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Post subject: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Spkrs?
Posted: Fri Apr 11, 2014 11:45 am
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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Fri Apr 11, 2014 12:00 pm
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What is a miod? For that matter what is a Twin reissue? I don't believe the Twin was ever reissued.

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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Fri Apr 11, 2014 4:24 pm
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Mod - and I'm referring to 65 Twin Reverb re-issue.

Everyone already knew that but thanks for pointing it out.


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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Fri Apr 11, 2014 4:43 pm
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R36 is the tail resistor for the phase inverter circuit. Someone has removed the negative feedback resistor and capacitor (R39/C39) and replaced them with this resistor to provide negative feedback from the secondary side on the OT to the phase inverter. I can't tell what the resistor value is to say what the feedback percentage is.

http://support.fender.com/schematics/gu ... ematic.pdf

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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Fri Apr 11, 2014 4:47 pm
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Ryan41018 wrote:
Mod - and I'm referring to 65 Twin Reverb re-issue.

Everyone already knew that but thanks for pointing it out.


Actually, no. A Fender "Twin" is a completely different amp from a Fender "Twin Reverb", that's why shimmilou asked. But you obviously didn't know that. It helps to use the correct name so that we know which schematic to look at.

http://ampwares.com/?brand=fender&line=twin

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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Fri Apr 11, 2014 4:53 pm
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I believe that R36 is the "tail" resistor in the long-tailed paired phase inverter section. Seems that the mod is going from OEM 22K-ohms to 47K-ohms.

Doubling this resistor's value should halved the current flow. All other things being equal, this will significantly reduce the idle bias of the PI tube. Anyhow, good bit about long-tailed PI's used by Fender in Valve Wizard's site:


http://www.valvewizard.co.uk/acltp.html



http://bmamps.com/Schematics/fender/65_ ... erb_sm.pdf


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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Fri Apr 11, 2014 5:25 pm
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BMW2002Ti wrote:
I believe that R36 is the "tail" resistor in the long-tailed paired phase inverter section. Seems that the mod is going from OEM 22K-ohms to 47K-ohms.

Doubling this resistor's value should halved the current flow. All other things being equal, this will significantly reduce the idle bias of the PI tube. Anyhow, good bit about long-tailed PI's used by Fender in Valve Wizard's site:


http://www.valvewizard.co.uk/acltp.html



http://bmamps.com/Schematics/fender/65_ ... erb_sm.pdf


BMW, read my first post above and look closely at the photo and schematic. R36 is still in place. The modification is the resistor suspended above R36 (I cannot read the value). R39/C39 are what have been removed and replaced by the new negative feedback resistor. The amount of feedback provided by the new resistor can be calculated as follows:

http://ampbooks.com/home/amplifier-calc ... /feedback/

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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Fri Apr 11, 2014 6:38 pm
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Bluesky, ok thanks!

I haven't heard of many mods to the tail-end of a long-tailed paired PI. But, it is one of the big differences between late model SF and earlier BF amps.

Mods to GNFB is much more mainstream.

:D


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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Fri Apr 11, 2014 8:44 pm
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BMW2002Ti wrote:
Bluesky, ok thanks!

I haven't heard of many mods to the tail-end of a long-tailed paired PI. But, it is one of the big differences between late model SF and earlier BF amps.

Mods to GNFB is much more mainstream.

:D


You're welcome.

The previous owner (?) apparently didn't like the amount of feedback provided by the stock circuit and decided to change it. It would be nice to know what the new resistor is so the new value could be calculated. The stock TRRI (and original) used a lot of negative feedback to help maintain the clean sound of the amp at high volume levels. I'm betting the amount of feedback was decreased (larger NFB resistor) in an attempt to get more "grit" out of the amp.

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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Sun Apr 13, 2014 5:52 pm
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Bill - you are exactly right! The amp jumps into overdrive quick with the volume at 4 and sustains nice the higher the volume.


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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Sun Apr 13, 2014 6:23 pm
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Ryan41018 wrote:
Bill - you are exactly right! The amp jumps into overdrive quick with the volume at 4 and sustains nice the higher the volume.


What is the resistor color code?

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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Fri Apr 18, 2014 6:46 pm
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Bill - Just read your last comment. In not sure but I'll post it when I get my ipad open.

OH and bias (MA) would also be affected right?


Thanks!


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Post subject: Re: Is this a Miod? 1997 Fender Twin Reissue R36 Down to Sp
Posted: Sat Apr 19, 2014 5:17 pm
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Ryan41018 wrote:
Bill - Just read your last comment. In not sure but I'll post it when I get my ipad open.

OH and bias (MA) would also be affected right?


Thanks!


Nope. The negative feedback loop has nothing to do with power tube bias.

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