The Dude Posted August 15, 2015 Share Posted August 15, 2015 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. Quote Link to comment Share on other sites More sharing options...
Luv_sum_Horns Posted August 15, 2015 Share Posted August 15, 2015 Are the pin sizes the same on the 803's and 12ax7's? I've seen 12ax7's but not familiar with the 803's. 1 Quote Link to comment Share on other sites More sharing options...
jimjimbo Posted August 15, 2015 Share Posted August 15, 2015 If you are using a mouse, turn off your touchpad on the laptop. Or, adjust the sensitivity of the touch pad 1 Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 15, 2015 Author Share Posted August 15, 2015 Are the pin sizes the same on the 803's and 12ax7's? I've seen 12ax7's but not familiar with the 803's. I don't know, I wouldn't think so but you never know. How could you tell? Quote Link to comment Share on other sites More sharing options...
Luv_sum_Horns Posted August 15, 2015 Share Posted August 15, 2015 Just sayin if the pins are bigger on one set than the other set they may stretch the pin contacts so when you switch back the contacts are loose. If you had calipers you could measure the pin diameters, if not just use your calibrated eyeballs. Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 15, 2015 Author Share Posted August 15, 2015 If you had calipers I do have access to a pair, maybe I will give that a try next week. thanks for the idea. Quote Link to comment Share on other sites More sharing options...
Luv_sum_Horns Posted August 15, 2015 Share Posted August 15, 2015 Looked up the ECC803 and it looks identical to the 12AX7 and they are definitely listed as compatible. Just a quick SWAG. Don't imagine you have swapped tubes enough in the two years to have stressed out the connectors. Just sounds like a connection issue. Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted August 15, 2015 Share Posted August 15, 2015 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 2 Quote Link to comment Share on other sites More sharing options...
Luv_sum_Horns Posted August 15, 2015 Share Posted August 15, 2015 Tube fanatic to the rescue! Whew Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted August 15, 2015 Share Posted August 15, 2015 Can you post a schematic? Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 15, 2015 Author Share Posted August 15, 2015 Can you post a schematic? This is the best I can do. PH6 Schematic and board layout.pdf Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted August 15, 2015 Share Posted August 15, 2015 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. 1 Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 16, 2015 Author Share Posted August 16, 2015 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. Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 16, 2015 Author Share Posted August 16, 2015 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. Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted August 16, 2015 Share Posted August 16, 2015 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. 1 Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 17, 2015 Author Share Posted August 17, 2015 I was thinking between the phono board and selector switch. But if it wouldn't help and we are certain that it's just the output gain of the tube I will be good with that. Would there be another tube that would work good. Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted August 17, 2015 Share Posted August 17, 2015 (edited) 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 August 17, 2015 by thesloth 1 Quote Link to comment Share on other sites More sharing options...
The Dude Posted August 17, 2015 Author Share Posted August 17, 2015 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. Quote Link to comment Share on other sites More sharing options...
Guest thesloth Posted August 17, 2015 Share Posted August 17, 2015 thanks for all your help. You are very welcome 1 Quote Link to comment Share on other sites More sharing options...
mkane Posted August 17, 2015 Share Posted August 17, 2015 https://www.tubesandmore.com/sites/all/modules/custom/tech_corner/files/12ax7_comparison_of_current_made_tubes.pdf Quote Link to comment Share on other sites More sharing options...
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