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Old 15th October 2012, 04:24 PM   #11
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Hi Kevin

I actually use battery grid bias on the next stage in this circuit. Its similar to the ear 834p schematic... I was going to tear it apart but decided to tinker with it instead

I agree that battery grid bias is very good. Since the batteries only reference the grid they seem to last a VERY LONG time...

Ian
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Old 15th October 2012, 04:38 PM   #12
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by soulmerchant View Post
Hi Kevin

<snip>
I agree that battery grid bias is very good. Since the batteries only reference the grid they seem to last a VERY LONG time...

Ian
Theoretically they should last the rated shelf life of the battery and perhaps beyond. The batteries I purchase generally have shelf lives of 5yrs or more and the alkaline lithium cells much longer than that. I think the main concern would be the rising internal impedance, and I will just monitor the battery voltage periodically, they're cheap enough to replace..

In one application where I need about 1V of bias I am running them through a resistive divider with a standing current of about 1uA - even these should not need replacement for at least most of the rated shelf life of the battery. (8.76mAH per annum)
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Old 15th October 2012, 08:10 PM   #13
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Quote:
Originally Posted by DF96 View Post
That only works for transformer coupling. For RC coupling the ripples add, as 'jane' said.
If we assume there is no signal on grid, and part of ripple voltage is applied to cathode, than positive voltage change results in lower anode current and lower voltage drop across anode resistor. Then anode voltage rises. Rrrrright. Absent minded I was.
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Old 15th October 2012, 08:51 PM   #14
tinitus is offline tinitus  Europe
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Originally Posted by soulmerchant View Post
The problem with LED bias on ecc83 is that the current draw of this valve is too meager to induce any standard LED to conduct.
its possible to mount a small SMD voltage doubler directly on the LED (seen on AMZ blog)

but would that be any good at all for LED biasing
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Old 15th October 2012, 09:17 PM   #15
Bigun is offline Bigun  Canada
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Originally Posted by soulmerchant View Post
[...] I therefore bring the LED to continuously conduct by an additional connection, bypassing the valve (and annode resistor) entirely with a separate sufficient wattage higher value resistor directly to the HT.
Why don't you move the LED bias resistor from the B+ to the smoothed B+ at the top of the anode load resistor. This may buy you further improvement:
a) the B+ is smoother so marginally less noise
b) you will create a shunt, a constant current draw that will improve the regulation of the B+ at the top of the anode resistor.

You will need to lower the value of the 120k B+ dropper resistor of course.
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Old 15th October 2012, 09:52 PM   #16
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Originally Posted by Bigun View Post
You will need to lower the value of the 120k B+ dropper resistor of course.
If you put the resistor downstream then you have to be certain that any other resistors upstream can carry the additional current load.

I didn't want to create an incendiary device... (the schematic as drawn is of course not complete)

Also, my HT is well fitered with chokes so I didn't have any extra noise (in fact I experienced less noise). Otherwise I see no problem with what you suggest.

Last edited by soulmerchant; 15th October 2012 at 09:57 PM.
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Old 16th October 2012, 03:35 AM   #17
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Originally Posted by soulmerchant View Post
Dear All

I have used LED cathode bias on a number of valves, but only recently tried it on some ecc83. I am pleasantly surprised with the improvement.

The problem with LED bias on ecc83 is that the current draw of this valve is too meager to induce any standard LED to conduct. I therefore bring the LED to continuously conduct by an additional connection, bypassing the valve (and annode resistor) entirely with a separate sufficient wattage higher value resistor directly to the HT.

The value of this resistor is chosen to drop almost all the HT and deliver approx. 6mA current to the cheap red LED. For my test circuit I am using HT of 280 Volts so chose a 5 watt 47K ohm resistor that I had handy. I pre-selected the red LED's and chose two that elevate the cathode by 1.67 volts on each channel.

My method of testing for linearity is a crude point-based method. Nevertheless, I measure distinct improvments that I believe can be heard with my own ears...

The by-passed LED bias on e83cc also seems to remove any last vestiges of power supply noise. Perhaps there is a bit of aikido-stype ps noise injection in this method. Since I by-pass the LED to make it continuously conduct, the effect is electrically similar to filament bias.

When I have used LED bias on other valves the change is distinct and subtle, but the changes I am getting with this method on e83cc seem even more significant. Maybe it is just my own biased hearing though. Has anyone else done this before?

Ian
I've been doing it this way for about 8 years now.... I do it no other way...

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Old 21st October 2012, 12:19 AM   #18
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I found these low power LEDs from Vishay and used them on a cv4004/12ax7 stage with local feedback, just as is, led conducts and it sounds good. A bit better than resistor bias did, I think, but the difference was rather subtle. B+ is 300V and plate resistor 220k.

http://www.datasheetcatalog.org/data...4ia79lqgwy.pdf

Tried all three colours, I settled for the green one, partly because it sounded better, and possibly because it looked better on the datasheet.

As far as suppliers go, I only know of this page where I ordered them (swedish company).

LED grön 3mm lågström 2mA TLLG4401 | Electrokit
(there's a little english flag to the upper right on the page, click it for english version of the page)

Try them out and see what you think! They cost about $0.50/piece, so they're rather cheap.
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Old 21st October 2012, 03:48 AM   #19
wa2ise is offline wa2ise  United States
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Would using a separate low voltage power supply and lower resistance resistor to feed the 6ma thru the LED work just as well as pulling the 6ma via the bigger resistor off the B+ supply? Assume a regulated say 12VDC supply feeding a 1666 ohm resistor to the say a red LED. Idea is to avoid heavy loads on the B+ supply.
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Old 21st October 2012, 05:56 AM   #20
dgta is offline dgta  United States
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Yes that will work but if an extra 6 mA is bogging down your B+ supply, I think you may have bigger problems

Make sure the 12V is WELL regulated.
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