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Topic: Question regarding Mesa Boogie MKIII bias |
Marty Nemanick
From:
Madera, California, USA
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Posted 17 Jul 2013 7:01 am |
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I use one of these amps and have been told by a couple of people that the pair of EL34's can be replaced with 6L6's for a bit more power. If this is true, would it be necessary to also change the bias?
Thank you,
Marty |
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Michael Brebes
From:
Northridge CA
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Posted 17 Jul 2013 7:18 am
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If you're talking about the old 1-12 combo, they had fixed bias that was set for generic 6L6 tubes. The bias is a bit different for the EL34. It could have been changed with a fixed resistor or someone just put the EL34's in without the correct bias change. The only way to know would be to open up the amp and look at the resistor values for the bias voltage. On my brother's Boogie Mk3, I installed a bias switch to flip fixed bias between 6L6 and EL34.
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Michael Brebes
Instrument/amp/ pickup repair
MSA D10 Classic/Rickenbacher B6/
Dickerson MOTS/Dobro D32 Hawaiian/
Goldtone Paul Beard Reso
Mesa Boogie Studio Pre/Hafler 3000
RP1/MPX100 |
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Marty Nemanick
From:
Madera, California, USA
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Posted 17 Jul 2013 8:00 am
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| Michael, this is a 1987 1X12 combo and came stock with a pair of 6L6's and a pair of EL34's. Sounds like I will need to take it in and have it biased for the 6L6's. Thank you, Marty |
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Chris Reesor
From:
British Columbia, Canada
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Posted 17 Jul 2013 1:37 pm Mark III bias
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Marty, I believe you are talking about the SimulClass output section here, which uses a pair of triode connected EL34 tubes with fixed bias ( Class A ) to yield 15 watts, and adds a pair of pentode connected 6L6 tubes with, again, fixed bias for the SimulClass mode, producing 75 watts.
If this is the case, I think subbing 6L6 for EL34 in the outer pair of output tube sockets may change the tone character of the amp a bit, but will not yield a significant volume increase, bias change or not.
What speaker is in the amp?
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Excel Superb U12, MIJ Squier tele, modified Deluxe Reverb RI, Cube 80XL, self built acoustics & mandolins |
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Marty Nemanick
From:
Madera, California, USA
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Posted 17 Jul 2013 1:56 pm
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| Chris that is exactly what I have here. I swapped out the EL34's with 6L6's and I like the tone much better. Is there any risk of damage to the amp by doing this without a bias change? |
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Howard Montgomery
From:
Topeka, KS US
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Chris Reesor
From:
British Columbia, Canada
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Posted 17 Jul 2013 4:43 pm Simulclass bias
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Marty. I'd say 6L6's will probably draw a bit more idle current than EL84's in the triode connected slots, but that is unlikely to harm this amp which is built like a tank. Watch for red plates and excessive power trans. temperature rise, of course.
To be really safe, get a tech to check the idle current of that pair and adjust as required.
I had a straight 60 watt, no graphic Mk III with an EVM 12 L speaker, and it was the best amp I've ever had. Sounded pretty darn good for steel, too.
Compared to a couple of different Simulclass Mk III, my plain Jane sounded cleaner.I'd guess that is what you are looking for with the tube swap. Try leaving those sockets empty; you probably won't notice the volume difference.
If you want maximum clean volume for steel, make sure you have the most efficient speaker possible; an EVM or a JBL E120 is what I would prefer.
You will look long and hard for an amp with equal ruggedness and build quality. Wish I had mine back.
Chris
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Excel Superb U12, MIJ Squier tele, modified Deluxe Reverb RI, Cube 80XL, self built acoustics & mandolins |
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Marty Nemanick
From:
Madera, California, USA
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Posted 17 Jul 2013 5:43 pm
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| Thank you Chris for the info, I have tried the EVM, 1201-8 PV, SICA 12", and the best sounding speaker for this amp, playing PSG only, is the PV 8 ohm Scorpion hands down. I'll keep an eye on the tubes and monitor the heat. Thanks Chris, Marty |
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Brandon Schafer
From:
Indiana, USA
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Posted 18 Jul 2013 2:40 pm
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There is no bias adjustment on this amp. I've owned many Mark IIIs over the years (one of my favorite 6 string amps), and have had great success with using EL 34s in the outsides where the Class A happens on in the Simul-Class amps.
You need to be sure you have a Simul-Class amp, though. It sounds like you may since you said it had EL34s on the outside sockets from the factory. If you have a 60/100, then you should put all 6L6s in it in the first place.
If you wish to use all 6L6s in a Simul-Class amp, you may without bias adjustment. But, you won't really increase the power of the amp that much and you may sacrifice some reliability.
Keep your little fan it came with running and that will help! |
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