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300B SET Design Project
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699
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Author:  Suncalc [ 08 Jun 2019, 11:07 ]
Post subject:  Re: 300B SET Design Project

Woodo wrote:
should i move it to pin 4 or is either side ok.
It's fine. So long as the positive side of the filament supply is on the Rk-Ck side of the filament, it doesn't matter.

Author:  Woodo [ 08 Jun 2019, 21:58 ]
Post subject:  Re: 300B SET Design Project

Thanks much appreciated Matt.

Author:  Woodo [ 13 Jun 2019, 06:39 ]
Post subject:  Re: 300B SET Design Project

i am getting some buzzing with my setup. Attached is a sound file that i hope is playable. Recorded from iphone in the centre of the woofer. Is this sound typical of not enough filtration or something else? Thoughts appreciated. Regards.

Author:  ILoveHiFi [ 13 Jun 2019, 20:27 ]
Post subject:  Re: 300B SET Design Project

That sounds like poor ground wiering or layout, if you followed the original schematic PSU, it shouldn't buzz like that.

Layout in the sense that the audio path is too close to a transformer or valve rectifier and the magnetic field just leaks into the amp and gets amplified.
Also the valve heaters is importnat, if you did not follow the orignal correct one fully, it will buzz.

Author:  Woodo [ 14 Jun 2019, 01:39 ]
Post subject:  Re: 300B SET Design Project

Thx Koichi. It could be all those. I'm looking into how I could layout the wiring better. I'll have to move things around a bit.

Author:  Woodo [ 16 Jun 2019, 06:20 ]
Post subject:  Re: 300B SET Design Project

I am using the Hammond 266LA12 transformer to heat the 300b filaments. I struggled to get to 5v max was 4.8 adjusting the trim pots. So i thought that’s the best i could do. I took some measurements and i’m getting a little over 6v after the bridge rectifier where i would expect more ie at least 8v. I asked Hammond for advice and sent them the circuit. it has 2 A windings but maybe this is not enough? Any thoughts? There is a model with 6.3 @ 2.5A windings. Hammond has requested i measure the unloaded outputs which I haven’t done yet but will do soon.

Author:  Suncalc [ 16 Jun 2019, 08:37 ]
Post subject:  Re: 300B SET Design Project

Woodo wrote:
I struggled to get to 5v max was 4.8 adjusting the trim pots. So i thought that’s the best i could do. I took some measurements and i’m getting a little over 6v after the bridge rectifier where i would expect more ie at least 8v.
This is rather unusual for a 1.2A load on a 2.5A winding. Further, the LT1085 is a 3A LDO regulator. At 1.2A load, the voltage drop should be just about 1V. A 4.8v output voltage infers an input voltage of ≈5.8v at the filter input. So if you're getting a little over 6V at the filter output you should be able to get the required 5V at the filament with the trimmer. Something is not right here.

The buzzing you're getting could be due to inadequate filtering in the filter. Or the regulators may be struggling to properly start up into the load and going into an oscillatory state. Here a look at your waveforms would be very helpful. Can you scope the input waveform at the regulator input? I also suggest you make sure that the bridge rectifiers are properly connected and the filter caps are all per the specified design.

I have never had this type of issue before with the LT1083/4/5 regulators. Look carefully at the circuit to make sure that the rectifier bridges are wired correctly.

Also, look at the post here on the filament circuit design: http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699&start=127 Note that I said:

I chose the Hammond 266L12B (or the 266L12). This is a dual secondary 6.3v 2A (2.5 for the 266L12) filament transformer. The 300B heater current load of 1.2A with derating requires at least a 2A secondary.

It could be that the 2A secondary is marginal and a 2.5A secondary is really required to prevent startup problems. Hammond transformers tend to have higher secondary resistance than most other power transformers and this could be contributing to your issues.

Author:  Woodo [ 16 Jun 2019, 18:07 ]
Post subject:  Re: 300B SET Design Project

Thanks Matt for the response. I'll double check everything while I have it offline. The terminal strip the regs are on is quite crowded with parts so something could be amiss but the bridge rectifier looks straightforward. I used this transformer in my last project too and the voltage out from the BR on the pcb (there was no regulation for 5v) was about the same 6.x V rectified ac and I used 0.47 R dropping resistor on the on the input ac to get near 5v. I currently don't have my own scope (on the list of must haves) but can take it to a guitar amp tech who checked my work on the last project far to scope it. If I haven't made a mistake I may need to get the higher rated transformer.

Author:  ILoveHiFi [ 17 Jun 2019, 04:14 ]
Post subject:  Re: 300B SET Design Project

If you wanna see if the heater is giving you noise.

Adjust the voltage down so you have anywhere from 4 to arround 4.5v
This allows the voltage regulators to do its thing and regulate given the max possible is 4.8v.
If you have it at 4.8v, then it can't get rid of the ripple cause dosen't have enough input to output voltage diffrence to regulate properly.

Then you filter out if the valve heater is the prolbem of noise.

If you currently have maximum possible only 4.8V then your input voltage to votlage regulator is not high enough and you cannot get proper voltage regulation and there will be noise.

If your heater is currently AC, then some noise in the system is normal since it will just leak from to heaters into amp

Author:  Woodo [ 17 Jun 2019, 16:27 ]
Post subject:  Re: 300B SET Design Project

Thanks. I'll have a look at that. Before I adjusted the trim pot to get more voltage I don't recall the buzzing but there was a small amount of hum. Not sure I can trust my hearing these days especially left side is poor. Didn't get a chance to look at it last night as home from work late hopefully tonight.

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