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| PSU Design. |
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| KT88. 400R cathode |
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| 300B. 740R cathode |
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| KT66. 300R cathode. |
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| EL34. 400R cathode - can share the same plate as KT88. |
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| 6550. 400R cathode - can share the same plate as KT88. |
One of the projects that has been sitting on the shelf for some time has been the Aleph 30. I originally got the PCB from Chipamps about two years ago, and got most of the parts a year ago. Recently, due to my interest in the F4 amp, I decided it was time to finish this project sooner, rather than later. To that end, I purchased a Torodial transformer from Tortech, a 300VA dual 18V type. The only parts needed to complete this is a chassis and the heatsinks.
Regarding the heat sink issue, I have been exploring various options in my mind. Using the standard off the shelf heat sinks from someone like a Conrad Heatsink. Another option floating around is using heavy weight structural steel sections. I was thinking a 380PFC section cut to around 300mm in length. This would also form part of the top chassis. It weighs around 55.2 kg per meter, so a 300mm section would weight about 16.56 kg. Plenty of mild steel mass to sink the heat. But not sure if this would be a good thermal solution. More experimentation to come.
Currently looking around for the parts for the two F4 amps that I have on my mind. With the active devices already purchased, the choice is now narrowing for the remainder of the passives. I have elected to purchased 4x PSU PCB from Brian at Chip Amp as it allows a fairly straight forward build. I have use his kits in the past, quality and service is very good. The main question is how much to budget for this build. With the use of high quality resistors and capacitors, the passives alone for a 4 channel build will be close to the US$1,000 mark. The thought of diminishing returns start to play a part here.
PSU:
Signal:
Having set a path forward with the headphone side of my audio setup, I thought it appropriate time to look back at the stereo setup. I am quite satisfied at the current crop of speakers sitting in boxes, ready to be mounted onto cabinets. With a move to a home with a more fitting stereo room, I think it would make an ideal start in comparison of the speakers.
In this same vein, I have started to review the amps. I have a range of 1W and 8W tube amps ready to be assembled and tested. At the moment, most of my speaker drivers are in the 95 db+ range, and can be considered efficient, with the exception of the Jordan JX92s.
Reading through the various articles about amplifiers in the voltage and current domains, my conclusion (which is very similar to many others) is that tubes are excellent voltage gain devices. They work primarily in the voltage domain. To get them to power speakers, one has to convert some of the voltage into current, hence step down transformers. The output transformer takes, typically, hundreds of volts and milliamps, and transforms that into tens of volts and potentially tens of amps. This is then able to drive a speaker.
Though I have not much been interested in MOSFETS or transistors for amplifying devices, there seems to be a movement towards using them where their strength lies, in the high current domain. In a sense, they operating in tens of volts and tens of amps (or thereabouts). Most popular power amps are of the transistor type, and are able to produce hundreds of watts of power.
The debate between what is better; tubes or transistors, I think will never be concluded, as it is a function of taste. Each to their own. So far, I have experienced primarily tubes, and I like them not only for their sound, but their nostalgia and appearance. For me, a glowing red tube is so much more attractive than a massive heat sink. The fundamental problem, taking aside taste, is one of power. Tubes cannot really go above 100W, and if we limit ourselves to the single ended affair, probably not above 30W, sensibly.
I think what I, and other SET fanatics crave is more power. The only way to achieve that is to operate the tubes at an ever higher voltage and current. I have not found tubes that really work well in 16W plus in SE operation that operates at less than 800V. The 6C33 is close, but operates at a very high current of around 400 mA. The high current represents its own problem in transformers and chokes.
Fundamentally, what I have come to understand is nothing new, or earth shattering. However, it was important that one reach that conclusion in one's own time and way. My tastes at the moment favour; Single tube SE or one tube pair PP, both in Class A. Further, my goal would be 32 Wrms of clean power, irrespective of speaker efficiency.
Now that I am at this point, I find that I have two paths to move forward with. First path is with a high voltage SET, like the GM70, 211, 845 or similar transmitter tube. The other, a hybrid solution mating a tube front end with a transistor output stage.
Since I have ruled out high voltages, that only leaves the second path, one of a hybrid approach. Having only casually explored the hybrid path, I have come across the Moskido design, which is a hybrid of the Aikido design with a MOSFET output stage. The other design is one of a booster amplifier, after a tube output amp. Having further researched this idea, I came across two interesting solutions; DIY Hifi Supply Fusion module and the Pass Labs F4 impedance converting amplifier.
The Fusion module seems to be very easy to implement, with the addition of a low voltage power supply and the insertion of the module. It looks like a current amplifier that is probably in parallel with the speaker. It probably includes some kind of Class D chipamp operating as a current amplifier.
The F4 is more interesting. Now only has it got some really good reviews, it is a similar approach as with Andrea Ciuffoli in his "100W Hi End Hybrid" design. Any which way, they are both interesting approaches and designs. The F4 has a cult following in the DIY Audio community, and there are PCB boards available as well as volumes of forum postings about this. For me, that makes it a safer design, as it can be supported by a range of people who have built it.
Having come to this conclusion, I ordered 2 sets of the PCB boards from the DIY Audio Forum store. Also ordered the matching number of sets of the active devices from Tech DIY. I hope to start the build soon, and share what I have research and progress to date.
Here is a quick idea for a simple 6c19p SE design.
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.
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.
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:
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.
Total Heater consumption = 35 W (approx). More to come later.
Just wondering if I wanted to change the rectifier in my SE84 kit from the N4007 to a 5U4G the wiring would be as shown in your manual "ZSmanual.pdf", in which the following wiring is shown:
Question is, on the "ZSmanual" schematic, the first RC filter resistor is rated at 50W, whilst on the SE84 kit it is a 6W. Do I need to change it to a 50W dale, or similar?
[Steve/Decware] You are correct about being able to wire the tube rectification as per the SE84ZS schematic. The 50W Dale used in the SE84ZS is not required. We chose that particular resistor because it is a 1% tollerence and like the way it sounds. The 6 watt dale will be absolutely fine.
After looking around for simply HV tube kits, I came across Decware. They make the Zen SE83 tube amp in a range of configurations. As you may know (and can find out more from the site itself), the base model is around a 1.5W to 2W Class A design using a Triode connected pentode as the output tube. The arrangement is clever in that it can appear to bridge two amps together to form a higher power 6W or so SET amp. The claim is that the sonics are not reduced. However, of interest to me specifically, is that they offer a almost all parts (minus power switch/socket, chassis, etc) kit for the same amp design for a reasonable price. The kit comes with output transformers, power transformers (with international windings of 240V, a great move unlike some other kit manufacturers who refuse to sell and/or option the power transformer for anything but 110/120V). So like all good things one comes across, I ordered the kit and started to think about upgrading the kit already!
The question was why did I abandon the Uniamp and the DRD45 project for this? Firstly, the power transformers for the Uniamp are playing up, and may have been shorted or wound only for 110VAC. Which is annoying. Secondly, I am still somewhat hesitant in doing a point to point job on the DRD45 with 500VDC without first working on something somewhat safer (that was the point of the Uniamp). Since the Uniamp hasn't got around to working yet, but I am still keen on a tube amp, this seemed to be a good alternative.
Looking at the schematics, the circuit looks to be a fairly typical SET with a PI (CRC) PSU filter, AC heater windings, grounded cathode input stage with a capacitor coupled to the power stage, all of which seemed fairly typical. However, on closer examination it appears that the output stage is somewhat unconventional. The output transformer is not connected to the ground! The two output terminals are connected direct to the speaker. Thus, no connection to the ground at all. On the input side, one is connected to B+ (specified at 346V DC) whilst the other is to the anode of the output tube (6BQ5, etc). This feature, is, according to Decware, what allows two amps to be connected as a bridged amplifier. I would love to investigate this further. A true SE amp of 1.5W ish, can be instantly converted to a 6W SE amp! Sounds tempting.
After ordering and reading through some of the information on the site, I sent Steve of Decware some questions, here are his responses. (Reproduced with his permission)
1. Can I substitute the SS diodes in the kit for a Tube rectifier? Like a 5U4G or similar. Looking at the schematic of the kit it appears that there is a secondary winding on the power transformer that is unused, I assume it is a 5V designed for the rectifier?
[Steve/Decware] Yes, the power transformer has the windings for a tube rectifier.
2. To connect in series bridged mono mode, what is the process for series connection? Would it be a series connection between the two output transformers; connecting - of transformer 1 to + of transformer 2 and then the speaker to + of transformer 1 and - of transformer 2. And at the input end, both inputs would be connected to the same signal.
[Steve/Decware] Yes.
3. As a later date upgrade, could I replace the AC heaters with a DC regulated supply or similar.
[Steve/Decware] Possibly, but it’s debatable if it would actually be an upgrade. You best bet for future upgrades is to focus on trying different coupling caps. The board has been designed to accommodate both the small stock cap as well as large after market caps.
4. Does the point-point wiring have audible differences to the PCB?
[Steve/Decware] Yes, but this board is quality designed so the difference is minimal. The tube rectification would make a much more noticable change than point to point.
5. In the future, am I able to order just the transformers from you, where I can source my own parts elsewhere?
[Steve/Decware] Yes, I plan to make it possible to order just the transformers or board next time I edit the web site.
6. The 0.1uF coupling cap that forms a R-C filter, can I up the value to lower the f-3dB point?
[Steve/Decware] You can, but I wouldn’t recommend it. The bass is perfect as is, and the phase response is exactly where I want it.
7. Do I need a preamp? I would like to run this direct after a phono stage (with say 40 dB of gain)?
[Steve/Decware] No preamp needed, no. Your input sensitivity will be 2 volts. You can achieve close to that with a 5mv cartridge like the Grado and a stage with 42dB.
8. Can I replace the 1K/6W PSU resistor with a choke (I have a few Hammond 159P 10H/125mA/155DCR)
[Steve/Decware] You can, but the fundamental signature of the amplifier will change as will the clipping characteristics. Right now the amp clips gracefully. If your goal is to get the best sound possible you might try it both ways and see which you like better.
9. Can a cathode bypass capacitor be added to the 6922 tube as well? Perhaps something in the range of 220uF to 1000 uF to assist with reducing any chance of NFB.
[Steve/Decware] Again, it can obviously, but you’ll want to listen to it both ways on a variety of tubes and determine which you like best.
10. Will the board fit larger (more exotic) coupling caps? 0.1uF? Can I say put a Mundorf Silver/Oil or a Audionote/Jensen copper in oil, etc?
[Steve/Decware] Yes, the coupling cap area of the board was made with two different hole patterns so you can use either small or large caps.
11. There should be no issues replacing the PSU caps (33uF/450V) with a few 30uF ASC oil caps I have?
[Steve/Decware] Probably not, other than a small change in the power supply ripple.
Currently investigating the Decware SE84 kit. The tubes used are some fairly commonly available tubes with a number of interesting equivalents. I like the tube complement because you can get a huge range of NOS, current production and Russian/Chinese equivalents. Great opportunity for tube rolling!
Input: 6DJ8, 6922, 6N1P, E88CC, E88CC/01, ECC88, CV2493,CV2492, E188CC, 7308, E288CC, CCa, 6N23P or 6H23, 6N11, 7DJ8 (different 7V heater).
Refer to this link for more info.
Output: EL84, 6BQ5, SV83, 7189, 7189A, CV10321, CV2975, CV8069, N709, 6P15, 6P14P.
Refer to this link for more info.
