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Old 13th September 2018, 05:46 PM   #1621
Itsmee is offline Itsmee  England
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Maybe
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Old 13th September 2018, 06:23 PM   #1622
Zen Mod is offline Zen Mod  Serbia
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dunno ,in XA25 constellation (read - sans source resistors) pucks are just too beasty , to leave them without constant surveillance** , regarding output offset

**without NFB loop
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Old 17th September 2018, 09:23 AM   #1623
flancfuster is offline flancfuster  Europe
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I just did a couple of tests with tube front end. I'm using a PCC88 tube with a CCS loading the plate, operating at 85V and 8mA in anode follower configuration. Found out that I need a low impedance, low distorsion FE to drive the output stage, otherwise I'm getting HF attenuation.
XA25-ECC88.png
I need to add a cathode follower.
test.jpg
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Old 17th September 2018, 10:26 AM   #1624
analog_sa is offline analog_sa  Europe
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Quote:
Originally Posted by flancfuster View Post
I need to add a cathode follower.
For me sound-wise that would be the last resort option. Rather use a lower Rp valve or an srpp.
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Old 17th September 2018, 12:19 PM   #1625
flancfuster is offline flancfuster  Europe
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Modified the input stage, added quickly a cathode follower, now the frequency response increased to 100Khz(measurement taken at 21Vrms). Distortion wise I've got 0.5% at 21Vrms. Feedback resistor is 4.7Kohm.
XA25-ECC88-cathode follower.png
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Old 19th September 2018, 07:24 AM   #1626
flancfuster is offline flancfuster  Europe
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Did some listening tests and I find out that I need to carefully choose the GNF resistor, with 4.7K resistor the sound in bass and low midrange was shy, with 10K the sound in lower region had more weight. Did anyone else who built a version of the beast noticed that?
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Old 19th September 2018, 09:13 AM   #1627
generg is offline generg  Germany
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I had the same impression some time ago.

Changing the bias also influences the sound quality, stps of 0.1A are to be heard.
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Old 19th September 2018, 12:45 PM   #1628
Botte is offline Botte  United States
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Originally Posted by flancfuster View Post
Did some listening tests and I find out that I need to carefully choose the GNF resistor, with 4.7K resistor the sound in bass and low midrange was shy, with 10K the sound in lower region had more weight. Did anyone else who built a version of the beast noticed that?
So are you saying that you increased the gain of the amp and by virtue reduced the negative feedback by that amount?

I run 4.7K to the 330 for the feedback. If i changed that to 10K i would have a gain of 29dB and reduce the loop feedback by 6dB. This would definitely reduce the damping factor and increase the base sound but would also increase the DC output drift.

My two cents.
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Old 19th September 2018, 12:48 PM   #1629
flancfuster is offline flancfuster  Europe
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I had the same impression some time ago.

Changing the bias also influences the sound quality, stps of 0.1A are to be heard.
I remember you mentioned before that you have J2 at home, what is listening impression comparing with the beast?

On my amp I perceive a very "clean" and dynamic sound but not harsh, comparing with other amps I have.
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Old 19th September 2018, 12:54 PM   #1630
flancfuster is offline flancfuster  Europe
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So are you saying that you increased the gain of the amp and by virtue reduced the negative feedback by that amount?

I run 4.7K to the 330 for the feedback. If i changed that to 10K i would have a gain of 29dB and reduce the loop feedback by 6dB. This would definitely reduce the damping factor and increase the base sound but would also increase the DC output drift.

My two cents.
I see, what you DC output with 4.7K? Mine is around 80mV with any value of feedback resistor. The FE is capacitor coupled with OS.
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