When this was posted I recall Planet_10 showing interest. It also caught my eye as I had suitable trannies lying about. Thought it would make a nice little bedroom or workshop amp.
http://www.diyaudio.com/forums/showthread.php?postid=552739#post552739
Knocked it up last weekend with mods as below:
1) My power supply was a bit high so I cathode biased the output - individual 390R cathode resistors bypassed by 470uF/50V Blackgate standards.
2) 20% Ultralinear Taps connected via 150R screen resistors
3) 1K balance pot between the input triodes cathodes with wiper fed by a 1.2mA ring of 2 BC547B current source
4) No global feedback (Zout = 3.2 Ohms)
5) Did'nt have the separate 13V winding so just rectified the heater winding to give approx -7V for the current source supply.
Note: I could have (but did'nt) voltage doubled the heater supply and then used fixed bias on the outputs (about -11V required with V+ = 310V for 28mA idle) using NOS Philips 6GW8 (ECL86).
Sound is quite lovely. <10Hz to 35kHz -3dB points at 1Watt which given the quality of the trannies I had was amazing. (Ex an old Heatkit, originally driven by 6BM8s with 250V rails). The shunt feedback is doing a great job. Output a clipping 10.5 Watts.
A good "newbies" project.
Cheers,
Ian
http://www.diyaudio.com/forums/showthread.php?postid=552739#post552739
Knocked it up last weekend with mods as below:
1) My power supply was a bit high so I cathode biased the output - individual 390R cathode resistors bypassed by 470uF/50V Blackgate standards.
2) 20% Ultralinear Taps connected via 150R screen resistors
3) 1K balance pot between the input triodes cathodes with wiper fed by a 1.2mA ring of 2 BC547B current source
4) No global feedback (Zout = 3.2 Ohms)
5) Did'nt have the separate 13V winding so just rectified the heater winding to give approx -7V for the current source supply.
Note: I could have (but did'nt) voltage doubled the heater supply and then used fixed bias on the outputs (about -11V required with V+ = 310V for 28mA idle) using NOS Philips 6GW8 (ECL86).
Sound is quite lovely. <10Hz to 35kHz -3dB points at 1Watt which given the quality of the trannies I had was amazing. (Ex an old Heatkit, originally driven by 6BM8s with 250V rails). The shunt feedback is doing a great job. Output a clipping 10.5 Watts.
A good "newbies" project.
Cheers,
Ian
Cool.
I salvaged iron for mine last week (and put up a 2nd set on eBay)
Probably going to go with Russian "6BM8s" -- thet should just be coming out of cryo now,
I was going to order up some IXYS CCS chips & try those (1st on the front-end and then maybe on the output stage)
dave
I salvaged iron for mine last week (and put up a 2nd set on eBay)
An externally hosted image should be here but it was not working when we last tested it.
Probably going to go with Russian "6BM8s" -- thet should just be coming out of cryo now,
I was going to order up some IXYS CCS chips & try those (1st on the front-end and then maybe on the output stage)
dave
Update
I've been running this little amp for about 2 weeks now and am simply amazed by the sound quality - especially the bass strength. The amp is lightening fast and very open. The balanced shunt feedback scheme is (I'm convinced) doing something special. The 2 x 47K plus the 12K is adding some self balancing (AC balance) as well as lowering the rp of the output tubes which no doubt helps to better drive the modest primary inductance of the "cheap" output tannies. I also think the 20% Ultralinear I use is probably worthwhile compared to the straight pentode arrangement of the original circuit.
I'm so impressed I'm going to try scaling this up to EL34/6550/KT88. I've put aside my 845SET while listening to it and believe it or not it suffers very little in comparison - in fact its a lot more open and less "covered" than the 845. Considering that I could build 10 of these for what the 845SET cost thats saying something.
Anyone considering building a 6BM8/6GW8 or even EL84 based amp then I cannot recommend this design highly enough. I recommend you include the front end balance circuit and the Ultralinear connection discussed in the original post.
To reiterate something I said in an earlier post - This is a great amp for a first project for "newbies" to tube audio - the only trouble is that where most of us build 3 or 4 amps before we approach this sound quality you'll have it from day 1.
Its also a great project for the more experienced of you - like me, you'll wonder where the sound quality is coming from.
Cheers,
Ian
I've been running this little amp for about 2 weeks now and am simply amazed by the sound quality - especially the bass strength. The amp is lightening fast and very open. The balanced shunt feedback scheme is (I'm convinced) doing something special. The 2 x 47K plus the 12K is adding some self balancing (AC balance) as well as lowering the rp of the output tubes which no doubt helps to better drive the modest primary inductance of the "cheap" output tannies. I also think the 20% Ultralinear I use is probably worthwhile compared to the straight pentode arrangement of the original circuit.
I'm so impressed I'm going to try scaling this up to EL34/6550/KT88. I've put aside my 845SET while listening to it and believe it or not it suffers very little in comparison - in fact its a lot more open and less "covered" than the 845. Considering that I could build 10 of these for what the 845SET cost thats saying something.
Anyone considering building a 6BM8/6GW8 or even EL84 based amp then I cannot recommend this design highly enough. I recommend you include the front end balance circuit and the Ultralinear connection discussed in the original post.
To reiterate something I said in an earlier post - This is a great amp for a first project for "newbies" to tube audio - the only trouble is that where most of us build 3 or 4 amps before we approach this sound quality you'll have it from day 1.
Its also a great project for the more experienced of you - like me, you'll wonder where the sound quality is coming from.
Cheers,
Ian
Schematic?
Note to Gingertube:
Could you post the schematic you used for the ECL86 and any construction commetns you have time to offer. I have a recycled stereo console ECL86 donor that is looking for a first tube amp project. I seem to be in luck here. I hope to follow your lead and try this as my first amplifier project.
Thanks for the help
Note to Gingertube:
Could you post the schematic you used for the ECL86 and any construction commetns you have time to offer. I have a recycled stereo console ECL86 donor that is looking for a first tube amp project. I seem to be in luck here. I hope to follow your lead and try this as my first amplifier project.
Thanks for the help
Scott,
Here is one where he goes into great detail -- it has some "complex" bits thou
http://www.mcamafia.de/tubes/keksdose/cookiebox.htm
dave
Here is one where he goes into great detail -- it has some "complex" bits thou
http://www.mcamafia.de/tubes/keksdose/cookiebox.htm
dave
ECL86 diagrams
Thanks Guys:
I will study those two diagrams. I am interested in a pretty simple amp that has had the RH mods done to it. As I am a very junior amp guy I guess I need a bit of close on supervision. I will keep in touch with my progress.
I guess I need to come out of planning mode and try to make an amp to match up with my fleet of Fostex speakers.
Boy this is fun
Thanks Guys:
I will study those two diagrams. I am interested in a pretty simple amp that has had the RH mods done to it. As I am a very junior amp guy I guess I need a bit of close on supervision. I will keep in touch with my progress.
I guess I need to come out of planning mode and try to make an amp to match up with my fleet of Fostex speakers.
Boy this is fun
Scott i have a diagram of a fairly simple one that i was going to simplify further and add RH too... that is what you likely want to execute.
I just read the peter Wednt article i posted the URL of -- it is interesting that he was using the plate to grid feedback (ala RH) in that one. The grid leak boas on the triode is not best, but all the stuff in the CSS on the cathode of the pentode is interesting if you can't get reasonably well matched tubes (you'd want to get a working amp before you added that guilding thou)
dave
I just read the peter Wednt article i posted the URL of -- it is interesting that he was using the plate to grid feedback (ala RH) in that one. The grid leak boas on the triode is not best, but all the stuff in the CSS on the cathode of the pentode is interesting if you can't get reasonably well matched tubes (you'd want to get a working amp before you added that guilding thou)
dave
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