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6BM8 beginner tube amp video series

I finally got around to doing some final mod testing. I ended up changing the coupling caps from a "fast" Solen to a Mundorf Alum/Oil EVO of the same value, which was an improvement. I then swapped out the unbypassed 1Kohm cathode resistor on the triode, for a combo of a Cree C3D02060A, 2A 600V, Silicon Carbide Schottky Diode in series with 270ohm resistor, which gave the amp a huge increase in power from the standard line input off my DAC. The final mod I tried, that was a failure, was a 10pf cap across the Schade feedback resistor. While it cleaned up the square wave pattern on the scope, the amp sounded dull and lifeless and it wasn't a subtle change, so I took that back out.

Here is the final video and schematic.



6BM8-Single-Ended-Tube-Amp-Stephe-V4.png
 
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Nice update! I too am a fan of those Mundorf EVO caps for coupling. I was surprised to find I could hear no difference between the Aluminum in oil and the Gold in oil versions. My wallet is happy about it.
 
JohnT123 said:
Nice update! I too am a fan of those Mundorf EVO caps for coupling. I was surprised to find I could hear no difference between the Aluminum in oil and the Gold in oil versions. My wallet is happy about it.
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I found the same thing.
 
JohnT123 said:
O/T, Can that diode bias be used on the 6SL7 in my circuit?

View attachment 2284029
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Absolutely. Using a diode/resistor does introduce a little NFB but I've found the diode is brighter/more efficient than a resistor/cap, so using the combo helps tone down the diode back down and overall is an improvement. This diode + a 270 ohm resistor should end up with about the same bias voltage. Cree C3D02060A, 2A 600V, Silicon Carbide Schottky Diode
 
Lavane said:
Using tubes like like those, driver and output in one bottle, can keep the parts count and cost lower. One third of the cost for the amp I built using sweep tubes was for the chassis. I don't know about StepheK's build, but all the parts and two extra tubes for the one I built came to right at $200 without the fancy chassis.
These amps can be cheap and fun to build. And sound good too!
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Hi, did you by chance build this amp? I’m looking for the BOM. The BOM on Skunkie’s website is the wrong one. This is my first attempt at building an amp so if you have the BOM I’d be grateful.
 
@TheLardlad, I checked and you are correct that the BOM listed on StepheK’s website seems incorrect with a dead link. I suggest that you contact and ask her to correct the BOM link, as well as posting it here on AK
 
Thanks for replying. I tried that first and got an initial response about a month ago but think she must be pretty busy. Was hoping a member might have done the build and had the BOM somewhere. I can likely get parts from the schematics but since it’s my first build I was hoping to avoid picking the wrong size, etc. I have the chassis, transformers, tubes, switches. Just need electronics.
 
TheLardlad said:
Hi, did you by chance build this amp? I’m looking for the BOM. The BOM on Skunkie’s website is the wrong one. This is my first attempt at building an amp so if you have the BOM I’d be grateful.
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Sorry, no, I haven't built that amp. I would try contacting Stephe again.
I'm in the middle of building a simple 6BQ5 single ended amp at the moment using a single 6EU7, or 12AX7 to drive both channels.
 
Well after many requests, I sat down this morning and created a BOM and also added some minor changes I have done to the circuit since I built it. The main one was increasing the cathode bypass cap on the output tube to a 220uf one.

There is a XLSX version on my website. https://www.skunkiedesigns.com/6bm8-amp

I HIGHLY suggest you check this against the schematic before ordering. I honestly hate making up a BOM (most people have no idea how time consuming this is) and every time someone finds an error... Also, in a few months time these part numbers will change or be out of stock, please do not email or msg me asking me to look up replacement part numbers.

6BM8-Single-Ended-Tube-Amp-Stephe-Final.png

PS2 6BM8.png
 

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I'm curious how the feedback applied through the 220K resistor from the plate of the pentode section to the plate of the triode section works out distortion wise.

See pages 396 to 399 of this book, in which it is explained that using this type of feedback when using a triode as the preceding stage can cause more distortion than the feedback can diminish:

Fundamentals of Radio Valve Technique (Philips 1949)
 
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