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Old 21st February 2012, 01:56 PM   #1
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Default 300B Amp Testing

Hello, everyone!

I built my second audio project: a 300B amplifier along the lines of the new reference 300B amplifier, but using some cheaper PS caps and a different mains transformer. (Also, note that the D3a is not wired right on the schematic. I used the standard triode wiring.)

All the voltage measurements indicate that it's functioning normally.

I do not own speakers yet, so I bought a $10, 20W driver from Digikey for testing (http://parts.digikey.com/1/parts/1015-speaker-8-ohm-20w-102mm-square-gf1004h.html). While just sitting on the desk, it plays music at a modest volume, but distorts when pushed louder, especially the bass.

My first question is: Is it the speaker or the amp? The output of this thing should be 6W per channel, so I figured a 20W speaker should be able to handle full volume, but to be honest I have not been able to understand "audio power."

My second question is: how do I make sure this thing is fully functional before I spend money on expensive speakers. I only have the following test equipment: oscilloscope (with differential amplifier plug-in), function generator, multimeters. I did the obvious things (sent in test signals, compared output into the speaker), but are there subtleties?

Thanks!
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Old 21st February 2012, 03:19 PM   #2
a.wayne is offline a.wayne  United States
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Better to post up pictures of your waveforms ......
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Old 21st February 2012, 03:34 PM   #3
Yvesm is online now Yvesm  France
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Quote:
Originally Posted by mariner4 View Post
Hello, everyone!
. . .
I do not own speakers yet, so I bought a $10, 20W driver from Digikey for testing (http://parts.digikey.com/1/parts/1015-speaker-8-ohm-20w-102mm-square-gf1004h.html). While just sitting on the desk, it plays music at a modest volume, but distorts when pushed louder, especially the bass.
Any speaker needs some kind of "boxing" around'em to work.
As a fast test you may just try to punch an hole in an heavy (and large) cardboard box and attach the speaker to.
A simple 3 by 3 feet flat pannel with the speaker centered will already changes drastically what you hear.

Yves.
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Old 21st February 2012, 03:38 PM   #4
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Also post a pic of where the speaker is... Since it is errrr.... uh... "open baffle" at the moment, if it is lying there flat on the desk or backed up on a wall, then the bass travelling to the rear will get reflected back to the speaker and somewhat cancel out. Plus, without a box, the speaker will have very little in way of a waveguide or resonance box, so the bass will wimp out quickly.

See if you can get borrow a set of bookshelf speakers (or even better, some tall single driver full range speakers) to test.
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Old 21st February 2012, 05:10 PM   #5
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Thanks! I will try the speaker with a box. I don't see distortion on the scope (at least not with sign waves), but heard distortion. This may explain it.
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Old 21st February 2012, 06:52 PM   #6
tomchr is offline tomchr  United States
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First off, if the 300B is biased as shown in the schematic, I would turn the amp off immediately as it'll draw a few hundred mA of cathode current. This is NOT good for the tube. I assume the biasing is different than what's drawn, as the schematic as-drawn will result in Vgk = 0 V. You'll want to hook up the bottom end of the 200 kOhm resistor on the 300B grid to something like -80 V (or whatever gives you the plate current you're targeting).

I suggest inserting a 10 ohm resistor in series with the OPT primary. Measure the voltage across this resistor and divide by 10 to get the plate current of the 300B. Max current for new production tubes is around 100 mA. Most people seem to run in the 60~90 mA range.

Once you get the operating point of the 300B figured out, get an 8-ohm power resistor - I'd use a 20 W or 50 W type mounted to a heat sink. Connect that to the amp speaker output. Measure the output waveform using your oscilloscope.

If the amp is indeed built as shown on the schematic, I bet the operating point of the 300B is the cause of your trouble.
Other sources of trouble in your circuit includes blocking distortion caused by discharging the coupling cap. There's a good description of what causes blocking distortion on Tubelab's website. It's common to get blocking distortion on bass notes, but it shouldn't kick in unless you really crank the amp. But then a quick look on the oscilloscope will tell you what's going on. With blocking distortion the output voltage will remain constant for a while after a large transient. It's fairly obvious once you see it.

~Tom
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Last edited by tomchr; 21st February 2012 at 06:54 PM.
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Old 21st February 2012, 07:11 PM   #7
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Quote:
Originally Posted by tomchr View Post
First off, if the 300B is biased as shown in the schematic, I would turn the amp off immediately as it'll draw a few hundred mA of cathode current. This is NOT good for the tube. I assume the biasing is different than what's drawn, as the schematic as-drawn will result in Vgk = 0 V.
Hey there,

the drawing is a little misleading - the 300B seems to be in cathode bias, so everything is ok so far, the cathode resistor and its bypass cap are hidden in the PS part of the circuit diagram

Best regards,
Andreas
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Old 21st February 2012, 08:44 PM   #8
tomchr is offline tomchr  United States
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Oh, DUH! I didn't recognize the 1 kOhm Caddock MP816 as a power resistor. So even though I checked for cathode bias, I only saw 2x22 ohm and went hmmm...

1 kOhm is quite high, though. I guess it depends on the target operating point for the 300B, but I wouldn't expect much from 1 kOhm, 400 V. From the JJ 300B data sheet I'm guessing the operating point will be around Vak = 325 V, Vgk = -65 V with some 10-ish V across the OPT primary. That's not going to give you much power. Maybe 1 W at 3 % THD. That would explain why the amp distorts when the volume knob is cranked up.

With 360 V, 60 mA you'd get a few Watt. I'm running 400 V, 90 mA and get about 15 W at 3 %. 11 W at 1 %. But then I'm running a driver that can source some current. The 300B does draw a bit of grid current as Vgk approaches 0 V. Its internal capacitance also needs current to charge, discharge. Must .... have .... oomph!

~Tom
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Old 21st February 2012, 09:22 PM   #9
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Thanks, guys.

The 300B is biased correctly (60V at the 1K resistor, so 60mA). I also checked and the waveforms going to a dummy load are correct (no distortion visible to the naked eye on the scope for the sine wave).

I am guessing my problem is the speaker mounting. I'll try it in a box and see if things change...
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Old 21st February 2012, 09:46 PM   #10
tomchr is offline tomchr  United States
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With 330~340 V from anode to cathode and 60 mA I wouldn't expect a whole lot of power. A Watt maybe two... And fairly high distortion as well. Been there. Done that.

I've used PC speakers for test speakers before. That works pretty well.

~Tom
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Last edited by tomchr; 21st February 2012 at 10:02 PM.
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