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Eli Duttmann's phono preamp voltages
Eli Duttmann's phono preamp voltages
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Old 3rd February 2018, 09:47 AM   #1
lquintas is offline lquintas  Spain
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Default Eli Duttmann's phono preamp voltages

I am going to build Eli Duttmann's RCA modded phono preamp but my limited knowledge doesn't allow me to estimate the anode and cathode voltage. I need these voltage to undestand the circuit and mostly to build it correcty. Can anyone help me giving me these voltages in the 12AX7 valve pins?
Thanks

Luis Quintas
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Old 3rd February 2018, 10:01 AM   #2
Eli Duttman is offline Eli Duttman  United States
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For starters, compare the tweaked setup to RCA's original. A regulated 250 VDC rail is the starting point.

BTW, the tweaked setup's grid leak biased 2nd gain block has the cathode at ground potential.
Attached Images
File Type: jpg Full Tweaked RCA Phono Stage.jpg (45.4 KB, 1058 views)
File Type: gif Original RCA Phono Preamp Schematic.gif (116.5 KB, 1070 views)
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Old 3rd February 2018, 10:13 AM   #3
lquintas is offline lquintas  Spain
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Thank you, Eli. Sorry for my ignorance, but what are the working voltages in pins 1, 3 and 6?
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Old 3rd February 2018, 10:17 AM   #4
artosalo is offline artosalo  Finland
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At pins 1 and 6 something between 80 V and 130 V.
At pin 3 about 1 V.
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Old 3rd February 2018, 11:31 AM   #5
lquintas is offline lquintas  Spain
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Thank you Artosalo
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Old 3rd February 2018, 11:11 PM   #6
PRR is offline PRR  United States
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Quote:
Originally Posted by lquintas View Post
....what are the working voltages in pins 1, 3 and 6?
The gain-device "fights" the plate resistor.

If the tube were open, Vp would be Vsupply. If the tube were shorted, Vp would be near zero. Neither condition works good.

For "a fair fight", you expect similar voltages, so Vp is about half of Vsupply.

Like pro wrestling, we never get a fair fight; and don't need exact fairness. Also here V1a has the large 39K dropping resistor. My pencil-check of this stage suggests the plate may sit near 180V (due to large-ish Rk). Artsalo may have actual data. However for a smoke-test, anything from 1/4 to 3/4 of Vsupply is probably a working condition, especially in a phono preamp with small signal levels.

The total drop across the plate resistor can be used to estimate the cathode resistor voltage. Or the estimated plate-cathode voltage divided by Mu for a rough guess of cathode voltage. Vpk= 180 and Mu=100 gives 1.8V. Actually probably ~~0.6 times this or 1.1V.

These thoughts will work for 90% of the circuits you will meet. (Well, maybe 70% of the stranger things seen on DIYA, with added fancy-bits and odd details.)
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Old 4th February 2018, 11:20 AM   #7
lquintas is offline lquintas  Spain
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Simple, practical, and therefore very useful. Thanks PRR
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Old 24th January 2019, 12:21 AM   #8
gregas is offline gregas  Canada
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Hey guys just wondering if anyone has built this modded
Phono stage?
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Old 25th January 2019, 10:18 AM   #9
lquintas is offline lquintas  Spain
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I have had no time to build it..
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Old 26th January 2019, 04:55 AM   #10
gregas is offline gregas  Canada
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I'd like to build a phono preamp and and I'm looking at building it. I was hoping someone had built it. I'm a newbie to building electronics and tubes, and was hoping to get some advice.
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