New KT120 PP In The Style of the Leben CS1000P

[Cressy Snr]

I need a T shirt made that says, "Pentodes Rock"

[Cressy Snr]

Yikes!
I've got to be more careful with the volume. I was enjoying led Zep's "Moby Dick" at realistic levels, "because I can" when there was an almighty crack from both of my FF225WK wide range drivers, as Bonzo hit his kick drum dead centre of the soundstage.

Luckily it appears I haven't damaged the drivers. The useable dynamic envelope of this amplifier is wider than what I have been used to over the past eight years, so a bit of re-learning is in order I think.
The last time I had an amp of this power, I was running KEF Q55s at 88dB/W efficiency, not quarter wave speakers with a 95dB/W sensitivity.

Efficient, eight inch widebanders are not built for the kind of hammering this amp is capable of giving them.

[JamesD]

Its called running them in!

Get the drivers properly loosened up

[Paul Barker]

I love Moby Dick, actually the whole of Zep II

[Cressy Snr]

Running in?
Hearing the drivers bottom out was a brown trouser affair I don't want to repeat in a hurry.

Led Zeppelin, ZZ Top, Canned Heat, Deep Purple, Camel, Hatfield and the North, Stevie Ray Vaughan, Jimi Hendrix, Tony Joe White, Elvis Presley, Chuck Berry, Little Richard and The Rolling Stones have all been played and thoroughly enjoyed, this weekend.

I've not played this much rock'n roll in years

Push-pull pentodes seem to have a wonderful way with rock music, that's for sure. Other types fare quite nicely too. Large scale classical is also a good match providing care is taken to recognise the wide dynamic envelope and go with it, rather than turning up the wick too far on the quiet passages. Climaxes are way too loud if this regime is not adhered to.

There is a loss of refinement on loud orchestral crescendos; for example the massed choir, organ, brass and rolling timpani finale to Mahler 8 loses a bit of focus and becomes congested, but it could easily be down to the FF225WK widebanders suffering power compression, as I might have found their limits with this powerful amplifier.
Expecting an eight inch single driver and supertweeter to cope with the might and majesty of this gigantic symphony, at anything approaching realistic levels is perhaps simply asking too much. With this amp I'd probably need a pair of big Tannoys or Mr I's ESS for large scale classical at high levels.

Smaller scale chamber music is divine, as is choral and early music of the "ye consorte of viols and sackbutts" variety. Piano trios of both classical and jazz type are lovely, with the piano taking on a realistic size and perspective, due to the controlled deep bass performance.

What this amp absolutely does not have, is the airy sparkle or that special something that SE DHTs possess in spades. The sound is altogether darker, creamier, more muscular and soulful, with a nice, big soundstage to explore and listen right into the back corners of.

The refinement and sophistication of a good SET is completely absent; no use pretending otherwise and it is no doubt a trade-off too far for SET fans; but for me it works.

[Cressy Snr]

For those interested, here is the circuit:



As can be observed from the output stage currents, we have class AB1 operation. Heat at idle is manageable in a
domestic environment and the high energy storage power supply copes with the power demands, class AB
operation places on it.

What I would do differently if I were to build this amp again, would be to use a different mains transformer, to raise
HT to around 500V, with 250V on the screens, which you can do with no problem with class AB1 pentode operation; result, more clean power at higher levels than I can get at the moment, still with low heat and high efficiency.

Driver stage would be valve rectified and the screen supply would be an adjustable valve regulated affair. Fixed bias would also be incorporated, as it is better than cathode bias, when class AB is being used.

[Cressy Snr]

Made a few minor alterations this morning.
The changes have been added to the circuit diagram above.

1. I have added in the 100R screen stoppers, as these had been missed off the earlier schematic.

2.The screen volts have been increased to 214V using 2 x 0B2 in series.
This has raised the quiescent current to around 120mA per output pair, giving a tad more class A wiggle room.

I haven't auditioned it yet but I don't expect there will be much change at normal listening levels.

Valves are dissipating 21W each so the amp is running at a comfortable idling temperature.

[Mike H]

Great, except you've legended them as '100K CC'

[Cressy Snr]

Bloody hell!
Senility approaching fast.

Corrected

[Mike H]

Nah always takes another pair of eyes. At Maplin we followed the traditional publishing trend and had a permanent proof reader who read all our stuff and corrected it, also when it was being typeset (in house). It's the only way to be sure. Nowadays staff writers are expected to make do with spell checkers then chuck their electronic copy straight down to the print room.

Sorry that turned into a bit of a rant

Anyway I was going to say one of my back-burner projects is to do something like this, I recently got a PDF copy of the GEC book 'An Approach To Audio Amplifier Design', it features various balanced input drivers ideas, using centre-tapped IT's or chokes. Worth a play around with.

[Mike H]

Was there a new post here?

[Cressy Snr]

I downloaded the 1957 edition from Pete Milletts site.
Some interesting bits in there about push-pull amps.
I truly believe, that true balanced topology is the way to get the best sound out of a push-pull valve amplifier. It is great, if you have balanced XLR outputs on your DAC. If you need single-ended inputs, then transformer phase splitting is the next best; not as good, but the least compromised IME.

Balanced pp pentode operation for me, gives an honest sound; no frills, no magic, no romance, it just plays.

[Cressy Snr]

The final part of this amp, which is heavily based on Harvey Rosenberg's musings on push-pull pentode amplifiers fell into place this afternoon,
with the addition of the, "sweet" bit to his "Sweet Slam" aural matrix, as he called it.

I was determined, with this amplifier to follow as closely as possible, the Gizmo philosophy and with the installation
of a 40V toroidal transformer, followed by a PY500A, TV damper diode, in line with the output of the driver branch of the power supply,
the final realisation of his ideas was achieved.

The man was a genius!
With Lynn Olson on the push-pull DHT side and Gizmo on the re-imagination of the pentode amplifier,
we have two philosophies that complement each other very well. The elimination of the on-board phase splitter
by using true balanced operation and the use of plate to grid feedback is the icing on the proverbial.

This is an amplifier I am looking forward to bringing to Owston in a fortnight's time.
I have no qualms about playing it to you guys. None at all, and that's a first for me.

Here's the final PSU:

[Cressy Snr]

And here is the final glow pic:



Niiice

[IslandPink]

What does the PY500A do, Steve ?
Also, where is the 40V toroid ?
It's on my list of essentials at Owston to listen carefully to this amp !