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PostPosted: Mon Apr 14, 2014 9:25 am 
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Mark III
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Joined: Fri Aug 29, 2008 2:00 am
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Location: Melbourne, Australia
I'm hoping someone can shed some light here.... I searched the net and got conflicting answers.

I have a Rectoverb with a matched pair of power tubes. I'm thinking of buying a Roadster which obviously has a quad of them.

Is it possible to use 2 sets of matched pairs of tubes in the Roadster provided I can bias each tube or each pair of tubes? I'm thinking of having a bias mod done.... so this leads me to the next question....

Is it possible to modify the Roadster to bias each tube (4 x bias pots) or at least the pair of tubes (2 x bias pots)?

Reason? I have a pair of EL34 Winged C's at the moment. I'd like to get another pair of Winged C's but it's unlikely all tubes would be a 'matched quad'. Considering Winged C's are becoming scarce, I have the opportunity to buy a pair. Ultimately, I wouldn't mind total control over the bias of the amp anyway.

Thoughts / opinions?

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If you're interested in the chicken head knob replacement on my Roadster amp, go here:
viewtopic.php?f=24&t=69901&p=476098#p476098


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PostPosted: Mon Apr 14, 2014 3:26 pm 
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Mark II

Joined: Mon May 18, 2009 10:41 pm
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The diagram in the owner's manual shows that the inner tubes need to be a matched pair and the outer tubes need to be a matched pair. That strongly implies that you do not need a matched quad.


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PostPosted: Mon Apr 14, 2014 7:29 pm 
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Mark II

Joined: Sun Jan 05, 2014 7:19 am
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Only the pairs need to be matched.
You can have different color codes in the same amp as long as they stay in their matched pair sockets.

Roadkings have the EL34s both on the far left side looking from the back)


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PostPosted: Thu Apr 17, 2014 6:18 pm 
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Triple Recto

Joined: Fri Feb 22, 2013 7:31 am
Posts: 3049
Location: North Carolina
Like others have stated, all you need are matched pairs. Even matched pairs are not perfect. The matching requirement is to ensure both positive and negative signals of the same amplitude generate the same amount of plate current on each tube. Typically most amps are Class A/B where the A circuit will amplify the positive signal and the B circuit will amplify the negative signal. If the tubes are not matched, one tube may peak higher than the other will may result in poor sound quality.

I have three amps that require a full set of quads. Only one can operate with an integrated quad (EL34 mixed with 6L6) Mark IV). The two other amps use the same type of tube in all 4 positions. I have bought Matched quads (SED 6L6GC and SED EL34) Never had an issue. I have also mixed different tubes (a matched pair) with the other tubes to obtain a unique tone.

When using tubes of other brands, verify that the tubes will be used with a fixed bias Mesa amp, also indicate the model type and your vendor will select tubes suitable for your amplifier.

Also, with the SED =C= EL34, you could also run a pair of 6CA7, KT77 or another brand of EL34. As long as the SED tubes are in the primary tube positions (remain active in all power modes) the main tone and character will be that of the primary tube. You may get some characteristics of the other tubes running in parallel while operating at full power.

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Current amps: TC-100, TC-50, JP-2C, MK V, Roadster, RA100


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PostPosted: Thu Apr 17, 2014 11:15 pm 
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Mark III
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Joined: Fri Aug 29, 2008 2:00 am
Posts: 384
Location: Melbourne, Australia
Thanks all. Appreciate the informative feedback.

It makes life a little easier without having to worry if matched quads are a must. It'll make shopping for tubes easier or at least open up other options.

Have a great day/night wherever you are.

_________________
Gotta love tubes!
Image

If you're interested in the chicken head knob replacement on my Roadster amp, go here:
viewtopic.php?f=24&t=69901&p=476098#p476098


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