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Old 3rd August 2012, 02:37 PM   #1941
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Here's a rough sketch of the design I was using. Andy
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Old 3rd August 2012, 04:01 PM   #1942
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Quote:
Originally Posted by merlin el mago View Post
Hi Rod, what schematic do you use?
Hi Felipe, my personal taste is to drive cables strongly (unless they have very low capacitance).

So I like high quality transformer output, or low impedance drive circuit.

I like filament bias best - nothing is worse than electrolytics in the cathode circuit! But, the filament supply must be highest quality for this architecture.

Here's another schematic - this time the low-impedance - but low-cost solution!
This is the version of filament bias we developed, using a IDHT cathode follower - with its heater soaking up the filament bias heat - and saving the trouble of an extra heater supply!

The 6H6 has a 6.3V 750mA heater, and 2 in parallel give 22mA/V, which translates into a cable-drive impedance of 45 ohms !! Enough to drive the worst kind of long cable.
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Old 3rd August 2012, 04:08 PM   #1943
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Rod, Andy, thanks for posting.

Any more schematics?
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Old 4th August 2012, 12:19 PM   #1944
euro21 is online now euro21  Hungary
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Quote:
Originally Posted by merlin el mago View Post
Any more schematics?
Output to S&B TX-102 TVC.
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File Type: jpg HT raw supply.jpg (29.2 KB, 660 views)
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Last edited by euro21; 4th August 2012 at 12:36 PM.
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Old 4th August 2012, 01:35 PM   #1945
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Thanks euro21 with all schematics we can open a Wiki page.
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Old 4th August 2012, 03:59 PM   #1946
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Hi Andy,
In post 1941 above you set the anode voltage to 98V, with 12 VDC as a heater voltage input to the Coleman reg and a 5 ohm/40W resistor.
Last year you mentioned for the 26 an anode voltage of 138V, with 16 VDC as a heater voltage input to the Coleman reg and a 10 ohm/50W resistor.
Can you please expand on the difference, as both designs used the double 156C? Was it just a difference in available mains transformer voltages, or is the one above a further development with audible improvement?
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Old 4th August 2012, 05:20 PM   #1947
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Hi!

Quote:
Originally Posted by euro21 View Post
Output to S&B TX-102 TVC.
you were going with 7k source impedance into a TVC?
Even with a TVC for high impedance drive, I would not consider this as optimal

Thomas
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Old 4th August 2012, 07:49 PM   #1948
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av-trouvaille -

Yes, I preferred the sound at around 95v with 5 ohm resistor. It was also really useful because of the 12v supply. I was running the 26 with slightly starved filaments also.

Andy
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Old 4th August 2012, 08:21 PM   #1949
euro21 is online now euro21  Hungary
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Hi Thomas!

I now, it's not optimal.....but sounds amazing.

Tx-102 (400H, 100pF) simulated impedance graph (as TVC).

Some discussion:
how to build a pre using S&B TX-102 transformers

My measuring: upper -3dB point above 300kHz!
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Last edited by euro21; 4th August 2012 at 08:24 PM.
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Old 10th August 2012, 07:42 PM   #1950
Magz is offline Magz  United States
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Quote:
Originally Posted by Vinylsavor View Post
Hi!



you were going with 7k source impedance into a TVC?
Even with a TVC for high impedance drive, I would not consider this as optimal

Thomas
I'll be trying something similar using Slagle autoformers in a parafeed setup. The nickel laminations can be stacked to give up to 190H inductance. I plan to try out the 26 plate output directly into the autoformers as well as the mu output from the plate CCS (should be ~1500ohm). I figure that it can't hurt to try running directly into the volume control before sticking a transformer in between.

I've read that others have tried and failed using a CCS mu output into a parafeed output due to oscillations, but my sims show that with the right "lossy parafeed" setup the LF peaking can be very well controlled. I've attached a plot showing the LF response and the core drive figure of merit, using the autoformer stacked 3x3 (~150H) for regular parafeed (dark blue trace) and lossy parafeed. The extreme LF peaking is greatly reduced using the LPF setup, and the -3dB point is lowered. We'll see how that works out in real life...

P.S.: My two aluminum chassis (PSU and Preamp) are finally in the manufacturing process at Front Panel Express and should be done within 5 working days - Yeah!!

.
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Last edited by Magz; 10th August 2012 at 07:45 PM.
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