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6BM8 beginner tube amp video series

I finished assembling the outside of the amp, I think it turned out nice. This is only a 9" X 5" X 2" chassis. I hate the silver machine screws and white washers they use on these transformers. I made a trip to the hardware store and picked up some black ones.

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StepheK said:
I finished assembling the outside of the amp, I think it turned out nice. This is only a 9" X 5" X 2" chassis. I hate the silver machine screws and white washers they use on these transformers. I made a trip to the hardware store and picked up some black ones.

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That looks great!

You're brave building in such a small chassis. I've restored lots of vintage amps that either use a very small chassis or are tightly packed with parts, but when I build I always try to give myself some extra room.
 
Very nice. It would be a great little desktop amp. Thanks for posting the link to the machined rings. I like that there are several size options for the 4 and 8 pin rings. Making a template for the mounting holes would be easy using gasket material and a small ball pein hammer. I used to make carburetor, water pump, thermostat and other gaskets this way. Just gently tap around the object.
 
I used the largest size ones on a KT88 build and drilled some vent holes between the ring and the tube base. The medium size ones fit well with an EL34. They make "thick and thin" medium size ones to choose which you think will look best.

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This is a pretty cool looking amp. I like that even though there's the technical aspects, you take time to make it look like it should sound good.

It also looks like it wouldn't break the bank to build one. Hmmm.
 
JohnT123 said:
This is a pretty cool looking amp. I like that even though there's the technical aspects, you take time to make it look like it should sound good.

It also looks like it wouldn't break the bank to build one. Hmmm.
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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!
 
I got the pentode section powered up tonight, and was wondering why I was seeing no voltage at the cathode resistor, then I realized somehow I was looking at the wrong datasheet when I wrote down the pin numbers lol. Luckily no harm and just had to rewire a few pins. The voltage B+ on the plate is a little lower than I expected but 200V across the tube should still be fine. I have it biased at a bit over 80% at 30ma as I don't want to stress the power transformer and this wasn't being built as a high power amp. I hope to get the driver section wired up in the next day or two and toss it on the scope.

When I'm all done, I'll add up the parts total, it should be under $300 with iron, chassis and everything.
 
6DZ7 said:
What gender "barriers?" That's your, imagination. People create their own barriers, male or female in their own minds. Some people jump over them to discover it was just their own lack of confidence that was their "barrier." This happens to everyone across all of humanity. Nature constricts all of us until we each rise to overcome our own natural, "disabilities."
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o_O Oddly, I don't think I've ever heard a woman utter the phrase "What gender barriers?"
 
OK here is an updated schematic with the pin numbers corrected, a power supply schematic and actual measured voltages. I had to cool down the output tube a bit to get it within spec of both the tube and the power transformer. I still plan to do some tuning by ear to the amp, mainly playing with the triode cathode bias scheme (I simplified this initial design to just have an unbypassed resistor) and the Schade feedback value so I am not calling this a final design yet. I've had a few busy days so it will probably be a bit before I upload another video.

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Well I got this project finished and have spent several hours listening to it.

It sounds fantastic! I still have a few mods I want to experiment with, especially on the input section cathode bias scheme, but I am super pleased how it sounds as built in that last posted schematic.

This last build video has the final wiring details and talks about voltage testing and biasing the output tube.


And the final one shows testing the amp on the scope and the visible difference with the Schade feedback setup.

 
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Nice series and great logo.

I've always wondered why the Schade feedback resistor doesn't take over the job of being the Plate resistor for the driver section. Do the two resistors in parallel create the actual plate resistance? Also curious what happens if you add a cap to the Schade resistor to tune the feedback.
 
JohnT123 said:
Nice series and great logo.

I've always wondered why the Schade feedback resistor doesn't take over the job of being the Plate resistor for the driver section. Do the two resistors in parallel create the actual plate resistance? Also curious what happens if you add a cap to the Schade resistor to tune the feedback.
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Someone on another forum ran a sim on this amp, their results came out exactly what I saw on my scope, a slight overshoot/ringing on the square wave. He suggested I try a 10-20pf cap across the shade feedback resistor to clear it up. I'm gonna see what that does.
 
The presence control on a guitar amp is a feedback loop with a filter, as you know. It would be cool to have a hi-fi amp with real-time adjustable feedback. I don't know of any.
 
JohnT123 said:
The presence control on a guitar amp is a feedback loop with a filter, as you know. It would be cool to have a hi-fi amp with real-time adjustable feedback. I don't know of any.
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On my KT88 amp, I had a DPDT switch with two different Schade feedback resisters so I could A-B them. That's where I learned how much impact on the tone those have.

And on another forum someone breadboarded this amp with a variety of output loads to simulate different primaries, with the 5K load it made the highest power at 2.65W per channel.
 
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