New KT120 PP In The Style of the Leben CS1000P

What people are working on at the moment
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#136

Post by Cressy Snr »

pre65 wrote:
Paul Barker wrote:I refuse to get involved. Sorry I mentioned it. :lol:
:lol: :lol:

I'd give it a try Steve.
Oh..I've done much better than that :wink:

Image

Image

A little Christmas Day diversion whilst the rest of 'em watched "Toy Story 3".
Fitted a set of 5K a-a, output transformers, courtesy of our Martin.

Leben, with a bit of classic Mc'Intosh thrown in to make things interesting :D :D :D :D
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#137

Post by Cressy Snr »

Talking of CCS (no not the early 70s Alexis Korner project)

Did you do the Lazy Man LM371 one Phil?

I have a few LM317s in stock. Probably as noisy as fook on an input stage, but maybe worth trying.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#138

Post by pre65 »

I use LM317 on the cathodes of my 832a PP amp, and LM317T (the higher powered version) on the cathodes of a KT66 PP amp I tried some time ago.

I use the Ixys 10M45s as a CCS device on the anode of the 12HG7 used to drive a GK-71 on my current project. This was instead of an anode choke as I had not got any to hand at the time, seems to sound OK. :)

http://ixdev.ixys.com/DataSheet/98704.pdf
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#139

Post by Cressy Snr »

Well it all started with the fact that with the triode-strapped EF184 front end, I still had a bit too much gain, despite the increase in the feedback levels.

You know how these things snowball, well this did, but in a good way; eventually.

OK...below is the loadline I decided on, in order to drop the gain of the stage.

Image

Looking at Tom Schlangen's 30K loadline, it gives a gain of around 40 before feedback.
The red loadline is the one I decided to use. At 12K It produced marginally more 2nd harmonic,
but this would be cancelled by the balanced operation; so far so good.
My loadline gave a gain of around 30.

To get roughly the same dissipation, the HT to that stage needed lowering. I know! What about an inline gas tube to do the job? Yeah that'll do nicely.

So a 7 pin socket was installed in line with the HT to the driver stage.
An 0B2 was plugged in and the amp was fired up.

Things then got quite interesting; no bangs or anything like that, but nevertheless, something puzzling did happen. And it is essential to the plot, that I chose the gas tube rather than a dropper resistor. :)
Last edited by Cressy Snr on Sun Dec 29, 2013 8:32 am, edited 1 time in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
little eddy
Old Hand
Posts: 693
Joined: Sun Nov 09, 2008 2:06 pm
Location: Manchester

#140

Post by little eddy »

pre65 wrote:I use the Ixys 10M45s as a CCS device on the anode of the 12HG7 used to drive a GK-71 on my current project. This was instead of an anode choke as I had not got any to hand at the time, seems to sound OK.
Does anyone know the max power dissipation of the IXYS10M45 without a heat sink?
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
User avatar
pre65
Amstrad Tower of Power
Posts: 21373
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#141

Post by pre65 »

little eddy wrote:
pre65 wrote:I use the Ixys 10M45s as a CCS device on the anode of the 12HG7 used to drive a GK-71 on my current project. This was instead of an anode choke as I had not got any to hand at the time, seems to sound OK.
Does anyone know the max power dissipation of the IXYS10M45 without a heat sink?
Not a lot I would think. :(

At 35ma on my GK-71 amp quite a big heat sink is necessary.
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#142

Post by Cressy Snr »

SteveTheShadow wrote:And it is essential to the plot, that I chose the gas tube rather than a dropper resistor. :)
Right! Where were we?

Ah yes; the gas tube. Well therein lies a tale.

So I fired up the amp. Now remember, the input stage was fed from a soft start mechanism,
courtesy of a 6CJ3 damper diode, before the CLC filter, leading to the tops of the EF184 anode resistors.

The output stage screen supply gas reg shunts usually strike immediately because the output stage power supply is fed
straight off the solid state rectifier bridge... and so they did. However the inline gas tube feeding the input stage,
also struck instantly; which was strange to me, since the soft start diode had not even had time to warm up.
The reason why was hidden in plain sight, but I will get to that soon.

Well of course, you can imagine the head-scratching and chin-rubbing that ensued,
because there's more. During the warm-up period, the input stage gas tube went out,
then after another 20 seconds, it struck again, glowing steadily from then on. The amp played fine.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#143

Post by Cressy Snr »

Now for the life of me, I couldn't figure out what was going on.
Clearly there was current flowing around the input stage from somewhere causing the reg to strike.
Then there was no current, then there was.

First there is a mountain,
Then there is no mountain,
Then there is


I don't know, that snippet from Donovan seemed appropriate :)
But on with the tale.

I was still perplexed after an hour, so I took the dog out and had a think.
Eureka! it was the feedback!
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#144

Post by Cressy Snr »

So on power up, the output stage HT appeared instantly, which meant that the output valve anodes became live straight away.
The connection between the output stage anodes to the input stage anodes, provided a path to ground, via the
feedback resistor, the input stage plate resistors and the input PSU cap bleed resistors, all of which, set up an opposing current, backwards through the inline gas tube.
As we know, gas tubes have a preferred way round to connect them, but they are not strictly polar devices,
so a reverse current through them will strike them, provide there is sufficient emf behind the current.

Then as the input stage comes up, current starts to flow through the input valves causing an equilibrium
and no net current flow for a while, which extinguishes the regulator. Then as the current and voltages to the input stage ramp up further, current once more flows through the gas tube, but this time, in the right direction, striking the tube again.

Well I think that's what happens, because I'm buggered if I can think of anything else.
Last edited by Cressy Snr on Sun Dec 29, 2013 8:38 pm, edited 6 times in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#145

Post by Cressy Snr »

OK, so that puzzle solved, but now we have a bigger problem.
A problem which calls into question the twin power supply thing.

You see operating plate-to-grid-feedback, by anode to anode connection via a feedback resistor,
allows each separate power supply to talk to the other one rather too easily.
This is about as much use as a chocolate fireguard and has caused me to have to have a major rethink
about the wisdom of separate power supplies in a Schade type pentode amp setup.

Triode connection of the output stage, zero feedback, no probs with twin PSUs as there is no easy mechanism for them to talk to each other.
Pentode or Ultralinear with GNFb from the OPT secondary...again no problem, but plate to grid feedback? .....
Ah!.... Now we have an issue...at least I think we do (my brain hurts)
Last edited by Cressy Snr on Sun Dec 29, 2013 1:03 pm, edited 4 times in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#146

Post by Cressy Snr »

So.. the upshot is that, yesterday evening I tore down the entire power supply setup and rejigged the whole thing.
It's lucky I made the interior of the amp so roomy.

Below is the power supply after rebuilding it from the ground up.

Image

The 6CJ3 gives the soft-start and tube rectifier sound to the input stage, whilst the 75C1 gas tube drops the HT voltage
sufficiently to accommodate the revised load resistor values.

Why was the gas tube significant?
Because if I had simply chosen a dropper resistor to give the correct input stage HT voltage,
I would never have discovered the folly of using twin power supplies in a Schade type amplifier..

Surely there is a providence which looks after the non-engineers among us. 8)

Anyway here she is playing away,
Image
no renegade striking of the voltage dropper, until it is supposed to, relaxed and silky sounding.
Once more sporting EL34s, with Martin's WAD 300B OPTs at the rear.
Last edited by Cressy Snr on Sun Dec 29, 2013 12:04 pm, edited 4 times in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#147

Post by Cressy Snr »

And finally Cyril?
And finally Esther....

Here is the graph showing how the output stage is working at the moment.

Image

Not bad at all. Clearly, the 5K output transformers suit the EL34/KT120 family really well.

Phew...(mops brow, then dons dunce's cap and sits in the corner sucking thumb)
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8874
Joined: Mon May 21, 2007 9:42 pm

#148

Post by Paul Barker »

It is a strange time in the forum where power supplies are concerned.

Some are finding excellence in their amps from refined excellent regulators (definitely not referring here to simple solid state regs made capable of valve voltages by adding a valve to the circuit. or simple 3 legged solid state regs with a built in voltage capable mosfet, so that we can use them on B+) is the ultimate in perfection.

Some are saying because they have so many chokes, (and they have a power supply per element) their amp is now the best it has ever been.

And now, if I read it right, you are saying, your VR tube has taught you that you don't need monoblock power supplies.

I can't be the only one who has noticed the many directions power supply discussions are going in is opposed to your direction.

It is may be not possible these days to use the old adage that it was better to make one power supply out of really good components, than to make two power supplies out of lesser components. I am aluding here to Black Gates. those of us who went that way could barely afford Black Gates for a single power supply.

But now that the majority of people involved in these thoughts can't get or never had Black Gates, the common opinion seems to be migrating toward separate supplies.

But I thank you for sticking your neck out. No argument from me.

I could go on endlessly about power supplies, but I think we are all learning at our own pace what is appropriate for what we are doing, with the materials which we can afford.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#149

Post by Cressy Snr »

Hi Paul,
Yes, in the direction I am going in at the moment, which is towards, push-pull pentodes with partial feedback, the single power supply solution seems to be the way towards maximum sound quality from the topology.

If you had asked me two days ago I would have still raved about twin power supplies, but the results I get with the single PSU are better than with dual power supplies......only with this particular circuit topology, I have to emphasise that.

With hindsight, the mistake I made was generalising from the particular.
My first foray into dual power supplies was with the Harvey Rosenberg inspired, twin PSU, triode strapped EL84 and 6AQ5 amps, which then went Ultralinear; this then led to more powerful, triode-strapped KT120s.

It was when I went pentode with partial feedback that the wheels came off.
I found as explained at length earlier, that you can't, (if you are using plate to grid feedback, anode follower, Schade, whatever you want to call it topology on your output stage) use multiple power supplies, because the necessary plate to plate connection, firmly connects the two supplies together, destroying any benefits you might have gained, and possibly causing who knows what other hidden problems.

You're right though, I seem to have found myself going in a direction, that is diametrically opposite to everyone else. Oh well, never mind, You have to go with what floats your boat. Pentode push pull amps seem to do it for me. I can't help but love the sound they make :)
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
Paul Barker
Social Sevices have been notified
Posts: 8874
Joined: Mon May 21, 2007 9:42 pm

#150

Post by Paul Barker »

Well I am out in the cold with you having one power supply bravely soldiering on. :D
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
Post Reply