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Post subject: Headroom in a Blues Jr.
Posted: Thu Feb 05, 2009 12:08 pm
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I am thinking of trying to get a little more headroom with my Blues Jr. If I were to replace one of the 12AX7s with either a 12AT7 or 5751 would the amp need to be re-biased? Would I put it in v1 or v2? Any other suggestions?
Thanks.


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Posted: Thu Feb 05, 2009 12:51 pm
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With a preamp tube replacement there's no need to "bias". The Blues Jr. and Pro Jr. are biased hot from the factory. The only way to alter that would be to get a tech to add a bias pot and adjust from their.
a 5751 should lower the gain and give you a bit more headroom but you may also wish to buy a "matched" set of EL84s that have a lower gain rating than the stock ones. Again, an experienced tech can help you out with that but it's not something you should try on your own unless you have a background in schematic reading, voltage measurements and safety regarding dealing with such items. An amp can kill you even if it's unplugged.
HTH.


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Posted: Thu Feb 05, 2009 2:42 pm
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Thanks. Here's the deal: I usually play with the volume set at 12 oclock and the master volume wherever I need the sound volume to be. If I want more overdrive I can turn the volume knob higher, hit the fat switch or if for some hard to imagine reason wanted even more drive I could hook up my Bad Monkey. If I still didn't have enough sound volume I could turn up the master volume which of course would not reduce the drive. On the other hand if I want to play clean I need to set the volume knob at 9 oclock. Then if I turn the master volume all the way up I get a clean and fairly loud sound but not anywhere near the potential volume of the amp. So it seems to me the "problem" is with the preamp tubes overdriving too soon, not the output tubes. Am I getting this wrong?

You mentioned trying a 5751 but not an AT7, why is that? Eurotubes uses the AT7 in its low gain Blues Jr. retube packages, but Dave Hunter of the "The Guitar Amp Handbook" says the AT7 is not really a direct replacement for a 12AX7 (different impedence). Any input is welcome.


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Posted: Thu Feb 05, 2009 3:22 pm
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The AT7 is a different preamp style tube I think. It's usually in reverb circuits and the 5751 is a lower gain than the 12ax7. Check out the tubestore.com for a better explanation. Bob @ Eurotubes knows which end is up and I recommend him from personal experience. He's helped me retube my 1968 Princeton and my Pro Jr.
Try this for kicks:
Set the drive aat 5 and the Master at 7-8 and see if there's headroom that you can clip when you have the guitar's volume wide open. With my PJ I tend to have the Volume @ 7-8 and the tone 9-12 and clean up my tone with the guitar's volume rather than relying on ODing it with a pedal.
Hope this works and I'm interested in what you do. Keep me posted.


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Posted: Thu Feb 05, 2009 4:42 pm
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Grant1 wrote:
Thanks. Here's the deal: I usually play with the volume set at 12 oclock and the master volume wherever I need the sound volume to be. If I want more overdrive I can turn the volume knob higher, hit the fat switch or if for some hard to imagine reason wanted even more drive I could hook up my Bad Monkey. If I still didn't have enough sound volume I could turn up the master volume which of course would not reduce the drive. On the other hand if I want to play clean I need to set the volume knob at 9 oclock. Then if I turn the master volume all the way up I get a clean and fairly loud sound but not anywhere near the potential volume of the amp. So it seems to me the "problem" is with the preamp tubes overdriving too soon, not the output tubes. Am I getting this wrong?

You mentioned trying a 5751 but not an AT7, why is that? Eurotubes uses the AT7 in its low gain Blues Jr. retube packages, but Dave Hunter of the "The Guitar Amp Handbook" says the AT7 is not really a direct replacement for a 12AX7 (different impedence). Any input is welcome.
If you really want to maximize your headroom you should try to put the master at 12 and your volume to taste.This is what i do and it works really good.I use my tubescreamer as my od.


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