CCS Loaded Parafeed Output - Design Considerations?

[L0rdGwyn]

Would it be overkill to use one 6BX7 per channel with a resistive load and parallel the two triodes?

[Paul Birkeland]

I don't think that would be a problem.  The two triodes in parallel will let you use a lower resistor value, but will be somewhat demanding on that B- supply.

[L0rdGwyn]

Yeah it would be demanding, always a compromise to be made.  I was planning on a TL431 shunt regulated B- supply, could probably spec it for 30-40mA.  I'm trying hard to use the 6BX7 since I have a bunch of them in pairs, but if it came to a point where I was using a single 6BX7 for both channels, I think it would make sense to switch to the 6AK4 or 6CK4 to take advantage of the higher single triode plate dissipation.  Also, maybe it is total tube amplifier voodoo, but in my experience I have found equivalent single-triodes to have better soundstage and imaging than equivalent dual triodes (6J5 vs 6SN7, for example), although I have not found a objectively measurable way to explain that (crosstalk, distortion, etc.).  Not sure if the same applies to cathode followers, haven't tried it yet.

I really like the second triode as a current sink if I can keep the 801A happy on startup, might really go with the time delay idea.

[Paul Birkeland]

Check out the 12GN7.

[L0rdGwyn]

Hey PB - I'll check out the 12GN7, thanks.  I'm still brainstorming some ideas on the 6BX7 cathode follower feeding 801A startup biasing issue.

What about this - placing a high-value resistor between the 801A grid and negative supply.  At startup before the 6BX7 is biased, the 801A will see the full negative supply and be placed at cutoff, then the grid will rise to its 0V bias point as the 6BX7 come up to bias.

Any reason you can see that wouldn't work?  My thinking is this would also protect the 801A in the event of a failure of the 6BX7, although the 801A will be fused as well.  I might be making a mental error though, I am experiencing post night shift brain.

[Paul Birkeland]

The problem with the high value resistor is that just a whisper of grid current will shift the bias substantially.  You can place a transistor between B+ and the primary of the OPT and have it sense current draw on the top 6BX7 in order to turn on and allow the output stage to work, but I favor a chunky resistor under the cathode follower as the simple/reliable solution.

[L0rdGwyn]

I gotcha PB, thanks for the input.  I think I am going to go with the resistor load and parallel triode sections.  I crunched the numbers, a 6BX7 as a current sink with a 160ohm cathode resistor only represents about a 2.6K AC load, not great!  Compare that to a 6K resistor on a single triode or a 3K resistor on parallel triode 6BX7 cathode follower.

Check out these curves PB - I recently purchased a curve tracer.  The first set are both triodes traced and overlaid on eachother.  With a single triode resistively-loaded cathode follower, operating point would be around 330V, 15mA, -35Vg1.

Compare to the second set of curves, this is both triodes in parallel, operating point would be 330V, 30mA, -34Vg1.  The linearity is much, much better!

So, think I will use the 3K resistor on the parallel cathodes of the two sections running at 15mA a piece, just have to spec my negative supply for it.

[Paul Birkeland]

Fancy.

[L0rdGwyn]

Random question for you PB - what did you think of the sound of your 801A A2 design? Was it worth the effort?

[Paul Birkeland]

Yes, the current owner is quite happy with it. 

[L0rdGwyn]

Thanks, still plugging away at mine, I am hoping the payoff is worth it, it has been a lot of work with much more to be done.

[L0rdGwyn]

Here is my recent iteration PB, haven't prototyped this yet, but I feel pretty confident this is close to the final design.

I dropped the CCS load on the pentode and the screen DC feedback.  What I found was I was getting an H3 dominant THD spectrum and the sound suffered for it.  Instead, what I am going to try is using a high-value resistive load with a VR105 glow tube regulating the screen at 105V.  Inspired by some other DIYers, going to use 1.5V battery on the EF37A g1 and buffer the feedback loop with a PMOS on the EF37A cathode.

This gets a ton of sand out of the gain stage (less opportunity for oscillations), simplifies it significantly, gives me an H2 dominant THD spectrum and I believe the gain will be enough to get a good Zout.  I think I've pushed the resistor load as well as the 6BX7 grid leak (max 2.2Meg) to their limits while maintaining a good operating point on the EF37A to maximize the gain.

I feel pretty good about this, I will pray to the oscillation gods before I prototype it and take some real-world measurements.

[Paul Birkeland]

Let us know how the breadboarding goes!

[L0rdGwyn]

Got another random question for you PB, hope you don't mind.

Have you ever thought about loading a 6080/6AS7G cathode follower with an output transformer?  Think a Bottlehead Crack with an OPT load.  I recognize finding a transformer that is right for the job would be challenging as it would need to be gapped for 80-100mA at a low turns ratio, could do 1:1 but if you are going to the trouble, perhaps 2:1 or 4:1 to drop the output impedance.  These are like, interstage and line output turns ratios, which typically don't come gapped for 100mA.  Also would likely need to put a resistor between the primary and ground to bias up the cathode.

Something like this, gyrator loaded, direct coupled with an OPT load with a theoretical 1:1 OPT.  Ever seen anything like that?

[Paul Birkeland]

Are you talking about doing this for a headphone amp or for A2 drive?

I have thought about using a small push-pull output transformer in place of the center tapped choke to do an Altec 1570 variant, and possibly using the secondary as part of a feedback network of some sort.