Trying to Replicate a Triode Push Pull Build

[DummyLoad]

Ok, so rather than do some tree work this past weekend, I was musing about this amp - What are we hearing and what does it really sound like with the lower mu cascaded gain stages. I have a breadboard that was unused, so I cobbled together something similarto your Weekender. I used SSR and have a PT that got me close enough to 300V B+ with SSR. I used a NOS Stancor A-3830 15W output transformer strapped for 8Ωvc:10KΩa-a.

Other changes were bias points for V1 and V2, I left V3 (6BL7) relatively same. Initial impression was it sounds decent with the 6SN7, but kind of anemic with single coil pickups (Fender Strat w/ Seymour Duncan Antiquity), and I omitted the Master Vol.. I used 3.3KΩ cathode resistors which were a better match for the 100KΩ plate load with 280V B+ with the 6SN7 - Note that the 3.3KΩ cathode resistors also work well with 6SL7. In fact with 6SL7 in V1 and V2 it woke up with much more positive results. The entire amp with 6SL7/6BL7 draws about 37mA.

I have stash of 6DZ7 - I plan to replace the 6BL7 with one of those just to satisfy my curiosity - More on that later. In the meantime I attached a schema of what I built - Link to pics as well.

--Pete

https://www.dropbox.com/scl/fi/vfnkp58428jj38ln1d3ph/20230925_015324.jpg?rlkey=ym5y4111j5btbjcxxjfjtc683&dl=0

NOTE: Schematic is wrong - States V2 is 6SN7GTB, it is actually a 6SL7GT and the telemetry indicated reflects that.

[scstill]

I also have been busy...
I redesigned each stage of the PhuelCel8 (Philco Celestion 8") so that they are balanced (ie equal voltage swing) which I understand will provide a proper clean headroom. I discovered that there were some issues with the balance of the original design, maybe why it was "ODing". This would include my earlier amps Weekender, Westy as well as the Moonlight (someone else's design http://www.frontiernet.net/~jff/moonlight_bl7/Moonlight_AllOctalDoublePower.html).
But the Weekender and Westy sound good so they are staying for now, https://stillampd.com/westy-02
 
Remember my complaint was that the amp had no clean headroom. Seemingly always in overdrive.
I am pleased to say that the new design is great. It behaves as I think it should. Clean until about half volume then a nice crunchy overdrive increasing as the vol is rolled higher. To me it sounds good. I still can't figure out why Weekender and Westy sound good with the original design (lifted from Moonlight). I will create and post the PhuelCel8 design picture essay to stillamp'd site soon.

Changes include;
1) Change the plate resistor for each 6SN7 from 100k to 30K
2) Change Cathode resistor on LTP from 470 to 800
3) Change Tail resistor on LTP from 22k to 10k
4) Change the Cathode Bias resistor on 6BL7 from 470 to 360
5) Get supply voltage for 6BL7 from first node (rather than 2nd)
6) Get supply voltage for 6SN7 LTP from second node (rather than 3rd)

The revised schematic is provided below
in addition for those interested, I have included the load line calculations for each stage showing design before (unbalanced) and after (balanced). Appreciate any comments as this is the first time through this design for me. Much easier just to copy someone else's design I suppose.

[scstill]

I used a NOS Stancor A-3830 15W output transformer strapped for 8Ωvc:10KΩa-a.

Other changes were bias points for V1 and V2, I left V3 (6BL7) relatively same. Initial impression was it sounds decent with the 6SN7, but kind of anemic with single coil pickups (Fender Strat w/ Seymour Duncan Antiquity), and I omitted the Master Vol.. I used 3.3KΩ cathode resistors which were a better match for the 100KΩ plate load with 280V B+ with the 6SN7 - Note that the 3.3KΩ cathode resistors also work well with 6SL7. In fact with 6SL7 in V1 and V2 it woke up with much more positive results. The entire amp with 6SL7/6BL7 draws about 37mA.

I have stash of 6DZ7 - I plan to replace the 6BL7 with one of those just to satisfy my curiosity - More on that later. In the meantime I attached a schema of what I built - Link to pics as well.

10k load for the 6BL7 is also on my radar to try (my OT has multiple taps). It will lower the load line slope (from my 8.8k design) and should stay balanced with same cathode resistor. BTW - I had another "stupid things you do" entry as initially I thought I had an 8ohm speaker and designed for 8800:8 haha; it was 4ohm so I really had 4400 which also contributed to some of my issues (load line was extremely sloped, 263ma class B for 290v!!). Of coarse it has been corrected.

3.6k cathode resistor for 100k is what I calc (orange dot on pre amp load line) so 3.3k is a good choice. But the load line was so low and so flat. That is why I went with 30k plate resistors.

Thanks for the 6DZ7 intro, did not know about it, but I want to also try this dual pentode in a future project.

[DummyLoad]

Please excuse the threadjack - The following evolved from the experiments of the mockup of your amp. I have a small stash of the 6DZ7 and have always wanted to fool around with one or a pair in a P-P arrangement. I was pleasantly surprised with the results - Should have fiddled these long ago.

Spec sheet says about 12W with roughly same conditions presented.

https://frank.pocnet.net/sheets/093/6/6DZ7.pdf

Schama attached. Link below to the experiment.

https://www.dropbox.com/sh/3tkvhtgpsb6wcdo/AABl8SOl0wDfI9EShnGAXCWba?dl=0

Regards, 


--Pete