Unfinished WAD 300b PP kit

[cc454]

Originally Posted by Richard

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Also mentioned (twice) is that the circuit is "pure Class A self bias". The bias is set with the cathode resistors and runs the 300Bs quite hard. Most of us removed/left out the 6k8 resistors and so used 1k5 as the cathode value instead of 1k2 to lower plate dissipation. Whatever you choose check the voltages once up and running and keep the 300Bs under 32W plate for longer life. I used 4 makes of 300Bs without any failures. Popular were the EH Gold Grids, about £140 per quad at the time off eBay or Hotrox (UK).

Except for the DC filament power I am going to leave the circuit from T1 to T2 as per the WAD schematic I have.

I am gathering components to flesh it out, still looking for a good deal on for a quad of 300bs.

[Richard]

Hi Chuck,

You may get a better deal on valves from a US supplier. I bought a set off eBay from a non-Uk supplier, China-arts or similar iirc, and they were fine,

and others from Mark at Hotrox here in Nottingham,

https://www.hotroxuk.com/matched-val...0b-valves.html

https://www.hotroxuk.com/electro-harmonix-300b-eh.html

https://www.hotroxuk.com/electro-har...gold-grid.html

[cc454]

Does anybody have have the power transformer's original spec sheet? I am curious how much current can the HT source.

[Richard]

Hi,

I’ve not seen the TX spec anywhere but the 300Bs are drawing;

95/1200 = 79mA each or 316mA for the set of 4

In power terms;

Plate dissipation is (95/1200) x (550-95) = 36W each or 144W

The cathode resistors burn another 30W and the small valves a few more W.

In use it runs warm-to-hot with no hum so it seems happy enough with no failures reported I can remember. I wouldn’t push it much further as there would have been little point the designer using anything huge as it is a completed design for 300Bs and isn’t regulated. You could try removing all the 300B to check the HTv loaded and unloaded to get an idea how much the load pulls it down.

[cc454]

Thank you for your post! I am ever the librarian and like a well populated tech sheet.

I am still missing a few parts to complete the BOM, primarily the 2nd set of chokes and the output tubes along with the filter caps I want to use.

I do have some vintage chokes that could be used in a pinch, but at 80 some years old, they may rattle a bit.