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300B SET Design Project
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699
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Author:  Woodo [ 18 Aug 2019, 00:20 ]
Post subject:  Re: 300B SET Design Project

Woodo wrote:
Chris65 wrote:
I've used Triad Magnetics transformers in similar applications, much cheaper than the equivalent Hammonds.
Mouser has F-344X 12.6V@2A A$22 and F-326X 12.6V@2.5A A$28.


Thanks for info Chris. Will check it out.

I ended up going for the 6v 2.5A toroidal with twin secondaries. Haven't quite worked out how to mount it yet. A vertical L mount looks the best approach. Connected it up to check voltages and seems ok. Got 7.5 v at the inputs to the bridge rectifier. Was able to adjust regulators to 5 v no probs although will test and readjust when the tubes are back in.

Author:  ILoveHiFi [ 19 Aug 2019, 00:59 ]
Post subject:  Re: 300B SET Design Project

Doing some simple calculations, assuming 0.7v drop on diodes.
6-0.7 = 5.3V AC
5.3*1.41 = 7.473V at the tubes, ignoring the fact that there is also some voltage ripple due to capacitor discharing.

I haven't noticed that you asked a question about weither or not the transformer you chosent will work or not.

Your using a center tapped transformer so there is only 0.7V losses
If it was a single 6V winding and using a full bridge rectifier you will have 1.4V losses
6-1.4 = 4.6V AC
4.6*1.41 = 6.48V to regulator

Author:  Woodo [ 19 Aug 2019, 06:18 ]
Post subject:  Re: 300B SET Design Project

Yes I posted on 8 August. On RS components they had a nuvotem talema toroidal with dual 6v secondaries which seemed like they might be ok for about $24 AUD. I have only tried it unloaded yet as i have to figure out how best to fasten it to the underside of my chassis. It looks promising with 7.5v at the bridge rectifier. I only managed 6.1v with the 266LA12. laying flat is a problem as I have some bolts in the way which hold the transformer and choke above. I looked at the triad magnetic suggested by Chris65 which also looked good and much easier to mount, but I could only see single secondary therefore I may need to get 2 of them if i searched the models right.

Author:  ILoveHiFi [ 19 Aug 2019, 20:15 ]
Post subject:  Re: 300B SET Design Project

Look for steel braces, or sheet metal, then with a vice or some how. You can cut to correct size or ask people at the store to sell you them.
Then you can have a very long bolt and hook up the trasnformer.

Full bridge whenver the diodes conduct, two diodes are conducting in series. 2 x 0.7V losses
Center tapped, whenver diodes conduct, only one diodes are condcuting 0.7V losses

Author:  ILoveHiFi [ 20 Aug 2019, 03:30 ]
Post subject:  Re: 300B SET Design Project

Also there should be standard parts for mounting transformers toridal ones.
I have no exprience but search them up.

Author:  Suncalc [ 20 Aug 2019, 09:46 ]
Post subject:  Re: 300B SET Design Project

Woodo wrote:
I have only tried it unloaded yet as i have to figure out how best to fasten it to the underside of my chassis.
Just remember to NEVER put a conductive path fully around the core. For example, like an "L" bracket over the top of the transformer held with a bolt through it's center. This constitutes a shorted loop around the core (which fully encloses the magnetic flux path) and will sink high current levels. This is why you see so many toroids mounted with a bolt through the center and a big washer on top. This method means no conductive path around the core.

Author:  Woodo [ 20 Aug 2019, 21:47 ]
Post subject:  Re: 300B SET Design Project

Thanks Matt. I have the bolt and washer that came with the transformer. I will have to move things around a bit to make use of it.

Author:  Woodo [ 23 Aug 2019, 08:41 ]
Post subject:  Re: 300B SET Design Project

6v 30VA transformer mounted and working. Filament voltage is at 5.02,5.01v. the noise is almost non-existent. :thumbsup: :horseshoe:

Author:  Woodo [ 29 Aug 2019, 22:15 ]
Post subject:  Re: 300B SET Design Project

Suncalc wrote:
Wow! :shock: Interesting by-play on this thread.
Attachment:
Power Stage Design 3.jpg



Comments anyone?

I am trying to plot out my 6sn7 and 300b loadlines for the operating points I have got. I think I understand the preamp and driver load lines and 300b saturation point ok. My question is how is the cutoff point for the 300b determined? Do you project the voltage and current from the point where the grid swings 71v more negative than the bias point? Earlier in the thread I may have recalled Matt mentioned 142v full swing is required to drive the 300b to max power. Thanks.

Author:  ILoveHiFi [ 30 Aug 2019, 04:01 ]
Post subject:  Re: 300B SET Design Project

Using the loadlines from data sheet, you know apparoximatley the operating charateristics, then you can annalyze which bias point yeilds best results.
But bias point is generally fixed when your biasing for maximum power, for the output stage.

Theres no point in plotting the loadlines for your current valve in the circuit.

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