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300B SET Design Project
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699
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Author:  ILoveHiFi [ 17 Jun 2019, 18:53 ]
Post subject:  Re: 300B SET Design Project

Sorry to hear that, good luck

Author:  Woodo [ 18 Jun 2019, 19:59 ]
Post subject:  Re: 300B SET Design Project

No worries. Just old (er) age and a misspent youth. :blush: I was able to reach 5.02 v. I may have been turning the trim pot the wrong direction. Hum/buzz still remains. Not sure how to go about tracking down hum. There must be a logical approach. I don't see any significant ac v at the volume pot, grids but on the output signal there's about 34mv which must mean something. Still searching when not playing music :)

Author:  Woodo [ 21 Jun 2019, 04:35 ]
Post subject:  Re: 300B SET Design Project

The 250k volume pot arrived from china with a bent pin but straightened up ok. The slightly odd electric bass sound I raised with the 100k pot has gone with the bass and other frequencies sounding amazing to me now. sitting at about 3m away i hear a light hum between songs not as buzzy as before but still around 150hz peak according to an iphone frequency analyser app, probably not very useful. I am working through Matt’s power supply design after the event to estimate the ripple i have and whether enough filtering :confused: . I have a xpwr149 on the 380-0-380 secondary, a 5u4g rectifier, 39uF cap, 8H 200mA choke, 100uF cap giving 403v b+. Then there’s a 15k resistor and 47uF cap for the 6sn7 gtb supply at about 245v (a bit low). I have the JJ 6sn7s too but these old GE ones sound good too. the power transformer resistances are pri-7 ohm, sec-117 ohm, gsxe 15-8-3.2 pri- 91,93 ohm (l,r). I see on @Mwhouston original transformer an additional cap was required to deal with lower mains frequency.

Author:  ILoveHiFi [ 21 Jun 2019, 04:51 ]
Post subject:  Re: 300B SET Design Project

You can look at the voltage ripple with a oscillie scope, but it will be hard to trigger when your at 10s of mV ripple.
Use ac couppling and use hf rejection functions on the scope.

Try multiple trigger modes, if you cannot get a still waveform.
Allot of people just pause this scope when they cannot trigger and take the one as the reading. This is verybad and highly NOT accurate

I have no idea on how much ripple the one can accept but this will tell you first if your power supply is clean enough

Author:  Woodo [ 21 Jun 2019, 06:44 ]
Post subject:  Re: 300B SET Design Project

Thanks Koichi but i still need to buy and learn how to use the scope. Birthday coming up soon, i might have to treat myself.

Author:  ILoveHiFi [ 21 Jun 2019, 15:53 ]
Post subject:  Re: 300B SET Design Project

If don't plan to make much amps in future, I would just save the money and straight away buy extra filtering capacitors for the amp.
After adding the capacitors if the noise goes lower, then you know the supply dosen't have enough filtering.

A scope is allot of money

Author:  Woodo [ 24 Jun 2019, 04:55 ]
Post subject:  Re: 300B SET Design Project

Woodo wrote:
I have a xpwr149

My apologies it’s actually an xpwr144.

Author:  Woodo [ 24 Jun 2019, 16:41 ]
Post subject:  Re: 300B SET Design Project

Following Matt’s power supply design guideline except i already have the transformer hardware on hand, my target ripple should be 12.2 mV. there’s a lot of steps so i may have made a mistake-i’ll check it again. With the filter i have the ripple comes out at 22mV so more filtration is needed. I am still absorbing the information in the procedure to estimate the size of the RC i need to add to do the job. I don’t really need to drop voltage so a resistor is not required but the procedure says the reactance of the series element should be about 20 times the shunt element. A further choke and capacitor could be used but i am a bit short of space under the chassis. I have 2 small chokes for a prospective 6as7 set.
At the speaker binding post i measure 28 mV ac with no music input. I haven’t worked out the ripple on the 6sn7 supply yet. it has an extra 47uF cap.

Is it correct the average r-p is larger than peak r-p?

Author:  Suncalc [ 24 Jun 2019, 21:14 ]
Post subject:  Re: 300B SET Design Project

John; How large is the first reservoir capacitor after the rectifier?

It almost sounds like you are getting too short a conduction angle which is driving up the voltage spikes. This can be a problem if the secondary resistance is small. Sometimes this can be remedied by adding resistance to the outer legs of the secondary or shrinking the size of the first capacitor to increase conduction angle and shrink the current spikes into the first capacitor.

B.t.w... Power supply ripple is noise so reference the peak value.

Author:  Woodo [ 25 Jun 2019, 04:01 ]
Post subject:  Re: 300B SET Design Project

Suncalc wrote:
John; How large is the first reservoir capacitor after the rectifier?

Matt, I installed these caps.
https://au.mouser.com/ProductDetail/661 ... 1ELL390MK5
https://au.mouser.com/ProductDetail/871-B43501A6107M000

I was hoping to get a 40 uF given the recommendations for the rectifier tube but the closest at the time was 39. From memory there wasn’t too many to choose from Mouser with the 500v rating. Regards.

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