I decided to make a video series of my next project, a small 6BM8 SE UL scratch built amp that is perfect for a beginner or anyone looking for a fun project to build. You should be able to get all the parts new for under $300, it's a simple design and from what I have read, these are great sounding tubes.
I plan to post a final schematic and a BOM once I finish the amp and get everything sorted out. I am explaining in fine detail my thought process, tips on chassis fab work etc. so anyone with basic tools and are good with their hands should be able to build one. I also plan to show how to test it on a scope and the process I use for tweaking different things to get the best sound out of it.
I have warned folks, if you think you might want to build this with the 10W 5K Edcor UL output transformers I am using, you might want to order them now. It's taking 8-10 weeks to get these. The rest of the parts are pretty easy to find.
Anyway, I wanted to share it here. I'm about half way done, all the fab work is finished and the layout decided on, just waiting on a few parts to finish wiring it all up. Hope you enjoy following along as I build it.
6BM8 DIY Stereo Tube Amplifier Series - YouTube
I plan to post a final schematic and a BOM once I finish the amp and get everything sorted out. I am explaining in fine detail my thought process, tips on chassis fab work etc. so anyone with basic tools and are good with their hands should be able to build one. I also plan to show how to test it on a scope and the process I use for tweaking different things to get the best sound out of it.
I have warned folks, if you think you might want to build this with the 10W 5K Edcor UL output transformers I am using, you might want to order them now. It's taking 8-10 weeks to get these. The rest of the parts are pretty easy to find.
Anyway, I wanted to share it here. I'm about half way done, all the fab work is finished and the layout decided on, just waiting on a few parts to finish wiring it all up. Hope you enjoy following along as I build it.
6BM8 DIY Stereo Tube Amplifier Series - YouTube
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Thanks. I've been buying those for all my builds as it becomes the "nut" for the screws that hold the tube sockets to the chassis. I've been getting those + volume knobs from these folks.
Tube Shield Ring Washer
Tube Shield Ring Washer
They have two threaded holes in the bottom and (in my understanding) are retained by the same screws that hold the socket
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A combined triode/power pentode amplifier imho isn't the best project for a beginner. It needs some experience to take care of oscillations. Remember the signals at the triode grid and the pentode plate are in phase, with lot of gain in between!
Best regards!
Best regards!
Kay Pirinha,
what would you consider an appropriate amp project for beginners? Make a video series of it.
Let us encourage Stephe! We need more, not fewer beginners and I think Stephe’s videos are great to draw in more beginners, especially female by example.
what would you consider an appropriate amp project for beginners? Make a video series of it.
Let us encourage Stephe! We need more, not fewer beginners and I think Stephe’s videos are great to draw in more beginners, especially female by example.
Maybe if it was being run as a pentode with global NFB. It's not.
This is the schematic I came up with as a starting point. I will adjust some things once I have it built and running. I don't see where oscillations are going to be an issue here, but maybe I'm missing something.
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I'd suggest an amp with two power tubes, such as 6AQ5 or 6BQ5, and a double triode. Maybe this would be not as cheap, but suitable output transformers are readily available for these tubes. 5 kohms primary impedance would fit for both, but it doesn't fit for the 6BM8 pentode running at 250 V plate voltage.
Best regards!
Best regards!
Kay Pirinha,
You still mean SE, right? I don’t see how a 3-tube stereo amplifier (still one triode driving one pentode) will be simpler that this 2-tube amplifier.
The prescribed 5k primary SE Edcor transformer is readily available and not expensive (at least in the US). Are you saying the primary impedance is wrong for the 6BM8 in UL SE with ~240V on the plate? What would you suggest as a better fit for this purpose?
You still mean SE, right? I don’t see how a 3-tube stereo amplifier (still one triode driving one pentode) will be simpler that this 2-tube amplifier.
The prescribed 5k primary SE Edcor transformer is readily available and not expensive (at least in the US). Are you saying the primary impedance is wrong for the 6BM8 in UL SE with ~240V on the plate? What would you suggest as a better fit for this purpose?
The 6BM8 pentode's dissipation rating is 7 watts. As a common rule of thumb, the load impedance's optimum is plate voltage ^2/plate dissipation, hence 240²/7 = 8.2 kohms.
Best regards!
Best regards!
Then every 6BM8 schematic I have seen online is wrong. Everyone I have seen is showing using them with a 5K OT.
I'm really starting to feel like maybe I shouldn't be posting to this forum, I didn't realize there was this level of hostility here. 🙁
Classique – 6BM8/ECL82 SE – 3W – White Paper – Caledonian Audio
Needing OT for SE ECL82/6BM8 amplfier
6BM8 Valve Audio Amplifier
Well, indeed the 6BM8 would fancy ~7k a bit more, but I don't see anything horrible with 5k either. I doesn't really make that much of a difference in practice, and 5k OPTs are widely available (unlike the 7-8k ones).
Need a 7k primary?
Get close to 7k with a 5k transformer, and a 6 Ohm loudspeaker on the 4 Ohm tap.
. . . Where do you get a 6 Ohm loudspeaker?
. . . Most 8 Ohm loudspeakers drop to 6 Ohms (or less) at some frequencies.
Just measure the 8 Ohm loudspeaker DCR, and see what the loudspeaker impedance will be at very low frequencies, and also often at some mid frequencies too.
5k primary x (6 Ohms/4 Tap) = 7.5k
So connect your 8 Ohm (really 6 Ohm) loudspeaker on the 4 Ohm tap.
Then try using the 8 Ohm tap, and note the difference.
Yes, the primary inductance will limit the very low frequency response, but that is true of most inexpensive output transformers anyway.
And with most SE Edcor transformers, you have the UL tap.
You can try UL, and try Triode wired modes with the same transformers.
Mono-Blocks or Stereo:
I would prefer the EL84 or 6BQ5 with individual self bias resistors and individual bypass caps; and a 12AY7 dual triode, with individual self bias resistors and individual bypass caps. And I would start with IXYS current sources in each 12AY7 plate (set for 3mA); and use RC coupling to the output tubes.
Start with no negative feedback (except technically, UL and Triode wired each one really are negative feedback).
Simple but effective.
That is just one way to do it.
Have fun building and listening!
Get close to 7k with a 5k transformer, and a 6 Ohm loudspeaker on the 4 Ohm tap.
. . . Where do you get a 6 Ohm loudspeaker?
. . . Most 8 Ohm loudspeakers drop to 6 Ohms (or less) at some frequencies.
Just measure the 8 Ohm loudspeaker DCR, and see what the loudspeaker impedance will be at very low frequencies, and also often at some mid frequencies too.
5k primary x (6 Ohms/4 Tap) = 7.5k
So connect your 8 Ohm (really 6 Ohm) loudspeaker on the 4 Ohm tap.
Then try using the 8 Ohm tap, and note the difference.
Yes, the primary inductance will limit the very low frequency response, but that is true of most inexpensive output transformers anyway.
And with most SE Edcor transformers, you have the UL tap.
You can try UL, and try Triode wired modes with the same transformers.
Mono-Blocks or Stereo:
I would prefer the EL84 or 6BQ5 with individual self bias resistors and individual bypass caps; and a 12AY7 dual triode, with individual self bias resistors and individual bypass caps. And I would start with IXYS current sources in each 12AY7 plate (set for 3mA); and use RC coupling to the output tubes.
Start with no negative feedback (except technically, UL and Triode wired each one really are negative feedback).
Simple but effective.
That is just one way to do it.
Have fun building and listening!
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"Experts"
There are a lot of schematics on the internet, lots of pictures of amp projects. If you look at some of the projects you can see that some of the project can not work! I trust my own ears. There are lot of these experts out there. They use output transformers built by Nubian virgins at 20 degrees celcius, when the moon is full. They do not use normal resistors and capacitors, but uses resistors and capacitors also made by these Nubian virgins! They(The experts) have their own fans who adore them. Thrust your own ears, listen to your music and enjoy live.
There are a lot of schematics on the internet, lots of pictures of amp projects. If you look at some of the projects you can see that some of the project can not work! I trust my own ears. There are lot of these experts out there. They use output transformers built by Nubian virgins at 20 degrees celcius, when the moon is full. They do not use normal resistors and capacitors, but uses resistors and capacitors also made by these Nubian virgins! They(The experts) have their own fans who adore them. Thrust your own ears, listen to your music and enjoy live.
Stephe,
Yes, sorry.
There are always lots of opinions on Tubes / Valves.
(my opinions included).
Negative feedback, even if it is Schade (plate to plate) can be tricky sometimes.
What kind of signal sources do you have?
Most CD players put out 2.1Vrms, (3V peak), more than enough for most simple 2 stage SE amplifiers.
What kind of speakers do you have?
Efficiency will affect the maximum sound from a low power amplifier.
Low power amplifiers can be simple, and effective, and are a good start for beginners too.
I mostly do near field listening, with the speakers about 2/3 meter away (2.2 feet).
Do not get discouraged.
This thread should be able to help you build an amplifier that will be satisfying.
But beware, you may get addicted to building another amplifier after that.
Yes, sorry.
There are always lots of opinions on Tubes / Valves.
(my opinions included).
Negative feedback, even if it is Schade (plate to plate) can be tricky sometimes.
What kind of signal sources do you have?
Most CD players put out 2.1Vrms, (3V peak), more than enough for most simple 2 stage SE amplifiers.
What kind of speakers do you have?
Efficiency will affect the maximum sound from a low power amplifier.
Low power amplifiers can be simple, and effective, and are a good start for beginners too.
I mostly do near field listening, with the speakers about 2/3 meter away (2.2 feet).
Do not get discouraged.
This thread should be able to help you build an amplifier that will be satisfying.
But beware, you may get addicted to building another amplifier after that.
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