CCS Loaded Parafeed Output - Design Considerations?

[Paul Birkeland]

Yeah, you're running up against transformer issues there.  The good news is that there are tons of 5K transformer options that aren't so limited. 

The amps in the photo are 100TH with a 6BL7 as a directly coupled cathode follower to drive the grid of the 100TH.  The extra half of the 6BL7 regulates the B+ for the cathode follower, which is something PJ suggested when he was visiting last year.  The input tube is an EL41, which was about the only pentode I could find that didn't care too much about high plate voltage and had the middle of the road transconductance that made it work properly in the circuit. 

[L0rdGwyn]

Funny to think I am running up against transformer issues, these are from a reputable builder, although I suspect the designer may have sacrificed some HF extension for a high primary inductance, 100H on a 6.5K primary, these can be rewired for 11K and 23K as well, I thought I would be using them at 11K.  The thought of replacing them isn't super appealing, will have to do some soul searching and decide if 17kHz is good enough, certainly beyond my hearing ability.  I'll think about it, but the sound I am getting right now is sensational, the bass is jaw dropping...worried if I replace them I could end up with something less satisfying, "the devil you know is better than the devil you don't."

The 100TH...that one is out there, never even heard of it!  Very cool looking tube though.  I imagine when you have been building tube amps as long as you have PB, you have to get more and more esoteric to stay occupied.  I also imagine they keep you nice and warm in the cold winter months

[Paul Birkeland]

When you make a transformer that is either 6K or 12K, depending on how you wire/load it, that is already a bit of a compromise.  I think this is more doable with a 5K/2.5K, as it's easier to land right in the middle and make a fully optimized 3.75K:6 transformer and ratio it accordingly.  It's still a compromise though!

You are using the big Lundahls right?  I have been pleasantly surprised by the performance of the 5K One-Electron output transformers, especially up top.  They aren't insanely expensive and are plenty big to get all the power out of an 801 that you could possibly get.  The datasheet says you'd be -1dB at 30kHz with this iron, but the last amp I made with them was -1dB at 42kHz, which was rather surprising.  Said amp was -1dB at 22Hz, so about the same down low as your current transformers.

[L0rdGwyn]

Interesting, always compromises in this hobby.  Well thank you for the recommendation, I'll check out those One Electron transformers, -1dB at 42kHz, that's pretty impressive.  That's right, I am using the Lundahls, these are the LL9202.  I've used Sowter as well, tried reaching out to Magnequest recently for a potential project I am cooking up but never heard back.  I've liked the sound of the Lundahls so I have stuck with them, but always open to trying something else.  Thought about trying Electraprint as well, they seem to have a good reputation in the DIY world.

[Paul Birkeland]

If budget allows, Sowter and Monolith are really solid options.  For a ~10W amp, the prices aren't completely insane either.

[L0rdGwyn]

Thanks, I'll give the OPT changeout some thought, I have an order pending with Sowter right now actually...it might not mean much considering I am only three scratch builds into this thing, but this might be the best amplifier I have ever heard...no kidding, it's that good.  So perhaps it is worth squeezing out some more performance with a wider bandwidth OPT, assuming the LL9202 are not a large part of why it sounds so great.  I'll sleep on it!  For a week.

I think the most I have ever heard of someone getting out of an A2 801A is 8W, you didn't mean to imply you could get 10W out, did you PB?

[Paul Birkeland]

I got 8.5W.  Perhaps with Sowters you could go over 9W.

[L0rdGwyn]

Wow that would be interesting.  I am in the process of reworking my 6A5G amplifier, transplanting into a new chassis.  It uses 3.3K 60mA OPT, maybe I will throw them in this prototype and see how it works out.  Only thing is I would have to rethink the bias supply, it uses a LND150 CCS and a 56K resistor to ground to set the bias.  If I just used a resistor to ground instead, the bias would be set by the gate-to-source voltage of the FET buffer, something like -4 to -5V.  Wouldn't be adjustable though, have to look into it more deeply and think it through...

[Paul Birkeland]

Just be sure your loadline still makes sense in terms of giving you the power you are expecting.  I ran the 801A at 300V/60mA (the 60mA was measured as plate+grid current) and got about 8.5W. 

[L0rdGwyn]

For sure, would have to alter the B+, bias supply, etc. to give this a try, but shouldn't be too bad I don't think.

I have to admit PB, I am not seeing how you got 8.5W out of the amp with a 300Va / 60mA bias point and a 3K load, I believe it, but something is missing.  I've attached a load line, I am seeing at max something like 360Vpp into 3K, limited at the positive peaks, that's about 5.5W by my math.  What am I missing here?

[Paul Birkeland]

I went back over my notes and it looks like I was more like 330V/60mA.

[L0rdGwyn]

Hmmm still don't see 8W at that load line, but maybe it is the real world difference between the datasheet and live implementation?  You aren't the only person I have heard that from, I know of another builder who used a nearly identical bias point, 300V 60mA, and got 8W output.  Maybe there is some magic in that bias point.

[Paul Birkeland]

Also the curves are hand drawn off a "bogey" valve, so some variation isn't surprising. 

[L0rdGwyn]

Fair point, well over the next week or so, going to see if I can throw these 3.3K 60mA OPT in the prototype and see what kind of sound / power output we get!

[L0rdGwyn]

PB - I tried out my 3.3K transformers this morning.

I nixed my bias supply and just used 520K to ground, not ideal since the grid voltage isn't really adjustable, it is fixed by the gate-to-source voltage of my FET buffer, so -3.1V on the grid.

I was able to adjust my bias point to some degree using the trimpot on the Maida regulator I am using.  I dialed it in to a 335Va / 57mA / -3.1Vg bias point.

With this, I am getting 156V to 524V peak-to-peak swing, with the 801A hard clipping beyond 524V on the positive peaks.  This results in 16Vpp into 8ohms, about 4W out.  This is pretty darn consistent with the datasheet if I draw out the load line.

Ideally, I would get to a true 0V bias point and 320V with a 3K (rather than 3.3K) primary impedance, it is doable, have to put +3V on the gate of my FET.  At that bias, if the datasheet is to be believed - and so far has been consistent with my measurements - even assuming no copper losses, that makes 5.5W into a 3K load.  Even if you did something crazy and biased at +10V on the grid at 280V on the plate, that is 400Vpp into 3K, makes a 6.4W.

So, I am just not seeing how it is physically possible to get 8W out of an 801A A2 amp at any 0V bias point.  Am I missing something here?

I added additional NFB given the reduced gain needed from the driver stage, got the output impedance down to 2ohms.  FR of the LL1620 below, think I am hitting transformer limitations on the high end again.

Thanks for your thoughts.