Rob’s micro deluxe with a 6bm8

Dylanmarleyyoung

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Hello everyone I’m new to the forum and would like to know if you guys could help? I have a build in progress of rob’s deluxe micro and would like to use a 6bm8 as the power tube simply because I have a couple. I am using the power transformer from an old rheme roberts 1725 reel to reel and the out put transformer (5k to 8ohms I believe)as well. The power transformer out puts 230-0-230 and gives me a b+ of 310 after diode rectification. It also has 2 6.3 outputs without center taps and one of the two 6.3s has thicker wire. I already have all the components like caps, resistors and wire and would like to make a single ended amp with my parts but not to sure were to start in adding the 6bm8 tube any advice would be appreciated.
 

King Fan

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Welcome! You obviously know a lot already, and in fact I can't answer your good question about the 6BM8. Smarter folks will be along shortly. In case it can help anyone, tho, I'll attach a 6BM8 data sheet as a PDF (click to view).
 

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dan40

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The 6bm8 is a dual section tube containing a triode section and a pentode section. They work great for freeing up space in a push pull circuit because you can use one section of the tube for one half of the phase inverter and the other section for the power tube. The triode is similar to a 12at7 and the power tube is like a smaller EL84 good for about 9 watts. I recently built a Tweed Dumble inspired circuit using that tube and the amp sounds wonderful. The 6bm8 can get quite loud and pushes out a lot of low end for a smaller tube.

You could definitely use it in a SE circuit and use the triode section as one of your gain stages if you wanted. I suppose that you could also avoid the triode section of the tube and just use the pentode section if you didn't want a 12at7 gain stage.

Here is the thread by our member "nickfl" that inspired my build.

 

dan40

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One last thing...with the 6bm8 as your power tube, the amp won't actually be a "micro" anymore. As I said before, they can get pretty loud and have a robust low end.
 

Dylanmarleyyoung

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I actually built the deluxe micro with two 12at7 that I got with the reel to reel and it sounded bad so I swapped out the preamp with a Jan-12ax7wa from my fender super reverb and it was very different but also was not amazing now I don’t know if it’s because I’m using old 60s /70’s tubes but ya, the bass was just terrible. I did end up just putting in the 6bm8 as the power tube and it was loud but very clean. I am using 3 22uf caps and I have read the voltage at each b+ Is there any b+ for the 6bm8 that I should be aiming for and do you know where I would grab the screen power from ?
 

dan40

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300vdc is what most of the builders using this tube aim for. For the screen supply, you would need to create an additional node in the power supply with a resistor and filter cap to feed the screen.
 

dan40

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How did you end up playing the amp with no screen supply voltage? If you connected it to the plate supply, you are running it in triode mode and not really hearing the true sound of the little pentode.
 

Dylanmarleyyoung

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I first found a person who made a diagram of how to make an adapter so he could swap a 6bm8 with a 6v6 and that gave me a layout of were the wires for the pentode went, then I stole the b+ 3 that gave me around 205v and taped in a wire for the screen.
 

Dylanmarleyyoung

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So for instance on the champ layout from stewmac the screen grid on the 6v6 gets its power from the 8uf cap on pin 4
 

dan40

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So for instance on the champ layout from stewmac the screen grid on the 6v6 gets its power from the 8uf cap on pin 4

Correct. You would just need to add the extra dropping resistor and filter cap right after the main cap to create a screen node. You could try something like 1k/10uf or so. Raising the resistor value will drop a bit of the screen voltage but it also affects voltages to the preamp if you go too high. Raising the filter cap value will stiffen up the screen supply a bit more and give you a slightly stiffer feel from the power amp. Some folks like the lower value caps for a squishier more vintage feel while others would prefer a higher value here.
 

Dylanmarleyyoung

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Ok so the screen voltage should be around the 300 \- 10 v. I would still use the cathode bias cap and resistors at the same values ?
 
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Dylanmarleyyoung

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good day , when putting a single 12at7 in parallel in accordance with robs micro deluxe does the b+1 become divided by 2 or no , because I can get a b+ of 310 tops after my diode rectifier, I am thinking maybe that’s why it sounded bad.
 

dan40

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Ok so the screen voltage should be around the 300 \- 10 v. I would still use the cathode bias cap and resistors at the same values ?

Plate voltage should be around 300v with the 6bm8. Some builders have gone as high as 350v but that is pushing this little tube pretty hard. Screen voltage will normally be close to plate voltage or a few volt lower depending on the screen node resistor you choose to use. The cathode resistor for the 6bm8 will need to be adjusted for optimum performance and the bypass cap value can stay the same or you can go higher for a slightly firmer low end. In the Tweed ODS build that I linked to above, with 300v on the plates, a 300 ohm cathode resistor worked well and the amp's bias came in right at 100% which is perfect for a cathode bias circuit.

good day , when putting a single 12at7 in parallel in accordance with robs micro deluxe does the b+1 become divided by 2 or no , because I can get a b+ of 310 tops after my diode rectifier, I am thinking maybe that’s why it sounded bad.


No...the plate voltage on each half of the 12at7 will still be the same value of 310v on each of the two plates in the 12at7 if it's wired in parallel.
 




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