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PostPosted: 01 Jan 2020, 12:28 
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Joined: 08 Jul 2019, 06:05
Posts: 8
Location: Ottawa, ON, Canada
Hello:

Happy new year folks!

My 6N1P’s showed up. As it turns out, they are not a drop in replacement because pin 9 is connected to a single ended screen that divides the two triodes. It seems most common advice is to ground this.

I am considering not grounding the screen, to avoid disconnecting that pin in case I end up going back to ECC8x tubes (which is likely). So I would just disconnect my heater supply from pin 5, and install a jumper from 9 to 5. This would have the effect of leaving the screen elevated which seems fine to me. Beneficial maybe?

Any opinions on this? Should I leave pin 9 elevated or ground it?

Best,

David


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PostPosted: 01 Jan 2020, 13:10 
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Joined: 08 Jul 2019, 06:05
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Location: Ottawa, ON, Canada
Here is the ECC82 or ECC802S pinout:

Attachment:
12AU7A_diag.JPG



Here is the 6N1P pinout:

Attachment:
6N1P_diag.JPG


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PostPosted: 01 Jan 2020, 20:26 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, They are more like 6DJ8/ECC88 than the ECC802/82/ 12AU7 and will probably work ok. They do have the different use for pin 9 as you discovered and different heater voltage between pins 4 and 5.

Good listening
Bruce

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Some of my DIY Tube Amplifier Projects:


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PostPosted: 01 Jan 2020, 22:26 
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Location: Ottawa, ON, Canada
Thank you Bruce.

I left the screen elevated, and I have listening to them today. I like them more than the RCA cleartop 12AU7A’s I had in there. The system sounds very nice.

The increase in gain is minor, the treble very similar to the RCAs (a touch more subdued in comparison), the bass is more prominent, they are quieter, and (less importantly) not microphonic at all (the cleartops were very much so).

I will be leaving them in!

Best,

David


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PostPosted: 02 Jan 2020, 20:28 
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Joined: 07 Jan 2018, 07:13
Posts: 50
Hi,

I built a Forewatt a couple years ago per the latest schematic with the regulated B+ with great results driving a pair of tube monoblocks I built as well.

My question is how does this preamp do powering SS amps. I’m planning on making an amp using class D Icepower modules as the output stage. Should anything be tailored in the Forewatt to drive SS amps?

Thanks


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PostPosted: 03 Jan 2020, 00:51 
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Joined: 23 Feb 2017, 02:02
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You need to account the low output impedance by using the correct output capacitor sizing so the base is not cut off.

Only concern


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PostPosted: 03 Jan 2020, 01:08 
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Location: Ottawa, ON, Canada
The latest Forewatt schematic shows the 1 MOhm resistor to ground on the output. The note says the resistor might be required if the amplifier has an internal blocking capacitor.

I omitted this resistor but I have been wondering if this might explain a wicked buzzing through the speakers when I connect the Forewatt to my SS amp. No issues when I connect the Forewatt to the tube amp but the buzz on the SS amp makes it unusable.

What would the symptom be if the SS amp had a internal blocking capacitor and the 1 MOhm resistor on the Forewatt was omitted?

Best regards,

D


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PostPosted: 03 Jan 2020, 06:52 
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Joined: 07 Jan 2018, 07:13
Posts: 50
Another question I have that goes along with my last question in my previous post is the Ice Power modules I plan to drive have a max signal input voltage of +-12V. The specs in the Oddwatt page state that the output into a 50k load is in excess of 20V. The Icepower modules have an input impedance of 47k.

Will this overvoltage pose a problem?
Could the 1k resistors in series with each output of the Forewatt be increased to drop the output voltage?
Or would I need a line output transformer in the Forewatt to drop the voltage of one of the outputs?

Your help is appreciated!
Thanks


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PostPosted: 03 Jan 2020, 17:13 
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Joined: 23 Feb 2017, 02:02
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1M ohm on otput to ground is mandatory for any amplifier.

Don't have it and the capacitor will directly pass DC through it, can fully destruct a amp if you connect this to an input, though it will very quickly discharge to zero volts when plugged in.

All amps need less than 6V pk-pk of input voltage, normally arroun d 0.5-2V pk-pk.

20V pk-pk is impossible unless you delibratley crank up the volume so much it will damage the amp.

Theree is no such thing as over voltage and it will not damage the valves, those boys can drive allot of power and is unlikley to die.


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PostPosted: 03 Jan 2020, 17:49 
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Joined: 07 Jan 2018, 07:13
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Thanks,

How does the 1M keep the cap from passing DC? I thought all caps blocked DC.

The Forewatt specs on the Oddwatt website state the 20V output. That’s where I got that from. I was concerned that that amount of voltage would create a problem for the IcePower class D modules I plan to drive with the Forewatt. Ice power states 12v max for signal input voltage.

Is there a way to measure the output voltage of the Forewatt? I’m assuming it’s not as simple as measuring voltage with a meter at the output jacks.

Thanks


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