CCS Loaded Parafeed Output - Design Considerations?

[L0rdGwyn]

Interesting amplifier, would love to see it.  I was thinking more along the lines of a headphone amp.  I have seen the idea discussed somewhat for speaker amplifiers, with a high-gain input tube, but obeying Vhk could become a problem at high outputs.

With the 6AS7 OTL headphone amplifiers like the Crack being so popular, I'm surprised I haven't found anyone trying this, but could be due to the oddball transformer that would be needed, probably a custom job.  And who knows if it would really sound better than a simpler, cheaper output cap.

[Paul Birkeland]

When you start your design assuming that you'll use an output transformer, you'd then really free up your choice of tubes to drive said output transformer, and there are many candidates that are far more linear than a 6080.  Edcor makes some small 1.7K output transformers that would work for such an experiment.

If you aren't using an output transformer, then you want a tube with a good amount of transconductance that can also pass a pretty heavy amount of DC current. 

[L0rdGwyn]

Good point PB, I'll take a look at those Edcors.  The EC360 might be an interesting choice, I'll see what's out there.  I think this could make for a cool design, going to build on the idea over time and see what I can come up with, thanks for the $0.02.

[L0rdGwyn]

Hey PB - going back to when you were using the 3C24, did you find an efficient way to degas them?  I am attempting to trace their curves, have five different samples, all appear gassy and will not conduct on my pulse tracer.

I have read anything from running the filaments for five hours to running the plates red for 10-20 minutes since the tantalum plates are self-gettering, although for the latter I would concerned about arcing given the gas...

I have a pair of HK54 which I was able to trace A2 curves on no problem.

[Paul Birkeland]

I degas them by throwing them away and using a different one when they are gassy.  That's the big downside to the 3C24.

[L0rdGwyn]

Yeah seems to be an issue, thought one out of five would pan out, bummer.  Seems many sellers don't have the means to test them, so buying is a crap shoot.  Alright well I guess I will chuck these and keep an eye out for a tested pair, thanks.

[Paul Birkeland]

It's worth it to buy NOS in box with the 3C24.  The ones with black plastic bases seem to be the best bet.

[L0rdGwyn]

Sounds good, I was able to find a tested pair, black base like you suggested, thanks.

Can I ask your thoughts on a full A2 3C24 output vs. a partial A1 + A2?  The below schematic seems to be the most popular design choice with the 0V grid being the positive swing boundary.  Attached a 3.3K load line, wonder if it is worth adding a low voltage negative supply to get the additional 10-15V of negative grid swing?  Thought about choke-loading the cathode follower for the negative swing, but might cause feedback issues.

[Paul Birkeland]

I've built one or two of those.  They sound decent enough but they don't make a lot of power.  This is what I ended up doing instead, and it makes about twice the power with no GNFB.

[L0rdGwyn]

Thanks for the schematic PB, choke loading the cathode follower with plate-to-plate local feedback, nice.  What is the purpose of the 2.5V bipolar supply?  I'll keep this in mind, think I would like to do something similar i.e. choke loading the cathode follower with local feedback and avoid a negative supply.

[Paul Birkeland]

That was there as an experiment for tweaking the cathode follower bias to dial in the plate current perfectly.  I didn't end up liking it so the power supply it there but that's no longer reflected in the schematic. 

[L0rdGwyn]

Got it.  Well thanks for sharing, appreciate the input as usual, will report back when all of these ideas become real world things!

[L0rdGwyn]

I got my functioning 3C24 PB.  Traced the curves too, all looks well.  Plate resistance around the bias point I am thinking is around 9.4K.

I might breadboard this with an E80F and 6AH4 with a negative supply, could be low voltage since the 3C24 grid only needs to swing down to around -25V.  I have a spare Lundahl mains transformer, would only need one of the four 6.3V 3.1A windings, was thinking I would place the other three in series and use a voltage doubler for the negative supply, should get me something like -50VDC. 

Did you really leave the EF86 cathode degenerated in your design, still had enough gain?

Also, just wanted to say, I know I am talking about my own designs on this thread (which might be renamed to "L0rdGwyn Pesters PB"), but I am a super BH ambassador, probably responsible for 10-15 BHC sales people often ask me how to get into tube DIY.  I tell them to read Valve Amplifiers and build a BHC.  Excited about the YouTube channel announcement, will follow along.

[Paul Birkeland]

What you're messing around with is something PJ and I have been working with for a while, and to be completely honest there was a good 6 month period where it was me pestering PJ and seriously trying his patience, so this thread feels rather appropriate. 

One thing PJ told me was that the unbypassed cathode resistor on the first stage pentode would improve performance of the amp.  While this is absolutely true, I eventually made a really gnarly spreadsheet on the computer (that started out as a pentode operating point performance predictor from PJ) and this was able to show what bypassed vs. unbypassed cathode resistors looked like.  The performance differences between the two were small enough that I don't worry as much about squeezing out that aspect as much as I did before.

[Paul Joppa]

The unbypassed driver cathode resistor does not provide very much feedback - around 6dB or so, I think.