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Old 13th March 2014, 02:48 AM   #11
kevinkr is offline kevinkr  United States
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Dinner is served, so I'll be brief.. Try 1N4148 cathode to cathode of pentode cathode. Delay B+ until filaments are hot.
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Old 13th March 2014, 02:49 AM   #12
AJT is offline AJT  Philippines
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1n4004 will work...

is that opt 5/8 inches CL?
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Old 13th March 2014, 04:37 PM   #13
kevinkr is offline kevinkr  United States
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1N4004 has too much non-linear capacitance (and some leakage current as well) when reversed biased imo, that's why I recommended the 1N4148.. Voltage rating should be a non issue when forward biased, current will be limited by the high resistance in the EQ network, and reverse biased with tube conducting it will see <2V across it.
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Old 13th March 2014, 11:38 PM   #14
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i have used those 1n4004, and i can not hear any difference....

this is a neon NE2...
Click the image to open in full size.
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Old 14th March 2014, 12:39 AM   #15
kevinkr is offline kevinkr  United States
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On a high impedance node with very low signal levels and a lot of gain to follow? Pehaps some 1N400x are better than others in regards to their parasitics..

I use NE2 to clamp cathodes in long tailed pairs to <90V, this might still be too high a potential grid to cathode, I believe the limit is somewhere around +/-30V.
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Old 14th March 2014, 02:14 AM   #16
coffin is offline coffin  Taiwan
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Thanks!

I'll try to add an 1N4148(I have quite a lot).

If I don't add soft-start, when powering up, the current will flow from 27K to 130K to 20k to 1N4148 to 12K, right? Then when the heater warms, current of triode will be drawn to triode so the path thru 1N4148 will be cut, right? I think so....

cheers

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Old 14th March 2014, 02:26 AM   #17
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Yes the EQ string of resistors and the triode plate load are the source, and it will rise quickly to the supply rail while that 330uF cap holds the pentode cathode close to ground. It is the cathode bias in the pentode that will cut off the diode by rising above the voltage on the grid. Make sure that you orient the diode so that the cathode (banded end) is connected to the cathode of the pentode.

I think you might want to use a smaller cathode bypass cap in the second stage. I should think 47uF would be adequate and would reduce stress on the tube as it is warming up.

Consider a LED for biasing the first stage, they come in all sorts of forward voltages, IR leds typically from about 1 - 1.3V, and red leds from 1.7 - 2V.. Other colors offer higher vf, but also higher dynamic impedances in most cases. Very large electrolytics with very little voltage across them don't seem to perform well with audio.
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Old 14th March 2014, 02:28 AM   #18
coffin is offline coffin  Taiwan
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Thanks Kevin!
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Old 14th March 2014, 02:34 AM   #19
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kevin, how do you see the grid resistor on the pentode side?, looks lowish to me....

@coffin, can you put dc voltage readingss on plates, cathodes and screens on the schematics?
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Old 14th March 2014, 03:28 AM   #20
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There is no classic grid resistor on the pentode per se as it is direct coupled to the plate of the front end triode through the EQ network resistors, there however is a grid stopper resistor of 150 ohms which should be fine. I tend to overkill, but in noise sensitive locations am using values as low as 47 ohms to reduce johnson noise in low level circuitry. The key is close to the pins and sufficient in value to isolate external reactances from the pin. (And check for oscillation of course.)
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