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300B SET Design Project
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699
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Author:  Woodo [ 10 Oct 2019, 07:37 ]
Post subject:  Re: 300B SET Design Project

“While I had Qians amp I increased the snubbing around the filement rectifiers. Though the hum from the amp was only low I thought it worth while to see if I could reduce it more. I put 0.01uf polys between all legs of the rec. bridges. Qian tried serveral times to find a hum and could not. “

@mwhouston, Hi Mark. I still have (just) audible noise I suspect from the heater circuitry. Above do you mean you placed the caps across the + and - of the bridge rectifier? Are there any other sites you have snubbed that you found necessary?

Author:  gyrene [ 08 Nov 2019, 15:25 ]
Post subject:  Re: 300B SET Design Project

May I ask the board about the 300B schematic?
I see it is one channel.
How do I tie the second channel to the first?
Thank you.

Author:  Suncalc [ 08 Nov 2019, 18:55 ]
Post subject:  Re: 300B SET Design Project

Both channels are shown in the schematic in this post: http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699&start=131

Author:  gyrene [ 08 Nov 2019, 19:05 ]
Post subject:  Re: 300B SET Design Project

Thank you very much.

Author:  Woodo [ 09 May 2020, 22:48 ]
Post subject:  Re: 300B SET Design Project

Woodo wrote:
Thought I would post the circuit as currently stands for my build of this great sounding amp and prepare to move on to next project albeit slowly which I hope will be simpler and cheaper on vacuum tubes at least. Some differences to the circuit on page 33 are noted in the schematic. Have to order a new filament transformer that will reach 5v quietly. May try going back to the capacitors in the original circuit after the transformer arrives and put into the box . Thanks for all the input everyone. :up: If mistakes are spotted, happy to receive input.


A neater version of the circuit in microcap. 5v filament circuit as before. Still to locate the bridge rectifier and LT1085 in the component library. Still sounding good. I still get some buzz on startup which dissipates after tubes warm up, circa 2 min.

Author:  trunopercasi [ 30 May 2020, 05:37 ]
Post subject:  Re: 300B SET Design Project

Woodo wrote:
Woodo wrote:
Thought I would post the circuit as currently stands for my build of this great sounding amp and prepare to move on to next project albeit slowly which I hope will be simpler and cheaper on vacuum tubes at least. Some differences to the circuit on page 33 are noted in the schematic. Have to order a new filament transformer that will reach 5v quietly. May try going back to the capacitors in the original circuit after the transformer arrives and put into the box . Thanks for all the input everyone. :up: If mistakes are spotted, happy to receive input.


A neater version of the circuit in microcap. 5v filament circuit as before. Still to locate the bridge rectifier and LT1085 in the component library. Still sounding good. I still get some buzz on startup which dissipates after tubes warm up, circa 2 min.


can I get wattage detail rates for all resistors used?

Woodo wrote:
Thanks Koichi. The buzz is almost inaudible now and the amp is sounding great to me now so I might leave things be until I procure a filament transformer that can do the job as intended. I have emailed Hammond to get an opinion on whether the one I have (266LA12) is up to the task or is perhaps faulty and should be replaced but no word as yet. The only other change I have made is to remove the 100 ohm resistors in the secondary wires, as this didn't improve the buzz anyway. I might have mentioned it but I put one of the 100 ohm resistors and spare 39 uF capacitor for additional filtering of the b+ to try and reduce the hum (this was before I found the regulators tone making the noise so made no difference to the buzz). I may return things as they were to be as faithful as possible to the original. The b+ is about 420v now. I replaced the dropping resistor with a 10k to the 6sn7 supply so this is 300v now.


Is your amplifier quiet and free of hum? which solution did you use?

Author:  Woodo [ 02 Jun 2020, 16:38 ]
Post subject:  Re: 300B SET Design Project

I generally followed the circuit as posted At the beginning of the thread for resistor ratings. I’ll update my circuit with those used. I had a look at my empty mouser bags and the plate, grid and driver cathode resistors are 1 watt. The 300b cathode resistor is 10 watt, I had difficulty finding a 7w as per circuit. That one gets very hot. I didn’t check power dissipation but I used 1/2w resistors in the filament regulation circuit. As for hum there’s a bit of a buzz at startup but around 2 minutes subsequently it becomes inaudible. I ended up replacing the filament transformer with a toroidal 30VA 6v dual secondary unit. Seems to work well. I haven’t tried adding any bypass caps as suggested yet so that’s next.

Author:  Citation19 [ 28 Aug 2020, 18:42 ]
Post subject:  Re: 300B SET Design Project

Well...
Decided to throw my hat in the ring....
Unit is playing .... I haven't buttoned it all up yet as where many things worked exceptionally well .. I did have a couple issues.

In short amp uses Edcor XPWR077 PT / Hammond 1630SEA OPT / Hammond 193M choke / Hammond 166N12 Filament xfmr /5U4GB / 6SN7 / Genelex 300B Gold Lions / K40Y-9 coupling caps / DC Filament Supplies per schematic with Tantalum cap added at output / Panasonic Film Cap 40uf first PS cap / 25w wirewound Cathode bypassed by Panasonic Film Cap / Nichicon Muse bypass caps for input stage / .25a SloBlo pigtail fuses in line before OPT.

The good... Unit plays beautifully , no pops / clicks / hums / blue flashes or smoke. It's DEAD quiet.

The not so good... B+ voltages were HOT...481vdc under load. Resulting in cathode voltages 75v plus a bit (100ma plus a bit). One 300b started to red plate pretty much immediately. The two output tubes had identical cathode readings yet only one was red plating. I used a variac to drop the A.C. voltage into the unit. Everything seemed to stabilize around the 420v B+ range. Since all voltages were dropping at once, I could only drop just so far before the filament supplies started creating a hum due to the regulators losing headroom.

Based on these results and hours of checking / rechecking the circuits/ researching consensus of 300b biasing.... I have endeavored to drop B+ to 400 - 415 vdc. Started by adding resistance to the PS (replacing 47R between First Cap and Choke). I am up to 500R placing B+ at 418v. 500R added as an array of 7watt metal oxide resistors. This array gets smoking hot even though power is spread across 5 large resistors in series. I will have to bring the voltage for the 6SN7 back in line after settling on the 300b voltages. Chassis mounted wirewounds are the next step. A 1/2 hr session of fairly loud listening was about all I have dared so far. All went well.

Depending on how things go with the power resistors installed, I may settle for a bit. I understand that this is not an ideal solution. Possibly adding sag , Dorking up the balance of the PS circuit, as well as creating a hot plate... I get it... but my limited amp building chops are holding me back from some more adventurous fixes (zeners in the primary ct ?? ). I may invest in a PT with a lower primary rating 400-0-400 or 375-0-375 instead of the existing 450-0-450.

Have a looksee at the attached image more will be forthcoming .
I welcome advice and suggestions on the Red Plate / bias issue.I

Take Care All...

Author:  Citation19 [ 30 Aug 2020, 17:23 ]
Post subject:  Re: 300B SET Design Project

Input section included in the attached image.

Also forgot to mention these build differences ... Snubber circuit added to Filament PS inputs / backup diodes added to input of the 5U4GB (no difference in voltage due to this addition... I removed and checked and then reinstalled ) / X2+100R added between circuit and chassis(earth) ground.

Author:  Woodo [ 31 Aug 2020, 16:28 ]
Post subject:  Re: 300B SET Design Project

Hi. Project looks like it’s going well. Watching with interest. I have been enjoying (Matt’s monoblock variation on page 33) mine for over a year now although I reused a lower voltage power transformer and smaller choke and output transformers from an earlier project. A little bit of buzzing that comes and goes still to be sorted. Elpaso on his YouTube site built a 300b recently. Not sure which circuit but used A.C. heaters with humpots!

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