A budget 300B build based on Edcor Iron

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Stephe_K
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Joined: Wed Mar 03, 2021 6:46 am
Location: Atlanta GA

A budget 300B build based on Edcor Iron

Post by Stephe_K »

I have to say, this amp turned out super clean. I actually robbed all this iron from a "BlueGlow" KT88SE build and IMHO suits these tubes better. It sounds AMAZING! You can watch the video series on building this amp here.

https://www.youtube.com/playlist?list=P ... kE3jm9xnd9

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Bevan
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Joined: Sat Jul 10, 2021 6:10 pm

Re: A budget 300B build based on Edcor Iron

Post by Bevan »

Looks Great. Why the topside caps?
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Stephe_K
Posts: 36
Joined: Wed Mar 03, 2021 6:46 am
Location: Atlanta GA

Re: A budget 300B build based on Edcor Iron

Post by Stephe_K »

Are you talking about the gold fined things? Those are the cathode resistors and I wanted to get that heat out of the inside of the amp.
Stephe
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Stephe_K
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Location: Atlanta GA

Re: A budget 300B build based on Edcor Iron

Post by Stephe_K »

I wanted to do an update on this. I did a video series build based off this "classic 300B" schematic that has been floating around for decades, using a direct coupled 6SN7 2 stage front end. After I got finished, did some more scope testing and then spent more time listening to it, I wasn't as impressed. Thin bass, overly bright top end but mainly saw weird asymmetrical clipping on the scope out of the front end to the point the amp would go into heavy distortion at anything past 2W RMS output. It did have a very high input sensitivity but this clipping before the output tube grid crippled it to flea watt status.

Given I planned for this to be just a starting point for this amp, I gave this some thought knowing I wanted to try using these 6SN7 tubes. I thought about trying a cap coupled twin triode two stage, but then wired up the frontend very similar to the mods I did to my Nobsound 6P1 amp: as a Cascode with a voltage divider feeding the upper grid. I also increase the DC offset on the heaters of the 6SN7 tubes from 35V to 50V. This mod fixed the asymmetrical clipping and now can be driven to 6W RMS. It lost some of the input sensitivity but most of that was wasted on a normal line input, as the amp went into heavy distortion so easily. It now sounds 100% better and I feel like with a few minor tweaks (I plan to try a 0.33uf coupling cap etc) will be a great amp.

Where I am baffled is I have seen several articles about builds based on this design, using some high end transformers etc., praising how great it sounds. I was also told some of the current production ChiFi 300B amps use this design. The problems I saw were easily visible on the grid of the 300B, so I don't see how $1500+ OT would fix those fatal flaws. I suppose if I had never heard a good tube amp and had very sensitive speakers I might have been impressed, but seriously my little 6BM8 amp sounds much better, more solid bass response and has almost as much drive! Rewired as a cascode, this 300B amp now sounds more like you would expect.

Anyway, if anyone ever plans to use that schematic, I would suggest they not bother, and instead try to this cascode front end design I came up with. I'm also interested in other designs using a pair of 6SN7's to drive a 300B.

https://youtu.be/Ws_P1hdMAwY

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svante
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Location: Sweden

Re: A budget 300B build based on Edcor Iron

Post by svante »

Hi Stephe_k.
The amp locking very fine! have found your YouTube channel recently, Excellent videos, many thanks!
I uses an 300b with 6sn7 driver (Audio Note kit 1), its sounds quite good, but uses couplings caps, clips almost equal on both + and - side.
Br Svante
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Stephe_K
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Location: Atlanta GA

Re: A budget 300B build based on Edcor Iron

Post by Stephe_K »

I ended up running the cascode with a higher plate load and got a lot more drive out of it. This is the final schematic, at least for now, and it sounds really nice to my ears now :) The BOM etc is at my website https://www.skunkiedesigns.com/300b

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