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First DIY amp FINISHED - 300B SET - works!
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Author:  sampleaccurate [ 15 May 2010, 17:27 ]
Post subject:  First DIY amp FINISHED - 300B SET - works!

Last night I finally finished my first DIY amp, a single ended triode 300B stereo amp.

I used SMPSs for the heaters, and a direct coupled 6SN7 driver for the 300B tubes with Hammond audio output transformers..

Brought it up slowly this morning on a variac, no smoke, voltages looked about right. Finally plugged in some speakers and a CD player and it worked - first time!

I was missing some resistors and had to make a modification that is limiting the power right now, but at low volume it sounds sweet. Very clear, nice highs. I LIKE!!! Next up is a KT88 stereo oddwatt. And today from Ukraine my GM70s arrived in the mail for a future high power SET amp.

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No pictures of the inside yet. I confess - it's messy. I need to clean it up.

Author:  sampleaccurate [ 15 May 2010, 22:03 ]
Post subject:  Re: First DIY amp FINISHED - 300B SET - works!

The inside. It sounds better than it looks. I know it's not very neat. It had some hum at first, but shielding the cable from the input to the first stage of the 6SN7 eliminated all hum and buzz. It's now dead quiet, despite not rotating the output transformers with respect to the two power transformers. The OTs stay completely cool after several hours. The power trannys get slightly warm.

The 300B tubes have a cool blue glow that pulsates with the music. I now can't wait to finish my oddwatt.

I'm sold on tubes. I wasn't sure what to expect when I built this thing. Now I'm really glad I did, and I'd like to thank everyone who answered my questions when I was (and still am) learning how to do this.

It's star grounded which is one reason it has so many wires for such a simple circuit.

The two empty 9 pin sockets are for an input preamp to increase the sensitivity if I need to.

The power trannys are 120 volt isolation transformers rated at 300 watts each, the two in series being rated for 600 watts. It's insanely oversized as are the OTs at 30 watts, but the power transformers were very inexpensive compared to ordering a much smaller specialty transformer.

I don't normally plug products, but I really like the paint I used. Rustoleum Hammered Finish spray paint. It has a glossy texture that hides fingerprints very well. It looks much better than straight gloss IMHO and the pictures don't do it justice.

There are two very sloppy but effective "turret boards" made out of strips etched on PCB for the ground bus and the power supply connections.

Image

Author:  azazello [ 16 May 2010, 03:23 ]
Post subject:  Re: First DIY amp FINISHED - 300B SET - works!

Congratulations!
I don't doubt that it sound is amazing!
I like this 3D wiring ! /what kind of output trany Hammond You use?/.
Later You can compare SE 300B and PP KT88 and SE GM70!

Author:  Bluesman [ 16 May 2010, 09:10 ]
Post subject:  Re: First DIY amp FINISHED - 300B SET - works!

Congrats on the build!

As for the wiring, trust me, it could be much worse. If your soldering is sound and you have no hum and nothing dangerous going on, it's all good.

I have some factory point to point wired vintage amps that are not nearly as neat as yours.

Cheers,
Greg

Author:  sampleaccurate [ 16 May 2010, 11:45 ]
Post subject:  Re: First DIY amp FINISHED - 300B SET - works!

azazello wrote:
Congratulations!
I don't doubt that it sound is amazing!
I like this 3D wiring ! /what kind of output trany Hammond You use?/.
Later You can compare SE 300B and PP KT88 and SE GM70!

Thanks.

The optimal position for every wire was carefully calculated to minimize noise and induced interference and each was cut to precision within 1 mm before being carefully placed in their 3D configurations as shown above!

:wizard:

I wish. Actually this wire I bought is finely stranded and won't hold a bend, I need something with fewer strands or even solid that holds it's shape - lesson learned for my KT88.

The trannys are Hammond 1627SEA, 30 watt, 2500 ohms - 4/8/16 ohms. I would recommend a slightly higher primary impedance for a single 300B tube, although with the secondary taps on this one there is some flexibility in changing the reflected impedance of the secondary as seen by the primary. I was contemplating making or later modding this into a parallel dual tube SET which is the reason for the lower impedance and high wattage. It can handle the bias current for two 300Bs in parallel if I ever decide to go there.

The only thing I wish it had was more power. My speakers are not very efficient (91dB/1 meter), and I didn't expect it to be very loud, but it's about 4 to 6 dB lower in volume than I expected before it noticeably distorts. I need to put my scope on it and see what's going on to make sure it's properly biased. However, at low volume it really sounds good. I think this might be the first amp I've ever even listened to in my entire life with no global feedback. I can't put the sound into words. It's different, but definitely in a good way. I need to get a pair of efficient speakers and listen a lot more.

I can say with confidence that the solder joints are all solid and it's "safe" in terms of solid chassis grounding, fusing, and insulation.

BTW, I plugged a guitar cabinet into it and tried it as a guitar amp. It distorts VERY NICELY on electric guitar. I don't know how good that is for my 300B tubes, but it really sounds nice. It very gently "breaks up" with a bluesy sound at first, and when you really drive it hard you get full blown sustaining distortion that sounds like a quality guitar amp.

Also, the blue glow is completely gone. I'm guessing the getter absorbed some gas that was in the tubes that was ionizing. I miss my blue glow.

Author:  cbj591 [ 16 May 2010, 12:58 ]
Post subject:  Re: First DIY amp FINISHED - 300B SET - works!

Geez guys....I had to almost go to the hospital from choking on my lunch after reading about the 3-D wiring....Too Funny.

I'm glad it sounds great. That is all that matters. :thumbsup:

Brian

p.s. It could always be worse. :firefighter:
Attachment:
wiring.jpg

Author:  tombethe [ 16 May 2010, 13:19 ]
Post subject:  Re: First DIY amp FINISHED - 300B SET - works!

Brian,

this air wiring (on your posted pic) is some kind of real sonical improvement Image.

I love it!

Have fun!

Author:  azazello [ 16 May 2010, 16:28 ]
Post subject:  Re: First DIY amp FINISHED - 300B SET - works!

In India, maybe....?
/Why main transformers in USA are on the top of wood pillars?? :shock:.

Author:  cbj591 [ 16 May 2010, 18:24 ]
Post subject:  Re: First DIY amp FINISHED - 300B SET - works!

Azazello - India is correct. you win the prize!!! Oops there is no prize. ;)

As far as the mains transformers, the poles that are commonly used were treated with creosote which is a very effective wood preservative.

The prevailing use of creosote to preserve wooden utilities/telephone poles, railroad cross ties, switch ties and bridge timbers from decay. Due to its carcinogenic character, the European Union has regulated the quality of creosote for the EU market and requires that the sale of creosote be limited to professional users.

I guess the USA is primative in certian aspects. At least the poles last for many years.

Bottom line, I guess they are cheaper :$: :$:

Take care
Brian

Author:  azazello [ 17 May 2010, 03:07 ]
Post subject:  Re: First DIY amp FINISHED - 300B SET - works!

I mean in Europe /in Bulgaria/ 1 big transformer /in building/ between some avenue and all cables are closed under ground never mind in towns or villages /sorry for off!/.

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