New 300B Amp

[Shannon Parks]

I've wanted to do this as soon as I did the original 300B mod to my get*set*go and I finally started working on prototypes four or five months ago. Anyhow, the goal was for 12W which calls for high voltage, high current and a super stout driver with lots of voltage swing due to the Miller Effect from the 300B. I'll post the schematics after I finish tweaking the design in the next few weeks, but the first fire up tonight worked great. Hit 10W easily without any adjustments.

The circuit is rooted in the GSG with the same PA774-type transformer rectifying the filaments. We can use space saving bridge rectifiers since we only need 5V now (used Schottkies in the GSG). Used fixed bias straight from my Eiclone as it's a proven design and stable, and this helped us use a lower B+ as we aren't wasting voltage due to a cathode resistor. Also eliminates hifi issues with the electrolytic bypass on the cathode resistor. The 300B cathode is connected to ground through the hum voltage divider to a 10 ohm resistor (bias sense) and a 125mA FA fuse (will probably switch to 250mA FA). The driver circuit is a 6GH8-type using a near clone of the ST70 VA stage. Note that this biasing scheme is pretty much out of the era's datasheets and it works great. The triode half is a current sourced DC cathode follower biased at 7.8mA using a cascode CCS: the DN2450 and LM334Z I posted earlier (search for "EZ sink").

Shannon

[TerrySmith]

This should be an interesting project! I probably have a dumptruck load of 6GH8 / 6U8 / 6EA8 etc types to burn up so I will be building one of these.

Are those outputs the Edcor CXSE-25's?

[Shannon Parks]

Yep! Those are CXSE25-8-3K outputs. I had spec'd 2.5K, but they had these in stock. The power tranny is the Edcor XPWR005-120. For those wanting a tube rectifier, I would just spec Edcor this PT with a 5V tap and use a 5AR4.

The 6GH8/6BL8/6U8/etc tube is beautiful as we have scads of NOS available, it it linear well over 100Vpp and it has high gain and impedance required for plate feedback from the 300B.

Shannon

[EWBrown]

Excellent! Looking forward to a great new300B amp project

Is that an "Uncle Ned" wood base ? Looks very similar to one I got from him a few years ago, on which I built up an ST-35 Rev D.

Ditto the 6U8As, 6GH8s, 6EA8s, etc, I also have a dump truck load of these, never assigned them to the "plinker"
pile, they're definitely still useful Also have lots of the 5V versions, I suppose that I could really "cheap out",
and use the same 5VDC filament power as for the 300Bs
That would also put DCon their filaments, which should eliminate any vestigal 60 Hz
hum that could be induced by AC powered filaments.

/ed B

[Shannon Parks]

Yes, that's an Uncle Ned base that I 'borrowed' from my Clementine ULtra for prototyping. I need to repay him somehow one day as he was a big help from day one of me entering the DIY hobby.
I'm thrilled to be using a "junk" tube, and basically every flavor of that tube tested very well. Now I only feel compelled to use a 6AU6 one day!

Shannon

[EWBrown]

6AU6s / EF94s... The Great Tube Haul, I ended up with over 550 of them, all loose pulls / used / unused, who knows... I cherry-picked out the best, about 40 of them, and then gave the remainder of them (along with thousands of "plinkers") to my friend Dave in NH, who already had a very impressive tube collection / hoard , stored in his basement, and two outdoor storage buildings.

So, one of these days, I really need to put some of these 6AU6s to good use... For the cathode follower, probably any old cheap twin triode would work nicely,
no need to spend big bux (or Kroners) on "audiophile" quality glass for that function. Or if havingan extra tube sticking out of the board, then a 6C4 CF for each 6AU6.

Oh, yeah, I like the friendly smiley tubes on the topside of your board

/ed B