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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 13 Apr 2014, 20:16 
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Joined: 11 Oct 2011, 19:39
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I have built the following amp using the schematic below:

http://diyaudioprojects.com/Schematics/ ... Amplifier/

I breadboarded one channel of this amp prior to starting my build and it worked well. Since building it onto a plate I am seeing the following problems:

1) The feedback resistor on one channel will scorch within a matter of about 1 minute.
2) No audio output

I tested both the 6sl7 tubes and the 6v6 tubes.
1) The 6sl7 tubes test as new
2) The 6v6 tubes test low to replace. One or two of them test as "ok"

I printed out the schematic and numbered the tube pins on the schematic. I color coded the wiring and marked such on the schematic.

The only difference that I made to the build is to make this an integrated amp. I built the 6n7 "heavy metal" preamp into the same box and that is actually working properly. I saw the amplified signal on the output of the pot that I was running into the amp (cd player). That feeds directly into the 10k/470k input of the 6sl7.

The output transformers are old hammond output transformers. I used one of them when I breadboarded the amp (1 channel) and it worked fine. I should have tested the other.

I went back over the schematic/build tonight. I verified the pin numbers from the datasheets to the schematic, and then traced each connection from the schematic to the amp build. There were no issues there that I found. I checked each solder connection and resoldered any that looked remotely questionable.

I am wondering if I am actually seeing the result of two different issues here...... a bad OPT and bad tubes in the output. I can't think of a reason that the feedback resistor would be drawing current like that unless there were a shorted winding. Then again, the 6sl7 cathode in the driver doesn't have a direct path back to ground for DC, only AC signal.

I have no more 1k resistors to replace the bad component with at the moment, so I will need to order some.

Anyone have any ideas as far as the feedback resistor? Do I sound like I am on the right track for t/s the problem?


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PostPosted: 13 Apr 2014, 20:53 
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The circuit must be oscillating.

In normal operation, that resistor will dissipate 197mW at full output clean sine.

Cheers!

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PostPosted: 13 Apr 2014, 21:28 
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Hi Geek, thanks.

I put the grid resistors butt up to the pin on the socket as well as the other resistors. The PS is clean, so what else would make it oscillate?


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PostPosted: 13 Apr 2014, 21:38 
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Do you have the primary of the OPT backwards? Another problem maybe layout..... been guilty of both myself :blush:

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* Ratings are for transistors - tubes have guidelines*
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PostPosted: 13 Apr 2014, 22:06 
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More information.....

Just now I was tracing the signal through one side of the circuit (the one without the burned feedback resistor..). I was getting amplified signal from the preamp to the 6sl7, but not out of the 6sl7. I started hearing a faint popping sound coming from the 6sl7 socket and then the cathode cap from pin 6 to ground blew.

One thing that I did see was noise in the driver circuit, but I am wondering how much of that is coming from the ac heater. I had planned to go with dc but changed my mind mid build.

The sockets that I am using are the flat mount Russian military sockets that use the retaining ring of spring steel to hold them in. I have built pre-amps with those before and never had an issue.

The driver/power tubes that I am using are ebay purchases. The 6sl7 tubes tested 100% on my eico 666 tester, so I believe that they are good. Obviously as far as that one channel goes I have wired something very incorrectly, but I am just not seeing it right now.

I will take some pics and post them.


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PostPosted: 13 Apr 2014, 22:19 
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Hi, If you leave the feedback resistor off, does the amp work. If so it would indicate that the phase of the output transformer could be backwards and cause oscillation. But still it should still not fry the resistor. My guess is that the output transformer has failed and the B+ is getting into the circuit. There are other possibilities but few would fry the resistor.

Good listening
Bruce

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PostPosted: 13 Apr 2014, 22:26 
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I'm thinking that is the case. I will order a set of new 7.6k Edcors and rebuild the circuit from the ground up. I still want to debug this one just for personal edification though. I will start taking it apart tomorrow and testing out the segments to see where some of the issues lay.

The power supply and the heavy metal preamp worked perfectly though.... heh... so at least that's something. :-\


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PostPosted: 13 Apr 2014, 22:40 
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#1 DO NOT DISCONNECT THE FEEDBACK RESISTOR! In this design the feedback resistor and the transformer secondary resistance (usually just a few ohms) serve at the driver cathode biasing circuit. Disconnect it and your driver will go into runaway. (Which seems to be happening anyway!)

The resistor is burning out because it is handling too much tube current. If both plate resistor are properly connected, the max current it could be seeing is 6mA (assuming a 300V B+; 300v/50kΩ) This is only 36mW, so I doubt that your resistor is seeing current from the driver stage alone. High current is coming from somewhere. This may sound strange, but make very sure that the ground at the base of the output stage biasing network (250Ω-47µf) is good. If it is built like it is shown in the schematic (people often do this even without realizing it) then a bad ground there will positively bias the off side driver grid and allow output stage current through your feedback resistor. How is the 100Ω between the driver cathodes holding up?

This is a strange design. The attempted balancing of the offside grid with no AC coupling to ground makes me nervous. Take another look and see what you can find.

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PostPosted: 13 Apr 2014, 23:05 
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Will do Matt.


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PostPosted: 13 Apr 2014, 23:18 
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One thing that is different between the breadboard build and the actual build was that I used tag board for the section between the driver and power tubes. I also used Kester solder whereas before I used Chemtronics. I doubt that the solder made any difference, but maybe the tag board instead of the tag strips (like those found in old radios, etc..)...... dunno... it's late and I am sort of guessing. I will go through the circuit and check for bad solder connections, etc...


Matt.... the 100 ohm resistor is fine. No issues with that. The cathode cap on the driver did blow out though on my last test though.

I am really wondering what that popping sound is coming from the driver socket(s).....It doesn't really sound like a high voltage arc or anything.... sort of like when your stove top is heating up when you first turn it on. That sort of sound. I thought maybe it was just the tube itself heating up right up until the cap blew.....


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