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Oddwatt Push Pull Tube Amplifier Projects
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Author:  Danny [ 06 Dec 2018, 12:20 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce & Team,
The latest findings. Sorry to say that:
1) grounding to the signal grounds the metal cases of the coupling caps did not make a difference,
2) placing a short between the grid of the lower triode section of the driver stage and ground, did not make a difference also. The noise is still there and at the same level.

Danny

Author:  gofar99 [ 06 Dec 2018, 21:26 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, Hummm. What I would like you to do is describe exactly (as possible ) what is still going on as I have lost track of all the things we have tried and we should start to trouble shoot from the basics. Clearly we are missing something. Additionally please tell us about the set up / configuration and everything that is connected to it. I don't know about anyone else, but I have lost track of what has been tried and what happened. I appologize for that, but with four fourms, 5 email accounts, a business web site and bunches of individual inquiries I sometimes get lost. BTW, where do you live? There may be a forum member that is nearby and can assist.

Good listening
Bruce

Author:  poty [ 07 Dec 2018, 03:30 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Maybe contradicting with the previous message:
- taking out the driver tube eliminates the noise;
- switching off the block (working just on the capacitors charge) eliminates the noise;
- blocking input signal (grounding input RCA or the lower tube grid) does not help;
- grounding FT-3 capacitors does not cure the problem;
- changing ground layout does not help also.
What may I suggest to try: remove the LR8 and replace it with a resistor (the value of the resistor can be estimated from the previous versions of the power secion of the driver tube or just measuring the voltage between in and out of LR8 and count).

Author:  Danny [ 13 Dec 2018, 11:57 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

What may I suggest to try: remove the LR8 and replace it with a resistor (the value of the resistor can be estimated from the previous versions of the power section of the driver tube or just measuring the voltage between in and out of LR8 and count).

Poty,

OK. Thanks.
See the before and after figures. I want to make sure I get it right. Is this what you are suggesting to do?
What are you trying to determine? If the LR8 is introducing oscillations?
What do you mean by "count" in "measuring the voltage between in and out of LR8 and count".

Bruce,

I live in NJ.

Thanks,

Danny

Author:  Jim_O [ 13 Dec 2018, 19:11 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

I've been following this thread and the troubleshooting steps. The one thing that stood out for me was that the noise disappeared immediately when Danny switched off the AC power. The audio continued, noise-free, until the capacitors discharged. Switching off the AC removed whatever ripple content that remains in the rectified and filtered DC. Is it possible that the 120 hz ripple could be exciting some weird oscillation in the LR8? Maybe just a freak thing with just the right amount of capacitance around the LR8? I don't know if that is even possible, but....
The test that Poty suggested, removing the LR8 from the circuit will be interesting....

Author:  poty [ 14 Dec 2018, 02:19 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Danny wrote:
See the before and after figures. I want to make sure I get it right. Is this what you are suggesting to do?
Yes, but if you have a capacitor on hand (voltage - more than 350V, capacitance 40-100uF) please put it across the 0.22uF. I also doubt you'll have to remove 910R/220k divider
Danny wrote:
What are you trying to determine? If the LR8 is introducing oscillations?
Any weird thing with LR8. As Jim_O already mentioned - all the experiments points to the power supply circuit.
Danny wrote:
What do you mean by "count" in "measuring the voltage between in and out of LR8 and count".
Sorry, my English is far from perfect. There is a voltage which is dropped on LR8 (let's call it Vio). There is a current for driver part (Id = 1mA for SRPP + 1.3mA for 910/220k divider + 1mA for heaters' lifting). You can use the Ohm law to count the resistor with the same voltage drop: R=Vio/Id. Alternatively you can determine Id just by measuring the voltage across the 33k resistor (Vr): Id=Vr/33k.

Author:  Danny [ 22 Dec 2018, 16:47 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Well, I am happy to finally report that my white noise/hiss problem has been resolved.
As suggested by poty, I replaced the LR8 circuitry with a 33K resistor in series and an additional 47uF cap in parallel and the amp is now dead quiet. I can finally rest...and enjoy the music.
I don’t have the proper equipment to fully analyze this issue. So, I leave it to you, the experts, to discuss why the LR8 would cause this. Oscillations issues?

Also, I while ago I reported that I had a problem where the voltage across the test points at start up would hover up to 240mV for several minutes and then settled to the correct setting of 92 mV.
On further investigation it turns out that it would do this routinely and not just at start up. Anyway, I thought that I had solved the problem by removing a terminal block that I was using to connect the LM317 to the mother board. But, this was incorrect. The problem was sporadic in nature. Thus, difficult to observe. Finally in desperation I replaced the LM317 and the problem went away.
Thus, LM317 can also fail in this peculiar fashion. Where they work sometimes and sometimes they decide to go crazy.

Thanks to all for the support. Especially poty and Bruce. Now, I need to go and replace the LR8 on the second monoblock.
Thanks to all,

Danny

Author:  gofar99 [ 22 Dec 2018, 20:06 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, I kind of wonder if you were sold either seconds or bogus parts. I have probably personally used 100 of each type and never had a problem that was not caused by some "oops" on my part. In the comercial kits they are virtually never at fault for an issue (except for someone elses oops). Having several go "bad" in your builds is extremely unusual. Most curious.

Good listening
Bruce

Author:  Danny [ 23 Dec 2018, 11:27 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Bruce,

You may be right. But I get my parts from Mouser. I would think that they are reputable. I have purchased lots of stuff from them and never had problems. However, I am intrigued to find out if the LR8 you use would work. Can you tell me the part number?
The one I got from Mouser is https://www.mouser.com/ProductDetail/68 ... 89-LR8N3-G.
The LM317 is https://www.mouser.com/ProductDetail/92 ... HVT%2fNOPB.

One thought - these parts are static sensitive. Maybe I did not handle them correctly?

Thanks,
Danny

Author:  gofar99 [ 23 Dec 2018, 15:49 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, I have used their parts often and never had a problem. The LR8 is slightly sensitive to static, but not like a CMOS chip. I don't handle them with a lot of care. They will fail if improperly connected or shorted. I have never seen a noisy one or one that oscillated if used correctly. The LM317 is nearly bullet proof. I don't think you can harm one easily. Over votlage is the only way I have found. Even then they will usually put up with votlages about twice the rating and survive fine. If you over heat them they can fail though. Your problems and findings are very strange. That is why they took so long to discover. Things that don't normally behave that way are not generally suspected as cause factors. Maybe they got some duds.

Anyhow I'm glad it has finally and is working now.

Good listening
Bruce

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