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Old 8th January 2004, 02:08 PM   #1
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Default Running tubes at lower voltages

Hello folks,

I was thinking - would it be possible to run tubes at a lower voltage than what they're designed for (not heater, but grid(s))?

I am making a preamp, which needs to have a gain of only 2 (or something around there).

Can I use an EL84 or similar, and run it at maybe 100V or even lower - and of course still have things sound as bright, only not as loud?

Greetings,
-- Charl
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Old 8th January 2004, 02:15 PM   #2
mcs is offline mcs  Denmark
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You can run tubes at low voltages, but you will probably get higher distortion. Try having a look at the datasheets. But just try! If you like the sound it's good - if not try something else.

The playback amp of my r-to-r deck (5842 and 6N1P) runs well on voltages as low as 70-80V for instance - it normally runs on 330V.

You can get tubes meant for running on low voltages (ECC86 and EF98 for instance). I have also seen circuits running 6J5 triodes on 30-40V...

Best regards,

Mikkel C. Simonsen
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Old 8th January 2004, 02:20 PM   #3
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Hello,

30-40v would be abolutely awesome, but 80 will do as well. Did you lower the voltages on your tape deck yourself or was it manufactured that way? And did the sound quality drop?

It's probably best to just make a test setup as you said. However, being relatively new to tubes, I don't know whether I can mess a a tube up by lowering the grid voltage(s). Is this so, or could it do no harm?

Thanks,
-- Charl
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Old 8th January 2004, 03:14 PM   #4
mcs is offline mcs  Denmark
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Quote:
Did you lower the voltages on your tape deck yourself or was it manufactured that way?
Yes, I manufactured it that way (read more here: http://stiftsbogtrykkeriet.dk/~mcs/Tandberg/index.html

I designed it to run on 330V, but while testing it (with an adjustable supply), I tried running the circuit on lower voltages.
Quote:
And did the sound quality drop?
It did at some point (40-50V I think), but on 70-80V it sounded just as good as on 330V, but the output level dropped a bit.
Quote:
It's probably best to just make a test setup as you said. However, being relatively new to tubes, I don't know whether I can mess a a tube up by lowering the grid voltage(s). Is this so, or could it do no harm?
I guess you mean plade/anode voltage? A standard grounded cathode triode stage has no grid voltage (the DC voltage on the grid is 0V). But you do no harm by lowering the supply voltage - but at some point it will stop working properly. Be careful with DC coupled circuits though. The stages may depend on certain voltages being present at other parts of the circuit. But in AC coupled stages or single stages there are no problems.

Best regards,

Mikkel C. Simonsen
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Old 8th January 2004, 05:19 PM   #5
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Default Low voltage operation

Hi,
One low voltage tube is the 6DJ8 or ECC88 or 6922 ( all basically the same ). You can run it well even at 30 volts.

I tried the ECC82 at about 40 volts. It works OK with some feedback in the circuit. It does have higher distortion than higher voltage operation or the 6922 series. But it sounds good. This however is not a "good" operating point and you could possibly hear the improvement in sound at higher operating voltages.
No harm experimenting and if it sounds Ok to you , its OK !
It shouldn't affect the tube.
The 6DJ8 can be used with ZERO bias voltage (grid cathode voltage) for very low signals like from an MC cartridge !
Cheers.
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Old 8th January 2004, 06:06 PM   #6
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Hello folks,

Thanks ashok - I'll try to find some datasheets on those.

I just did some experimenting with the EBC91. Clipping occurs if the input voltage(of audio) - anode voltage ratio exceeds a certain level.

Ua of 140 of 120 makes no difference. When I lower it to 100, I can see that the output level begins to drop. At 80 volts, it's still dropping, and at 60V, clipping begins to occur. So, judging by ashok's reply, this isn't the tube I want!

Greetings,
-- Charl
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Old 14th January 2004, 08:25 PM   #7
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Hello,

I have done some experimenting with since my last post, and would like to share the results with you and also ask another question.

I went out and bought an ECH83 low-voltage tube. Its frequency response drops dramatically from 15KHz and up. I might look in the future if I can prevent this somehow.

However, what I'm experimenting with at the moment is a good ol' EL84. Running the heater at around 3V, I have gotten some pretty good results - for my purpose that is (making a preamp). The gain is about 3.

There is one problem though. When I tried running the heater at 6V, the tube got very hot in about 30 seconds. I didn't think it was good for the tube to run this hot so I turned it off. (N.B. Va = 12V)

Thinking about this, it would be obvious - not 'enough' electrons are being pulled towards the anode, so they stick around at the heater - causing heat. Does anyone have an opinion on this?

Greetings,
-- Charl

PS. I know that I'm being rather silly using an EL84 as a preamp, but it has to look good as well If this doesn't work out I'll switch to something more common or maybe a higher anode voltage.
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Old 14th January 2004, 08:43 PM   #8
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Default EL 84 at low voltage

Look if you can at the last elektor (november, december, and this month).
You can find many information about that,and some other tube at low voltage and also some "theory".
Good luck !!!
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Old 14th January 2004, 08:54 PM   #9
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Hey Dedessus,

Could you please give me the title and writer of the article? I live in Holland so it might well be possible that they've published it in another issue.

I have about 150 old Dutch Elektors lying around (from 1984), and only ONE of them has an article about tubes in them! It's good news that they decided to do another article on them ;)

If I can't find the article, would it be too much trouble to scan it for me?

Greetings,
-- Charl
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Old 14th January 2004, 10:03 PM   #10
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Hi,

Quote:
However, what I'm experimenting with at the moment is a good ol' EL84. Running the heater at around 3V, I have gotten some pretty good results - for my purpose that is (making a preamp). The gain is about 3.
Running a 6.3V heater at 3V, he?

Quote:
There is one problem though. When I tried running the heater at 6V, the tube got very hot in about 30 seconds. I didn't think it was good for the tube to run this hot so I turned it off. (N.B. Va = 12V)
@3!)*&&^?

Quote:
Thinking about this, it would be obvious - not 'enough' electrons are being pulled towards the anode, so they stick around at the heater - causing heat. Does anyone have an opinion on this?
Yeah...I do. Go through some books first or stick to candles...They light up nicely too.

Good luck,
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