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Post subject: Valve question
Posted: Tue May 10, 2011 1:18 pm
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Hi All,
My 15 year old son has a Twin Reverb, someone kindly spilled Vimto over it and one of the valves blew. My question is do we have to replace the valve exactly like for like? The blown valve is a GT-6L6-R 3 Blue; could we replace it with a GT-6L6-R 7 White? If not, then why not?

Apologies for our ignorance but even our local Fender stockist couldn't answer our question


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Post subject: Re: Valve question
Posted: Tue May 10, 2011 1:29 pm
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Theoretically, power tubes should be replaced as match pairs (or in the case of the Twin Reverb, with a matched quartet). That said, it's likely possible to replace the damaged bottle with a new one of the same brand and similar performance specifications. The bias should be checked (and if necessary, adjusted) once the new tube is installed.

The "someone" should be taken to a public loo and receive a chocolate swirlie as a reward for his "kindness".

:lol:

Arjay

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Post subject: Re: Valve question
Posted: Tue May 10, 2011 1:33 pm
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Thanks retoverbial, I've read similar statements about pairs and quartets and checking the bias elsewhere; but can someone tell me why?? What'll happen if we put in a valve with a different rating?


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Post subject: Re: Valve question
Posted: Tue May 10, 2011 3:03 pm
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The output stage of your amp relies on symmetry within and between the two matched pairs of output tubes -- each matched pair "pushing" then "pulling" against one other through the primary winding of the output tranny in theoretical equilibrium. All tubes possess slightly different transconductance specs and conscientious vendors try to match them as closely as possible using complex test equipment. Once a pair (or quad) is matched, for optimum performance they should always be used together as a set.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Valve question
Posted: Tue May 10, 2011 11:23 pm
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Arjay, you could have been a politician; plenty of information without answering my question :)

If we put an incorrectly rated or odd valve into the amp, what effect will it have? Will one of the lower rated valves quickly fail? Will the sound be obviously affected? Will the amp even work??

I know we could just try different things but we haven't really got the cash to waste on experimenting


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Post subject: Re: Valve question
Posted: Tue May 10, 2011 11:32 pm
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Baggins wrote:
Arjay, you could have been a politician; plenty of information without answering my question :)


What makes you think I'm not?

:mrgreen:

Baggins wrote:
If we put an incorrectly rated or odd valve into the amp, what effect will it have? Will one of the lower rated valves quickly fail? Will the sound be obviously affected? Will the amp even work??


If the replacement tube is correctly operating (regardless of brand or measured transconductance) the amp will work. Likely it will sound fine. It's possible that the service life of all the power tubes may be compromised but this scenario is impossible to precisely calculate.

Baggins wrote:
I know we could just try different things but we haven't really got the cash to waste on experimenting


I understand -- I hope I've allayed your concerns.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Valve question
Posted: Wed May 11, 2011 12:35 am
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+1 and Additional to what Arjay said, as an example to what may happen - if the single replacement tube is unmatched to the others by a decent margin, it "may" in fact draw more current and run much hotter and wear much faster (hopefully short of red-plating!) at the same bias setting the other 3 matched tubes were quite happily running with. So by the time you bias to get that one new tube running at a reasonable idle dissipation, the original 3 tubes may then be running as cold as a mother-in-law's kiss. So with the one tube not being matched it can be difficult/impossible to get the bias set optimally for the 4 tubes as a set (or in pairs), hence matching = good.


Snowy


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Post subject: Re: Valve question
Posted: Wed May 11, 2011 9:29 am
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Baggins wrote:
I know we could just try different things but we haven't really got the cash to waste on experimenting


Bottom line: No experimentation is necessary. Simply replace all four tubes with a properly matched quad. If you need recommendations for tubes, just ask.

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Post subject: Re: Valve question
Posted: Wed May 11, 2011 12:18 pm
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Needless to say we'll be doing as we're told and getting a new full set of matched valves. Many thanks for all your advice and info folks


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