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Old 1st March 2013, 11:12 AM   #1
12B4A is offline 12B4A  United States
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Default 6S31B-P Grid current

I've been searching and I haven't come across general information about small signal tube positive grid operation that would give me a ballpark idea about this specific tube. The 6S31B data sheets (here and here) have a +.5V grid plate curve drawn. I also see a nominal operating point is listed with a 0 volt grid bias. It seems like going positive on the grid of this tube is part of its design intention but I can't make out what kind of grid current it might draw since I recognize very little Russian.

In general, if there's a positive grid voltage plate curve drawn for a small signal tube, is it reasonable to assume that going positive on your input is alright provided the previous stage can provide the power?
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Old 1st March 2013, 11:39 AM   #2
DF96 is offline DF96  England
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If the only positive grid line is for +0.5V then I think you can assume that the valve is not really meant for grid current use. Compare this with, say, ECC82/12AU7 (or EC90/6C4) where some datasheets give lots of positive grid lines. Most small signal valves have grids which have no easy means to dissipate heat. A high gain small signal valve might allow bias at 0V to maximise gain, perhaps in grounded grid mode in a receiver front-end where signal levels will be small.
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Old 1st March 2013, 01:49 PM   #3
12B4A is offline 12B4A  United States
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So the purpose behind the positive grid plate curve (and the nominal 0V grid) is to inform for grounded grid applications where the input signal may be in the microvolt to single millivolt range?

I think I'm just going to try a -.7v bias with a 2.4v p-p input on one of these to see if it works and if not, learn the hard way about fragile grids.
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Old 1st March 2013, 01:56 PM   #4
DF96 is offline DF96  England
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Just had a quick look at one of your links, where it is described as a stabiliser triode. Not RF, but DC power supplies. I suggest you try it, and see what happens. The main issue will be supplying power from the previous stage.
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Old 2nd March 2013, 01:53 AM   #5
12B4A is offline 12B4A  United States
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I had it up to 2.83Vp-p in on -.65 bias and it was only drawing about 1mA of current. Now I'll just continue on with the rest of the circuit to see how it sounds.
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Old 2nd April 2013, 10:22 AM   #6
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Default 6S31B - headphone driving amp

Hi,
sorry - if I now possibly post to a dead thread, but I noticed it some minutes ago.

You took the link to the tube's datasheet from my website, but maybe you did not notice the hybrid headphone amp "Aquavite" from my website, where I used exactly this tube. Please have a look...

What I had to put in about this tube is, that it is perfectly suited for what you intended it for: As the output tube, driving the headphone - biggest part of the game is its extremly big transconductance. You better not think about its grid-to-cathode voltage, but you should couple the grid of the 6S31B "hard" to the SRPP and define its plate current only by chosing an appropriate cathode resistor. So the triode will "swim" on the output voltage of the SRPP while not loading it. I used a JFET to drive the triode, what functioned absolutely fine - anyway - pls. have a look at the schematic and please have a look at the article.
I have to say, that my website is not commercial, so you may not buy any parts from there. A description of this tube may be found here.

Best regards, Barbara
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Old 2nd April 2013, 12:30 PM   #7
12B4A is offline 12B4A  United States
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Gr Gott Barbara! Your site was one of two sites that had good info to share about this tube. The other design I've seen is from this DIYaudio poster and his his blog. The design I'm using is just 2 6S31B as a grounded cathode direct coupled to a cathode follower configuration with constant current through each tube (Broskie/Tubecad CCDA). I am not finished since I have to track down a weird problem in one channel's output transformer.
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Old 2nd April 2013, 12:58 PM   #8
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Hi again,
I had a short look at the website you posted. Looks somewhat complicated to solve such a tiny application! Anyway - what do you think this configuration will do better, than a configuration built from a triode/cath.follower combination?
I ask, because I have some concern about SRPP and other "tube-on-top" applications, since I made scientific research some 20 years ago about this chapter. If you investigated this problem you would always find, that a (at least small amount) of even order harmonics will be deleted by such configurations. But - I think you know - all PP arrangements are prone to this behavior.
What is your problem with the OPT? May I possibly help?

BR, Barbara
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Old 2nd April 2013, 02:26 PM   #9
12B4A is offline 12B4A  United States
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Yeah, his circuit was a bit more than I wanted to do since I didn't have a PCB I could just stuff with parts. The OPT in one channel starts to roll-off in the bass (high pass) from about 900hz on down. The primary is getting the full bandwidth and voltage level but the secondary drops bandwidth when connected to the headphone load. My next step was to swap out the OPT but with other things taking priority in my life I just haven't gotten around to it yet. Here's the schematic (90V B+, tubes PK get ~45V):
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File Type: gif 6S31B test circuit.GIF (4.5 KB, 118 views)
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Old 3rd April 2013, 01:17 PM   #10
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Hi,
according to my experience you have a winding-short in the bad OPT. You can check by swapping the OPTs.
This is a little off-topic, but maybe will help.
BR, Barbara
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