My Cathode follower with active load.

[bikerhifinut]

Hi Bob,
How are you getting along with the ACF now it's been in a month or so?
I'm about to do a job on the big case and output transformers from pre3 in order to make my own take on pre3. I'm curious about how it sounds. I have a couple of toroid mains transformers and bits to make a PSU inside the larger case which will be my intention.
Andy.

[bob orbell]

Hi Andy, in a word, excellent, just as JB said, no hummer no bummer, very neutral sounding but with a bit more grunt low down, and when the music stops, total silence, I now have no wish to change owt. BOB

[bikerhifinut]

that was my finding too Bob.
I do think it was worth the dosh for the JB PCB as although I successfully knocked up a prototype "Birdsnest and tagboard" circuit, the PCB was a doddle to populate and solder up and worked first time no dramas. It's reassuring to know someone else has done the hard thinking on the earth returns! I also like the extra info he gives on wiring selector switches and Volume pots etc as although none of it was new to me, it would keep a novice well out of "Hum loop" trouble.
And for anyone else contemplating this experiment, can I reiterate that you don't need fancy "Boutique" valves. Any decent pair of ECC82 will work a treat so you can use cheap readily available varieties.

A.

[bob orbell]

I have to question your last comment about valve choice Andy, I set my circuit up to run 6DJ8 tubes ( ECC88 E88CC 6922 ), I am using Mullard E88CC gold pin, but out of interest I tried some Sovtek 6922, no adjustment to circuit required, and they sounded harsh in comparison to the Mullards, I remember my Pre II, when converted to CF and using a Sovtek 6922, also sounded harsh. BOB

[bikerhifinut]

Question away Bob.
I haven't used ECC88 and its variants so you have the edge on me there.
Be careful though with Russian "equivalents" as many are not identical valves but may be "Similar enough" to work in a circuit. I have to say I haven't bothered with the ECC88 on the basis that it's an expensive valve compared with the ECC82 and 81 types that most of my stuff uses.
I have tried a few different ECC82 types in that circuit and I'll accept that perhaps my ears aren't as perceptive as others, but I really couldn't hear that much if any difference using that valve type. I wouldn't expect to either as the point of broskies circuit is to use the double triode's non linearities against itself in order to correct any distortions caused by such non linearities.
I often think that what we perceive as one valves "superiority" over another is more a function of a circuit that fails to compensate for such things as a valve ageing or other factors that will alter the sound quality obtained.
This is of course only my opinion based on what I think I hear. So don't shoot me!

Andy.

[bikerhifinut]

Another thing springs to mind Bob.
You are using "gold pin" ECC88, I guess that means they are specially selected or even (if the older mullards) specially manufactured to close performance tolerances and matching between the triode halves. I think some of the older E88CC were like that too although now i feel the "special quality" claims are bandied around fairly randomly on newer production valves.
Anyway, I think this means your ECC88 are actually working bang on their design parameters, the resistor values are right on the money, broskie supplies exact values not just preferred values. Whereas the Russian valves may be a rebadged 6N1P or something and also perhaps not so well matched between triode sections.
Something to ponder on Bob and would explain the evidence of your ears?

As I remember it, the ECC88 was never designed to be an audio valve, it's intended home was in the front ends of VHF receivers. This would suggest matching of triode sections was fairly low down on the priorities although it was designed to be suitable for cascode use? And again, a valve with capabilities at VHF frequencies could well be oscillating up there and perhaps this is why your russian ones sound harsh?

I believe your findings Bob, and I hope my amateur attempts at explaining your Experience make some sort of sense?

Andy.

[bob orbell]

Yes, I think your thinking is correct Andy, the tubes I am using are new Mullard E88CC GP, supplied by a very kind member of this forum, they are old stock white print, I also fitted gold valve sockets at the build and used the recommended resistor values, I don't think that I will change anything now, it just sounds fab. BOB

[bikerhifinut]

And that's all that matters Bob. If it works for you then its right.
I am no fan of the ECC82 used as a straight preamplifier valve as i think it has a few shortcomings, but as a cathode follower, where it's working nigh on 100% neg feedback and combined with the extra correction from the broskie active load idea, I really like the basically neutral sound quality coupled with the bass heft, i assume from the valves ability to deliver a highish current from a low output Z. Not to be confused with bass boost, just nice solid bass extension giving added depth and spatial dimension to the sound. I guess that's what you are getting too bob.
I should add that I use very long (6metre) interconnects from pre amp to the mono power amps which feed the speakers via very short wires. Thus low output resistance and good drive are important for me.

Andy.

[bikerhifinut]

A thought just sprang to mind Bob.
You are using that board configured as a White CF? I see the latest design of it (mines an earlier one) allows you to configure the circuit as white CF rather than Aikido.
In that case my personal observations on the circuit being fairly valve brand "agnostic" would not apply to your particular circuit? And go some way to explaining why you hear distinct differences between brands of ECC88/6922?
I kept mine totally standard and tweaked the resistor values to be spot on in the important potential divider to the grid of the lower triode.
I'm currently using it with my pair of very cheap and cheerful Edicron branded ECC82 and I can't really say i hear a difference between them and a pair of Raytheon 5814 that I scored at a swapmeet. Which is what I want of course.

A.

[bob orbell]

Andy, three years, so sorry, how did I miss your post , no, I did not build it as White CF, it is the same as yours, ACF, in the book that JB supply's he makes a point of telling the builder the best option and ACF with an impeccable PSRR is the option he recommends. By the way, my PS-1 has arrived at Heathrow. Bob