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Old 3rd April 2013, 05:47 PM   #13011
rade95 is offline rade95  United States
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Quote:
Originally Posted by 6L6 View Post
Change the (4) feedback resistors to 220 ohm 3W. That seems to be the magic number to get it to a 'normal' gain.
Hi Jim, I remember us talking about it. Will try 220 ohm x 4 instead of original 100 ohm x 4. Increasing JFETs source resistors should decrease the gain, according to the formula so I'll leave them @ 10 ohm. Any idea how would this change affect the bias? FB was briefly discussed on page 1112 of this thread. thanks
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Old 4th April 2013, 12:32 AM   #13012
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There are a few dedicated gain stages developed just for the purpose of driving the amp to full power output with a standard signal input - "preamp for F5 thread", I think.
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Old 4th April 2013, 12:57 AM   #13013
6L6 is offline 6L6  United States
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I would turn the bias down or off, then replace the feedback resistors. I wouldn't try to keep the same bias setting when changing the components.

Re-bias and see if it comes up properly... It most likely will be fine.
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Old 4th April 2013, 01:57 AM   #13014
rade95 is offline rade95  United States
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Originally Posted by jameshillj View Post
There are a few dedicated gain stages developed just for the purpose of driving the amp to full power output with a standard signal input - "preamp for F5 thread", I think.
I think this is the thread you are referring to:
Preamp ideas for F5

But, I don't like the idea of pre-amping the F5 for more gain. I already have a pre-amp that can drive 8-10W out of a stock F5. I will stick with changing FB resistors to 220 ohm, like Jim suggested and leave it like that. Then I'd rather design a new amp with a BJT driver stage, bump up PS to +/- 50-60V and push F5 to class AB. This could produce 50W with a single MOSFET pair or approx. 100W with 2 MOSFET pairs. Have the circuit ready. Oh and I am replacing the IRFs with Toshiba K1058/J201 laterals. More to come...
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Old 4th April 2013, 02:14 AM   #13015
rade95 is offline rade95  United States
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More feedback and gain discussions on page 1154, for those interested...newbies like myself.
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Old 4th April 2013, 09:38 AM   #13016
Zen Mod is offline Zen Mod  Serbia
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page break is browser and user dependable

post exact # of posts you referring to
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Old 4th April 2013, 01:46 PM   #13017
rade95 is offline rade95  United States
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page break is browser and user dependable

post exact # of posts you referring to
Really, so if I send a link with the page number: F5 power amplifier

This will surely take you to page 1154, but you may not see post # 11531 at the beginning? I am using Safari 602.
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Old 4th April 2013, 02:04 PM   #13018
labjr is offline labjr  United States
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My user settings on this forum are set for 5 posts per page so I can view pages easier on my phone. Thus page numbers are meaningless. When I click on your link I see post #5766 which is about half of #11531.
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Old 4th April 2013, 02:08 PM   #13019
labjr is offline labjr  United States
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I think if you do something like this. It will take you to the exact post and not a virtual page.

F5 power amplifier
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Old 4th April 2013, 04:36 PM   #13020
rade95 is offline rade95  United States
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Originally Posted by labjr View Post
I think if you do something like this. It will take you to the exact post and not a virtual page.

F5 power amplifier
Yes, just figured out you can copy a link to the actual post #. thanks
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