Future plan for 300B XLS amp

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Ali Tait
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#181

Post by Ali Tait »

I've a selection of 6n6p I could bring for you to try if you like Mark?
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andrew Ivimey
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#182

Post by andrew Ivimey »

who hasn't ! and some 6N6Ps I have some where that were advertised as being special for some reason ... then there are ECC99s. I could dig out some of those too ;-)
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Paul Barker
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#183

Post by Paul Barker »

I don't have any :(
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IslandPink
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#184

Post by IslandPink »

I might buy a few old-ish 6N6P's for the phono , yes. Currently I'm not using any, anywhere in my amp or phono, but the LCR phono will use some.
ECC99's I used in the Aurora and didn't like - moved to 5687, then 6N6P .

On the 4P1L's, just tried another one in the left channel . It started ringing at roughly the same frequency as the other one, after about 10 minutes !
Then I got out another one, put that in, and it has been fine over about an hour of playing . Fingers crossed !
With this one in, it's sounding really :D nice, should be good at the meeting. I have a couple of demo records in mind to bring, now .

I have a load of Dave Elson's valves I should bag-up and bring to Owston . I have been secretly getting a few for myself , Nick , Len etc, but there are lots left .
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andrew Ivimey
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#185

Post by andrew Ivimey »

sfunny, I didn't like ECC99s (-> Amity) and concluded that perhaps I wasn't doing them right. When Max waltzed along with his own version of a balanced push pull amp (300b) being driven by 5687s I went out and bought some!

Relatively imho 6N6Ps sound er, 'dark' but I am halfway through a build to use them to drive GM70SE.
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
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IslandPink
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#186

Post by IslandPink »

I tend to think that the 'good' 6N6P's are 'not bright' , ie. correct !
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We'd better not have our amps too close together, yours will detect all the RF in my power supply!
I'll bring a roll of mu-metal, and build a wall around my amp, just in case .
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#187

Post by IslandPink »

Still messing about with filament supplies here.
Gone back to the V-reg supplies for the 4P1L's after using the Rod Coleman ones for a while.
The 'floaty' spacious feel, realism, and great upper midrange are there again , which none of the current-reg supplies could quite give. I think it's a lot to do with the elusive 'low-level detail' .

I also tried ( along the way ) and reported to the guys ( on e-mail ) that if I swapped out the Guido Tent current-reg supply for the 300B in favour of a Thorsten Loesch ( design) passive supply, as per the 'WE91B' amp and 'Ladyday' mods ; basically Rect>R>C>L>C>CMC ; then I got more 'space' and realism, at the expense of less focus and extension in the bass. Similar to the V-reg vs. I-reg supplies applied to the 4P1L really .

So, there are more options to try, which we have been discussing . The key one is to try the V-reg supply and take the cathode connection off a virtual centre-tap , NOT one end of the filament supply . I will do that one tomorrow or Wednesday .

This seems to be really crucial, the difference between getting it right and not right is really significant , and not always apparent immediately .
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IslandPink
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#188

Post by IslandPink »

Just done a modification ( voltage divider off B+ ) to provide a small adjustable voltage to the suppressors ( G3 ) on the 4P1L valves, to evaluate the sound of moving G3 up slightly :
http://www.audio-talk.co.uk/phpBB2/view ... 3&start=45

Unfortunately after laboriously connecting up everything and switching on , I have no glow on the right-channel VR tube. Something must have gone wrong in the rat's nest once I put the driver board right side up and connected all the wires.
I will say the good (left) channel sounded a bit sweeter on the one track I played , but it's early days, and I've got to go under the hood again later this afternoon . Damn !
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JamesD
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#189

Post by JamesD »

Damn! Good luck sorting it out!

really looking forward to what you find after a proper listening session!

J
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IslandPink
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#190

Post by IslandPink »

Ah, I saved myself some work by swapping another OA2 into that socket - bingo, it lit :D Whether it's a failing OA2 or a difference in how thing 'strikes' caused somehow by the G3 being altered, I'm not sure.

Anyway , have done some listening . I would say there's a small but useful improvement in tone on vocals and some instruments. There might be a slight loss of detail too - maybe that is also a result of taking out a little 3rd-harmonic . On the other hand , there was a slight loss of detail when I did the virtual centre-tap mod on the filament supply for the left channel , so maybe I'm still hearing that .

I need to do more listening obviously . Things are becoming a bit confusing at times, and it hasn't helped that there's an earthing problem ( probably crappy RCA sockets on the input ) from the phono causing intermittent hum .

Currently I should have about G3 at +10 or +11V above cathode.
I will check this, then try +15V too .
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Paul Barker
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#191

Post by Paul Barker »

You are of course playing with the Timbre in the driver by changing the distortion profile which means best looking curves may not sound best.
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IslandPink
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#192

Post by IslandPink »

Well yes, but from the LTSpice sims, I think the main thing you get from the soft bend is a bit of third harmonic. From the basic pentode curves ( straight, but bunching-up towards the bottom ) you get 2nd-harmonic, which I'm less bothered about, and in fact I believe you need some of that in an amp .

What the G3 mod should do is allow me to more or less tune out the H3 without much affecting the H2 .
All I'm doing effectively is straightening-out the pentode curves of the 4P1L a bit, so it looks more like a C3m, or a 2E24 ( which I haven't tried ) . I felt I'd lost a bit tonally in going from the C3m to the 4P1L, which I didn't initially expect . I think this may be the explanation now.

Anyway, just been measuring the G3, and despite my simple-minded voltage divider calc ( to give about 20V on G3 ) I'm actually testing about 11V , with 9V on the cathode . No wonder the effect was subtle !
So, I'm guessing there must be appreciable current out of the G3 which is affecting the DC bias point .
I just twiddled the carbon pot on there a fair bit, to get me a higher voltage, but I've only got to about 12V now, ie. +3V rel to cathode.

It does sound very nice though - worth pursuing .

What I need to do next is increase the flow through this divider a bit . I've only got ~1mA through it ( & supplying 2x G3's ) so I probably need to get some lower resistors ( power ones , for the voltage ) . I currently have 2x 200K , then a 50K pot set to ~20K . Need a pair of 100K's or lower . Another rummage in the box then .
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IslandPink
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#193

Post by IslandPink »

Actually, thinking about this more ; it's an ideal application for a small 9V battery to supply G3 relative to cathode. No current drawn when it's off.

The voltage divider at present is non-ideal in several ways -
1. Not the right voltage ( yet )
2. Not 'stiff' enough
3. Referenced to ground, not cathode.
In fact at present with G3 3V over the cathode at nominal, the cathode may go above G3 at times . Certainly the potential between k and G3 is very variable. I think this explains some loss of dynamics and bass focus I can hear as well .
However, it's worth the effort as the tone is very nice.

I will have a look on RS for small 9V batteries, need a few odds and ends currently .... or Rapid .
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JamesD
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#194

Post by JamesD »

Agree that G3 needs to be referenced to the cathode - I've been looking for typical G3 currents when held positive to the cathode but I haven't found anything yet. Certainly some current must flow but it would be nice to know if its 1mA or 10mA or more!!!

J
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#195

Post by JamesD »

Okay,G3 will draw little current at these sorts of voltage - typically less than 1mA.

Besides straightening the anode curves it also ups the valves transconductance but typically by 10-15%..

And it reduces g2 current draw - not sure by how much but significantly enough if using a resistor divider on g2 to move the op point - should be OK with the VR setup on g2...

Its also been used on the SV83 and by Decware on the EL34...

J
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