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300B SET Design Project
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699
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Author:  Geek [ 26 Dec 2012, 10:30 ]
Post subject:  Re: 300B SET Design Project

The peak transient current I read in a transmitter catalog is around 2 amps, IIRC.

Author:  mwhouston [ 26 Dec 2012, 16:16 ]
Post subject:  Re: 300B SET Design Project

Geek wrote:
The peak transient current I read in a transmitter catalog is around 2 amps, IIRC.

For a 300B?

Author:  Geek [ 26 Dec 2012, 22:39 ]
Post subject:  Re: 300B SET Design Project

I sold the book and can't verify. It spoke of when using the 300B as a PSU pass tube.

Cheers!

Author:  mwhouston [ 26 Dec 2012, 23:07 ]
Post subject:  Re: 300B SET Design Project

I have replaced the fuses with 160mA. I'm wondering if I should not be using slow blow. There is a surge when I take the amp from standby to on.

Author:  Suncalc [ 27 Dec 2012, 01:14 ]
Post subject:  Re: 300B SET Design Project

Mark;

Take another look at this post (http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2699&start=310) then see if you can find the specification on the fuse you're using. The key here is transient current limits of the 300B. Most tubes with heavy plates can take substantial currents for short periods of time without problems.

The differences between fast acting and slow blow fuses is the overall I^2*t value of the fuse. And in reality, the values run in a continuum from very fast acting to very slow blowing. If you look at the data sheet for the series of fuses I recommended (http://www.alliedelec.com/images/products/datasheets/bm/LITTELFUSE/70184296.pdf), you'll see the differences in I^2*t go from 0.216 for the 250mA fuse to 464 for a 7A fuse. I also think that if you look at the DC output impedance of the power supply that the max current you can feed the 300B plate is rather limited. Here the 250mA fuse will take 200mA for 5.4s before blowing (or 400mA for 1.35s). However, I should think that the 300B would shrug that off without any problem.

If the fast acting fuses keep blowing, then definitely go to something with a higher I^2*t rating.

Author:  mwhouston [ 27 Dec 2012, 01:41 ]
Post subject:  Re: 300B SET Design Project

So are you saying 1A fuse?

Author:  Suncalc [ 27 Dec 2012, 13:47 ]
Post subject:  Re: 300B SET Design Project

mwhouston wrote:
So are you saying 1A fuse?
No. Actually what I'm saying is that a slow blow fuse of 250mA with an I^2*t rating of 0.15 to 0.25 should be acceptable. The equivalent 250mA fast acting fuse has an I^2*t value of only 0.036. which means that a large inrush could make them blow. I'm just giving options.

If the bypass cap shorts then the 300B will try to run at the intersection of its resistive load line and the Eg=0v plate characteristic curve. Because the DC resistance of the transformer is small, this will equate to many amps. However, the power supply is going to current limit because the transformer secondaries will bog down. So the current will probably be in the range of maybe half an amp to an amp for a very short time. The 300B should be fine with this surge so long as it's just a single shot. If you go with a 250mA fuse with an I^2*t value of 0.216 (i.e. a slow blow fuse) the half amp current spike will blow the fuse in ≈0.9s. If the current actually got to one amp, the fuse would blow in ≈216mS.

Personally, I would use the 250mA slow blow fuse in this application and not worry about it.

P.S. I would also check the ripple current rating on those non-polarized caps you're using. That may be the reason that they are blowing.

Author:  mwhouston [ 27 Dec 2012, 17:25 ]
Post subject:  Re: 300B SET Design Project

Thanks Matt. It took week for the 100mA quick blow to pop. The amp is up going now but I will have some 250mA slow blow on hand should the fuse pop again.

Author:  mwhouston [ 15 Jan 2013, 07:37 ]
Post subject:  Re: 300B SET Design Project

Recently I replaced the bypass electrolytics on the 300Bs with bipolars. To protect the 300Bs from a shorted bypass cap I put a fuse in. The original 100mA fast blow popped after a few weeks. The current 160mA has lasted a lot longer under a lot of use and so far has not blown. Looking good. Just thinking about the EL34 amp I made some months back and sold shortly after, it has the same bipolar caps and the new owner has not reported any problems.

I took the Silver Dragon to a friend's (Dave) house with $26K, 6', 400lb speakers with all Focal drivers (2X 12" woofers). The Dragon drove the speakers well and loud and though Dave has just bought $13K Audio Space 300B monoblocks he felt the Dragon was more than comparable. In fact both his and my amp had a very similar sound. I guess that must be the "300B" sound. My amp was dead quiet (according to Dave) his has a slight hiss.

Only one track with strange phasing which throws a female voice 1 meter to the left of the left speakers the Dragon not play well. Too much Xtalk?? Apart from that I heard Dave mutter, "it makes me want to make a high-end SE 300B". I already have one (lol).

Author:  mwhouston [ 22 Jan 2013, 01:52 ]
Post subject:  Re: 300B SET Design Project

I performed a health check on the Silver Dragon under load conditions. I was concerned some voltages were not correct. See below.

HT to the primary of the OPTs: 420V
Filament voltages measured at the sockets to the 300Bs: 4.93V
Grid to cathode voltage -70V.

This means the the 300Bs are conducting about 93mA.

All looks good to me. And sounds good too.

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