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PostPosted: 30 May 2020, 17:56 
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Kornbread wrote:
The adult beverage's purpose is twofold. There are times, especially after a rough day at work, that good music and a cold beverage hit the spot and when sitting beside the speaker in a picture, it serves as a size reference.
Thanks for the compliment. Colter Wall and Norah Jones sounds pretty special on those speakers with the 4s and a small TriPath chip-amp driving them. Probably have a half dozen, if not more speakers in various stages of completion sitting around, and a closet full of drivers to build more. Most my own design, others not.
Should get in some fancy wirewound resistors and Jantzen blocking caps for the 4s today. Well see what kind of difference they make, if any.

Personally I prefer the look of these.

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PostPosted: 18 Jun 2020, 11:04 
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I've just tried to build this and I'm having some trouble... which seems funny because its way simpler than my last two builds. But, those two were my first builds, so there is every chance I am making some bonehead mistake.

Basically I am getting no signal output. I have a B+ of 260v and with either of two brand new 12au7 tubes, I see 68v at pins 1 and 6. And I see roughly 2.6v at pins 3 and 8.

This is almost exactly what user mtlsir reported on page 8 of this thread. But i never saw a post concluding how or if he fixed the problem.

I can post some photos later if no ideas are forthcoming. I did rebuild it once in an effort to locate the issue and not surprisingly, it dosn't look as tidy is it did the first time.

As always, thanks for any help.


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PostPosted: 18 Jun 2020, 13:44 
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The 68v plate voltage implies 1.92mA of plate current (i.e. (260v-68v)/100kΩ). The design point for the 12AU7 is 1.88mA so this is pretty close. That's good. This means that your DC biasing is likely correct.

I noted that there were several possibilities back in 2015 and I reiterate those now:

1) The coupling caps are conducting DC. So check the caps for leakage. If the caps were not rated for at least 400vdc they could have been blown by the higher than anticipated B+. Not all caps fail with a bang. This can cause all kinds of bad behavior.

2) Some components are not the values marked. This is a long shot, but it has been known to happen. However, like problems in both channels make this unlikely.

3) There is something wrong with the way voltages are being measured or problems with the voltmeter itself.

At this point it would be a definite help if you could post some detailed pictures of your build and layout. Sometimes just getting more sets of eyeballs on something can help find something that has been overlooked.

Let us know what you find.

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PostPosted: 18 Jun 2020, 22:54 
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Ok, well... I really tried to triple check all the basic stuff before taking up any of your time but... sigh*... after all that, the problem was, I had the input and output RCAs mislabeled on the housing. How embarrassing.

Thanks for the reply, in any case!


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PostPosted: 19 Jun 2020, 08:58 
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Minarai wrote:
How embarrassing.

Don't feel bad. We've all done things like that from time to time. Here's an example: https://www.cascadetubes.com/2017/05/18/one-little-transposition/

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PostPosted: 19 Jun 2020, 19:44 
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Suncalc wrote:
Minarai wrote:
How embarrassing.

Don't feel bad. We've all done things like that from time to time. Here's an example: https://www.cascadetubes.com/2017/05/18/one-little-transposition/

Over the last fifty years - way to many times.

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Projects:“Salt Cellar” - Mono 807 triode integrated amp |
"Rosso" - E88CC SRPP preamp | ”Salt Cellar II” - 6EM7 mono wine box triode amp | “MWH” - 1920s UX-171A preamp |


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PostPosted: 12 Aug 2020, 16:03 
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Hola!

Since my last post, I have built several 4S preamps. All with solid state power supply.

Now I am building one for a friend. Not finished yet, but in the photos you can see how the project is doing.
Attachment:
IMG-2839.jpg

Attachment:
IMG-2738.jpg


I added an old General Electric voltmeter to indicate the plate voltage (a decorative touch, nothing more).

But I have a question: I have a 2M Ohms volume potentiometer. Can I use it at the input and a fixed 250kΩ resistor at the output?

It is the only good quality potentiometer I have for now, so I would like to use it if I can.

I await your comments. Thank you very much!

Sebastián


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PostPosted: 12 Aug 2020, 18:52 
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2M is very high when generally 250K is used. Try it but you may get hum. You could put a 470K resistor across the pot but it may make the pot action a little strange. Again an easy try.

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Projects:“Salt Cellar” - Mono 807 triode integrated amp |
"Rosso" - E88CC SRPP preamp | ”Salt Cellar II” - 6EM7 mono wine box triode amp | “MWH” - 1920s UX-171A preamp |


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PostPosted: 12 Aug 2020, 21:26 
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Sebastián wrote:
But I have a question: I have a 2M Ohms volume potentiometer. Can I use it at the input and a fixed 250kΩ resistor at the output?
No. The maximum grid circuit resistance is 1MΩ with cathode bias for these tubes. You risk thermal runaway with a grid circuit resistance this large. The fixed 250kΩ on the output is fine.

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PostPosted: 13 Aug 2020, 07:04 
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Thanks for the reply, Matt and Mark.

I'll get a suitable potentiometer.

Best regards,

Sebastián


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