M70 EL34 PPP (Parallel Push Pull) Mono Amps

With all the custom amps here I thought I would add these that I recently completed to the mix.  These use original Dynaco A430 output transformers with 4 EL34 tubes per channel.  The Mark III chassis, from Triode Electronics, was laser cut for 4 tube positions so it seemed a natural to make this into a PPP (Parallel Push Pull) amp.   I run 8 ohm speakers on the 16 ohm tap on the output transformers to match the impedance.

The output transformers, while rated for 60 watts are more like 100 watt size.  I'm getting 72 watts output at clipping with none (as far as I can see) any saturation at 20 Hz at full power.

Also unique is bias adjustments for all 4 output tubes.  This required doubling the number of coupling capacitors, bias pots, and grid leak resistors and fitting them all on the tubesforhifi Octal driver board for the Mk III amps.  It required a couple of small holes to be drilled; relatively easy.  This way the bias can be precisely adjusted for each output tube.  Notice the jacks for the meter probe were added topside adjacent to the output tube that is getting adjusted.  I color-coded the bias pots to match the color of the bias jacks.  The black colored jack is for meter negative probe.

A couple of other details.  I redesigned the circuit to run a 6SL7 in the front tube position.  I also changed the wiring on one of the boards so one channel always runs one section of the 6SL7 and the other channel runs the other.  Originally as designed, one triode section of the 6SL7 is not used.  This way, when the active section "wears out" simply swap with the other amp's 6SL7 and you will be using the previously "unused" section.

Nice setup....thanks for the Pic









Thanks for the site Bob...............

Nice looking build, Tom.

-Pat

excellent, nicely done!

Hey! Great work! Do you have a schematic of that you could post?
And of course, how's it sound? Go ahead...brag!

Blitzen, I have the M70 EL34 PPP Mono Amps at my house right now. I have a Rogue 60 watt 6550 amp which sounds wonderful. Upon my first hearing of Toms' amps I was impressed. They sounded like really great EL34 amps. But after listening for a few days the amps kept getting better and better. I've listened to them for almost 2 weeks and I can tell you I'm impressed. This is not the first amp I've auditioned of Toms'. And I have been impressed with the quality of work as well as the sound quality. The M70's are quiet and I'm driving a pair of Apogee Centaur Majors with them. The M70's don't miss a beat and best of all it has very good control of the woofer. Yes, the bass control is very, very good. I have turned off my two Dali Suite 12" Acoustic Suspension Subs. I actually don't need them with the M70's. The 40" ribbon is very revealing. When Tom heard them he was impressed with the articulation of the ribbons. His M70's do a wonderful job on my power hungry Apogees. My listening room is only 13' x 14' and I have 14.5' ceilings. The Rogue did a wonderful job as well. But EL34's always give me more inner detail than KT88 or 6550's. So I'm a little biased for EL34's. I'd have to do some tube rolling as I like the EH EL34's. I have them in my Dynaco ST 70 and I've only heard one other ST 70 that out performed it. Tom's ST 70 that had Gold Lions in it had the best soundstage height I've ever heard from a ST70. If it had more power I would have bought it. But for now the M70s may have a new home. I like what Tom has done with this design. Other than some settling in of components the M70's has been much quieter than I ever expected. They look great too and have excellent features. Great Job Tom.

As I do the same, I like how you color coded your VRs/pots to your TPs.  

I build them that way more for the next, possibly (probably) technically challenged person who may acquire the unit.  It makes it a no brainer for them unless they're color blind.