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Old 17th September 2020, 11:03 AM   #741
rock12 is offline rock12  United States
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Is it ok to use carbon film resistors
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Old 22nd September 2020, 09:09 AM   #742
itsikhefez is offline itsikhefez  United States
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Carbon film is probably fine in most positions.. theoretically the circuit will be quieter with metal film.

George, hoping to get your thoughts on the compensation cap value.
I've tried 120pF, 270pF, 390pF and 470pF.
As you increase the value, the square wave gets nicer.

The issue though is that beyond 120pF, the amp distorts at about 3W of output.
I think this could be similar to an issue rkinze mentioned with new 12AT7's, although I've tried 3 different NOS types, a Siemens & Halski, Brimar and GE 6201 and all of them have the same issue.
I've also tried various output tubes JJ and NOS types, as well as both UL and Pentode connection, with the issue the same.

Technically, I can keep the amp with 120pF as I can drive the amp to clipping without observing this distortion, but the square wave is pretty ugly.
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Old 13th October 2020, 08:07 AM   #743
itsikhefez is offline itsikhefez  United States
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I still have the instability issue as demonstrated above.
I have a different thread with more detailed information, the latest in this post: EL84 measurable distortion between different makes

If anyone has an idea of where to start looking, please let me know.
Thanks!!
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Old 17th October 2020, 10:09 PM   #744
itsikhefez is offline itsikhefez  United States
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If I'm not using a pot, can I replace the 220K resistors across the input to something like 50K ?
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Old 7th November 2020, 08:58 PM   #745
itsikhefez is offline itsikhefez  United States
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I'll give this thread another shot, as most of my questions above were left unanswered.

With UL and feedback the input sensitivity is about 0.5V for max output.
Is it OK to replace the 12AT7 with a 12AU7 ?
My DAC outputs 2V so I'm throwing away alot of signal and don't have much usable range on the pot.
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Old 7th November 2020, 09:02 PM   #746
Duke58 is offline Duke58  United States
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itsikhefez-- not being a smart aleck- why not go ahead and try it? What's the worst thing that can happen?
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Old 7th November 2020, 09:13 PM   #747
itsikhefez is offline itsikhefez  United States
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Yea agreed, I am going to go ahead and do that. Still wanted to hear from someone with the expertise if there any issues of doing this.
I don't trust my ears enough to know if this is can cause some subtle problem with the amps operation.
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Old 7th November 2020, 09:18 PM   #748
Duke58 is offline Duke58  United States
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Tubelab says he blows things up so we don't have to. I doubt that you'll blow up any tubes switching out the 12AT7.
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Old 7th November 2020, 09:59 PM   #749
Einric is offline Einric  United States
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I've rolled SO many different 12AU7 types into my SPP.
My favorite is the 6414.
The 12AT7 Baldwin Japan Blue Tip is really nice too.
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Old 7th November 2020, 11:28 PM   #750
Tubelab_com is offline Tubelab_com  United States
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Quote:
If I'm not using a pot, can I replace the 220K resistors across the input to something like 50K ?
Yes, any resistor less than 220K is fine as long as your source can drive it. I have used 10K with a solid state source.

Quote:
Is it OK to replace the 12AT7 with a 12AU7 ?
I have also tried lots of different tubes in the 12AT7 socket. Some work, some don't. Any tube that has the same pinout should do something, and not blow any parts. Something with largely different characteristics may need some resistor value changes.

I wouldn't use a 12AX7 since they don't like to run over 2 to 3 mA.
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