Have anyone built this 6V6 amp?

[lawbadman]

Im looking for an easy to build, great sounding 6v6 amp since I already have the tubes in hand.

I am thinking of building thisDIY Push-Pull (PP) 6V6 / 6V6GT Tube Amplifier Schematic 6v6 amp but I want to know if anyone has tried it and what was the result.

Anyone?


Thx

[Cassiel]

if anyone has tried it and what was the result.

Fantastic result. Go ahead.

[kevinkr]

There is an error in the schematic; the feedback resistor should be 1K, it provides both feedback and cathode bias to both triodes in the input stage.

This is the correct schematic: Push-Pull (PP) EL84 (6BQ5) or 6V6 (6AQ5) Tube Amp Schematic with Dynaco A-410 Transformers Note differences between 6V6 and 6BQ5 versions. I'd be inclined to run the output stage a lot hotter than in the project that references this original schematic.

[Tom Bavis]

Overall a nice circuit - uses self-balancing paraphase inverter. Cathode resistor for 6SL7 is 10K (feedback resistor, via OPT secondary). Seems a MITE high... a poor operating point. I'd add a ~1.2K cathode resistor to ground, and adjust feedback resistor for desired gain.

[Cassiel]

Right, I didn't notice.

It oscillated with 1K so he tried 10K. LOL. DIY power.

[kevinkr]

Originally Posted by stalker

Right, I didn't notice.

It oscillated with 1K so he tried 10K. LOL. DIY power.

That's because it was not properly compensated for the transformers in use.. DIY power indeed.

[lawbadman]

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.

[Cassiel]

Originally Posted by kevinkr

That's because it was not properly compensated for the transformers in use

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.

[Ty_Bower]

Here's the "original" Dynaco schematic, in case the OP is looking for it:
http://www.triodeel.com/dyna6bq5.gif

Doesn't the Magnavox 8802 have a similar self-balancing paraphase inverter?

edit: Oops, sorry Kevin. I missed you already posted the Dynaco schematic from another source.

[Miles Prower]

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: out of five