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
- 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.