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why only JJ ecc803s's work


The Dude

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I finally have a chance to post in this new forum, I guess this could be a technical question for the technical section, but I would like to think we would like to keep the technical section open for technical question about speakers and what not.  So here goes.

 

I have had Tubes4hifi pas3 z mode running for about two years or so.  It actually is in a homemade chassis but none the less its the 12x4 regulated power supply with the PC5, and PC6 line stage and phone stage.  Up until this last Christmas I have been running an EH 12au7 in the line stage and a pair of JJ ECC803s's.  For Christmas I received some EH 12ax7's from my lovely mother in law(yeah shes awesome).   I couldn't wait to swap the JJ's out for the EH's, though it had taken me a day or two, after I finally got around to listening, I noticed the output wasn't there.   I thought maybe I had some bad tubes, so the following week I had taken them down to the local tube tester fellow, and they tested fine.  I emailed Roy, the desinger and seller of the preamp amp, and the only thing he recommend at that time was to try and reflow the solder joints.  I finally was able to get around that tonight.  Nothing, same thing pretty much no output. 

 

I got to thinking, lets try some different tubes.  So I popped in some Siemens 12ax7's and the output was there, but not up to par.  So I finally decided to pull the original JJ ECC803s's form my Scott and reinstalled them.  Now the phono section seems to be working fine.  I am currently spin vinyl and all sounds great

 

I have emailed Roy again to see what he has to say but I thought check with you guys here.

 

by the way I hate laptop keyboards, its consistently moving my cursor and causing me to type in different areas, but thats a whole other story all on its own.

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It seems that you are simply experiencing gain differences among the different tubes.  Although they are all supposed to meet the same criteria, in actual practice there can be fairly large variations.  So, if your JJs are running on the high side, and the EHs on the low side the difference can be striking.  You can verify that it's not a socket issue by simply rocking the tubes slightly while listening (with no music playing).  You will know quickly if there's a problem.

Maynard

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Guest thesloth

This is just from observation of datasheets.

 

 

Here is the JJ: http://www.thetubestore.com/lib/thetubestore/JJ-ECC803-S.pdf

 

Here is the EH: http://www.thetubestore.com/lib/thetubestore/eh-12ax7eh.pdf

 

 

Tubes can be defined by their properties at a given operating point. mu=gm*Rp 

 

You will see in the JJ datasheet the operating points are 250 plate volts, 1k6 cathode resistor, 1.2mA plate current, 1.6mA/V transconductance, 62k5 plate resistance, and a mu of 100. Take note that the grid voltage here is -1.92V, 1600*.0012=1.92

 

The math works out: 100=62500*.0016

 

The EH datasheet says at the very bottom NOTE: Plate volts, 250V; grid bias, -2v;  So they are testing the tube at a basically the same operating point as the JJ. You can see that the amplification factor is 92 because the plate resistance is lower than the JJ at this operating point. Transconductance is 1.7mA/V and plate resistance is 54k1.

 

So check the math: 54100*.0017=91.97

 

The gain is lower because the plate resistance is lower. This lower plate resistance will also effect the time constant in the RIAA passive EQ. Not to mention if the valve tested low for transconductance, the gain would be even less. Say the transconductance was 1.4mA/V then you would have a gain of 75. This is why matching tubes in critical circuit is important and can account for the biggest reason you hear a difference in tube rolling. You can have tubes with varying specs from the same assembly line.

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This is what the designer Roy had responded with, maybe it makes since.  I was wondering if I should install a resistor some where to reduce the load on the phono board.   I have checked the B+ and filament voltages, they are the same to that board as they are to the line stage board.  I will ask Roy, what exactly they should be. 

 

that's a very strange issue, the board was designed to use 12AX7s.   It has 40db of gain, standard for phono.
Have you checked your B+ and filament voltages?
So I don't have an answer for you.    The only other possible reason I can think of is that maybe you have
too much of a load on the board, like under 100K ohms.
Since the board doesn't have an output buffer, it's designed to work with the standard PAS3, going into a 500K or 750K balance control, and then thru a 250K volume control to the line amp (PC5) board.
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I will say, I hope it doesn't offend anybody that I have asked for help on this.  As I am planing on selling these boards to fund a different project.  I really wanted to have an answer to whoever purchase these. 

 

Thanks for all your help.

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Guest thesloth
This is what the designer Roy had responded with, maybe it makes since. I was wondering if I should install a resistor some where to reduce the load on the phono board.

 

Adding a resistor where? And how would that help with loading the board? The output impedance for that board is 70k, if you adhere to the 10x rule of thumb to not load the board down the input impedance of the following stage would have to be 700k or higher. Don't use long cables with this design.

 

 

I never use EH tubes so I have not tested their 12AX7 but the datasheet shows a lower plate resistance which would effect the gain, hence why they wrote 92 for the amplification factor in the datasheet.

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Guest thesloth

I just found this: https://www.tubesandmore.com/tech_corner/12ax7_comparison_of_current_made_tubes

 

 

As you can see the Electro Harmonix on average don't have as much gain as say a Mullard reissue on average. If you want the most in gain try some other brands at the top of that list. A lot of times you can talk to a tube vendor and ask them to for tubes that tested high for gain and low noise, matched.....etc........

 

What I do is try and buy them in lots then use my tube tester to match them up before I put them in circuits.

 

I also have a good habit of looking at tube datasheets before I start throwing money out the window.

Edited by thesloth
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Well I guess that about sums it up, its funny how Roy recommends the EH's for the z mod board.  But again the board is designed for more of a Dynaco PAS3, I don't know what that has to do with it.  But as long as the JJ's seem to work I will stick with it.

 

thanks for all your help.

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