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Post subject: grrove tubes?
Posted: Wed Feb 26, 2014 5:54 am
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I am going to buy some new power tubes for my amp which is a 93 Blues Deville. There is no bias pot on the board, so rather than have one installed or taking it to a tech I was thinking of buying groove tubes. I was under the impression that if you replaced the stock tubes (no idea if the ones in my amp are original or have been replaced) with the same type of groove tubes then there was no need to bias the tubes. Now I have recently discovered that you have to replace the tubes with ones of the same rating and colour...is this correct? So if I swapped high rating tubes with low rating tubes i'd have to bias them? If so, could it be possible that the person who previously owned my amp did not know this and could have potentially put the wrong rated tubes in? I am half thinking perhaps I should have a bias pot installed and just put non groove tubes in, then the next tube change I make I can do myself and buy cheaper tubes.

My questions are;

1: If I replace the current tubes with ones of the exact same type, is there still a chance my bias could be off due to previous owner/s not using exact same tubes as replacements? (ive been thinking the amp may be getting too hot but its my first tube amp so not sure).

2: Would I be better off just getting a variable pot installed so I can buy non-groove tubes and then bias myself in future?


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Post subject: Re: grrove tubes?
Posted: Wed Feb 26, 2014 7:24 am
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lodgi wrote:
...I was under the impression that if you replaced the stock tubes (no idea if the ones in my amp are original or have been replaced) with the same type of groove tubes then there was no need to bias the tubes....you have to replace the tubes with ones of the same rating and colour.


Correct on both counts.

lodgi wrote:
...if I swapped high rating tubes with low rating tubes i'd have to bias them?


Correct again.

lodgi wrote:
....could it be possible that the person who previously owned my amp did not know this and could have potentially put the wrong rated tubes in?...


Who knows what the previous owner did?

lodgi wrote:
...2: Would I be better off just getting a variable pot installed so I can buy non-groove tubes and then bias myself in future?


Well, you could be certain of the bias if you check it yourself, and a bias adjust pot would certainly allow you more flexibility in tube choices.

It is always a good idea to check the bias of the output tubes when you get a new (or new to you) amp, and also when installing new tubes. Even if you install the exact same tubes, they should be checked to be sure that the current is where it is supposed to be, and to be sure that the tubes are matched. Tubes aren't perfect and sometimes you can get a dud, or even a mismatched set. No way to know unless you check.

Typically, many Fender amps are biased cold from the factory, including the BDvl, and can benefit from a bias check and adjustment.

So to sum up, I think that a bias adjust pot is important, and knowing the bias is important also.

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Post subject: Re: grrove tubes?
Posted: Fri Feb 28, 2014 5:12 am
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Thanks for the reply. It is as I thought, i NEED to get the bias pot installed :(


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