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Old 17th January 2013, 10:07 PM   #21
rrrs is offline rrrs  United States
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Hi, thanks a lot to all for comments, really appreciate this feedback!

I will try to answer/clarify some of the points that came up in the posts:

At the moment I got a cathode resistor bias on 2C51, bypassed with 220uF, electrolytic. On 5687 I currently got 6.4V battery bias in line with the grid, bypassed 0.1uF film cap.
In general, 5687 driver sounds better, especially at higher volume, but there is something nice with 2C51 at low level; it sort of brings voice a bit forward…
For that reason I wanted to eliminate cathode resistor/cap from 2C51 and see if it gets better.
Cathode resistor bypassed with film cap to B+, as suggested by Thomas would certainly be good option, but would require extra HV film capacitor of 5-10uF.
Especially, this would be problematic with my output tube as large, 1000V+ rated cap would be necessary.
Since I already got negative supply and am using input transformer I considered fixed bias as a better option.

All the capacitor in my B+ are Solen film type, so once cathode is grounded there is just a IT primary, valve and B+ film cap in signal path; or I might be missing something here?

5687 version sounds really good now, but with the current battery bias I do not have needed flexibility to adjust right plate voltage; now with 6.4V bias I got approximately 20mA current at 175V on the plate.
My IT is gapped for 18mA, so 20mA is pushing it a bit; switching to adjustable bias should enable me to dial up operating point to 17-18mA at 190-200V on the plate which should be better.

So, based on all the feedback I think I will settle to fixed (but potentiometer adjustable) bias, as in my first schematics with clear advantage over battery version of being adjustable as well as not having to worry about battery life.
The question that remains is: are those negative supply bypass caps on the driver necessary and, if yes, how large they need to be (on my schematics they are marked as 0.22uF)?
As mentioned before I did test with and without 211 negative supply bypass cap on IT secondary and got very interesting results; see attached below.

Interesting is that I tested 0.47uf, 1uF and 50uF cap in this position and could not see any difference between them, but as soon as I would remove it visible distortion on the 211 grid would happen as soon as Vpp came close to 2xVbias and 211 would start to draw some grid current.
Also, you can see very strange frequency response with this cap removed; this measurement is taken at 1W output.
Attached Images
File Type: jpg With bias cap.jpg (52.6 KB, 154 views)
File Type: jpg No bias cap.jpg (52.6 KB, 149 views)
File Type: jpg 118Vpp on grid with cap .JPG (308.8 KB, 150 views)
File Type: jpg 100Vpp on gridd-no cap.JPG (317.9 KB, 150 views)
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Old 18th January 2013, 05:57 AM   #22
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Hi!

Quote:
Originally Posted by tomchr View Post
Oh. I guess I'm not up on the latest GeekSpeek.
The term 'ultrapath' was inroduced by Jack Elliano about 15 years ago. As far as I'm aware the first time it was mentioned was in the Vacuum Tube Valley magazine.

The concept goes back to what Western Electric did in some of their amps in the 1930ies.

Check Lynn Olsons site, for example this article:

Western Electric - Rosetta Stone for Triodes

Or his presentation about current loops which he gave at the ETF 2004:

ETF Presentation

Nothing really fancy about this. A simple concept which works very well for me. There are different ways to do it. Pretty much every circuit which is published on my log uses this concept.

Best rgeards

Thomas
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Old 18th January 2013, 08:43 PM   #23
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Vinylsavor View Post
The concept goes back to what Western Electric did in some of their amps in the 1930ies.
I know. I've read the original article by Loftin & White. I believe they were the ones who came up with it.

~Tom
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Old 18th January 2013, 10:05 PM   #24
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Hi!

The Loftin White circuit is a DC coupled amplifier. While this might also have a cap from B+ to cathode, the oveall concept is a bit different

Thomas
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Old 18th January 2013, 11:07 PM   #25
rrrs is offline rrrs  United States
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Hi, I am also thinking about possibility to “step-up” and use DHT as a driver.
Would need to add DC power supply, bat that should be doable, especially if I use 801A as Thomas recommended in other thread, I could probably add another CRC filter to my 211 fillament supply and use R in the filter to drop from 10 to 7.5V; my transformer for enough reserve to handle additional 1.25A current.

Thomas, I understand you used 801A with IT coupled to 211, but I am wondering a bit about right operating point. As 801A got relatively high R-plate (around 4.5K) it would need high IT inductance. My IT has around 100H inductance when set as 3.5:4, but can take max. 21mA in that configuration.
It looks like 801A should be run at higher current, around 30mA.
I can wire my IT as 2:3.5 where it can take 35mA current, but offers only 35H.
What is the operating point/IT configuration that worked well in your 801/211 amp?

Was also considering 1608 tube which should have very similar plate resistance as 801A, but mu of 20.
Does someone have experience with 1608?
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Old 19th January 2013, 07:28 AM   #26
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Hi!

Don't use a step up IT ith the 801A, drive impedance for the 211 would become too high.
I used it with a Tango NC20 run at 600V plate voltage and about 20mA. I used it wiht a step up transformer at the input to get enough gain.

The 801 will need a very clean filament supply to sound good. I used LCL passive filtering for each filament. Alternativey you can use Rods regs, ideally fed from a choke input filtr supply.

1608 is meant for use in Class B. The curves for class A a probably not very linear (but I have not looked at them yet)

Best regards

Thomas
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Old 20th January 2013, 04:07 AM   #27
rrrs is offline rrrs  United States
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Hi, thanks a lot for the tip; might try that down the road...
As I need a minimum gain of about 20 before the 211 grid I would really need more gain than my input transformer offers (it is 1:2 ratio).
Other way I might be able to do it would be using 46; with slightly less mu but much lower R-plate I could possibly wire IT as 2:3.5 and, together with 1:2 input transformer get to just 20. For my application max. plate voltage of 250V should be fine. As well, prices and availability of 46 looks really good while 801 and 10Y seem to be much more expensive.
For sure, 1608 with mu of 20 would be really interesting, but could not find any info on its linearity in class A...

All the best, Robert
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Old 20th January 2013, 09:47 AM   #28
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Hi!

The primary impedance of the IT might get a bit low for the 46 in 2:3,5

The gain of your proposed solution is a bit less then 20. With the 801 and 1:2 input transformer it is 16. That is around 2dB difference, barely noticable.

Best regards

Thomas
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Last edited by Vinylsavor; 20th January 2013 at 09:57 AM.
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