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PostPosted: 04 Aug 2011, 15:04 
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Location: Santander, Spain
Hello,

I've been working in a diy audio proyect that is described in this post http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3269. I managed to solve the issue about the relay that was causing the problem, so the PSU is now working perfectly. I finally received the shorted valve replacement and I was able to made some tests. The preamplier works but not properly. I noticed a problem in the upper tube of the mu-follower stages (the setup in the preamplifier is the following: a solid state based buffer + mu-follower + Baxandall tone control + mu-follower). I used a couple of E88CC bottles: one for the lower tubes and the other for the upper tubes. I used an elevated supply for the heaters of upper tubes so that cathode to heater voltage rating is not exceeded (the supply is elevated like 220 Volts). All measured voltages seems to be OK but one: upper tube grid voltage. I am getting 64 Volts instead of about 217 Volts, so the upper tube is off. I swapped the bottles to check that is not a problem in a tube. Apparently, all conections are done properly. The problem is observed in both mu-follower stages. The design is almost equal to the one that can be found in Morgan Jones book. I used PSpice to make the simulations. I attached a schematic with the measurements so that you can compare them with theorical values.

Did anybody attempt to build a mu-follower stage? Do you have any ideas of why the grid of the upper tube is getting that low voltage?
Attachment:
measurements.png

Thanks for your help!
Jesús.


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PostPosted: 04 Aug 2011, 15:18 
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Hi, The circuit looks OK and the SIM values are probably about what I would expect. I have concerns about the size of the grid resistor on the upper triode. It may be a bit too large. I would have expected to see about 220K not 1meg. With the voltages you show on the cathode and anode of the upper tube, there is no way the grid can be at 64V. I suspect that where you are measuring it is not a good connection. With 64 on the grid and 218 on the cathode the tube would be so far cut off that it would never conduct. What is the amplifier doing that is incorrect?

Good listening
Bruce

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PostPosted: 05 Aug 2011, 09:26 
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Rather odd that 64V is seen at the upper grid even though the voltage at the plate of the lower tube is 82V . Check your wiring . A much more elegant solution to the mu follower is to use a solid state CCS which avoids the elevated heaters , enables much higher output swing and simplifies chassis layout . A depletion mosfet strapped as a CCS can also provide a low impedence 'mu' output from the source pin

BDA


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PostPosted: 05 Aug 2011, 16:47 
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Hi,

Are you breadboarding this? May be a bad pin on the board. Move it over one.

Cheers!

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PostPosted: 07 Aug 2011, 03:54 
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Thank you all for your answers. We agree that the situation is not possible unless the circuit is oscillating. My suspection is that either the multimeter is not working well when measuring high voltages at high impedance grids because of bootstrap operation (mutimeter probe may have 1 MegaOhm internal impedance) or the 10 nF capacitor is leaky.

The amplifier seems to work ok, so there may not be a problem in mu-followers. It is fairly quiet, no hum/noise at all. I sometimes listen some quiet discrete random noises. They can be compared with the noise that is produced when turning on a dirty/old potentiometer but more quietly. Initially, I thought that was caused by electrons that escape from rectifier tube (one E88CC tube is fairly close to the rectifier bottle). To overcome this I wrapped the rectifier with a grounded aluminum foil to screen it. Noises didn't dissapear, so the rectifier is not causing that.


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PostPosted: 07 Aug 2011, 16:23 
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Hi, The noises can be from several sources. My best guess is that the individual tubes are noisy. I buy tubes in groups of 4-10 and all the ECC88s and 6DJ8s have noise levels above what I would expect. Several brands as well. So even though they have nice sound I find them unsuitable for high gain low noise circuits. For your use they may be fine, but given a choice I would use different tubes. If gain is not a big issue then 12AU7 or one of its variants (I particularly find ECC802S to be quiet and good performers). If you need gain then a member of the 12AT7 family would be the choice. They can deliver a fairly large amount of power. 6 or 12SN7s would also be good picks.

Good Listening
Bruce

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PostPosted: 07 Aug 2011, 17:00 
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Thanks for the answer, Bruce. From reading I understood that this tube is particularly quiet because of its high transconductance (low plate resistance). The tubes that I am using are from Tube Amp Doctor (http://www.tubeampdoctor.com/). The TAD tubes are highly appreciated over guitarrists' community due to their performance and results. TAD people sell tested tubes in their premium brand for low noise/hum/microphonics. I think that a non-tested low quality tube can be noisy, but that means a continuous background noise, i.e. what you listen in a radio when no radio station is tuned. What I get is some discrete random noises; sometimes I listen nothing, sometimes I listen something quietly that can be compared with the sound produced by a dirty volume potentionmeter. In conclusion: I think that E88CC tubes are fairly noiseless and TAD tubes are trustworthy.


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PostPosted: 07 Aug 2011, 17:15 
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Hi Another possibility is EMI from either a local radio/TV station, appliance, florescent ligh or perhaps even something like a PC. I have to shut off my UPS in my workshop to do serious S/N measurements as it radiates at several frequencies. At one time before I completely separated my audio system from the TV I was getting odd noises. It turns out that the cable TV system was using the wires to send other signals (phone and data) and they were getting into the audio. Your solution might be as simple as a few ferrite beads or some small RF chokes at the inputs.

Good listening
Bruce

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PostPosted: 16 Aug 2011, 05:01 
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Sorry, I've been busy lately and wasn't able to continue with this thread. I've played this amplifier with several instruments and the sounding is absolutely wonderfull: it has plenty of details and its harmonics are awesome. It has very high gain, so I need to turn the volume down so that not to overdrive the power stage. I find it fairly noisy when I connect it to a mixing console (I can listen clearly a background noise). When I connected it to a tube based guitar amplifier I didn't listen any background noise. As I stated I don't think that the signal valves are noisy. EMI from other electronic sources in addition to the high gain can explain the background noise and the random noises that I sometimes hear, as Bruce pointed out. The amplifier is built inside a steel-aluminum box which is connected to earth via the mains earth wire in a single point near the mains inlet, so this acts as an electromagnetic screen. I also used an aluminum plate connected to this chassis to screen power supply circuit from signal amplification circuit. The connection between chassis and signal ground is made at the input jack socket. Actually, output jack socket is making a second ground connection to chassis, but ouput jack socket is fairly close to input jack socket. I wonder if that small ground loop is the source of noise.

Following Bruce suggestion about adding some ferrites beads to inputs, where do you mean to add them? To the input jack signal contact?


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PostPosted: 16 Aug 2011, 11:48 
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Hi, Generally the right stuff to do. I would separate the grounds as follows. Not have either the input nor output directly connected to the chassis ground. I find that placing a type X2 capacitor of about 0.1 to 0.22 uf with a parallel 120-150 ohm resistor between the chassis and the circuit ground eliminates virtually all ground loop problems and still retains the shielding and safety from shock ability of the chassis. The ferrites would go as close as possible to the input jack. They may also be of benefit on the output one, but this is seldom the source of EMI. Another possibility to interface the two units is to add a "level" control at the input so the relatively high output of the mixer is reduced to a better level for the amp. Alternatively you could have it connect after the first preamp stage of the amp.

Good listening.
Bruce

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