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Old 16th July 2013, 03:18 PM   #61
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Quote:
Originally Posted by jazbo8 View Post
Thanks for the tip, I think this is the one you were referring to, got some good design examples and even some listening impressions...
Most people fail to adjust their feedback resistor to account for the much higher plate resistance of the pentode driver. they consequently up the feedback and tighten up the bass considerably. Adjust the feedback resistor appropriately and the sound will be less different to the original RH design - but the distortion will be lower. Most casual experimenters are not comparing like with like when they try a pentode driver because they failed to understand the basic design principle.

In most of my designs I hang the feedback resistor off the plates of the output tube and feed all of the driver current through it. i learn't this technique from Gary Pimms Tabor amp. This gives very high levels of feedback and turn's almost all of the output tube mu into gm. The effect is to turn the output stage into a Current dumper for the driver. This will not be to many peoples tastes - but it produces wonderfully tight and detailed bass which is what I want from an amp.

Shoog
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Old 16th July 2013, 10:32 PM   #62
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Have you got a quick schematic of that Shoog?

I'm always looking for a better high-feedback output stage to control the transformer better,
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Old 17th July 2013, 05:17 AM   #63
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Quote:
Originally Posted by Shoog View Post
Most people fail to adjust their feedback resistor to account for the much higher plate resistance of the pentode driver. they consequently up the feedback and tighten up the bass considerably. Adjust the feedback resistor appropriately and the sound will be less different to the original RH design - but the distortion will be lower. Most casual experimenters are not comparing like with like when they try a pentode driver because they failed to understand the basic design principle.

In most of my designs I hang the feedback resistor off the plates of the output tube and feed all of the driver current through it. i learn't this technique from Gary Pimms Tabor amp. This gives very high levels of feedback and turn's almost all of the output tube mu into gm. The effect is to turn the output stage into a Current dumper for the driver. This will not be to many peoples tastes - but it produces wonderfully tight and detailed bass which is what I want from an amp.

Shoog
Ran a quick sim on the Pimm's 6AU6 DC-coupled plate-to-grid FB circuit, it sure needs a lot drive with so much feedback, so I guess a pre-amp would be needed. Yet another design to try when I am done with my move...

Also, you were kind not to correct me earlier, the thread you mentioned should be this one instead, not the one I posted.. the "why pentode?" thread contains some measurement data, thanks!
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Old 20th July 2013, 01:04 AM   #64
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Quote:
Originally Posted by jazbo8
Ran a quick sim on the Pimm's 6AU6 DC-coupled plate-to-grid FB circuit, it sure needs a lot drive with so much feedback, so I guess a pre-amp would be needed. Yet another design to try when I am done with my move...
You might check your sim. I've never used a preamp on any of the local feedback amps on my webpage.

The various sources used over the years have all been in the 1.5V to 2V range for 0dB.

The only amp I have sensitivity measurements for is the 47-PP. 350mv gets you 1 watt @.4%, 537mv gets you 2.2 watts @3%.

2.2 watts may not seem like much but when it's connected to a 112dB/watt CD+horn it's plenty...
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Old 20th July 2013, 03:41 AM   #65
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Yup, the 6AU6 was not bypassed, duh, it's all good now.
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