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Post subject: Fender 57 Twin-Amp Custom (Changing tubes need an advice)
Posted: Thu Apr 19, 2012 3:29 am
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Hello to everyone :D !Need a professional advice.I am going to change tubes in my amp,usually i havent done it by myself yet.Now I am using in the amp - Power tubes are a matched set of vintage USA Phillips 6L6GC's . The four 12AX7's are a mixed group with two Sovteks, a Ruby Tube and a Fender Groove Tube and Rectifier tubes are a matched set of Russian Electro Harmonix.
So now i want to put in a power section 2 matched Vintage Tung Sol 6L6WGB/5881 ( the year is 1962 i think)and 4 matched 12ax7 RCA(1957 year).

The question is- Can i plug them in and play without biasing the amp(and other manipulations (checking voltage etc) and without damaging the amp.

Thanks for Answering!


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Post subject: Re: Fender 57 Twin-Amp Custom (Changing tubes need an advice
Posted: Thu Apr 19, 2012 5:17 am
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Even if there is a low risk of damage I say that it is a bad idea because you will not adapt your valves to your amp.

It can give a good bias by pure chance given the expected sound (best case), a cold bias given a bad sound due to distortion at cross zero, an over heating bue to hot-hot-hot bias reducing the livetime of your valves.

With care, you can do it yourself with a voltmeter.
But you have to open your amp, and be prudent with high voltage that can be lethal for human.
http://support.fender.com/schematics/gu ... ematic.pdf
On TP2 you have to measure -50V by adjusted by R46 (bias adjusting).

If you don't feel it, it is better to make it by a good amp tech.
8)

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Post subject: Re: Fender 57 Twin-Amp Custom (Changing tubes need an advice
Posted: Thu Apr 19, 2012 5:27 am
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Thanks for the answer :D
so its impossible just to plug them out and put any NOS tubes without biasing?


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Post subject: Re: Fender 57 Twin-Amp Custom (Changing tubes need an advice
Posted: Thu Apr 19, 2012 6:25 am
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Best practice is to check/adjust the bias when changing output tubes or rectifier tubes. You get the best sound, and longest life from the tubes when properly biased. As Tis-san stated; you might get lucky and install a set of tubes and the bias might happen to be close enough to work fine. And as Clint Eastwood stated; "Do you feel lucky....? Installing these tubes without checking the bias is risky, despite the stories about "the old days when we would just replace one tube of the set of output tubes without worrying about bias, blah, blah". Things are different now, we know better, not all tubes are made by RCA now, and many amps now have a bias adjust, unlike the "old days". The only time that you can change output tubes or rectifier tubes without checking the bias, is if you use the exact same tubes for replacements. And even then, it's a good idea to check the bias to be sure that they are exactly the same and that the tubes are functioning properly.

Rather than rely on the TP voltage for bias setting, I would want to check the Plate current (or Cathode current) and Plate voltage for a more accurate bias adjust, especially in this particular amp because the TP is only the grid voltage, not a current TP. The voltage at the TP for this particular amp will result in different current for different tubes, and the TP voltage is often considered to result in too cold a bias for many Fender amps. But, using the TP voltage and adjusting per the schematic is better than just putting in the tubes without a check.

No adjustment or check necessary when changing preamp and PI tubes, they are plug and play.

There are plenty of amps that get new tubes installed without a bias check, and many of these amps have no bias adjustment pot. Some work fine and some do not. The BJr being a good example, no TP and no bias adjust pot, so many people just change the tubes and most work fine, as they are running the tubes very hot anyway. If there is a bias adjust on the amp, best to use it.

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Post subject: Re: Fender 57 Twin-Amp Custom (Changing tubes need an advice
Posted: Thu Apr 19, 2012 6:40 am
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Appreciate your answer!Thanks a lot!


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Post subject: Re: Fender 57 Twin-Amp Custom (Changing tubes need an advice
Posted: Thu Apr 19, 2012 3:29 pm
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As long as you are replacing tubes AND properly biasing the amp, you should replace V1, V2, and V3 with NOS 12AY7s like the original 57 Twin vice 12AX7s like the reissue uses.

http://ampwares.com/schematics/twin_5e8a.pdf

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