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Oddwatt Push Pull Tube Amplifier Projects
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Author:  Danny [ 05 Nov 2018, 15:14 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce,
Ok. Thanks. So, basically what you are saying is that I should have separate driver stage and output tubes stage grounds returning to the power supply, correct?
Thanks,
Danny

Author:  gofar99 [ 05 Nov 2018, 18:13 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, Yes, sort of. Think of the ground system as both a reference point and a way to conduct "used" current from the tubes back to the power supply. If you run the current from the power tubes through the same conductor that is used as as ground reference for the signal then it will cause noise. Any wire regardless of size will have some resistance and the current flow will cause a noise voltage in it. Just physics. This current caused noise is effectively added to the input signal and amplified by the circuitry. What I prefer to do is a sort of modified star arrangement. I attach the signal ground, the power supply ground and any others like from the cathode circuits to a common point at the ground lug of the input signal jack. From there I run a large conductor to the ground terminal of the output transformer. That way none of the conductors will cause current flow in any other and the input and output signals have very close to the same reference point. If you choose to use a "buss" arrangement then the way things are arranged on the buss matters. At one end would be the power supply ground, next the cathodes of the output tubes, then the cathodes of the drivers, then speaker ground terminal and finally the input signal ground. Both ways work but I personally find the modified star to work best in my builds.

Good listening
Bruce

Author:  gonzalo guerrero [ 05 Nov 2018, 20:08 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

gonzalo guerrero wrote:
my stereo is coming!!!

Author:  Danny [ 06 Nov 2018, 14:48 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce,

OK. Thanks.
Do the 2 diagrams below correctly depict what you are describing? Want to make sure before making any modifications.
Note that at the far end of the Power Supply GND (where the AC comes in) I connect PS GND to Frame GND using a 120 ohms resistor and a 0.1uf type X2 capacitor in parallel.
I assume that this is the only place where I should connect to Frame GND.
Thanks,

Danny

Author:  Danny [ 06 Nov 2018, 14:53 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

In case you do not have MS-WORD.

Author:  gofar99 [ 07 Nov 2018, 17:29 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, Yes. Either can work well. My preference is the "star" sort of arrangement. Hoever, the chassis ground should be isolated from the signal ground by either a reverse pair of rectifiers (in parallel with each other and facing in opposite directions) or a type X2 capacitor with a parallel resistor of around 120-150 ohms. In days long gone by the signal ground and chassis were often the same. With current electrical codes and a better understanding of ground loops nearly all gear now has them separated. Just watch for duplicate paths that could cause ground loops both inside and outside of the gear.

Good listening
Bruce

Author:  Danny [ 12 Nov 2018, 12:26 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce,
Hi. Sorry to say that changing to the "star" grounding system did not make a difference.
The background/white noise is still there. If you recall, with the 5751 driver tube removed the noise disappeared.
Thus, should I reconfigure the amp to use a 12SL7? Would the 12SL7 be quieter?
Another option would be to use less-efficient speakers. Currently mine are 96 db/watt.
Thanks,

Danny

Author:  gofar99 [ 12 Nov 2018, 17:02 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, My experience is that the 5751 and 12SL7s are about equal in back ground noise. Considering that some of my speakers that have been used on the amps are only a few dbs in sensitivity below yours (My Altecs are about 94-95 dbw) and they are quiet I suspect we are missing something in the source of the noise. To refresh my mind....what happens if you place a short between the grid of the lower triode section of the driver stage and ground? Does the hiss go away or stay the same? What is the brand of the 5751? If you have a12AX7 or one of the EC or ECC equals you can try it in the amp. Do they have more or less hiss? Is the hiss lower or higher when the NFB is on or off? Does the NFB alter the gain? Is there anything near the amp tha might introduce noise into the tubes by direct radiation? Have you scoped the output? Check at least to 250KHZ. Hiss could be a byproduct of oscillation or EMI. Scope the B+ and see if you are getting something like switching noise from the rectifiers.

Good listening
Bruce

Author:  poty [ 13 Nov 2018, 10:52 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

I'd add another "test": you can switch off the block and listen to the noise. If the noise stops abruptly - the problem is in power supply circuits, otherwise - it can be any other mentioned areas.
I'd also disconnect the LED illuminating the driver tube, just for test.

Author:  Danny [ 19 Nov 2018, 12:41 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce & Potty,
Thanks for the replies.
The 5751s I am using are made by Sovtek.
I tried an EH 12AX7, but that did not make any difference. The hiss is the same.
With the NFB on the gain is lower. Which I believe is what's expected.
The noise stops instantly when I turn the power off. However, when the B+ kicks-in (after the time delay) it takes a few seconds for the noise to come-up to full level.
I do not have an oscilloscope. Thus, I cannot scope the output or B+. I just have a digital multi-meter.

Can you elaborate of the switching noise from the rectifiers issue? And how would I go about resolving it?

Next things I will try and let you know:
1) disconnect the LED illuminating the driver tube,
2) place a short between the grid of the lower triode section of the driver stage and ground.

Thanks,
Danny

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