6B4G - The Resurrection Shuffle

What people are working on at the moment
User avatar
IslandPink
Amstrad Tower of Power
Posts: 10041
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#121 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

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"
User avatar
Nick
Site Admin
Posts: 15709
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#122 Re: 6B4G - The Resurrection Shuffle

Post by Nick »

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.
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#123 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Nick wrote: Fri Aug 25, 2017 6:25 pm The amp doesn’t know what you have written on a bit of paper.
Neither do I half the time. :lol: :lol:
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.

#124 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Here's the layout to scale:
Image

All this, for 4W :)
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Ant
Shed dweller
Posts: 2332
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

#125 Re: 6B4G - The Resurrection Shuffle

Post by Ant »

Suppose id better build a chassis for it.......
Also starring Rex Hamilton as Abraham Lincoln

www.bte-designs.weebly.com
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#126 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Ant wrote: Mon Aug 28, 2017 3:56 pm Suppose id better build a chassis for it.......
Yep.
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.

#127 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

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 :mrgreen:
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.

#128 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

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:

Image
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.
User avatar
Nick
Site Admin
Posts: 15709
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#129 Re: 6B4G - The Resurrection Shuffle

Post by Nick »

Looking at the operating conditions I had in the 2011 amp driver stage, I could see why; the bias was too cold.
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.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
IslandPink
Amstrad Tower of Power
Posts: 10041
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#130 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

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.
"Once you find out ... the Circumstances ; then you can go out"
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#131 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

Nick wrote: Tue Aug 29, 2017 6:04 pm
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.
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)
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.
See below Mark:
Image
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.
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.

#132 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

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.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
User avatar
IslandPink
Amstrad Tower of Power
Posts: 10041
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#133 Re: 6B4G - The Resurrection Shuffle

Post by IslandPink »

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"
Cressy Snr
Amstrad Tower of Power
Posts: 10552
Joined: Wed May 30, 2007 12:25 am
Location: South Yorks.

#134 Re: 6B4G - The Resurrection Shuffle

Post by Cressy Snr »

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:

Image

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.
User avatar
Nick
Site Admin
Posts: 15709
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#135 Re: 6B4G - The Resurrection Shuffle

Post by Nick »

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.
Post Reply