Monday, 12 December 2016

Circuit as built and tested

As the build phase of the amp is completed, this seems a good opportunity to publish revision 13 of the circuit schematic. 13 seems to be the lucky number in this case as this is the "as-built" version, complete with reference voltages as tested.
Initial testing (with a loudspeaker as well as an oscilloscope) shows this working well.

The resistors in the bias section may perhaps need changing later, if I can't achieve 50mA quiescent through the KT88s with the resistors I've got. This would be a case of increasing the value of R26 and R27.

Other thing this schematic does not show is the delay turn on for the HT with the 555 timer IC. I'll get around to drawing that later.

This schematic is for the amp proper,

Note the power supply the diodes are paired in series, this is because PSUD2 warns the PIR voltage across them would be exceeded - albeit only slightly. So I decided to play safe and pair them, with balancing capacitors.

Note also the main electrolytics for the power supply are connected in series, not a usual configuration. This is because these caps are rated at 450V and my B+ is 560V.

So they are series-connected with balancing resistors. Voltmeter tests show the balancing resistors all have the same voltage across them: 280V plus or minus a couple of volts. So they are doing their job.

So this is a fairly conventional Williamson design with KT88 finals in ultralinear configuration. Gain and inverter are provided by a 12AX7, driver stage is a 12AU7. I have not used a CCS.

Click as necessary to see full size

As a passing comment, I was derided for choosing an excessively ambitious design as my first valve amp design and build. I took that as a challenge, rather than the discouragement it was intended as. I'd even considered naming this amp the "Defiance" in reference to it, but instead I have called it "Waru Titiwai" which is a Maori phrase (Maori are the indigenous people of New Zealand in case you didn't know) which means "Eight glow-worms"

New Zealand is full of glow worm caves, some of which are tourist attractions, others more out-of-the-way. All are sacred places to Maori.

Apart from possibly changing the values of the resistors in the bias section, I don't anticipate any further circuit changes.

[EDIT] This schematic is out of date, as I've subsequently fine-tuned the NFB which resulted in some changes, there's a current schematic here

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