Completed the Crack!

After some waiting and a few false starts, the Crack has been completed. It was done over three sessions. Firstly, I must say that the instructions and build went very smoothly. The instructions provided by Bottlehead is first rate. The photos included were first rate, and I used them mainly to do the work and to check the work. The written instructions are very clear, but I found them actually a little verbose and the layout could be improved. In addition to the Crack, I also purchased the Speedball upgrade and completed that at the same time. The one and only problem was I soldered the wrong Transistors on the Speedball update. I did not pay attention that there are actually 2 types of small transistors.

The only other initial upgrade I applied was to change the output coupling capacitors from the supplied electrolytic to a 100uF Aeon paralleled with a Russian K40Y 0.1uF. The caps are big and was a little tight to fit, but it does. After the above fix, the amp fired up without problems. The voltages and resistance all measured as per the kit.

The first session was with my usual setup, using source from an old iPod. On first impression, the bass is really present and in your face. Almost to the point that can be felt. Amazing. The sound was very good indeed. There was a slight amount of hum present, but its level is quite low, and I didn't hear it until I looked for it. Otherwise, the dynamics are quite awesome and was really an enjoyable listen. This was more impressive given that it was unbroken in and it was using 'standard' components like a stock attenuator, connectors, and electrolytic caps!

Having a quick comparison with my Mini Max Millet left a few impressions. The bass on the Crack is much better to my ears. More body and more present. The sound is more lively and seems to be more 'dynamic' and less compressed. As I run the amp in, in the coming months, it will be very interesting to compare this with the other headphone amps.

It is also time to dust off my Maple Tree Audio Ear+ and fire that up. In tandem, I also purchased a TubeCAD 9-pin all in one, with the intent of making another headphone amp with that. So in the next year or so, I will have supposedly four quality headphone amps to compare!

Operating Points for GU50 Triode

Here are some operating point options for 2k5, 3k5 and 5k loads.

6n6p Tube

• The pin connection is the same as the 6DJ8, and also has a screen on pin 9.
• Service life of 2,000 hours.
• Heater 6.3V at 750 mA.
• Gain of 22.
• Curves look very linear.
• Was the pre-cursor to the 6h30p tube.
• Still very readily available and cheap.
• http://www.klausmobile.narod.ru/testerfiles/6n6p.htm
  

6c19p SE Design Idea

Here is a quick idea for a simple 6c19p SE design.

• Operating point at 210V at 45 mA with -80V at grid.
• Say 300V at B+
• Swing from -5V @ 40V @ 70mA to -160V @ 380V @ 25mA.
• Loadline at 5k or 7k5 both have similar outcome.
• Require a grid signal of around 50Vrms to drive to full power, with a bit of headroom to avoid A2 operation.
• Assuming 2Vrms input, require a driver stage with a gain of around 25 minimum.
• CSS loaded input stage could use C3g, 6n6p, 6DJ8/family, 6/12SN7, etc, a lot of tube choices in the medium mu grouping. Given the B+ of around 300V, operating the input tube at around 200V would be ideal.
• Normal cathode biasing and capacitor coupling.

The great thing is that I have most of the parts lying around, with the exception of the power transformer. This can make good use of the Lundahl choke rated at 150mA, the CSS kit from K and K Audio and a few Jensen and Cerafine electrolytic capacitors. As usual, my enemy is time in getting around to hooking it all up. This would make a nice stereo amp. Maybe a schematic to follow.

This could even work for swapping out the output tube for a 45, by changing the bias capacitor and heater supplies. With B+ at 300V, the 45 could be 250V at 50V bias. Could work as a great convertible experimenter's amp.

Key Facts of the 6C33C Tube

Here are some key facts and information regarding this tube. The information below is gleamed from tube datasheets, websites, forums and playing with the tube itself (but not in an electrical circuit yet).

• In Cryllic, it is 6C33C. In transliterated English it is 6S33S.
• Comes in two flavours, the vanilla (6C33C/6S33S) and the rugged version (6C33C-B/6S33S-V).
• The main difference between the B and the normal version are the 'B' has lower Internal resistance and can withstand much more g forces in impacts and loads.
• Rate life is around 1,000 hours, but in a non-military/extreme environment and operated well below maximums, this could extend out to 10,000 hours as suggested by some.
• The tube is actually two triodes in one envelope. The cathode, plates and grid are internally connected in parallel. The heater is left separate, which can then be used singularly or both in a parallel or series connection.
• The tube is designed to run with lower voltage (150-250V range) and higher currents (150-250mA range).
• Very low internal resistance and a good candidate for OTL.
• The heater takes a lot of heater power; either 12.6V @ 3.3A or 6.3V @ 6.6A. That is 41.58W.
• The heater needs to turn on at least 2 minutes prior to B+ voltage. The longer the better.
• The tube itself feels very rugged, sturdy and well built.
• It was made in a number of different factories; Ullyanov, Svetlana and Poljaron.
• It was first discovered in the West via a MiG-25 from a defector's plane.
• Uses a special socket and has been suggested that it needs to be ceramic because of the high heat in the operation of the tube.
• The pins of the socket generally need to be well ventilated and clean to ensure good cooling and electrical contact.
• The tube runs hot and cooling and placement are critical to reliability and long life. Consider forced cooling, and considered passive cooling.
• A link from Good Sound Club with lots of advice on the tube. Rather biased opinions, but nonetheless valid observations from someone who has used this tube extensively.

Bottlehead Crack OTL

Just purchases the Bottlehead's new OTL headphone amplifier, the Crack . Seems like an interesting addition to my headphone system. It is relatively cheap, and looks to have plenty of room for upgrades/tweaks and expansion. Never used an OTL amp before, so it'd be interesting to build and compare this to the other headphone amps I have running.

The tube complement is very standard and readily available. The NOS prices on the two tubes, 12AU7 and 6AS7 are quite reasonable, moving up to the very expensive. But since it only uses one tube each, tube rolling may be an easy reality. Upgrades such as capacitor replacement (it looks 5 electrolytic can be replaced), CCS to the driver tube, resistor upgrades, sockets upgrades, etc. are possible.

Bottlehead seems to imply up to 4 weeks wait time, which is fine, as I am in no hurry to build. Over the last few months, I have come to the conclusion that headphone audiophile will be the main method of music enjoyment for myself. Mainly due to work and family arrangements. So, in the meantime, using my HD650, I will endeavour to build and listen to a wide a range of headphone amps as possible!

For reference, the following tubes may be suitable as substitutes.

12AU7 - ECC82, CV491, 5814, 6189, 5963, 6680, ECC802, E80CC and B749

6AS7/6080 - 7236, ECC230, 5998, WE421A, CV2523, 6336, 6520, 6h5c/6n5s, 6h13c/6n13s

6C33 Tube

Recently gained interest in Russian military surplus tubes, and inevitably the power tubes that tops the list include: GM70, 6C33 and GU50, etc. Here are some info on the 6C33 tube.

• http://digilander.libero.it/paeng/russian_numbering_system.htm
• http://www.tubes.ru/techinfo/AmateurRadio/6c33c-b.html
• http://www.jogis-roehrenbude.de/Russian/6C33C.htm

It appears there are two kinds of the 6C33; 6C33C and 6C33C-B. From reading various tech info, the 'B' designation seems to imply 'vibration proof, high reliability'. On the website above, the operation time (presume rated life) is shorter for the 'B' version at 750h rather than 1000h. The 'B' version seems to have lower internal resistance does have better acceleration and impact ratings. The curves for the two appear to be near identical.

Lowther DX3

Recently, I purchased a pair of second hand Lowther DX3 from eBay and they arrived over the weekend. They arrived in good condition and appeared to be in very good shape. After a cursory checked, I hooked them up to my Zen EL84 amp. This was the first time that I turned the Zen on in my new house. The first thing I noticed was that the mechanical hum that was present in the power transformer was gone! This was a good start.

One of my debates in my head has always been, is 1 watt really enough? These Lowther were certainly going to confirm or deny this. Just using my iPod as the source, the amp was near silent with no music. A very faint hiss was heard only when you were so close to the speakers that you'd almost hit the cones. This was also good confirmation. I still noticed that the EL84 were running very hot on the heaters, with 7V plus. This still needs to be fixed.

The first cue of music, was my usual mix of favourite tracks that I was very accustomed to. The sound wasn't great. But mind you, they were simply placed on my dinner table. I had no chance to build any baffles. The good news was that the speakers worked without a problem. As you'd expect, there was little or no bass, thanks for a complete lack of enclosure. Obviously more thorough review will need to be done once they are in a proper enclosure.

My first impression was that piano notes and vocals did sound very natural and real. However, I noticed that there was something that I could only say was a 'Lowther Shout'. Now, I am no expert, but there are so many variables that may have caused this. To note, these Lowthers were the older styled without the rolled whizzer cone.

More thoughts and experiences to come.

Different 45 Operating Points

Working out the operating points using the traditional ruler method and the RCA charts has been fun, but using TubeCAD's SE Amp CAD was not only much faster but produced far more information. The parameters are using a Tamura F5003 (Amorphous core US$600+ each transformer) with the plat at 202V biased at 35.8mA, results in an output of 1.3W and a distortion profile of; 2nd @ 1.7% and 3rd @ 0.1%). The input voltage swing required is 60Vp-p. If we wanted to maximise power without going into Class A2 (positive grid current), then the input swing would be 66V, and this results are 1.6W and a distortion profile of; 2nd @ 2.1% and 3rd @ 0.2%. According to SE CAD, %I max is 100%, %V max is 73% and %W max is 91%. Running the 45 reasonably hard. This will also be run with the Lundahl Amorphous core LL1620 or LL1623 output transformers. See the curves below.

Working on the assumption that we keep the 45 in Class A1 (i.e., no positive grid current), then the driver stage will need to deliver 23Vrms or 66Vp-p. If we assume that a 2Vrms signal is capable of being delivered by the preamp, the driver stage will need to have a gain of say 12x. This brings into contention a large range of tubes beyond the higher mu and high gm types (presently been considering C3g, D3a, 5842, etc.). Using a 1:2 transformer, either in the grid position (i.e., Lundahl LL1670) or as the interstage, would mean that the gain required by the driver itself is lowered to 6x. Opening up a range of DHT tubes. On the other hand, if a 2:1 interstage transformer was being used, then the driver stage will need a gain of 24x, BUT the impedance driving the 45 would lowered.

In one thought exercise, I might be able to get away with a 26 Tube into a 1:1 interstage transformer, and using the 1:2 step-up function of the Lundahl grid choke to provide a gain of around 15~16x. This would necessitate another Tentlabs Filament heater (to reduce hum) and probably far more attention to reducing microphonic and hum effects on the 26. Considering I already have starting a small stash of 26 tubes, this may be an interesting option. With various reports about the sound of the 26, this may be worth the effort.

At present, if using a 26 driver, the B+ for the 26 would be less then 200V, which means I can use a pair of 0C3/VR105 regulator tubes in series to provide a stable voltage for the 26. Perhaps a pair of 0D3/VR150 might be better if we were willingly to drop some voltage over a divider network. Consideration will be made to regulating the B+ for the 45 tube.

The 45 Concept

For those who have read some previous entries in this blog, you will no doubt have read my ramblings regarding my directly heater triode amp. It began as the 300B story, and thus far, ended at the Electra Print DRD45. Having gained more insight into amp design and building, as well as more reading into the various standard reference books and Internet forums, I have concluded that the 45 is the tube to stay with. Helped in no small part to my 20+ 45 tubes. And in some twisted logic, I still hold that the 45 is a good tube for those wishing to trying different NOS brands and new stock. My reasoning to this; there are good range of NOS 45 available. They are easy to find and readily available on most stores and the price is still the sub US$100 per tube. The globle versions (UX145, UX245, UX345 and UX445) are much more sought after and range in US$150 to US$200 per tube. Whilst expensive they are still readily available. In terms of new production 45, I have only come across the Emission Labs and TJ brands. The range of available 45 is great and the NOS supply is still relatively affordable. This is in contrast to the 300B, 2A3, PX25/60, AD1, 50, 211, 845 and all the other DHT power tubes.

The 45 amp in broad design will be:

• C3g driver tube. Either Triode of Pentode mode. To be reviewed.
• Interstage Coupling. Using 1:1 or near interstage from either Sowter, Lundahl or Tango-ISO.
• Fixed grid bias for 45. Using Tentlabs Negative Bias Module
• Filament heating through Tentlabs filament module for 45.
• Output Transformer is Electra Print 5K with 8R and 16R output. May consider changing to Tamura Amorphous core when the finances allow.
• Passive PSU with 2 choke stages. Using a Lundahl LL1673/140mA common mode choke and an Electra-Print 5532 choke for each channel. All capacitors to be ASC Oil X386 types.
• Rectifier tube 2x6AU4GT.

Most of the parts will be recycled from the DRD45 build that has yet to be done. This will be constructed as a stereo block with no attenuator.

Operating point of the 45 would be at the standard 220V with a -40V bias at 36mA onto a 5K load. Output should be around 1.3W, and will require the driver stage to provide around 40V of swing, or 80Vp-p or 28.4Vrms. If we have a sensitivity of 2Vrms then the driver stage needs to provide a gain of around 15x. An easy task for the C3g. The mu of the C3g is 40, so it might be worthwhile to consider using a 2:1 step-down interstage transformer. This may help reduce the reflected load and increase bandwidth. Being my first self designed amp, the above operating points may be completely wrong. However, it is a start, nonetheless. The other interesting note is that with the gain requirement of the driver stage of 15, it may open up possibilities of using an alternate driver tube, even using another DHT. Nothing ground breaking here.

Below are the heater requirements.

1. 45 Tube: 2.5V @ 1.5A = 3.75 W per tube total 7.5 W.
2. C3g Tube: 6.3V @ 370mA = 2.33 W per tube total 4.7 W.
3. 6AU4GT Tube: 6.3V @ 1.8A = 11.34 W per tube total 22.5 W.

Total Heater consumption = 35 W (approx). More to come later.