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PostPosted: 30 Nov 2015, 07:10 
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Joined: 09 Jul 2012, 14:49
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This is a new version of my Forewatt to match the Groovewatt I posted a while back. Thanks again Bruce for a great design. It is dead silent.
I am getting around 3.5v at the bottom cathodes with 820 ohm resistors and bypass caps. Is this ok?
I have set B+ at 225v from the LR8 regs. Filter string uses 1K5's and gives 254v before the regs. Valves are Jan6189W.

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PostPosted: 30 Nov 2015, 14:28 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Nice looking build. The voltage is OK. The one that is most indicative of proper operation is the one at the anode of the lower tube. It ought to be around 1/2 the B+ applied to the upper anode. I find that the individual triodes in the tubes are often not well matched and as long as the difference is not more than say 20% everything will be fine. Widely differing values will only limit the max output and in extreme cases result in non linear performance.

Good listening
Bruce

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PostPosted: 30 Nov 2015, 15:55 
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Thanks Bruce, I'll check that out. Both preamps sound great but I am interested to see if operating points are in the correct area. I was a little concerned about the relationship between B+ and cathode resistors as there have been a fair few options posted along the way on this thread. I figured that if there was an issue i could revert to the 215v B+ and 470R cathodes.
Regards
John


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PostPosted: 01 Dec 2015, 05:29 
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Joined: 24 Nov 2015, 04:55
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Location: San Francisco, CA
Thank you Bruce and Poty for your responses to my question. I'm on a pretty tight budget so I'm trying to get the best bang for the buck. So I take it that it would be ok to use the Russian hybrid caps and still get half-way decent sonic performance? My second choice would be Wima, or if the Dayton's from PE would work well, I could use those.

The OPA2134 phono stage I'm considering uses two stages per channel, uses passive RIAA equalization, and does not employ negative feedback. S/N is suppose to be better than -90dB. I haven't entirely ruled out the Groovewatt, of course, but there is so much positive feedback on the opamp version that I thought I'd at least give it a try. Here's a link to the project in case you're interested: http://audiokarma.org/forums/index.php?threads/another-super-high-end-phono-stage-no-expense-spared.371889/

Speaking of the Groovewatt, is it possible to use a 6DJ8 in the front end? The reason for asking is because I have the Amperex Bugleboy version in my stash. If not, then I have some NOS Amperex 12AX7 that I can use. These were intended to be used in the Tektronix o'scopes in our labs. One of the perks when I was working for a large high tech company that has been around since WWII is that I got access to a lot of interesting components when they got phased out, including various vacuum tubes, some mil spec.


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PostPosted: 01 Dec 2015, 08:32 
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Hi, The 6DJ8 doesn't have sufficient gain for this circuit. I just completed one using 6S52N Nuvistors and they work fine. They have a stated gain of 60. In the circuit the expected gain per stage (of the symmetrical SRPP) is about 1/2 the gain of a tube so in theory anything with a gain over 50 would probably be OK. Circuit values would have to be adjusted to account for voltage ratings (low for the Nuvistors), current flow and anode impedance (affects the RIAA section). Go for low noise and linear gain in the first stage especially. With the Nuvistors the overall gain is just below 40db. Just about perfect for MM/MI cartridges.

Good listening
Bruce

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PostPosted: 09 Jan 2016, 07:53 
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Thanks for your answer Bruce. I was just digging around my tube collection and realized that I have a bunch of NOS JAN tubes waiting for a home. Of particular interest are 5963, 6189, and 5814A. All of these are military grade 12AU7 equivalents. I have several dozen of the 5814A, so I'm hoping it will work in place of the 12AU7/ECC82. The majority of them are the RCA black plate with double mica, rated for very low noise. I also have a lot of mil-spec high-mu tubes too, in case I want to build your Groovewatt phono-preamp. Thanks for your time.


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PostPosted: 09 Jan 2016, 08:44 
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Hi, The 12AU7 and its variations are fine in the Forewatt. I'm not sure the lower noise will be obvious though as mine using JJECC82S are already at the bottom or what I can accurately read for S/N. Typical among the ones I have (and the commercial ones as well) is a S/N of about -92-95 dbv wideband no weighting. Everything else in the system will nearly certainly be far noisier. One thing may occur is a slight "flavoring" of the sound as different tubes while supposed to be the same sound different. It makes tube rolling fun. The JJs tend to be fairly neutral, I would suspect based on some others I have tried that the industrial types might be a tad warmer.

Good listening
Bruce

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PostPosted: 28 Jan 2016, 02:09 
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Location: Zagreb, Croatia
Hello everyone,
I am a forum and a tube virgin, so please be gentle. I am in a process of building, actually I thought I finished it yesterday however I have some issues.
My transformer secondary is 240V AC vs recommended 200V AC, and although I am using two 2,2K dropper resistors as in the original schematics, I have 320V DC before LR8N. When I tried LR8N without tubes connected I could easily regulate that input to 250V DC output. But now with tubes connected I am only getting 33V DC on one channel and 25V DC on the other. Is my in-out voltage difference to high, and LR8N are protecting themselves?


Kind regards

Marko


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PostPosted: 28 Jan 2016, 21:33 
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Hi, Welcome to the forum and tube diy. Yes it is very likely the LR8s are in protective mode. They have internal circuitry that can determine if the power level will exceed their max ratings and they will shut down to a low voltage. You will need to put more resistance in the power supply before the LR8s and get it down to something in the order of 275 or even down to 260. Below that they will not act as filters as well as they can, but the circuit will work. Another possibility is that there is a wiring error after the LR8s and they will sense it as a partial short circuit and also shut down. In any case be careful not to accidently short the output side as the IC will fail immediately. You can tell if one does as it stops regulating and lets nearly everything through in about 99% of the failures I have observed. During the design process for some of my projects I smoked a number of them. BTW if the LR8 is a standard "N3" series they can only handle about 3/4 of a watt. Replacing them with the "K4" series raises the rating to just over 2 watts. They are somewhat harder to use as they are designed for surface mounting. I use a "surf board" on mine to mount them. It is a little PC board with solder pads and has pins on one side. You can either solder the board in place or use standard in line strip pins to fasten them. There is another version that falls in between as well in a single in line case. IMO it is not enough bigger to justify it though and last time I checked they were more costly than the other types.

Good listening
Bruce

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PostPosted: 29 Jan 2016, 02:39 
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gofar99 wrote:
Hi, Welcome to the forum and tube diy. Yes it is very likely the LR8s are in protective mode. They have internal circuitry that can determine if the power level will exceed their max ratings and they will shut down to a low voltage. You will need to put more resistance in the power supply before the LR8s and get it down to something in the order of 275 or even down to 260. Below that they will not act as filters as well as they can, but the circuit will work. Another possibility is that there is a wiring error after the LR8s and they will sense it as a partial short circuit and also shut down. In any case be careful not to accidently short the output side as the IC will fail immediately. You can tell if one does as it stops regulating and lets nearly everything through in about 99% of the failures I have observed. During the design process for some of my projects I smoked a number of them. BTW if the LR8 is a standard "N3" series they can only handle about 3/4 of a watt. Replacing them with the "K4" series raises the rating to just over 2 watts. They are somewhat harder to use as they are designed for surface mounting. I use a "surf board" on mine to mount them. It is a little PC board with solder pads and has pins on one side. You can either solder the board in place or use standard in line strip pins to fasten them. There is another version that falls in between as well in a single in line case. IMO it is not enough bigger to justify it though and last time I checked they were more costly than the other types.

Good listening
Bruce


Hello Bruce,
thank you for your prompt answer and for sharing your project. I had a quick listen yesterday, a real gem you created, apart from that you dont know if it is off or on without pilot lamp :)
I never expected something valve based to be so noise free, prejudice I guess.

I lowered the input voltage to LR8 to 270V, no problem regulating to 250V now, however as a result I of that action my voltage via voltage divider for heater elevation also went down from 90V to 71V, should I do something about that? BTW I connected that voltage to my heater secondary center tap (3.15-0-3.15), I presume this is the best place to do so.

After LR8, I have 4,7uF Poly rated for 250V, I observed that on startup full B+ voltage is present on them for a short time, should I exchange them for higher voltage rated?

Currently my heater voltage is 6.88V DC, a simple C-R-C filter only, will the LM317 be able to regulate with only 0.8V droput voltage?

Lots of question, I apologize for that but I want to extract absolute best from you design, I think it deserves it.

Kind regards

Marko


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