DIY Audio Projects Forum

eBay 6SN7 / 300B Amplifier PCB Kit
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Author:  Woodo [ 24 Feb 2018, 09:12 ]
Post subject:  eBay 6SN7 / 300B Amplifier PCB Kit

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

Author:  Woodo [ 26 Feb 2018, 06:27 ]
Post subject:  Re: eBay 6SN7 / 300B Amplifier PCB Kit

IMG_0525 (1).jpg

IMG_0524 (1).jpg

Author:  Woodo [ 28 Feb 2018, 16:39 ]
Post subject:  Re: eBay 6SN7 / 300B Amplifier PCB Kit

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.

Author:  Woodo [ 11 Mar 2018, 04:40 ]
Post subject:  Re: eBay 6SN7 / 300B Amplifier PCB Kit

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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