The road to nirvana

free counters

My journey into Photography, Horology, and Audiophilia

Custom Search

AK-300 Schematic

Some technical notes:

The available parts from my bin will dictate the operating conditions of this amplifier. To start with, the Hammond 1628 I have is the older, smaller one and I was told it has a limit of 50mA. In addition, the LL1648 only has 350V HV secondary which will limit the B+ I can play around with, so I have to use a tube rectifier with minimum voltage drop possible. I could have gone fixed bias to compensate for the low HV of the LL1648, but then I will have to adjust the bias voltage every time I change the tube. Hey, I'm lazy so I'll stick with cathode bias.

I was hoping that I will be able to come close to the textbook values as specified in the tube datasheets given the design constraints. After building the amp, I measured the voltage and current and came up with this table.

Parameter 10Y Textbook 10Y Actual 50 Textbook 50 Actual 300B Textbook 300B Actual
Va 350V 386V 350V 336V 300 324
Bias -32V -34V -63V -54V -63 -62
Ia 16mA 15.4mA 45mA 41.5mA 50mA 45mA
Pout 0.9W not measured 2.4W 2.35W (simulated using SE Amp CAD) 6.1W 4.73W (simulated using SE Amp CAD)
Rk   2K2   2K2 || 3K3   2K2 || 3K3
Rectifier used   5AR4   5AR4   5AR4

As you can see in the table above, the 10Y configuration is closest to one of the textbook values. The 50 is acceptable, and the 300B is off already for getting the max power output at the textbook operating points. However, I'm more concerned of sound quality than power output as my speakers are sensitive enough to be driven by a 1 watt amplifier.

If I will be building this optimized for a specific tube, then I have to change a couple of parts.

Some notes on parts selection:

I'm not particularly picky about parts. I will normally use what I have or what is readily available and reasonably priced. For that reason, the power tube cathode resistors are 15W aluminum clad type. The 10K dropping resistor is a cheap 5W "white coffin" type. The 47K input resistors are precision metal film given to me by a friend. For the 417A/5842 cathode bias, I used the cheap variety Orange LED which should allow me to be as close to 2V cathode voltage as possible. The hum balance pots are the big 50W TDO wirewound because I can get them cheap from my source.

The CCS for the plate of the 417A/5842 is a pair of HV PNP configured for 10mA current.

The capacitors used were Solen 10uF/630V and Nichicon Muse 100uF/450 for the power supply, MultiCap RTX 0.22uF/630V for the coupling cap, and Black Gate standard 100uF/160V for the cathode resistor bypass cap.

Unlike my 45 amp where I used the teflon insulated hookup wires from Audience (I used them because they're cheap from a dealer friend) this amp will not be a beneficiary of that because I've used up all my Audience after I re-wired my speakers. So the hookup wires are those cheap ones, not even teflon insulated, that I can get from a local electronics store. The signal carrying wire is shielded and according to the label is made in Japan, which I took as higher quality wire.

For the irons, I used what I already have- Lundahl LL1648 for the power transformer, Hammond 1628 for the output transformer. Some custom ones were used for the choke (10H, 120mA), for the grid choke (5,000H), and for the pair of filament transformers.

While the chassis is a beautiful black powder coated 2mm steel, the power switch is one of those I rescued from the surplus. All connectors are brand new, gold-plated type. The IEC connector is the standard ones you can get from Farnell or RS Components. For the cathode resistor and filament voltage switch, I'd recommend the high quality APEM (France) toggle switches available from Farnell.

Update - May 17, 2007

A couple of weeks ago, I decided to "save" my Lundahl power transformer for another project since I'm not using its full potential, i.e., the other secondary windings.

In addition, I changed the coupling capacitor to Jensen Copper Foil which I have been using on my 45 amplifier.

I sold the Hammond output transformers to fund the purchase of James 6123HS output transformers. The James iron is sonically superior to the Hammond in this application.

Here's a shot of the guts.