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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Have anyone built this 6V6 amp?

kevinkr

Administrator
Paid Member
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I don't know what kind of results you'll get, but I don't like that design. Doesn't go with my design philosophy, and I see problems here.

*) Inferior phase splitter. It's a floating paraphase, and the main problem with these types of splitters is unbalanced harmonic distortion between phases. An LTP splitter avoids that problem, and is scarcely more complex than the paraphase (only requiring the added complication of an active tail load). The LTP also provides a ready NFB summing point, as it also works as a differential amp.

*) Inadequate drive: the 12AX7 or 6SL7 just isn't up to sourcing current into loads with significant capacitance, and audio finals will have larger input and reverse transfer capacitances than small signal types. I would like to see cathode or source follower grid drivers between the phase splitter/gain stage and the finals. If the current sourcing isn't there, you will get slew limiting at the high frequencies. That sounds nasty, and may cause stability problems when the gNFB is added. A cathode or source follower presents these high impedance voltage amp VTs with friendlier loads (Hi-Z, Lo-C).

*) Extremely poor power supply. A 100R resistor and a 47uF capacitor has a cutoff of: 33.9Hz. For a first order, that gives: 10.9db of attenuation of the 120Hz ripple. That's positively hideous! It might have been good enough for a cheap design back in the day. A proper second order, LCR ripple filter would be much better (but ripple chokes are more expensive than resistors). If you still want to go cheap, then an active decoupler made from MOSFETs would be a vast improvement over that obsolete design from the days before high voltage MOSFETs. I'd expect this thingy will Hummmmmmmmmmm a good deal.

This design looks more "quick 'n' dirty" than anything else. It might make for a low powered PA, an AM plate modulator, or a guitar amp, but that's about it.

I give this design a rating of: [IMGDEAD]http://img225.imageshack.us/img225/6457/pukeface3kz.gif[/IMGDEAD] [IMGDEAD]http://img225.imageshack.us/img225/6457/pukeface3kz.gif[/IMGDEAD] [IMGDEAD]http://img225.imageshack.us/img225/6457/pukeface3kz.gif[/IMGDEAD] out of five
 
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Cassiel

Disabled Account
2004-09-30 3:53 pm
Madrid
so I will be fine with this DIY Push-Pull (PP) 6V6 / 6V6GT Tube Amplifier Schematic version right??? It seems to have all the modifications you guys mentioned.

I would use a 470 Ohm resistor to bias the 6SL7 and then apply best feedback ratio for your taste, OT's, speakers, etc. If that's too complicated for you use 1K. Not the 10K in the schematic you posted. Is that clear now?

EDIT: Miles your theory is correct however the amp sounds great. I did try an LTP first. Talking about the phase splitter, not about the PS.

The Leben CS300 uses same splitter, maybe you have the chance to hear that amp one day.
 
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Well, could be. Maybe a bad layout or bad power supply design. Anyway 10K to bias the 6SL7 is ridiculous.
I didn't have oscillation using 1K feedback resistor with Hammond OT's.

BTW, the 6V6's sounded better in triode mode.

Not surprised to hear the 6V6s sounded better in triode mode.. As a general comment you clearly know exactly what you are doing and why - such may not be the case with a relative newbie, and as described that particular project amp is broken. :D
 
I'm wondering how serious could be the fact (pointed by Crowhurst, that hate this type of paraphase) that , from the point of view of the NFB signal, it passes two stages on the (-) phase, and three stages on the (+) phase (V1a + op tube vs V1a+V1b+op lower tube)
I'm wrong?

The NFB doesn't go through three stages, since there's that 100uF bypass capacitor to help prevent this. Still, the whole arrangement smells of "quick 'n' dirty", and I would expect that, at low frequencies where the 100uF bypass begins to lose effectiveness, you would have low frequency stability issues (and high frequency issues where the internal inductive reactance becomes significant).
 
ok hmmm im not getting a good vibe about this design from you guys. I did actually try the original version of the amp but with 12ax7 and 6v6 and a 6.6k hammond trafo. I did not like the sound at all so I dismantled it. I was going to give it another shot but.....
Soooo can someone point me to a well designed 6v6 amp???

I did find these though:
http://www.joeltunnah.com/images/jt_6SN7_6V6_v2.jpg
Frankenhouse


Any thoughts? ideas? suggestion?
 
Soooo can someone point me to a well designed 6v6 amp???

Poindexter's Musical Machine. (He's on here, so you might want to look him up, and there are just esssssss-loads of threads dedicated to this particular project.) The one and only "drawback" to this design is that the 6V6s are being used as pseudotriodes. That does compromise on the output power, but 6V6s do make excellent pseudotriodes. (They're also excellent as pentode finals as well.)

This also uncomplicates the design since you won't need screen voltage regulators if you're not running the finals as pents.
 
Poindexter's Musical Machine. (He's on here, so you might want to look him up, and there are just esssssss-loads of threads dedicated to this particular project.) The one and only "drawback" to this design is that the 6V6s are being used as pseudotriodes. That does compromise on the output power, but 6V6s do make excellent pseudotriodes. (They're also excellent as pentode finals as well.)

This also uncomplicates the design since you won't need screen voltage regulators if you're not running the finals as pents.


I did actually consider that amp but I got scared when I saw it needs a -160Vdc. That means I would need an extra high voltage trafo, and im trying to keep costs down.