Partial Choke Loading of Cathode Follower

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Paul Barker
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Partial Choke Loading of Cathode Follower

Post by Paul Barker »

Anyone remember the gm70 design in Glass Audio a few years ago? They had a White Cathode follower with a choke between both triodes. They preferred the sound with the choke in situ.

I was just wondering whether the choke has to be sized for all frequencies or whether 100hz upwards would do. i.e. is there a frequency which the choke benefits most. I am in the hope that with a fet ccs, like the cap on the poor 1:1 transformer, this choke helps the most audible range making it sound more like choke loading than fet loading, has anyone experimented with this?

Paul
JeffreyJ
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choke and ccs loading...

Post by JeffreyJ »

Greets, Paul... I love following your 212 explorations.. my amp sizes are growing.. we'll see of I get to 20 watts..

anyway, I have experiemented with CCS and choke loading, but is wasn't on the cathode.. it was on the anode.. I must admit that when a CCS plate load was immediately followed by a choke load it sounded so close to a pure ccs load that I couldn't reliably tell which was which.. when I added a small decoupling cap between them, it gave a small flavor of the choke loaded sound.. slightly smoother than the ccs alone, but still very dynamic.. I think I used one of those russian teflons, but memory is faded..

it did sound good enough that I would recommend trying it in your application.. I am thinking of using it again in a phono stage.. I really like teh thought of isolating the power supply as much as possible.. chokes in the ground leg as well as positive, a ccs before a choke, etc..

Peace,
Jeffrey
Paul Barker
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Post by Paul Barker »

Thanks Jeffrey,

In the event the fets blew and I didn't care enough about it to trouble shoot that one. I take it you prefer the sound with a russian teflon just decoupling the higher frequencies then. That would work for me I suppose.

I don't think I have an application any more for what I was up to when I posted the question.

Presently building a 112a IT to 112a IT to PX25. Well see how that sounds before I put up a schematic. I have some 01a's on order possibly to replace the first pair of 112a's, but the base roll off of the IT's would then be a problem at that stage. I shal;l still try it because it's no good just building amps on paper, it may sound better, so it has to be tried.

Will post results as and when.

Paul
JeffreyJ
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112A/01A

Post by JeffreyJ »

Hey Paul... shelving the 212 for the moment while playing with PX25's?

I built a linestage with the 112A that could directly drop in the 01A. I really like the sound of the 112A.. I used filament bias - highly recommended.

Have yopu ever played with the PX-4? I ran into a pair of Marconi's and I am maybe 75% done with a 417A > PX-4, but I got derailed. Customers pay the rent.. so you have to get them taken care of..

Peace,
Me
Paul Barker
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Post by Paul Barker »

Well the px25 is just a convenient valve that suits many occasions, I'm always playing with it. I use the px4 regularly it's more middy than the px25, the px25 appears more right. Unlike me many people prefer the sound of the px4 to the px25.

I'm not one fopr two stage amps, don't like high transconductance valves, very fast and seeming to do the job of two valves, but there is a trade off. A mix of medium and low transconductance valves works better for me.

Yes I am planning to use filament bias. One resistor shared across all four 112a's and to carry all their filament current. Something like 8.5 ohm is planned. As the filaments are in parallel I can sub in the 01a at any or both stages increasing resistor accordingly.

The 212 sorts itself out if you get the rest right, but one false move alsewhere and the 212 sounds bad.
Paul Barker
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Post by Paul Barker »

To catch up on this the amp described above worked well but was handicapped by something which I just didn't isolate at the time. Now 6 years later while building a cheap but excessive 6em7 botanchor

Image
Image

I discovere the problem. I had trusted the Soweter transformer attenuators and not eliminted them in my enquiries back then.

It just shows you cannot trust anything nomadder what your prejudices and opinions. everything has to be swapped out with an alternative when tracing a poor sound.

Presently just using the trusty Panasonic Pot.

At the moment I am just about having fun with cheap components.

But in combination with Nick Gorham we are hoping to present something which we expect to work. The reason we anticipate success is because of nick's driver design. I am coming to the party with the HT power supply and the output stage. Which is quite a lot to achieve in itself. As to what it is will be posted after the events of 18th June.
nickg
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Post by nickg »

Though in the interest of accreditation, its far from my design, loads of prior art on it :-)
Paul Barker
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Post by Paul Barker »

In the event it powered the 833a's grid perfectly. We watched the drive signal on a scope while it was playing and saw no flattening of the positive peaks whatsoever, there was an equal signwave positive and negative.

The 833a sounded better each time we raised the current. We finished up at Nick's power supplies design max which was 1,460v 160mA.

loadline was 20k with a likely power output from my rough sketching of loadline 74 Watts.

We stayed below the conservative max dissipation of 300 Watts but towards the voltage end of the loadline it tracked our loadline seperated only by 20mA.

With a steaper 10k load the two lines would have kept further apart, and of course more power would have been achieved.

My aim achievement was 1250v at 175 mA into a 10 k load to theoretically provide 98 watts at 2nd harmonic of 6.3% (roughly speaking) and I did keep egging Nick on for us to crank up the current but he was less enthusiastic about demanding more from his power supply that designed max, and anyway at 160 mA everything sounded great.

The greatest difference between 160mA and lower currents was the freedom of the music envelope, the lower the current the smaller the envelope of sound the higher, the less bound the envelope. More speed and clarity.

We were both very pleased.

Not necessarily something you would want in your living room though!
Paul Barker
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Post by Paul Barker »

nickg wrote:Though in the interest of accreditation, its far from my design, loads of prior art on it :-)


Ah but there is nothing new under the sun , like everyone's so called current inventions glancing through old books and articles one usually finds their source from 50 years ago which they haven't often accredited.

But as you say you have accredited this to an article from the 1950's. But you were the man who grasped the nettle and built it, and we have enjoyed the fruits of your labour. I shall certainly be building one.
Paul Barker
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Post by Paul Barker »

What the anodes look like when they almost kiss the 300 watt max dis curve Image
The drive
Image
nickg
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Post by nickg »

This is the source of the idea behind the driver stage

http://www.jrossmacdonald.com/jrmpubs/0 ... llower.pdf

In the scope picture, the center line of the display as the 0v line, idle bias was about -7v, I think we were looking at 60v P/P, going 25v into A2, the limit on volume was the loudspeaker at that point. I think there was another +20v in the driver into the grid.
Paul Barker
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Post by Paul Barker »

It certainly worked a lot better than an EL34 CF.
IslandPink
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Post by IslandPink »

There you are chaps !
I do wish I'd been there , the pics look great .
Been having a look at McDonald's website today - what a talented guy .
Paul Barker
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Post by Paul Barker »

hi
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