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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 19 May 2010, 12:50 
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Joined: 14 May 2010, 15:09
Posts: 30
Location: Toronto, Ontario, Canada
As for my project, thanks all for the info so far, i should be able to make it work.

sampleaccurate, ill keep you updated on my progress. I have the whole weekend to work on it, so hopefully ill be playing some music by Saturday evening! Ill take lots of pics and post em up on a new thread


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PostPosted: 19 May 2010, 17:05 
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Joined: 04 Jun 2008, 20:59
Posts: 3552
Location: Arizona, USA
Hi, 300 ohm ought to work, however I prefer about 1000 ohms on small power tubes.

Good listening
Bruce

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PostPosted: 27 May 2010, 11:01 
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Joined: 14 May 2010, 15:09
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Location: Toronto, Ontario, Canada
Almost done, just a few more things to take care of!

Just a couple questions before i fire this baby up

I used a separate transformer for the filament because i couldn't get a power transformer with quite the right secondary (or at least one that was readily available). Unfortunately, after rectification and the cap, and even 3 power diodes in series the voltage is still about 16 volts! (no load!) I'm sure the voltage will drop a bit under load, but will it drop enough to not exceed the voltage rating on the tube heaters?

I have the EL84's in series (6.3 volts each for total 12.6), and the 5751's in parallel (the 5751's can take 12.6v on the heaters)

i know that there is some tolerance in the heaters, say 10% but even 6.9 volts each (13.8v) is still quite far off from 16 volts....you think the voltage will drop by 2 or more volts under the load?

Second question - what value in mA should i be looking for when i bias the EL84's? i have the test points wired in as the schematic says with the 1ohm resistor across the terminals...i just need to know what value to dial in to....

Thanks, Matthew

p.s - i took your advice gofar99 and went with the 1000ohm resistor between the screen and the center tap/outer taps for triode/pentode mode (depending on how the switch is set)


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PostPosted: 30 May 2010, 12:09 
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Joined: 14 May 2010, 15:09
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Location: Toronto, Ontario, Canada
Amp is finished! I started a new thread about some specific problems im having, but just to resolve this thread, the voltage is not a problem! after installing the tubes and turning em on and testing the voltage, there is a 12.4 nominal voltage across each tube heater (6.2 for each el84) so its just right!

The new thread: Another OddWatt is finished - some issues


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PostPosted: 05 Jul 2010, 01:38 
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Joined: 05 Jul 2010, 01:33
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Hi. Where can I find 25R potentiometer? Thanks


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PostPosted: 05 Jul 2010, 12:21 
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Joined: 28 May 2008, 21:53
Posts: 4530
Location: Winnipeg, CANADA
Hi zz971073. From the project text: http://diyaudioprojects.com/Tubes/EL84- ... -Tube-Amp/
The 25-Ohm 3-Watt rheostat (variable resistor) can be difficult to locate - it is available in the United States of America from Radio Shack (Catalog #: 271-0265, US$3.99 each), in Canada from The Source (Catalog Number: 2710265, CDN$6.99) and in Australia from Dick Smith Electronics (Product Code: R6905, AU$8.98). The prices quoted are from October 2009.

Hope this helps. If someone has a European or South American source for these potentiometers, let us know and I will add it to the project page.

Cheers

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PostPosted: 07 Jul 2010, 15:52 
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Location: Arizona, USA
Hi , In many cases (all so far that I know of ) you can sub in a 20 ohm 1/2 watt linear trim pot. Both Vishey and Bourns make them for about $1. Check Jameco and Mouser for them. The range of adjustment has so far been suitable with EL84s, 6V6s, KT77, 6L6s, and EK34s. The math says they will work in the KT88s as well, but the range of adjustment may not be sufficient for all of the KT88s.

Good listening
Bruce

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PostPosted: 18 Jul 2010, 17:30 
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Joined: 15 Jul 2010, 20:49
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I had a few questions (noob) before i do my parts run. First my tubes, i have alot of nos 6n6p/6h6p tubes that i hear can sub for ecc99's. if thats true i can use them for a gain of 20 instead of the gain of 30 as orginal. I was going to steal the ssrp stage numbers from millet. Im looking to get full output from 2vrms (possibly 3vrms if i use my amb mini3)
http://www.pmillett.com/ecc99_srpp_headphone_amp.htm

For my main tubes i was going to try the 6P14P-EV to go along with my theme.
http://www.tubes.ru/techinfo/HiFiAudio/6p14p.html

My main question is the B+ voltages. diyparadise uses 300 volts, the odd watt orginal el84 uses 240 volts, and the pod watt uses 210 volts. Ive seen several el84 designs with 350 volts. It looks like typical voltage is 250 volts. So whats up with the 100 volt swing with the general el84 pp 8k primary theme? Is it just to go from 5 watts to 10 watts UL?


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PostPosted: 19 Jul 2010, 15:03 
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Location: Arizona, USA
Hi, Some very good questions in there. Also for the following discussion I'll stick to just class A amps. The EL84 (and many others) is quite versatile. It can sustain a wide range of B+ voltages. There are some factors to consider. Primary is the maximum dissipation (PD) of the plates. You must not exceed this or either you will have short tube life or if really high, immediate tube failure. So within the ratings you have to make a choice on both the B+ and the current. I spent a fair amount of time tinkering with the "PoddWatt" design and in doing so found that B+ in the 250 range makes for a reasonable level of companion current and will keep the dissipation sufficiently below the max rating. It also matched up with the output transformer impedance I wanted to use. You can use more B+ and less current and a slightly higher load impedance, or less B+, more current and a lower impedance. Within the 200-350 volt range there are many possibilities. The second sort of question is kind of true, but runs into the PD problem. If you increase the B+ and keep the current the same, yes there will be more power output, but unfortunately the tube will fry. That is why there are bigger tubes to play with. To double the power, the tube will approximately need to have a PD of twice the earlier one, and then you can safely use twice the input power.

You can get more ouput power from other classes of amplifiers. Notably class AB1. The output can be double to tripple the level of a class A running at the same B+. But then, it would not be a class A amp and depending on your preferences not sound the same or as good. Your call on that.

Good listening
Bruce

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PostPosted: 25 Jul 2010, 08:45 
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Joined: 15 Jul 2010, 20:49
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I think im down to 2 questions.

1. Can the input be dc coupled (ive seen some buffer beable to take the 2.5volt offset of dacs) or at least take 30mv or so offset?

2. What is the load resistance of the el84 section? The reason i ask is that my preamp volume control is a special burrbrown unit that has a built in 30db preamp that can drive 600 ohm loads at 75ma with a swing of +-14 volts. Im thinking that would be enough to drive the el84 section directly and bypass the ssrp.


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