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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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 Post subject: Re: Building a 300b
PostPosted: 01 Aug 2017, 22:45 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Does the fuse blow right away, or when the rectifier warms up?

If right away, you might have a bollox tube.

If when it warms up, maybe the peak currents are causing arcing.

Either way, try a different 5AR4 (The Sovtek's are good)

Cheers!

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 Post subject: Re: Building a 300b
PostPosted: 01 Aug 2017, 23:03 
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Joined: 24 Sep 2012, 22:42
Posts: 14
Hello Gregg,
Thank you.
Two out of 3, the amp turns on for a few seconds. Yes, i'm using a Sovtek 5AR4. I even tried a Shuguang 5U4G but it arced and blew the fuse also. Both rectifiers still work on my other amps.


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 Post subject: Re: Building a 300b
PostPosted: 01 Aug 2017, 23:41 
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Joined: 06 Apr 2009, 10:08
Posts: 1420
Location: US Pacific Northwest
clowkoy wrote:
3. The 5-volt supply for the tube rectifier has a center tap, I connected it to common ground. Is this okay?
NO!

The center tap of the 5V rectifier winding is the high voltage B+! You are shorting out the power supply by connecting it to ground!

With the 5v octal rectifiers, the B+ comes off of either pin #8 or the filament winding center tap (if the transformer has one). No part of the rectifier circuit should EVER be tied to ground. This is why you can't get the vacuum tube rectifier to work.

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 Post subject: Re: Building a 300b
PostPosted: 02 Aug 2017, 00:12 
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Joined: 24 Sep 2012, 22:42
Posts: 14
Hello Matt,
I've been thinking about that. It's probably because all my previous builds don't have center taps on the 5-volt tap.
Thank you very much for pointing that out.
Will a 3A fuse still good?


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 Post subject: Re: Building a 300b
PostPosted: 02 Aug 2017, 21:30 
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Joined: 06 Apr 2009, 10:08
Posts: 1420
Location: US Pacific Northwest
clowkoy wrote:
Will a 3A fuse still good?
With fuses it all comes down to the (i^2)t value of the fuse. If that's not right, "rating" doesn't matter. For guidance on fuse selection, see this link here:

http://m.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdf

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 Post subject: Re: Building a 300b
PostPosted: 03 Aug 2017, 02:08 
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Joined: 01 Jun 2013, 09:05
Posts: 601
As a helping hand,

The fault has already been sorted, for interest..
Look at the rectifier tube heater connection:
https://tubecad.com/2010/03/blog0183.htm

Its interesting to note the resistors on the anode and maximum dissipation against first capacitor charge.
NB the danger regarding voltage on the mains transformer HT winding are double that used on full wave.

Just out of curiosity,
Where is the HT fuse in the circuit?
NB its more difficult the "break" DC than AC under fault conditions. (sine wave)

Regards
M. Gregg

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 Post subject: Re: Building a 300b
PostPosted: 03 Aug 2017, 05:57 
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Joined: 01 Jun 2013, 09:05
Posts: 601
I haven't had chance to look at this yet so I can't verify the circuits,

However some of the concepts seem interesting (a blast from the past)
http://www.tubebooks.org/Books/Schure_Rectifiers.pdf

The industrial side of it reminds me of some of the early industrial equipment. (for fun).
Thought it might interest someone..

Regards
M. Gregg

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 Post subject: Re: Building a 300b
PostPosted: 03 Aug 2017, 21:41 
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Joined: 24 Sep 2012, 22:42
Posts: 14
I disconnected the 5-volt center tap wire from the ground and put back the rectifier. It worked when I turned the amp on.
Here are the measured voltages:
B+= 365
300B plate= 355
Cathode= 55 (73ma)
output power= 6 watts

OT transformer is 5K:4
cathode resistor: 750 ohms

Please give your opinion on those operating points.
Again, thank you for all your help.


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 Post subject: Re: Building a 300b
PostPosted: 04 Aug 2017, 19:55 
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Joined: 06 Apr 2009, 10:08
Posts: 1420
Location: US Pacific Northwest
clowkoy wrote:
Please give your opinion on those operating points.
This is not a bad spot to operate the 300B (≈22W). It's a little warmer that the higher bias point which, frankly, I think suits the tube well. 300Bs pushed to the dissipation limit curve can be a bit "cool" for my taste.

Enjoy your amp.

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 Post subject: Re: Building a 300b
PostPosted: 04 Aug 2017, 22:31 
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Joined: 24 Sep 2012, 22:42
Posts: 14
As always, thank you very much Matt!
I'm glad i'm not operating the 300B at a bad spot.
I got a Rigol scope last month and I'm still trying to learn how to use it.
I'm trying to measure the amount of hum in my amps but can't figure out how to do it. Voltages and frequencies are off.


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