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.
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.