Slowly making progress during the lockdown period. Over the past few weeks I managed to achieve quite more than I was expecting. The fixed bias and 300B filament supply board (board 1) needs to be wired. On the other hand, the driver filament supply plus the HT supply board is completed (board 2):
I tested the HT supply and can deliver over 430V. It has more voltage capacity, however the filtering capacitors are 450VDC. The DHT filament supplies can do 8V to 15V to accommodate different filament requirements (including some level of filament bias). It works beautifully!
Last night I did a quick job to build a pair of source follower boards. They will drive the 300B grid with DC coupling. The whole fixed bias arrangement will be as follows:
- Raw supplies for -200V and +25/50V for positive grid current supply and correct operation of the pass-MOSFET in the follower
- Swenson+ regulator for bias
- Channel trimpot and coupling capacitor for independent channel bias setting. This will be in one PCB
- Each channel has a source follower biased at 20mA
Here is a snapshot of the boards:
As I ran out of these PCBs, I decided to make a minor upgrade on the boards before requesting a new batch. I added a series resistor and parallel LED for either A2 operation or normal operation indicator. Also place underneath the board to solder a film/PIO 100nF decoupling capacitor as needed.
Hi,
Do you have a schematic or something for Swenson bias reg?
Regards
Ale
Any reason you have source followers biased at 20mA? In our past discussions, especially when used to drive the grid to A2, you mentioned that 10mA should be plenty
Thx
Sridhar
Yes, 20mA is an overkill. Likely to get it down to 15mA or even 10mA. That will save me space with the heatsinks. With 20mA I will need to dissipate about 5W per follower.
For the 300B, the input capacitance with socket included is about 100pF (80pF without socket). You may want to check my maths.
The PASS MOSFET (8nK80Z) in my case, has about Cout=140pF. Total is about 240pF. Let’s say 250pF to round up.
The maximum current to source/sink to charge/discharge both pass MOSFET and the 300B grid is Ic=C*2*pi*Vpeak*f.
For 20kHz and 100Vpeak (200Vpp), you need Ic= 3.14mA. With 10mA in the CCS you have about 3x headroom. Perhaps stretching to 15mA, but not more. I may change the resistors down to 15mA.
Read this if you are interested:
http://www.bartola.co.uk/valves/2015/12/21/slew-rate-in-preamps/
Can you post specifications on the fixed bias 50VA JMS transformer? What taps do you have?
Hi, it’s on the article: 300B SE Amplifier
Thanks Ale, I am asking a bit more about the 50VA power supply from JMS, what other output taps at what amperage do you have? I understand its a split bobbin type with copper screening.
Hi Paulius. I have the following secondaries: 0-25-50V and 0-200-300V. Both rated at 100mA. The idea was to have a flexible transformer to be used on multiple circuits given that I was having this made to order by JMS.
The pri-sec bobins are split, although not sure if really needed to keep mains noise to minimum. It’s a plus for sure. There is an outer screen shielding with copper on the bobin. Both transformer and shielding has earthing connections.
If you order from JMS, you will have to provide this information. They do outstanding work, although now with Brexit and shipment costs, it may be not be that cost effective.
Hope this helps.
ale
Thanks! You have documented this build in such a detail that it is tempting to build/or at least calculate the budget!