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300B SET Design Project
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699
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Author:  mwhouston [ 02 Nov 2015, 22:55 ]
Post subject:  Re: 300B SET Design Project

The AVAT power tranni I had wound for this project years ago always ran very hot. Yesterday the HT shorted, blew the 2A slow blow, music stopped.

I have contacted AVAT to ask if they could rewind the tranni. I'll keep you informed.

Author:  laurie54 [ 03 Nov 2015, 20:17 ]
Post subject:  Re: 300B SET Design Project

Just a note. I watched a vid the other night where the fella switched the ht heaters instead of the HT line. He was using 2x47's for his recs. The timer timed out, relay cut in and the voltage came up nice and soft. No flash no pops. Just a gradual rise to full HT.
On the other hand if your trany is done already it must have been on the edge and could have very well been your problem all along.
I'm still blown away by the fact that a design i had drawn up a year prior to your thread turned out to be excactly the same as my design in style and wood used.

Author:  mwhouston [ 03 Nov 2015, 20:49 ]
Post subject:  Re: 300B SET Design Project

Always had problems this amp flashing rec. tubes. Until I started using. 5U4GB. But switching to this tube a day or back may have tipped the tranni over the edge. 380V @200mA.

Author:  laurie54 [ 04 Nov 2015, 16:10 ]
Post subject:  Re: 300B SET Design Project

Ya 380 @200mA is a custom. Plitron has some @300mA with slightly lower voltage of 360vac. Problem is they are toroid.

Author:  wdecho [ 18 Mar 2016, 23:35 ]
Post subject:  Re: 300B SET Design Project

I just finished building a simple SE amplifier designed by George Anderson and now I am looking for a 300B build and ran across this build. I thought I would bump it up where I can look at it some more. Right now I have only looked at the 1st page and the schematics. Are there any changes to make in the schematics in all the threads I need to be aware of?

Author:  mwhouston [ 19 Mar 2016, 03:47 ]
Post subject:  Re: 300B SET Design Project

I think the original cct. Is the one. It's moderately early in the thread.

Author:  wdecho [ 19 Mar 2016, 06:36 ]
Post subject:  Re: 300B SET Design Project

mwhouston wrote:
I think the original cct. Is the one. It's moderately early in the thread.


After posting the last message I stayed up past my bedtime jumping around the pages and I saw it was a work in progress with a final schematic in a later page. Thanks

Author:  mwhouston [ 19 Mar 2016, 06:51 ]
Post subject:  Re: 300B SET Design Project

Great amp. I use it every day. It cost me AU$3.5K in parts. Worth every cent.

Author:  Paul [ 13 Apr 2016, 02:14 ]
Post subject:  Re: 300B SET Design Project

Matt, Mark, All,

on page 2 of this thread where the design was done, if you look at the load line BLUE, for the 3k5 primary transformer:

The calculation of 7.5W was done using anode current swing from 33 - 165 mA. The trouble is that this corresponds to
an anode voltage swing 121 - 578v. The anode can only rise to 435v in this amp. Thus 7.5w is impossible.

If 435 is the max anode volts then the grid can only swing negative -71 to -93 (22v). So for symmetrical swing the grid can only go
-49 -> -93 which corresponds to anode current swing from 70 - 119 mA only. Peak to Peak. This is a bit over 1 Watt RMS out.
This may account for the low power out of this amp.

Mark when you measured I think I read 5Watts out, was it into a dummy load and was the load watched on a scope
so as to check it wasn't distorting? I reckon it looks like even 5 Watts would only be possible with considerable distortion.

Matt , what are your comments?

Unless I missed something it's easy to see why the power is down here.

Paul.

Author:  Suncalc [ 13 Apr 2016, 06:36 ]
Post subject:  Re: 300B SET Design Project

Paul wrote:
The calculation of 7.5W was done using anode current swing from 33 - 165 mA. The trouble is that this corresponds to an anode voltage swing 121 - 578v. The anode can only rise to 435v in this amp. Thus 7.5w is impossible.
You are falling into a common trap. This is not a restively coupled stage it is a transformer coupled stage. The energy stored in the transformer core in the form of magnetic flux is what accounts for the upper voltage swing on the load. Remember that inductors cause voltage to rise when current falls. This is the way all transformer coupled output stages work. B+ is NOT the upper limit on the load voltage swing.

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