Cogito wrote:
David McGown wrote:
Be careful with this, I am not sure the power supply is designed for the voltage and current you will need. Be prepare to scrap everything but the chassis and perhaps the power transformer (I seem to remember that Rosenblit likes using 115V/230V transformers, though). Check current capability.
I replaced the supplied transformer with 2x200V 100VA toroid. Connected the two 200V winding in parallel. The PS circuit has CLC smoothing filter. 470uF caps and 10H choke.
https://www.antekinc.com/as-1t200-100va ... ansformer/It also has a 48V transformer which can be used for biasing the tubes.
As the transformer has 2 6.3V 3A windings, each heater can have its own power source. How much DC smoothing is needed for the heaters?
Any suggestions on soft start?
Well, with a 200V transformer and a bridge rectifier into a cap input filter, you can get a raw supply of 280VDC. There are couple of power supply design pathways.
1. Use a CLCLC filter with low R filter chokes, if you can keep the total drop in the filter to around 20V, it should put you in a good place for simple plate resistor loading of the 12B4, assuming you are dropping around 150V across the plate resistor.
2. Use a CRCRC filter with CCS loading of the 12B4. A DN2540 (or similar) cascode current source needs around 25V or more for good operation, so if you have 120V at the plate of the 12B4, that means you want to have no less than 145V at the top of the CCS. You need to drop about 130V across your filter network. Actually, the CCS is perfectly happy (given sufficient heatsinking) to absorb some of this voltage. That leaves more than enough voltage to add a shunt regulator for each channel, also fed off a CCS, something like a 0D3 or 0A2. So this looks to be a great upgrade path. Plan for two sockets for the regulator tubes. And space for heat sinks for the CCSs.
3. If you use a choke input filter, the raw DC voltage will be around 0.9x200V = 180V into the filter network. This is good for a choke loaded or transformer coupled 12B4, assuming your filter network drops the voltage down. May not be enough B+ for regulation, though.
So this transformer can definitely work.
I would not bother with fixed bias, it is an unnecessary complication. You can do fixed bias at the cathode of the 12B4 via a string of LEDs, you only need 10 to 12V, so that is 6 Red LED in series, or two 5.6V Zener diodes with a bypass cap for noise.
I do not think soft start is necessary for a preamp, particularly if you are NOT using separate fixed bias arrangement. Fixed bias via the cathode should be OK.
David