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 Post subject: Re: Mixing 6l6 and El34
PostPosted: Tue Mar 24, 2015 12:45 pm 
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Mark III
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I've always used EH 6CA7 tubes. When JJ came out with theirs, I read the reviews, and decided to stick with EH. They really do sound like an EL34 on steroids.

As for a "middy" 6L6, I've got a quad of Svetlana 6L6s in my old Carvin X100B. I had to do a double check to be sure they really were 6L6 tubes, because the midrange response was so strong, I thought for sure I had EL34s in the amp. I can't praise them enough, and they've lasted just as long and strong as any other power tubes I've tried.


MesaTone wrote:
I tried the 6L6/EL34 combo in my Mark IV A. I'm getting tired of the straight 6L6 tone. I want some more mids, and no one seems to make a 'middier' 6L6. Wasn't impressed with the 6L6/EL34 combo, at lower/medium volumes there was too much mid tone, and it sure didn't sound like a boosted Marshall. I'm not a fan of JP's tone, so I figured I was headed in the wrong direction.

So, JJ came out with their 6ca7, read up on that tube, and tried a set of those in the outboard sockets. WOW! Exactly what I was looking for, throughout the volume range. I ran that tube combo through EV12L's, Delta Pro's, MC-90's and a Weber Cali/Silver Bell mix. Just perfect. Still sounds like a Mesa, but with more clear mids.

I highly recommend this combo if your amp is designed to run different tube sets. If you're running an EL34 amp and want some 6L6 clarity, I'd try a set. I just love these tubes. I emailed JJ to see if they would make a 6L6 with more mids, along the lines of the 6ca7 tone, but not too interested. I think there would be a market for a 6L6 with more mids. Oh, well.

I don't work for JJ, but I also tried one of their mid-gain 12ax7's, and am digging that tone, too. It really brought the front end of my multi-decade old MKIV to life. And I won't use any other 6V6 than the JJ. Another great innovation, I think it's a copy of the 7591 tube with 6V6 pins, sounds much better in an old Fender tweed than the 6V6's.

Peace.

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 Post subject: Re: Mixing 6l6 and El34
PostPosted: Wed Apr 29, 2015 5:15 pm 
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Joined: Mon Jan 01, 2007 6:52 am
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Location: New Haven, CT U.S.A.
bandit2013 wrote:
............ For the full class A/B amps, there still is a slight bias offset but not enough to use an integrated quad unless the amp was designed to do such.........


Very interesting, I've never come across that (an intentional offset in the bias voltage for each side of the sine wave), either in an (Class AB) amp I've worked on, or schematic I've seen.

Any more info on this?

Dom

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 Post subject: Re: Mixing 6l6 and El34
PostPosted: Sun Nov 01, 2015 11:03 am 
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Dual Recto

Joined: Fri Feb 22, 2013 7:31 am
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Location: North Carolina
I can only reflect on the Mark V schematic since I do not have one for the other amps.

The outer tube pair has a -47V bias compared to the inner pair which is -43V (this is assuming the schematic is correct and not missing details) Bias source voltage -51v for 6L6 tube and -39 for EL34 tube.

Typical for a Simul-Class type amp where the one pair of tubes is biased hotter than the other pair. This is accomplished though a voltage divider network of resistors on the control grid of the tubes. For 6l6, source is -51V, passes though 220k resistor, 180k resistor and grounded though a 2.2M. The outer pair has control grid bias of (-51)*(2.2M + 180K)/(220K +180k +2.2M) = -46.6v, inner pair: (-51)*(2.2M)/(220k + 180k + 2.2M) = -43V. Note that there is a series resistor of 2.2k to the control grid on the hot biased tube vs the cold biased tube (having the higher -voltage).

As for the Mark IVb, the bias on the inner tubes is -52v and outer pair is -47v. Source voltage is -57V and replace the 180K resistor value with 220K in the equations above. One would almost think you could run EL34 without using the bias switch set for EL34. However I would not recommend this.

I have not seen any A/B schematics to compare too. My apologies if I stated something incorrect.

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