Friday, 26 August 2016

Slow progress

A couple of weeks of urgent family matters that needed attending to have put the brakes on the amp design, however things haven't slowed to a complete freeze.

Around one week ago, my custom-made power transformer arrived, and it is a beast. Just over 11kg and quite imposing on the bench

The digital caliper is just there for a size reference.

On this transformer we have a 230v primary
secondaries are:

  • 3.15 - 0 - 3.15v @ 8A (heaters)
  • 60v @ 200mA (for KT88 bias supply)
  • 400 - 0 - 400v @ 500mA (main HT supply)

Should be sufficient for the purpose. The challenge is going to be how to make it look nice in the chassis - it's a bit... naked.

Moving on, we are also near completion on the circuit diagram for the amp. A lot has been added and changed since the diagram was previously posted here:

(Click to see larger size)

  • Changed Circuit diagramming software to Tinycad (and made up my own parts library for the transformers)
  • Added KT88 final stage
  • Re-arranged 12AU7 driver stage layout in the diagram
  • Changed all resistors to preferred values, recalculated bias and voltages as required
  • Added theoretical voltage reference points through the circuit
  • Calculated power dissipation across all resistors
  • Calculated DC voltage across all capacitors
  • Added voltage dividers to power the gain, splitter and driver stages (modelled in PSUD2)
  • Added power supply (not yet completed)

Still needing to be done:

  • Add adjustable bias supply circuitry for the output valves
  • Add the negative feedback section

And as an additional safety feature to protect the KT88s, I have decided to have the 500V B+ switched through relays which are activated by the bias supply (one relay per valve). This way, if something catastrophic happens like a pot wiper lifting or a resistor burning out in the bias supply, the relays will open and the B+ will shut down.

Another subject I have been musing is the shutdown behaviour. There's going to be 2000µF of capacitance on the main HT which means it may take a few seconds to shut down when the amp is powered off. Potentially there may be a situation where the some of the B+ HT is still at the anodes of the output valves while the bias supply has shut down.

Given that there's no load on the bias supply this is unlikely, however it is a point I want to measure carefully in the construction.

Once I've completed the design, I'll post the circuit up on the forum once again and see what I get in the way of constructive criticism.

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