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 Post subject: 6J6 feeds my 6AK6-PP
PostPosted: Dec Wed 07, 2011 11:34 am 
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Thanks to TUBENUT I started fiddling with a 6j6 yesterday.
The two recent amps I've been discussing have been the tiny 5902 amp and most recently with tons of assistance from CDoose CHUCK,I've gotten a great little 6ak6 PP going so beautifully now!!

Both amps use the op-275 op-amp as a pre amp and I thought perhaps I should try to see what it will sound like to just go ALL tubes.

Then I remembered Tubenut's info about using a 6j6.
He linked a schematic from a web site article:
http://mysite.du.edu/~etuttle/electron/elect33.htm

So I ordered a few tubes and chopped off the SS front end to my 6AK6 PP and simply just tacked the 6j6 preamp on.
All I did was simply use the same values in the article... took an hour or so and it worked the first pass!... lol

Last night I first tried it and I thought it was pretty darn great... the only thing I wanted was a little more sensitivity.
My MP3 only puts out about 450mv
....and this 6J6 preamp has a gain of 21 ...so that is only about 8-volts or so... and I have to max out the volume on the MP3 player... even with the little passive amp thingie I just built.

Well I'll have to think about this.. it is pretty good volume now as it is...but a tad more might be nice.

Anyway.. the good news is that last night when I hooked it into the 6AK6 section I overlooked the 220k grid resistors that were soldered in there from when it was a 5902 tube driven by the op-275 op-amps.

Well when i discovered that ...I pulled them out tonight... and wow what a difference in the sound!!!!
I guess that really affected the (roll off? ) of the input filter...
Now you see it's just a .01 feeding into a 510k.

Anyway whatever... suddenly this thing seemed to "open up" holy cow!
astounding sound... ... and I'm only using a single channel.

Do any of you guys remember reverbs?
Well not that this sounds any thing like that, No ... but it just reminds me of the same wonderful strange feeling I had the first time I ever heard a reverb.. I couldn't figure out why it seemed to be "bigger than life"...
And I guess that's a perfect description for what I'm hearing here right now tonight with this amp... really full and alive.

I keep thinking I'm in some kind of a strange giant fishbowl of surround-sound!
It seems to be wrapping itself all around me.. really an odd feeling.. but pleasurable.... and I'm 20 feet away in the next room... great!
It really is pleasing. Hard to describe.

I don't know what to attribute it too.
It's almost...just like that stunning sound-feeling that I heard when I put those new Shuguang EL-34 B tubes in my ASL AQ1003dt amp on Saturday.
.. I'm having that same full-sound kind of experience here.

And I don't even have a "real" output transformer right now... lol
I need to get 16k- 8 ... but I had to put the 2 test speakers in series and get what I needed.
But ...I am only using a 6.3v filament transformer.
It's a 6va dual wound primary Tamura 3fd-412 with dual 6.3 volt secondaries.
Unloaded it puts out about 7.5 volts..
so 230v divided by 7.4 v = 31
That's a ratio of 31:1
31 squared =961 x 16 ohms = 15,376 ohms for a reflected plate load.
In PP this tube wants 16k.. so we are real close!

So that's close enough... but great sounding despite this.

I guess I have to really thank Tubenut for sending me that link.

And CDOOSE too for all he's helped me so far.

Chuck, what do you think about this schematic?
Does it need to be modified or is it good the way it is?

BY the way.. on a different thought.. sort-of ...
( I was trying to think of a good reason to justify building another amp or perfecting this one.. so maybe I'll send a tiny one as a surprise gift to my brother.. maybe get him hooked on tubes too.. lol)
To keep it tiny I like reducing tube count...
So...The only other .. sort of cool I idea I am toying with here is reducing the tube count even further by playing with another totally different amp that uses a pair of 6bm8 tubes ( triode & power pentode)
http://www.nj7p.org/Tube4.php?tube=6bm8

ASL amps make one called the WAVE-8
Claims to use dual 6gh8 tubes... but the parts list shows they used 6bm8s
Anyway it's a simple one too.

okay...
Getting back to this 6j6-6AK6 amp..
any interesting ideas or input or feedback is certainly welcome... it's all in bread board.
Here it is now:
Image

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Last edited by Pbpix on Dec Wed 07, 2011 12:34 pm, edited 4 times in total.

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Wed 07, 2011 11:50 am 
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How did you determine that a 20 volt zener at the cathodes was correct?


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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Wed 07, 2011 12:08 pm 
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Johnnysan wrote:
How did you determine that a 20 volt Zener at the cathodes was correct?

Well I was playing around and just trying a few bias points.. and one great guy here TOOBS.. ( what great teachers he and Chuck are) .. anyway he sent me a bunch of educational data and advice about plate curves and load lines and he suggested -15v if I remember.. ( you can read his great input on my other thread ;
viewtopic.php?f=13&t=179338

Then Chuck.. cdoose piped in and helped too.

Then I thought it looked like -18v would be good... and it seemed to be.. but then Chuck started running some curve tests and HE suggested the -20v point... and then I also went to a 100 ohm SG resistor.. and also pushed up the B+ to 270 to get a 250v potential from cathode to plate ... referencing from the cathode.

Now it runs so cool at only 8ma per tube at idol.

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Last edited by Pbpix on Dec Wed 07, 2011 5:42 pm, edited 1 time in total.

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Wed 07, 2011 3:51 pm 
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Peter,

The mu of a 6J6 is 38 so the gain of that stage will always be less than 38. Since right now the gain is 21, you could increase it by partially bypassing the 2.7k cathode resistor. This can be done by using a fullly bypassed (100 uF?) 2.2k in series with an unbypassed 470 ohm. This way you can use negative feedback from the output stage to the input stage to reduce distortion as well.

There are other options like higher mu tubes or an additional stage in front of the PI, But I'd have to check those out later when I get home from work.

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Wed 07, 2011 5:46 pm 
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Cdoose wrote:
Peter,

The mu of a 6J6 is 38 so the gain of that stage will always be less than 38. Since right now the gain is 21, you could increase it by partially bypassing the 2.7k cathode resistor. This can be done by using a fullly bypassed (100 uF?) 2.2k in series with an unbypassed 470 ohm. This way you can use negative feedback from the output stage to the input stage to reduce distortion as well.

There are other options like higher mu tubes or an additional stage in front of the PI, But I'd have to check those out later when I get home from work.

There you go Chuck!
wow.. right there to help.. How nice is that?

Wonderful.. I'll try it now and let you know later...
It's almost noon now.. only few hours sleep...and I didn't get to bed till 7:30 this morning... lol
See how much fun this is?

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Wed 07, 2011 7:14 pm 
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Ok Chuck:
I tried that.. first of all I think the numbers I posted earlier last night..were in error.. don't know why maybe I just mis-spoke as I was doing it from memory... lol
The output from my MP3 as I measured it today again.. is in fact 800mv pp
And that is producing about 17vPP... so that is a gain of almost 21... right?

BTW (How much pp signal should the grid on the 6ak6 be properly able to handle?)

Well.. then I did what you suggested and used a 2.2k with a 470 ohm in series and put a 220uf by-pass around the 2.2k only.

The increase in gain if any was very minimal.. the output only went up from 17vpp to 17.5v pp.

I also tried some negative feed back but have no clue as to what values to use so I just tried the same values the 5902 amp used and that's a .47uf in series with a 470k.
But that was an imperceptible change when connected to the junction of the 2.2k and the 470 ohm.

How do I know where to start with selecting values for the feed back?

.. Then.. I decided to experiment again with more gain and I just jumpered around the 470 ohm.
That helped... it brought the gain up to 25...
So now at full volume (input 800mvpp) I am getting 20vpp out.
That was actually noticeable to my ears.

How much can I play with this gain without screwing up the design?
Thanks again.

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Thu 08, 2011 4:39 am 
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Peter,

I gave you bad info this AM. Didn't realize that no bypass cap is needed with that type of phase inverter. That PI is called a paraphase and since the two triodes share a common cathode bias resistor, no bypass cap is needed because as the current goes up in one tube it goes down in the other to maintain a fairly constant current in the resistor.

So the only way to get significantly more gain is to use a higher mu tube. You could try a 12AT7, 12AY7 or 12AX7. I'm not familiar with any other mini 7 pin dual triodes, but if you could find one with a mu over about 70 that would work.

Ran some simulations with a 12AX7 (mu = 100), the gain should be about 56, so that would give you about 45 Vp-p on the g1 of the 6AK6 which is more than you need. To get full power you need 40 Vp-p since the grid bias is 20 V.

If you decide to use a 12AX7 use 250V B+ and a 1k cathode resistor. You will have to adjust the 100k pot do reduce the signal into the second section of the tube. You adjust that pot to get equal amplitude signals to the 6AK6s.

I don't think you can use NFB with the paraphase PI, but I'll check into that later.

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Thu 08, 2011 5:35 am 
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Why not use another 6J6?

If it is a stereo amp just simply connect the 6J6 and then connect the cathode directly to ground.

If the amp is mono just set up the cathode bias like your current 6J6 then run a coupling capacitor from each plate to the grid of the second 6J6. That way you now have a stereo to mono mixer and more gain.

That reminds me. I have a box of maybe 150 various tv and misc tubes I need to play around with some day.


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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Thu 08, 2011 5:53 am 
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Cdoose wrote:
Peter,

I gave you bad info this AM. Didn't realize that no bypass cap is needed with that type of phase inverter. That PI is called a paraphase and since the two triodes share a common cathode bias resistor, no bypass cap is needed because as the current goes up in one tube it goes down in the other to maintain a fairly constant current in the resistor.

So the only way to get significantly more gain is to use a higher mu tube. You could try a 12AT7, 12AY7 or 12AX7. I'm not familiar with any other mini 7 pin dual triodes, but if you could find one with a mu over about 70 that would work.

Ran some simulations with a 12AX7 (mu = 100), the gain should be about 56, so that would give you about 45 Vp-p on the g1 of the 6AK6 which is more than you need. To get full power you need 40 Vp-p since the grid bias is 20 V.

If you decide to use a 12AX7 use 250V B+ and a 1k cathode resistor. You will have to adjust the 100k pot do reduce the signal into the second section of the tube. You adjust that pot to get equal amplitude signals to the 6AK6s.

I don't think you can use NFB with the paraphase PI, but I'll check into that later.

Thank you very much Chuck:
I should have know that too because I just finished reading that cathode current info in the article from which I took the schematic.
"There can be considerable gain in this kind of phase inverter, so a preamplifier is not required. Also, there are no headroom problems when a considerable output amplitude is desired. A circuit is shown at the right that uses a 6J6. Note that the input to the grid of the phase inverter is taken from the output of the other triode, in the grid circuit of the following tube, where a trimmer potentiometer can be used to adjust the ouputs to exact balance. The gain of this circuit is about 21, so the resistance below the slider is set at about 25k. The plate currents of both tubes flow through the same cathode resistor. Since the signal currents are opposite in phase, no signal current appears across the cathode resistor, and it need not be bypassed. This circuit produces a 30 V peak-to-peak ouput with an input of about 1.4 V. "

BTW:
I couldn't figure out why mp3 output voltage that I originally stated was off by half... lol
Then tonight I see why... I took that measurement without the passive amp attached... so today's reading was double of course.
In any case.. the max output without changing the cathode bias resistor is 17.5 vPP.
I may try a 2nd passive amp stage as a test too.

I just received my stash of 9pin sockets so now I CAN try the 12aXX tubes too.
Did you not include the 12au7 because of its lower gain of only 20?

If I do get success using a 12ax7 or such... maybe the output will be nice and high and I can put a vol control on this too.

I stumbled on a sub-mini 6111 here tonight but that too only has a gain of 20.

Back on the 6J6... the 2.7k cathode resistor drop is 1.8v ... so that's about 1/2 ma.
I see the plate can take higher voltages too up to 300.
Wonder what led to the choice of 90v B+ ?

Thanks for the component values and voltages for the 12aXX tubes.
I'll keep you posted if I get the 12a tubes going here tonight..

I'm still so amazed with how great the sound is compared to the SS op-275.
Do you think that's due to the nature of the beast, or perhaps there is a component value that might improve it?
I never thought it was bad until I put in the 6j6 and its like a whole new "bigger" more full sound.
it is truly quite an amazing difference.

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" To be a man, Be a non-conformist, Nothing is as sacred as integrity of your own mind." Emerson


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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Thu 08, 2011 6:20 am 
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Pbpix wrote:
Cdoose wrote:
Peter,

I gave you bad info this AM. Didn't realize that no bypass cap is needed with that type of phase inverter. That PI is called a paraphase and since the two triodes share a common cathode bias resistor, no bypass cap is needed because as the current goes up in one tube it goes down in the other to maintain a fairly constant current in the resistor.

So the only way to get significantly more gain is to use a higher mu tube. You could try a 12AT7, 12AY7 or 12AX7. I'm not familiar with any other mini 7 pin dual triodes, but if you could find one with a mu over about 70 that would work.

Ran some simulations with a 12AX7 (mu = 100), the gain should be about 56, so that would give you about 45 Vp-p on the g1 of the 6AK6 which is more than you need. To get full power you need 40 Vp-p since the grid bias is 20 V.

If you decide to use a 12AX7 use 250V B+ and a 1k cathode resistor. You will have to adjust the 100k pot do reduce the signal into the second section of the tube. You adjust that pot to get equal amplitude signals to the 6AK6s.

I don't think you can use NFB with the paraphase PI, but I'll check into that later.

Thank you very much Chuck:
I should have know that too because I just finished reading that cathode current info in the article from which I took the schematic.
"There can be considerable gain in this kind of phase inverter, so a preamplifier is not required. Also, there are no headroom problems when a considerable output amplitude is desired. A circuit is shown at the right that uses a 6J6. Note that the input to the grid of the phase inverter is taken from the output of the other triode, in the grid circuit of the following tube, where a trimmer potentiometer can be used to adjust the ouputs to exact balance. The gain of this circuit is about 21, so the resistance below the slider is set at about 25k. The plate currents of both tubes flow through the same cathode resistor. Since the signal currents are opposite in phase, no signal current appears across the cathode resistor, and it need not be bypassed. This circuit produces a 30 V peak-to-peak ouput with an input of about 1.4 V. "

BTW:
I couldn't figure out why mp3 output voltage that I originally stated was off by half... lol
Then tonight I see why... I took that measurement without the passive amp attached... so today's reading was double of course.
In any case.. the max output without changing the cathode bias resistor is 17.5 vPP.
I may try a 2nd passive amp stage as a test too.

I just received my stash of 9pin sockets so now I CAN try the 12aXX tubes too.
Did you not include the 12au7 because of its lower gain of only 20?

If I do get success using a 12ax7 or such... maybe the output will be nice and high and I can put a vol control on this too.

I stumbled on a sub-mini 6111 here tonight but that too only has a gain of 20.

Back on the 6J6... the 2.7k cathode resistor drop is 1.8v ... so that's about 1/2 ma.
I see the plate can take higher voltages too up to 300.
Wonder what led to the choice of 90v B+ ?

Thanks for the component values and voltages for the 12aXX tubes.
I'll keep you posted if I get the 12a tubes going here tonight..

I'm still so amazed with how great the sound is compared to the SS op-275.
Do you think that's due to the nature of the beast, or perhaps there is a component value that might improve it?
I never thought it was bad until I put in the 6j6 and its like a whole new "bigger" more full sound.
it is truly quite an amazing difference.


You have just experienced why tubes are better than solid state.


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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Thu 08, 2011 8:55 am 
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Hey Chuck:
I started fiddling with using a second passive amp thingie...
I tried hooking two in tandem (auto transformer 6db mode) and that works perfectly and maintains the flat response and no distortion but require four transformers in total. 6db + 6db
...and it seems to be able to give me all the gain I need at the moment.
Almost 30vPP out of the 6j6 with about 1.4vPP in.
So the mp3 can naturally put out a max of only 450mv PP
With one passive amp it puts out 800mv PP
with two passive amps in tandem it puts out 1.4v PP

On the flip side I also tried the (17db) straightforward (8-ohm - 1k-ohm) step-up mode and that sounds ok .. but technically I "might" hear distortion according to the specs posted.

But the advantage is it only requires the two tiny transformers instead of 4.
I think I'll make-up another one of these passives things in a tiny box like I did the other day but this time I'll add a switch so I can use it in either the 6db or 17db mode.

In the mean time I'm also going to play, tomorrow maybe, with 12at7 or 12ax7

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Thu 08, 2011 4:09 pm 
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Here's a question for Chuck or anyone who can help enlighten me on this.

In the schematic above the 6J6 ( built exactly as the schematic I found) is used as an amplifier/phase inverter.
The design above uses a 2700 ohm cathode bias resistor and 90 volt B+ supply.
I measure 1.8v drop on the cathode resistor drawing a tad over 1/2ma plate current. With 100k plate resistor that puts the plate at about 40-50v... I think.. right

It certainly seems to operate perfectly. I don't know what the operating class is however, though I naturally assumed class A for linear performance.

However I found this GE data sheet for the tube that indicates a typical operation in class A as having 100v on the plate but it indicates the use of a 50 ohm cathode resistor with 8.5ma plate current. I calculate the grid bias at -4.5v right?
According to the "average plate curve chart" any grid bias more negatve than -4v @100plate volts shows the tube in cur off. So that confuses me a whole bunch.
I must be making a mistake reading the curves?

Can anyone help explain to me the reasons for the choice of component values in the design shown above and what class it is operating in so as to help clear up in my mind why the suggested "typical operating class A " component values should be so different?
Here's the data sheet chart I refer to:
http://www.mif.pg.gda.pl/homepages/fran ... /6/6J6.pdf

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Last edited by Pbpix on Dec Thu 08, 2011 4:43 pm, edited 3 times in total.

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Thu 08, 2011 4:20 pm 
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8.5 mA thru 50 ohms is 0.425 volts so it wouldn't be in cutoff.

I believe I have the same mp3 player as you, a Sandisk Clip+, but mine puts out 1.6 v pp. Under Settings>System Settings>Volume, do you have the "High" setting checked?


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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Thu 08, 2011 5:13 pm 
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Jon the Grimm wrote:
8.5 mA thru 50 ohms is .425 volts.

I believe I have the same mp3 player as you, a Sandisk Clip+, but mine puts out 1.6 v pp. Under Settings>System Settings>Volume, do you have the "High" setting checked?


Hi Jon:
AHHH ...lol
Thanks for catching the math error... I must have dropped a decimal point in my bit-bucket.
You're right!
-.425 volts grid bias... Thanks.

I just checked my Sansa Clip settings and yes, I do have "high" checked.

Are you measuring with a load?

I just picked up another MP3 player with 4 GB of storage (2000 songs) for $30 on clearance at RadioShack.
It's called GigaWare.
The output from it seems to be only 200mvPP

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Fri 09, 2011 4:40 am 
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Pbpix wrote:
Here's a question for Chuck or anyone who can help enlighten me on this.

In the schematic above the 6J6 ( built exactly as the schematic I found) is used as an amplifier/phase inverter.
The design above uses a 2700 ohm cathode bias resistor and 90 volt B+ supply.
I measure 1.8v drop on the cathode resistor drawing a tad over 1/2ma plate current. With 100k plate resistor that puts the plate at about 40-50v... I think.. right

It certainly seems to operate perfectly. I don't know what the operating class is however, though I naturally assumed class A for linear performance.

However I found this GE data sheet for the tube that indicates a typical operation in class A as having 100v on the plate but it indicates the use of a 50 ohm cathode resistor with 8.5ma plate current. I calculate the grid bias at -4.5v right?
According to the "average plate curve chart" any grid bias more negatve than -4v @100plate volts shows the tube in cur off. So that confuses me a whole bunch.
I must be making a mistake reading the curves?

Can anyone help explain to me the reasons for the choice of component values in the design shown above and what class it is operating in so as to help clear up in my mind why the suggested "typical operating class A " component values should be so different?
Here's the data sheet chart I refer to:
http://www.mif.pg.gda.pl/homepages/fran ... /6/6J6.pdf


Hi Peter,

Re amplifier class; a voltage amplifier must operate class A otherwise the distortion would be terrrible.

Don't know why the designer of that circuit chose those values, that's not what I would have done.

After looking at the plate curves for the 6J6 and running some simulations, it's clear to me that you should try a different operating point than shown in your schematic above. With it the way it is in the schematic, the distortion would be about 7% (with a 1.6 Vp-p input signal) due to the 6J6 operating at the low end of the characteristic curves.

Try this:
B+ 250 V
Plate resistors 20k
cathode resistor 100 ohm.

The above values should yield the following:
Plate to cathode 94 V
Cathode to ground 0.8 V
Plate dissipation per section 0.7 W
Gain 27
Distortion 0.5%

You would still need about 1.5 Vp-p input signal, but if your mp3 player does put out 800 mV and you used the autotransformer you'd have enough signal to drive the 6AK6 to full power.

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Fri 09, 2011 7:28 am 
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Cdoose wrote:

Hi Peter,

Re amplifier class; a voltage amplifier must operate class A otherwise the distortion would be terrrible.

Don't know why the designer of that circuit chose those values, that's not what I would have done.

After looking at the plate curves for the 6J6 and running some simulations, it's clear to me that you should try a different operating point than shown in your schematic above. With it the way it is in the schematic, the distortion would be about 7% (with a 1.6 Vp-p input signal) due to the 6J6 operating at the low end of the characteristic curves.

Try this:
B+ 250 V
Plate resistors 20k
cathode resistor 100 ohm.

The above values should yield the following:
Plate to cathode 94 V
Cathode to ground 0.8 V
Plate dissipation per section 0.7 W
Gain 27
Distortion 0.5%

You would still need about 1.5 Vp-p input signal, but if your mp3 player does put out 800 mV and you used the autotransformer you'd have enough signal to drive the 6AK6 to full power.


Hi Chuck:
Well I've been diddling with this for a few hours and finally go it at the values you suggested.. but I changed things to get there.
I am using 270v B+ else where so why go to 250 I figured I'd stick with the 270.

I now have 0.85v on the cathode resistor (I'm using 68 ohms)
and cathode to plate is +87v ( I used 27k plate resistors)
... so that seems like about 12ma going through the cathode resistor.

It's playing fine... i think.. haven't really scrutinized it all yet.
Except to say that there seems to be a bit of a hum somewhere esp if i push the mp3 vol to max.
I have a real rats nest of wires so I'm not surprised though.

But I didn't seem to hear it before.
However.. before my 90v B+ was coming from a separate regulated supply unassociated with the one on the 6AK6s.
Now they all share one supply.

EDIT:
I just went back to 2 separate supplies to make things simpler at the moment.. and it seems better because i was loading the 1st supply a little too much.
Now B+ is 270 for both sections... but separate.

It seems that we kicked up the gain by 67% ... from a gain of 21 to a gain of 31... (isn't that 67%)...lol

Yes now with 800mvPP in I get about 25vPP out.
That calculates to a gain of 31... right?

Well the Sansa MP3 I have only seems to only put out 400mvPP max.
So the passive amp is already what is kicking it up to 800mv. now.

The other mp3 I have put out 200mvPP

So feeding the 6AK6s with 25 volts doesn't really get full power out.
...but even at this lower power it sure sounds nice.

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Fri 09, 2011 11:19 am 
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Here's another question Chuck.. or anyone....lol

Why is it that the DC plate voltage at plate #1 on pin-one..... is different than plate #2 on pin-two?

Plate one has always been higher ( or so I think I remember) ... even before we changed the circuit tonight to the new, higher B+ of 270v from the original 90v

I measure from the cathode to plate #2 = +87vdc ..... but plate #1 =+107vdc.

The PP signals are tweaked by that pot to be identical.

The text of the article states that the two PP signals will be appox the same when the pot wiper to ground is about 25k ohms... and I measure 21k now with both PP signal matching.
So all that part is correct.
Plate # one must draw way-less current.
Right?

Yes... it does!
I measured the current there just now.... at idol
The plate resistors are precision 27k ohm
The one on plate # 1 drops 160 volts
The one on plate two drops 180 volts
so by calculation:
plate #1 draws 5.9ma and so plate #2 draws 6.6ma

So this confuses me .... and how can we determine the cathode to plate voltage for the plate-curve chart if each plate is different.... then which do we select?
We only have one cathode resistor.

Smarter minds here will clear this up... lol

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Fri 09, 2011 1:12 pm 
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Long as the push pull signals are the exact same don't worry about one part of the tube drawing more current than the other.


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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Fri 09, 2011 1:17 pm 
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Tube Radio wrote:
Long as the push pull signals are the exact same don't worry about one part of the tube drawing more current than the other.

Thanks but I'm still puzzled which grid bias to set up first... y'know?

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 Post subject: Re: 6J6 feeds my 6AK6-PP
PostPosted: Dec Fri 09, 2011 1:40 pm 
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Chuck:
In order to try to get measurements into a more sensible range... I decided to go back to your original settings and values and start from there.
So since I have an adjustable B+ I cranked it back down to 250v as you suggested.
Changed the plate resistors to 20k and used a 100 ohm cathode resistor again.

That made the drop on the cathode resistor 1.2v
And the two plates were still 20v apart and far away from the 94v you thought might be there.

So instead of testing a bunch of parts to get the right voltage on the plate... instead I tweaked down the B+.
I measured from cathode to plate #1 ... and reduced the b+ till I got 94v on THAT plate.
Then... the drop on the cathode is at .9v and the G1=.9v as well and the other grid is at 1v.

These numbers NOW look more like what you had anticipated.

However I had to back down the B+ to around 179volts.... ref to ground

At this point plate #1 =94v - ref to cathode
plate#2 = 78v

If I have .9v on the cathode resistor that means the total current is 9ma... right?

And plate #1 is at 94 volts ( relative to cathode)

If B+=179 then I am dropping 85 volts across the 20k plate resistor. That is 4.5ma

Plate#2 is at 78v... so that one dropped 101 volts across the 20k plate resistor = 5ma

So if each plate draws about 4.5ma that's = the 9ma at the cathode resistor... right?

Ok.. so what's the right B+ to use? ... if I'm at about 180v now?... guess it doesn't matter as long as there's 94 on the plate.
so does the bias point workout properly with .9voltes?
you were thinking .8 on the curve chart for 94v on the plate... right?

I'm finally going to bed...
it's 7:30 AM...lol
look 4 your answer later when I get up.
Thanks.

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