Normally they put off a reaonable amount of light, but not a lot.

More on the line transformers.
Line transformers are used in public address systems to distribute output from an amplifier to multiple sets of speakers. From what I see, line transformers do not have an airgap. If you go to the Edcor web site you will see that they come for use in 25, 70 and 100 volt systems, and that they have input taps rated in watts and output taps of usually 4, 8 or 16 ohms. This all confused me initially until I looked at the Mouser website and a pdf file that was included with the description of their 42KB001 Audio Line Matching Transformer. This sheet lists the number of input and output turns for the various taps. Suddenly it all made sense. I put together a spreadsheet for the 42KB001 with the 8-ohm tap.

8 ohm
/ power
10665 857.82 61.21   490490 
5 94085 11.06 122.30 978 980 
2.5 133085 15.65 244.83 1959 1960 
1.251880  85 22.12489.19 3914 3920 
0.6252660 85 31.29 979.32 7835  7840

 If the 4 ohm tap is used, the spreadsheet becomes:

4 ohm
/ power
10665 6011.08122.84 491490 
5 94060 15.67245.44982980 
2.5 133060 22.17491.363 1965 1960 
1.251880 60 31.33981.783927 3920 
0.6252660 60 44.331965.447862  7840

As you can see the load results are much the same.

The final column is simply resistance = (voltage x voltage) / power. Using this formula, we can produce the following table for the various voltage and power combinations.

watts 25
60 1082  167
3021  163 333
15 42 327667 
1063 490 1000 
5125 980 2000 
2.5250 1960 4000 
1.25500 3920 8000 
0.6251000 7840 16000 

As you can see, there are several potentially interesting combinations.

I decided to try out one of the low DCR, LSES powersupplies that was talked about on tubediy. I built on a 12" x 24" sheet of polycarbonate with a switch that enable me to have a B+ of 300v or 600v. I also can optionally lower the voltages by switching in some resistors on the 115v side of the industrial transformer. I keep track of this with some DC voltage and currrent meters. The voltage meters are actually a 1ma current meter in series with five 200k resistors (effectively 1m). Next to the meters I have some terminal blocks that I use to attach the removable amplifier modules. The meters are useful to make sure that the voltages are where they should be and that they are at zero volts when I change amp modules. While in use, I cover the terminal blocks with a strip of polycarbonate to keep prying fingers away. I would not recommend this method if you have small children around.
The 829B amp was rebuilt on a 12" x 12" sheet of polycarbonate as an amplifier module. There were no changes to the circuit. It sounded a little brighter if anything compared to the first version. Curiously, the light bulbs do not flicker as much now, except at volumes.

I then decided to try a Gary Pimm self bias ccs load on the 6C45PI tubes. I have them set at 15ma and after some months of listening, I think there is an improvement. It seems to be a greater sense of control and dynamics.

Bottleneck 6V6 Competition

6V6 Competition 

Results (photos and schematics at end of blog)

  • The competition was held at Steve's place on April 11, 2010

  •  The first round results were very close. Unfortunately, both Jim and Ray's amps only worked in one channel. The final round resulted in a clear win for John's amp. All of the amps were excellent and we can highly recommend using the triode 6V6 as an output tube.

  • Pictures and schematics (if available) can be seen below at the bottom of this post.

  • Ray's amp won the Ikea prize.

February 25 Update

  • Amps to arrive in the 10am - 11 am period, preferably closer to 10am

  • Since nobody is building a Push Pull 6V6 amp, I will bring the Poinz Music Machine that my daughter built to see if we can pick it out from all the SE amps - the amp will not be eligible for the prize

  • Each amp to be turned on and tested prior to 11 am to ensure that each amp works and the offical tester knows how to do it

  • Each amp to be given a name eg 'The Devastator'

  • The order of audition will be random

  • Everybody to be seated in their allocated chair by 11am

  • Ray to supply scorecards and explain the rules

  • First audition will be a short test run with the PP amp to get people used to the music and the judging procedure - that amp will be repeated later in the competition

  • Each audition in first round to have about 20 minutes of music (6 tracks)

  • Scoring to be be completed at end of each audition

  • Short break between auditions (hopefully we can do one amp every 30 minutes)

  • Slightly longer break at the end of the third amp for lunch

  • At end of first round, the top three entries to continue (or two if we are running short of time)

  • Fewer tracks in the second round

  • Ray to present prizes for Best of Competition and The Ikea Award

  • Event to finish at 4pm ( Ray flies to UK from Dulles at 18:40)

  • No food or drink in listening area

  • Send me an email if you need directions to Steve's place

January 3 Update:

  • The competition will be held on Sunday, April 11. The starting time has not been set in stone, but plan on 10 am.

  • If you are planning to submit an entry, please send me an email by February 1 which will be the cutoff date for entries. We have no idea how many entries we will have to allow auditioning time for, so we need to use this cutoff date for planning purposes. There is no entry fee.

  • I have an email account at gmail. Please use rperry13 and I will acknowledge your email with directions to Steve's place in Stirling, VA.

  • If you plan to attend but are not going to submit an entry, it would be helpful to send me an email prior to the competition so that we have an idea of how many people are turning up. There is no cutoff date for non competing attendees.

  • Please review the rules below, particularly rule 1 about carrying your amp in the door.

  • According to rule 7, there is no need to submit your schematic and parts list until the day of the contest, but for planning purposes, when you send me the email, could you let me know if the amp is single ended or push-pull.

Again, I thank all those who have made suggestions. 

Objective: to build an inexpensive stereo amplifier based on a common output tube, the 6V6. Participants may use any design, single ended, push-pull, whatever, including two or four 6V6’s. The use of driver tube(s) is optional and at the discretion of the builder but the output tubes must be 6V6's.

The Place: Stirling, VA (near Dulles airport)

The Date: Sunday, April 11 2010. Kickoff probably at 10am.

Building your own amplifier is very dangerous, particularly with the high voltages involved. We will not be held responsible for any injuries caused by you building your amplifier. This competition is not for novice builders. We highly recommend that novices build one or more Bottlehead kits before considering a scratch build like this.

1.  After considerable thought, our committee decided we will not accept entries that are shipped to the competition. You need to carry the amp in through the door at the competition. If you live in another part of the country, you might consider holding a similar competition in your own region. Our committee decided we did not want to be responsible for items that were shipped to us.

2.   Cost of components shall be $200 or less including the tubes, sockets, chassis, rca jacks, binding posts and fuse/power receptacles. Each 6V6 will be priced at $8.95 which is the cheapest in the AES catalog. We will use a more expensive set of matched 6V6's at the competition that will be used to audition all entries. For the other tubes, you may use your own special NOS tubes, but use the lowest pricing from the AES catalog. The model number of all tubes should appear in the AES catalog.

3.  Screws, bolts, nuts, wire, washers, lock washers, shrink wrap, cable ties, glue, adhesives and solder etc. are the only items not to be included in the $200. If in doubt, include the item in your $200. Do not include postage / shipping costs from the vendors in the $200.

4.    The design shall be based on the 6V6 used as an output tube. You may use any other tubes you like for input, rectifier, regulation, etc. Sakuma 6V6 - 6V6 style amps will be acceptable. You may also use sand in the driver circuit. Try to be innovative and use some techniques you haven't tried before.

5.    The amp can be any class you like - single ended, push pull, class A, class AB, class B, class C, triode, ultralinear, etc.

6.  For safety reasons, the amplifier will include an IEC connector, an on-off switch, a fuse, at least one power transformer and a bleeder resistor. No wiring carrying more than 1 volt (AC or DC) should be able to be touched from the top or sides. Plate dissipation for each 6V6 should not exceed 14 watts.

7.  A schematic and a list of all materials with part numbers and pricing needs to accompany the amplifier. Pricing shall be based on the quantity of parts shown on the schematic or, if greater, the minimum buy quantity. Components, both passive and active, must be current catalog items sourced from any vendor with a commonally available catalog that can be accessed using the internet. Examples are: Allied, Digikey, Mouser, Antique Electronic Supply, Radio Shack, Edcore, Antek and Parts Express. Components purchased from Ebay vendors, are prohibited. No component should be made from unobtanium.

8.  You may use parts you already own, provided each exact part is currently available in a catalog. Discontinued items should not be used. The intent is that somebody else could come along and build your design using the schematic and the list of all materials for the same price. As an example, I happen to have a 22uf 450 volt Nichicon capacitor laying around. I can use it provided that I can still buy that particular exact same cap at Mouser and I can show the part number 647-UCA2W220MHD6 and the current cost ($1.28).

9.  Pricing of parts will probably rise in the course of a year. The pricing of the parts for the winning entry will be checked against the online catalogs. If the cost for the winning entry exceeds $200, it will be disqualified. One approach is to design for $190 which would allow $10 headroom for price increases. Do not use 'sale' prices.

10.    Just for the fun of it, the case should come from Ikea.  Be imaginative. When we were first thinking about this competition, we thought the IKEA cost should be included in the $200 but on second thoughts, it would be more fun to see what our imaginations can come up with. So spend what you like on the IKEA stuff for the casework, but no IKEA stuff is to be used for electrical purposes.

11.    All amps shall be evaluated using the same tubes, preamp, cd player, speakers. We currently expect to listen to the amps at Steve’s place in Stirling, VA using his 95db efficient Econowave speakers with JBL 14A woofers and the Selenium D220 wave guide loaded drivers which are 8 ohms. At our last meet, we felt that the JBL was not quite efficient enough so we may change the woofer.

12. The speaker cables will use banana jacks to allow easy changing of amplifiers. You should include full function binding posts. RCA jacks will be used for input. 

13.    All amps shall be evaluated using a compilation of music on one CD. The selection will be eclectic and include classical, blues and other genres. Typically, the music will be from CDs that have been enjoyed at our regular meets. 

14.   We will provide a simple prize, possibly some little plate you can attach to your amp that you can treasure for the rest of your days.

15. We are still considering how to judge the entries. The following may change prior to April 11.
Set up:
  • As much as possible, the same setup and positioning for all equipment(CD player/DAC/Foreplay/ speakers/cables/power cable) including positioning of the amp on whatever support we use.
  • A single CD with the music selections preceded by a test tone at roughly 440 hz to set the volume with some kind of sound meter in a fixed position.
  • All amplifiers to be invisible to the audience, even while not playing.
  • All amps will be numbered. The order of playing the amps will be drawn from a hat.
  • One designated person to be responsible for hooking up each amp. This person will not take part in the scoring.

The judges:
  • Depending on the number of entries, we may need to limit the number of judges at the discretion of the organizers.
  • If the number of judges is limited, priority will be given to those who regularly attend our Bottleneck meetings.
  • Each judge will be presumed to have either cloth or tin ears.
The judging process:
  • Each judge to be given a score card (and pencil) and will sit in an assigned chair throughout the contest.
  • No discussion while music is playing or for 30 seconds thereafter when the score must be written down.
  • The scoring system is not yet finalized but a sample can be seen here
  •  There will be a preliminary round where all amplifiers are auditioned. The top three amps will go on to a final round with a different music selection.
  • Each of the three members of the organizing committee will select two short tracks (or excerpts)for each of the preliminary and final rounds. If there are a lot of entries and we are short on time, we may restrict the number of tracks to be played. The list will be published prior to the competition. Between us, we will cover different styles of music.
 John's winning entry used a cake pan.
In the schematic, The power transformer is an Edcor XPWR013,  L1 & L2 are the primary & secondary of the Edcor GXSE10-8-5k
David's entry used a wooden tray and pencil holders
 Fritz's entry used a child's push trolley. The amp is on the lower level and used a Dynaco chassis with the cover not shown in the photo. Fritz also  mounted his linestage on the upper level, which is really clever.
Jim's entry used a tray. The amp was working the night before but there was a problem with a capacitor that affected the small input tube. 
 Joel's entry used a cake pan 
Ray's entry used a candle holder.
The second photo shows the lightbulbs glowing in the dark. The amp worked fine during the warm up test and then failed for the first time since the amp was built. The cause of failure was a screw to hold one of the 2SK3564's to the heatsink had loosened with all the jolting on the trip down. I now use  a dab of nail polish to keep the nuts tightened.