Wednesday, February 8, 2012

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.

No comments:

Post a Comment