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Old 21st July 2008, 05:29 AM   #1
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Default 300B Schematic

Drew this up with the help of my pals TubeCAD and SE Amp CAD.

Click the image to open in full size.

Couple typos: The first 5687 section's plate resistor is actually 23K, and is biased at 10.9V. The second 5687 is actually drawing 13mA (not 12) and is biased at 12.8V.

Full RMS power output simulates at around 7.5W at under 5% total distortion, with 3rd harmonic at 0.5%.

I've heard the 5687 is a "dark" sounding tube, but it seems robust enough for this duty. And with so many other factors that might color the sound, I figured I wouldn't sweat it.

These are designed for use in monoblocks. I'd use SS rectification, with choke-input or something close to it off a Hammond 282X (500V 200mA). Comes with a handy 5VCT tap also. Max transient current draw from the circuit, basically doubling each tube's quiescent requirement, is 190mA, so I think we're covered.

The OPT, if I were to go through with this idea, would be a custom order 15W 3.5K from Edcor, at 20-20K. Should be about $140 for the pair, including design fee.

I'm probably going to use my two 282X's (and two 6.6K 60W OPT's) to rebuild some 6L6GC monoblocks I "unbuilt".

Anyway, what do you guys think? Nothing crazy special in that circuit but it seems reasonable. No crazy parts, not even any fixed-bias or direct coupling.
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Old 21st July 2008, 04:01 PM   #2
kevinkr is offline kevinkr  United States
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Hi Sorenj,
Most people don't use negative feedback with 300B as it has reasonable linearity without it.

I have not had good luck with AC heating on the 300B (too much hum on the output) and would recommend a dc supply probably using a ccs instead. You would unfortunately need a little more voltage than the 5V winding can provide, although you might do it with Schottky diodes and a RCLC filtered supply. (R there mainly to adjust filament voltage to correct value.)

Consider using the WE hum canceling connection in the cathode circuit of the 300B. Additional cap is placed between the B+ at the transformer and the cathode circuit. May be calculated by Cap= Cbypass/AV+1 where AV represents the actual voltage gain of the tube into its load rather than the unloaded mu.

I currently use the JJ 300B in my amplifiers, and earlier versions at least have proven rugged.

Inexpensive Valve Arts and Shuguang 300B may not last long at the proposed operating points, most of the rest should be fine.
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Old 21st July 2008, 07:21 PM   #3
jazzbo is offline jazzbo  Canada
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Second the DC on the 300B filaments. If you can swing it, use the DIY HIFI Supply board ($129USD a pair). They are a voltage+current regulator design. Works very well. I recently rebuilt a pair of Bottlehead Parabees and installed these boards. Very, very quiet, 300uV on the output, no hum pot required. I won't go back to AC.

Kevin, maybe he wants to use the feedback to lower the output impedance.
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Old 21st July 2008, 07:25 PM   #4
smbrown is offline smbrown  United States
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I'd say your 10w cathode resistor on the 300b is a bit low. You're at 6+ w as is. I generally try for 5x headroom for good low noise and reliable operation. In this case, a 25w part should be good enough.
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Old 21st July 2008, 08:43 PM   #5
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Quote:
Originally posted by jazzbo
Kevin, maybe he wants to use the feedback to lower the output impedance.

Then I'd use a 5K transformer instead of feedback. This strikes me as one of those schematics that will work, but that can be bettered with just a little bit of effort.
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Old 21st July 2008, 09:10 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by dsavitsk



Then I'd use a 5K transformer instead of feedback. This strikes me as one of those schematics that will work, but that can be bettered with just a little bit of effort.

I'm with Doug on this one.
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Old 22nd July 2008, 10:52 PM   #7
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Thanks for the "feeback," everyone. Sorry, I had to do that one. I see your point on the NFB for the 300B, I might as well not bother wiring it up.

Would a "budget" 300B tube really poop out at only 30W plate dissipation? That's at what seems a reasonable proportion of the max of 40W...

I'm definitely game for using DC on the 300B. I'll research some of the extensive threads about it. One thing, though: the voltages tend to run high on the 200 series Hammond transformers, so 5V is more like 5.75V. It's a 3A winding; is that enough current to feed a regulator? I might also just slap in a little toroid at 6V, however many amps I need.

If I ever build a 300B amp, unless I suddenly get entranced by more DC or IT coupling, it'll probably be this one.
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Old 22nd July 2008, 11:49 PM   #8
DougL is offline DougL  United States
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Quote:
If I ever build a 300B amp, unless I suddenly get entranced by more DC or IT coupling, it'll probably be this one.
If I was going to build something 300b'ish, there would be some things I would do differently.

If possible, I would only have a single voltage gain stage to drive the 300b. In order to get the voltage gain from a triode, I would CSS or Choke load a higher gain tube. Say 12AT7, 6H45pi or 6N1p, then use either a cathode follower or source follower directly coupled to the 300b grid. Alternately a small signal pentode with a follower was presented on DIYaudio a couple of years back.

Just random thoughts.

Doug
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