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 Post subject: Building a 300b
PostPosted: 14 Jun 2017, 17:03 
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Joined: 24 Sep 2012, 22:42
Posts: 14
Hello all!
This is my first post but I've been a member here since 2012.
I will be building a 300b using the schematic I found here. I have a new Edcor transformer that is 300-0-300 at 250ma from a previous project. Based on the PSU designer, I should be able to get a B+ of =360 volts using 5AR4 using CLC filter. Using the datasheet, I would like to run the tube at 300 plate voltage, 60ma on a 2.5K Edcor opt.
Will this be a good/doable operating point? I've never read a 300b operated with the above set-up.
Thank you very much.


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 Post subject: Re: Building a 300b
PostPosted: 15 Jun 2017, 13:02 
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Joined: 06 Apr 2009, 10:08
Posts: 1476
Location: US Pacific Northwest
clowkoy wrote:
Will this be a good/doable operating point?
It could be; however I normally like to run SETs closer to the plate dissipation maximum. This generally produces lower plate resistance and better low end performance without having to go to a massive output transformer primary inductance.

I suggest that you read the discussion of 300B bias points found here for a more complete discussion.

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 Post subject: Re: Building a 300b
PostPosted: 15 Jun 2017, 18:25 
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Joined: 24 Sep 2012, 22:42
Posts: 14
Matt,
Thank you very much.
Yes, I've read that thread and I get my inspiration to build the amp from that thread and from you.
I get lost when someone mentions load lines and ideal operating points.
My setup right now is a Bottlehead Stereomour with 45s that is tweaked to run at 250 plate voltage at 28ma (64% plate dissipation), producing <1.5 watts and i'm quite happy with it.
I will go ahead and build this one.
At what current should I run this one? 60 or 80?
Thanks again!


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 Post subject: Re: Building a 300b
PostPosted: 15 Jun 2017, 21:07 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Remember the tube voltage is across the tube, not from anode to ground. Anode to ground will measure 50-70V more than what's across the tube (depending on current and B+). So dissipation is measured E(across the tube) * I(through the tube) = Pa

Cheers!

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* Ratings are for transistors - tubes have guidelines*
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 Post subject: Re: Building a 300b
PostPosted: 16 Jun 2017, 08:11 
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Joined: 24 Sep 2012, 22:42
Posts: 14
Geek wrote:
Remember the tube voltage is across the tube, not from anode to ground. Anode to ground will measure 50-70V more than what's across the tube (depending on current and B+). So dissipation is measured E(across the tube) * I(through the tube) = Pa

Cheers!

Thanks.


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 Post subject: Re: Building a 300b
PostPosted: 05 Jul 2017, 22:35 
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Joined: 24 Sep 2012, 22:42
Posts: 14
Hello again!
I now have all the parts except the 300B tubes.
I'm attaching the layout of the amp.
I have a couple of questions:
a) Is it okay to put the input tubes close to each other?
b) Are the output transformers too close to each other?
Please give your comments or suggestions.
Thanks again.
Attachment:
FullSizeRender(1).jpg


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 Post subject: Re: Building a 300b
PostPosted: 10 Jul 2017, 05:05 
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Joined: 20 Apr 2015, 01:30
Posts: 83
The input tubes don't usually get very hot, but I usually keep at least a tube and a half's width between them. My preference is to orient the amplifier so the the long dimension is front to back. I usually put the rectifier tube furthest away from the preamp tube and next to the power transformer, power inlet and power switch. If the choke I am using is covered like yours, I usually still put it underneath the chassis, if it will fit, as I prefer a cleaner look. Either way, it should be in the back next to the rest of the power supply components. Power supply capacitors and resistors directly below. Next, I place the output transformers, with the speaker outputs nearby. Then I put in the power tubes, and the pre tubes are placed up front and furthest from the power supply. I normally put the RCA inputs and the volume knob as close as possible to the pre tubes. Building this way allows for the amp to be built point to point without using anything other than component leads most of the time. Power switches in the front are asking for trouble, and so are inputs in the rear. Attached is an example of the way I normally build. I usually put function over form, but the amps end up looking nice anyway.


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 Post subject: Re: Building a 300b
PostPosted: 10 Jul 2017, 12:14 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
clowkoy wrote:
I have a couple of questions:
a) Is it okay to put the input tubes close to each other?
b) Are the output transformers too close to each other?


a) Yes, OK.

b) Distance OK, but rotate that power transformer and choke 90 degrees.

The core laminations are inline with the output transformer ones as you have them now. This will make for magnetically induced hum.


Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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 Post subject: Re: Building a 300b
PostPosted: 10 Jul 2017, 21:04 
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Joined: 24 Sep 2012, 22:42
Posts: 14
Thank you very much!
I really appreciate your inputs.


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 Post subject: Re: Building a 300b
PostPosted: 01 Aug 2017, 21:51 
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Joined: 24 Sep 2012, 22:42
Posts: 14
Hello all!
I just finished working on the amp.
I can't get the tube rectifier (5AR4) to work. It blew the 3.15A fuse 3 times. The transformer is 300-0-300, CLC with 40uf cap-5H 65-ohm choke-33/40uf caps in parallel.
I bypassed the tube rectifier by using 2 UF4007 diodes and it worked. Hum level is very low, I have to get my ear very close to the speaker (96db) to hear the hum. Heater supply is 6.3V/2A Hammond dropped to 4.8 volts AC.
I didn't put a potentiometer on the input, I put 100K resistors to ground.
I have a few questions:
1. Is the 3A fuse not enough when using tube rectifier? I'm ordering a 4A fuse.
2. If I substitute the first cap with an ASC 50uf cap, I get rapid motion (oscillation?) of the speaker cone that comes and goes. What's causing this?
3. The 5-volt supply for the tube rectifier has a center tap, I connected it to common ground. Is this okay?
4. I didn't put a bypass cap on the 470 ohm resistor of the 6SN7. Is this okay?
Thank you.


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