CCS Loaded Parafeed Output - Design Considerations?

[Paul Birkeland]

I was only going to get something like 30dB PSRR from the EL83, so the performance hit was just too much.

How much did you calculate that you'd need?

30dB PSRR means 3% of power supply ripple is left, so a power supply with 2mV of ripple (noisy) becomes 60uV (inaudible).

[L0rdGwyn]

How much did you calculate that you'd need?

30dB PSRR means 3% of power supply ripple is left, so a power supply with 2mV of ripple (noisy) becomes 60uV (inaudible).

In my simulations, I think I was going to have around 6mV of ripple on the 45 B+, I just felt more comfortable with the FET option knowing it was going to be nuked into the noise floor.  Maybe some element of "fear of the unknown" as well.  I had a custom transformer wound for this amp, so it was one less heater winding too since it would have needed elevation.

Don't make me regret it, PB!  I have a quad of nice Valvo EL83 on hand, so maybe I will give it a try in a future amp.

And Derek - all of the caps were brand new and unused before the listening tests, other than the bit they were in the amp for the FR sweeps.

[L0rdGwyn]

I just wanted to thank PB and PJ for their input on the parafeed topology, this amplifier was completed in April.

Below is an FFT at 350mV into 300 ohms.  Not sure why I didn't measure at 1mW into 300ohms at the time, but this is what I have.  I think this is what I determined to be listening volume with my headphones.

Also the FR plot, I ended up using two 3.3uF Rike Audio S-Cap-2 aluminum foil PIO caps in parallel, after my experiments with the Solen caps, so 6.6uF total on the output.

I put some globe 45 tubes in this amplifier as well, they are very nice sounding tubes, I got them for a bargain on eBay since they were untested.  One had a filament to grid short, I whacked the tube a few times and broke the short, they test over 100% emission on my Jackson 648-R.

[Paul Birkeland]

Looks good!

[L0rdGwyn]

Thanks!  I'm very happy with it, I appreciate the help, every amp build is a learning experience.  I'm working on something truly insane now....

It is a three-stage all DHT class A2 hybrid amplifier.  Gain stage is a CCS loaded 841 (high mu cousin of the 801A) with filament bias, cap coupled to the second stage which is a transistor source follower buffer, which is direct coupled to the grid of the output stage, a fixed bias 801A with an A2 bias point (420Va, -20Vg, 40mA Ia) into a 11K:8ohm OPT.  The source follower drives the grid of the 801A far into A2 territory, the grid swings from around +50V to -100V for roughly 6W into 8ohms out of the 801A!  I just finished the prototype, sounds darn good too.

Anyway, thanks again.

[Paul Birkeland]

I thought it was a #45 amp?

[L0rdGwyn]

Oh yes the parafeed amplifier is a #45, this is a totally new design, sorry for the confusion.  I have the DIY bug, I probably should take a break.

[Paul Birkeland]

The 801 rewards a lot of effort!  I was wondering how those FR graphs could be from such a high impedance transformer.

[L0rdGwyn]

Wow, that looks incredible!  It is an A1 design with a choke input supply?

Yeah, not getting the same FR results with the 11K:8ohm traffo HA!  Bass extension is great but I do have some HF rolloff, -3dB at 16.5kHz, 20kHz is at -4.4dB.  Below is the FR at 1W into 8ohms.

Even with the HF rolloff, it sounds very good and the bass definition is fantastic.

[Paul Birkeland]

No, that's A2 biased (about 0V on the grid).   It is a choke input supply since the 801A doesn't need a whole ton of voltage to make a decent amount of power.  I ran it at 325V/60mA.

[L0rdGwyn]

Very nice, I bet it sounds great.  The 801A certainly does reward a lot of work, didn't realize how finicky a tube it is until I committed to using it.  Glad I did though, I have learned a ton working through the...peculiarities of an A2 biased amplifier.  My transistor buffer stage will hide inside the chassis, no one has to know I plan to trick some tube purists at future meets, certainly no one would put a transistor in the signal path of an all DHT amplifier?  Pure evil.

[Paul Birkeland]

The grid current demands of the 801A were modest enough in my design that I just used a 6S4 cathode follower.  It did require a negative voltage rail for the driver though.

[L0rdGwyn]

I was going to use a cathode follower at first, I had the 6BX7 in mind, but I had read of good results in A2 designs using source followers, so I decided to give it a try and I am happy with the results so far.  Going to try a few different chips and see how it affects performance.  I'll probably try a cathode follower A2 design in the future if the mood strikes again, but this is a ton of work.

[L0rdGwyn]

I'm curious PB what primary impedance you used with 0Vg bias point?

[Paul Birkeland]

3K.