Browsing my schematic archive I found this early implementation from some years ago on the 300B:
The amp design is straight forward. Let’s start from the output stage. The 300B is run hot at 33W (376V/90mA) with a fixed bias of about -78V. I used a pair of LL1623/90mA OPTs which I had wired on 3K:8 mode.
The filaments are driven by Rod Coleman’s regulators which are floating and connecting one end of the filament (cathode) to ground. This leg goes via a current meter (A1) with a pair of protection diodes (D1 and D2). There is a timed fuse which I believe I ended up with a 150mA and not 200mA as shown in the diagram. Should the fuse blow, R2 will force the valve to nearly cut-off and protect the valve and OPT.
The bias was implemented with a very early prototype from Rod Coleman of his fixed-bias regulator. The regulator was feeding a current of about 24mA through a 3K3 wirewound resistor to generate the bias voltage. I used a 4-wire Kelvin film cap (aka “Morgan Jones’ cap”) to provide a clean bias to the 300B via the secondary of the IT transformer.
The HT supply was properly “over sized” and you can see it here.
The driver was based around the 6e5p in triode mode. I wrote quite a lot about this and her sister 6e6p-dr. Both are great drivers. In this case, I biased it up to over 4.2-4.7V using a stack of my favourite SiC diodes. The IT was LL1671.20mA configured to 1:1 ratio. I used an HV Shunt Regulator set to about 40mA. Each channel had its own shunt regulator.
To get full output power of 8W you will need 52Vrms (146Vpp)drive. Therefore the driver should have at least 200Vpp/70Vrms capability as headroom is needed. The 6e5p will drive the stage nearly at maximum output level with a standard 2Vrms input. If you have a lower input source, you should look at a different driver like the ones discussed here.
Looking at this amplifier today, I’d probably change the input driver for a hybrid mu-follower (aka gyrator) plus a source follower instead of the IT. Also output transformer would be the Monolith Magnetics(Summit S-9) which are superior than the Lundahls.
Hi Ale, When I look at the schematic I don’t understand why you place the fuse parallel to the cathode resistor and I think it wil affect the cathode current. Maybe I am wrong but the dc resistance of the fuse will be much lower than the cathode resistor.
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Hi Alex, this bias regulator is ok to run 300b in A2 ? , I use similar schematic 2a3 LL1660 (Parafeed ) driving 211, when into in A2, the standard fixed bias circuit not work correctly