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PostPosted: 24 Feb 2018, 09:12 
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Joined: 13 Jan 2018, 21:33
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Location: australia
Hi everyone, Fairly new to the forum although I'm an avid reader of the valve pages. I would like if possible to seek advice here. Inspired by this forum I decided to take some baby steps to learning a little about electronics with the objective of building a valve amplifier. I likely put the cart before the horse and purchased from Ebay one of the may PCBs on offer. Hopefully the pics show enough detail. Having no circuit diagram (and little knowledge of circuits) I went about reverse engineering one from the PCB. I need to double check it because it does often get confusing to me which components belongs where and I need to go over it again and again. Happy1982 who provided the PCB kit has promised to send me the information but I have had it for a month now so not too hopeful on that.

Also I have no clue on the values of the voltages/current should be at the various point or for that matter the size of the power transformer, output transformers, chokes and 300b filament heater. I would appreciate any comments/advice on the circuit, if I have made mistakes - there's sure to be many, or maybe whether this circuit is based on something similar out there you have seen. I'm hoping to build knowledge so that I can purchase the correct transformers etc. I'm not quite ready to start putting it all together. I thought I would get some practice soldering and wiring first and how to safely test each stage before turning everything on (Might need to pay a specialist to help with that as my lab consists only of a multimeter and a soldering iron and I am a bit wary of electricity after getting a 240v shock from the toaster in my youth.

Also one thing I have noticed is that compared to the other circuits out there is the first stage of the 6sn7 triodes is the one having pins 4,5 and 6 but in my one it appears to be pins 1,2 and 3. Does that make a difference? Also Happy1982 Ebay provider, after I complained that I didn't have one of the caps and resistors of the right size, said there is a mistake on the PCB and I have all the right parts. I hope if I learn enough I can make the measurements and be sure myself.

Cheers woodo


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PostPosted: 26 Feb 2018, 06:27 
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Attachment:
IMG_0525 (1).jpg

Attachment:
IMG_0524 (1).jpg


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PostPosted: 28 Feb 2018, 16:39 
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Location: australia
happy1982 my Ebay provider of the board provided part of the schematic yesterday. I don’t see any models or ratings for the transformers and i have enquired further about that. one of their photos points to a 320-0-320v power transformer of what brand i don’t know. i seem to have drawn my circuit wrong so please disregard woodo amp schematic and i will try to figure out where i went astray. if anyone is interested i can send the schematic. it is the same as the one on happy1982 ebay site but the resolution is too low on the website to make out the component values.

Edit: 300B Schematic added to first post.

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PostPosted: 11 Mar 2018, 04:40 
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Attachment:
300b happyshop1982 pcb.pdf
I have been plugging away looking at all the tracks on the PCB and have come up with the attached schematic. i have had trouble reconciling this with the ebay schematic, it’s laid out quite differently and has some different component values. it’s quite possible it’s my error though. Although from the ebay pictures of the board, it has some different layout and physical component sizes. Possibly there has been some evolution in the design, up or down, which way for me i don’t know. appreciate your thoughts on whether or how this amp might perform given the schematic or not and any mistakes i have made in the schematic. i note that there is no potentiometer on the input so if one were to be used would that replace the resistors in the schematic? sorry if that is a daft question but learning on the job.


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PostPosted: 30 Mar 2018, 22:48 
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Hi, in this circuit there is a component (bridge rectifier?) to filter each filament supply for the 300b’s. These have the number rbv5006. could anyone advise on the transformer secondary voltage, current needed and the voltage drop of the bridge rectifiers. the data sheets are difficult to interpret for me. Thanks.

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PostPosted: 02 Apr 2018, 20:22 
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Location: Winnipeg, CANADA
Hi. Normally when you rectify AC, you get about 1.4 times DC.

Each 300B heater draws 1.2A at 5V (DC or AC). So for a pair of 300B I would pick a transformer rated at 3 amps to supply the heaters for a pair of 300B.

Also note that the 5u4g has a very large heater draw of 3A at 5V AC. I would choose a 3 or 4A rating for the rectifier tube.

Take a look here: http://diyaudioprojects.com/Schematics/ ... ematic.htm
and follow the links for more ideas about putting a 300B amp together.

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PostPosted: 03 Apr 2018, 07:11 
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Thanks Gio for your reply and the link. From the schematic it appears that a suitable transformer would have double 5v secondaries and current of 3 to 4 A as you suggest. i’ll have a look at the hammond selection to see what they have.

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PostPosted: 03 Apr 2018, 20:55 
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Yes, you will want two 5V secondaries. You could get by with two 3A secondaries. But if 4A is available you may want to opt for that as the transformer should run cooler.

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PostPosted: 08 Apr 2018, 17:08 
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Had trouble finding a transformer with exactly 5v unless I went with the ebay recommended transformer for the pcb. Thought I would stick with a hammond or edcor. There is a hammond 266 LA12 with 2 x 6v secondaries @ 4 A. Assuming Gios advice I should have 8.4v dc. Rectification should result in a loss but not sure how much for the rbv5006. The datasheet mentions 1v but not certain if this is overall or per diode. Other posts suggest 0.7 v per diode or 1.4v. So I would need to lose at least 2 v or more depending on the losses. This would mean a 0.5 ohm 8 watts Resistor. Does that sound reasonable? Woodo.

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PostPosted: 09 Apr 2018, 06:44 
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quite possibly the dropping R value should be worked based on 2A as there are two windings. not sure how the transformer ratings work. if so that would mean a 4 watt 1R.

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