Grid Bias Voltage

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Grid Bias Voltage

Postby vez on Wed Jun 21, 2017 7:50 pm

Where does the grid bias voltage for the phase inverter in my BFSR come from?
Thanks!
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Re: Grid Bias Voltage

Postby stratele52 on Wed Jun 21, 2017 10:06 pm

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Re: Grid Bias Voltage

Postby stratele52 on Thu Jun 22, 2017 8:24 am

Why are you asking that ? Amp do not work ?
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Re: Grid Bias Voltage

Postby pdf64 on Thu Jun 22, 2017 8:28 am

It comes from the voltage developed across R51 http://schems.com/schematicheaven.net/manu/fender/65_super_reverb.pdf
The dc reference point for the grids is at the node of R49, R50, R51 and R52.
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Re: Grid Bias Voltage

Postby stratele52 on Thu Jun 22, 2017 8:35 am

pdf64 this is not the Silverface shematic , you post the 65 reissue.

Here the right one ;
http://ampwares.com/schematics/super_reverb_ab763.pdf

from ; http://ampwares.com/?era=silverface&line=super
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Re: Grid Bias Voltage

Postby vez on Thu Jun 22, 2017 11:32 am

stratele52 wrote:http://www.aikenamps.com/index.php/the-long-tail-pair

Thanks Stratele52. I had a feeling it was the feedback loop. I've heArd of people leaving this open on some amps to get more distortion. That would really hose up this amp.
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Re: Grid Bias Voltage

Postby pdf64 on Thu Jun 22, 2017 4:30 pm

Sorry I should have explained that I used the RI documentation because the circuit is basically the same and the part number references may be beneficial in reducing misunderstanding when communicating with someone who, eg may not appreciate the difference between the tail resistor and the cathode resistor.
Note that due to the 2 ohm output but the same 820/100 feedback sample, the SR has a slightly lower degree of negative feedback than the 4 ohm 2 channel amps.
Also opening the feedback loop changes other system characteristics as well as increasing gain some, eg output damping/frequency response (more treble and bass), overdrive character (smoother onset)...

There are numerous forum threads about modding a CVR back into a VR, a major part of which is reinstating the feedback loop.
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Re: Grid Bias Voltage

Postby vez on Thu Jun 22, 2017 6:10 pm

Im posting the full story with pix in a new thread.
Thank you for the replies and to answer stratele52's question, the amp runs good now but I have a few questions that will be posted in the new thread.
Thanks!
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