6B4G - The Resurrection Shuffle
- IslandPink
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#121 Re: 6B4G - The Resurrection Shuffle
Takes 2 : No it does work as far as B2+ goes if you assume the cathode voltage is a bit approximate and take the anode / screen/ shunt contributions instead ; (ps. I forgot the screen/OA2 current goes through the cathode resistor, before - duh ) - anode resistor shows 35mA , sounds more like what I remembered. Add ~15mA for the OA2 shunt and some screen current , 45mA sounds right - for one channel.
"Once you find out ... the Circumstances ; then you can go out"
#122 Re: 6B4G - The Resurrection Shuffle
No it worked just fine. The amp doesn’t know what you have written on a bit of paper.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#123 Re: 6B4G - The Resurrection Shuffle
Neither do I half the time.
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#124 Re: 6B4G - The Resurrection Shuffle
Here's the layout to scale:
All this, for 4W
All this, for 4W
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#125 Re: 6B4G - The Resurrection Shuffle
Suppose id better build a chassis for it.......
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#126 Re: 6B4G - The Resurrection Shuffle
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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#127 Re: 6B4G - The Resurrection Shuffle
Been reading Merln Blencowe's preamp book and learning how to plot my own pentode curves for screen voltages different to the necessarily restricted range you get with data sheets, or alternatively how not to have to bother and educated guessing from the transfer curves instead.
Interesting for me because I can now understand how to specify your own gain requirements and plot curves that will fit those specs, rather than having to make do with what you're given. And you all know the problems/ mental blocks I have had in the past with gain
Interesting for me because I can now understand how to specify your own gain requirements and plot curves that will fit those specs, rather than having to make do with what you're given. And you all know the problems/ mental blocks I have had in the past with gain
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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#128 Re: 6B4G - The Resurrection Shuffle
So, exercising my newly learned skills, I worked out a scheme for the 12GN7A input stage for this amp. The original amplifier, despite sounding so good at Owston, all those years ago, did have a tendency to go hard at high volumes.
Looking at the operating conditions I had in the 2011 amp driver stage, I could see why; the bias was too cold.
So after much drawing, rejecting and drawing again, I settled on a gain of 120, which requires a load resistor of 3K3. With the HT I have, an appropriate loadline was drawn. To get the 0V grid line so the the loadline passes through the knee requires a screen voltage of 115V. Plotting a set of curves from that information gives the setup below:
Plenty of gain there and it should be good without a cathode bypass cap.
Distortion should be reasonable, with a small amount of second harmonic.
Looking at the operating conditions I had in the 2011 amp driver stage, I could see why; the bias was too cold.
So after much drawing, rejecting and drawing again, I settled on a gain of 120, which requires a load resistor of 3K3. With the HT I have, an appropriate loadline was drawn. To get the 0V grid line so the the loadline passes through the knee requires a screen voltage of 115V. Plotting a set of curves from that information gives the setup below:
Plenty of gain there and it should be good without a cathode bypass cap.
Distortion should be reasonable, with a small amount of second harmonic.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#129 Re: 6B4G - The Resurrection Shuffle
Remember with that much gain, its not going to take much to push the output valve into A2. It may not be the input valve causing what you heard.Looking at the operating conditions I had in the 2011 amp driver stage, I could see why; the bias was too cold.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#130 Re: 6B4G - The Resurrection Shuffle
So where was the operating point on the old driver ?
Looks good on the plot for this . Also you have Vg2/Va is approx. 66% , which is where Thorsten recommended and I have had plenty of success.
I would say you could go up a fair bit on the anode resistor, as I use 7.5k on the 4P1L and it drives the KR300B's very nicely ... but with 12GN7 you end up with silly amounts of gain if you go to high on the anode R.
Looks good on the plot for this . Also you have Vg2/Va is approx. 66% , which is where Thorsten recommended and I have had plenty of success.
I would say you could go up a fair bit on the anode resistor, as I use 7.5k on the 4P1L and it drives the KR300B's very nicely ... but with 12GN7 you end up with silly amounts of gain if you go to high on the anode R.
"Once you find out ... the Circumstances ; then you can go out"
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#131 Re: 6B4G - The Resurrection Shuffle
I'll have to be careful with the volume on this, but I've built so many amps with insufficient gain in the past, that I'd rather have too much and reduce, than not enough and weedy performance (yet again)
See below Mark:IslandPink wrote: ↑Tue Aug 29, 2017 6:08 pm So where was the operating point on the old driver ?
Looks good on the plot for this . Also you have Vg2/Va is approx. 66% , which is where Thorsten recommended and I have had plenty of success.
I would say you could go up a fair bit on the anode resistor, as I use 7.5k on the 4P1L and it drives the KR300B's very nicely ... but with 12GN7 you end up with silly amounts of gain if you go to high on the anode R.
As can be seen, the bias is far too cold.
I "got away with" the original amp by the looks of it.
The saying, it is hard to make a bad sounding 2A3 amp seems a sound proverb.
I wouldn't go by any of the currents on the original diagram; not on this evidence anyway. The bias voltage I had on the original amp corresponds, for the 150V screen I had, to the point I have shown on the mutual characteristics - unfortunately.
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#132 Re: 6B4G - The Resurrection Shuffle
OTOH the gm of these things is so high that the slightest difference in manufacturing tolerance could concievably put the actual characteristics completely out of kilter with the published averges. I don't know; clutching at straws maybe? It did have some kind of magic to its sound.
Anyway I know how and why with these things now, so let's see what happens with this new version.
Anyway I know how and why with these things now, so let's see what happens with this new version.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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#133 Re: 6B4G - The Resurrection Shuffle
Well if you put the same cathode resistors on, and the anode voltages are within a reasonable tolerance, then you should be fine.
"Once you find out ... the Circumstances ; then you can go out"
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#134 Re: 6B4G - The Resurrection Shuffle
I had made a slight error with the anode resistance on the vertical axis making the slopes a bit too steep, so below is the corrected plot:
Anode current is now slightly lower at the op point, but close enough. The truth will probably be somewhere in between.
Vg2/Va on this plot is even better at 65.71% so we are right in the Thorsten sweet spot ballpark there.
Anode current is now slightly lower at the op point, but close enough. The truth will probably be somewhere in between.
Vg2/Va on this plot is even better at 65.71% so we are right in the Thorsten sweet spot ballpark there.
Last edited by Cressy Snr on Wed Aug 30, 2017 2:39 pm, edited 2 times in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#135 Re: 6B4G - The Resurrection Shuffle
Well, if you come over some time to sort the transformer snubbing out, bring a couple of the valves and we can run them on the uTracer and get the actual curves.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.