Saturday, 4 May 2013

The Oarta - One Amp to Rule Them All

It has certainly been a quite a long time since I have done ANY DIY audio stuff. I blame this due to my relocation to Hong Kong for the last 18 months and having young kids. There just ins't time. Well, with the imminence of return back home on the horizon, I have started to dream up what I will finish up. In this long time away, I have realised that I like variety. But I don't want to build too many amps. So, I have taken a few ideas, based around a SET and merged it into one amp. Which I call - Oarta, or One Amp to Rule Them All, not exactly creative.

The basic premise of this amp is that the PSU will allow the supply of around 390V at 200mA max, with heater windings two sets of heater windings, 2x 6.3V at 2A and 1 or 2 5.0V at 2. This should allow me to use a large range of tubes that I have on hand: F2a, 300B, KT88 and EL34. And also open up a whole range of other tubes that can run at those voltages.

The second part is a universal output transformer. I have chosen a 5k tap, but may move to a 3.5k for more power. For this amp, it will be no compromise, so a amorphous Tamura F5003 or F5002 will be on order. The driver stage will be a medium mu (think around 20) that is interstage connected to the output. To accommodate the wide range of output tubes, I will probably want a higher gain stage, so C3g triode connected may well be the final choice.

The trick here is to allow the switching of the output tube. To this, I am thinking of an adaptor plate that allows the socket and cathode resistor to the changed. The bypass resistor will then be mounted on the chassis. In this way, I can have a large film cap that isn't changed with each socket. On each adaptor plate will be 5 connections; grid, plate, cathode, heater + and heater -. I need to find safe and reliable connectors that allow me to change.

The final trick is have a 6.3v and 5.0v winding, and finding a method to accommodate a range of heater suppliers. For the indirectly heated 6.3v tubes, this is no issue, but with the DHT tubes, like the 300B, PX25, etc., this will need some more work. There may need to be an allowance for something like the Tentlabs filament supply, that can run from the 6.3v to power a 5.0v and run from the 5.0v to power a 4.0v tube.

If this can be made to work, I have an amp, Oarta, that will allow me to change tubes, without having to change anything else. This will allow me to not skimp on the rest of the components, knowing that it will be universally applicable. At the moment, my current thoughts for the design are:

Amplimo 7N1474 Transformer (here - I may need to supplement with additional heater taps)
2x 6AU4GT diodes
Cap input 10uF ASC Blue line
Two LC stages with Lundahl LL1673 200mA (here) each with ASC 50uF.
I am thinking about a separate LC stage (15H with 40uf) for each channel and an additional LC stage for the driver.

The main issue with the PSU is finding space for all the ASC oil caps and the chokes.

This will give me around 386V of B+ at 190mA current draw (simulating with PSU Designer). At this B+, the following would be the power output with different tubes (from SE Amp CAD) using F5003 all running at 80mA per tube.

300B: 7.6W with 120 Vpp
KT88:  6.8W with 62 Vpp
EL34: 8.8W with 60 Vpp
KT66: 4.5W with 46 Vpp
F2A: 5.8 W with 32 Vpp (by hand)

And I would think that this would also work with tubes like GU50, PX25 and some of the other more rare European or exotic NOS tubes. And if I don't mind burning off voltage as heat, it could be used on 2A3, 45,  AD1, etc. - again, options. Beyond these, there are still a number of issues to work through, but in my mind, the concept already has been resolved. I am now thinking about cathode bias for the driver, and most likely will head down the LED route. Given the C3g triode seems to work well with around 2.0-2.5v bias. 170V at the plate, 2.5v bias 15mA. Nice and simple.

The main course of action now, is to determine how best to build the adaptor plate, this will make or break the design. It must be safe, easy and high quality to preserve the rest of the amp design. And the last question is how to deal with the different voltage swings required by the various tubes. Which differ by a factor of 4. I do not want to simply waste it by using an attenuator, and the first thought was to have taps on the interstage, which would allow me to choose the right ratio, 1:1, 2:1 or 4:1. This I think is possible with the Sowter 9525. But I probably want to avoid changing the IT ratios as that will change the frequency response characteristics.

Or I could just vary my source output - say between +0 dBV, +6 dBV and +12 dBV. (Which incidentally is possible with the Wadia 121 DAC that I am considering, see manual). Or, I could just accept it, and have to set the volume levels differently for each tube. This though, needs more thought and consideration.

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