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Old 17th February 2006, 06:11 PM   #1
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Question Mingda MC34B tweaks

Hi,

I bought this 6L6 tube amp last year and decided to tweak
it a little. Has anyone tried tube-rolling with this amp?
And how does one set the bias? I couldn't find a way
to do it.

Many thanks,

Mike
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Old 20th February 2006, 12:52 AM   #2
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Yes, and posted. You haven't searched the archives. When you do, you are in for a treat!
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Old 20th February 2006, 01:01 AM   #3
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Default Setting bias.

The amp uses fixed bias which may be adjusted at each output tube socket from under the chassis with great care using a mini slotted screwdriver. The pots are only supported by the circuitry itself. A DVM should be placed on millivolts DC range across each 10 ohm cathode resistor while setting the bias. I set for 35 mA quiescent (350 millivolts across the cathode R) when using Mullard 6CA7's. At that setting with these tubes the amp is capable of 28 watts RMS each channel and the power transformer does not overheat if the dress tube cover is left off. That cover causes a lot of radiant heat to impact the one side of the transformer enclosure from the output tubes. There is no ventillation to cool the transformers inside.
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Old 20th February 2006, 06:12 PM   #4
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Default Re MC34B

Hi

Ive had this amp for just over a year now and find its sound quality very good considering the price.

Ive swapped out the 6n1p's for Svetlana winged C's and what an improvement! Now i want to swap the Sovtek 6L6GC's for something of much higher quality...what do you suggest / recommend?

Ive been thinking about Svet 6L6 winged C's or Phillips 5781a's.

Do you recommend tube dampers like the ones made by golden Dragon?

This is my first post on DIYaudio.com, Im a valve newbie and will need to find an Amp technician in my area to rebias...

is there a lot of info in the archives on this little amp and indeed on rebiasing in general?

Best Regards

Jack.
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Old 20th February 2006, 07:53 PM   #5
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You live in England. The famous Mullard valves came from there. Get some Mullard 6CA7/EL34's. Forget 6L6's. I used 6BK7B's to replace the long tail phase splitters (outermost small tubes on the chassis) and picked up a noticeable improvement in fine detail.

I tried 6DJ7's and 6922's. They sounded lifeless in either holes. I now have 6CG7's in the innermost small holes (input amp) and although I lost about 3 dB sensitivity which is no problem I have very good tone. There is probably a more magical tube that will go here but I haven't found it although I haven't tried.
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Old 20th February 2006, 08:58 PM   #6
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I don't recommend substituting 6CA7/EL34 for 6L6 without some other changes to the amplifier and some good scope work to make sure the amplifier is still stable. The 6CA7 has considerably higher transconductance than a 6L6, higher filament current, and requires a very different bias point for proper performance. 6L6 bias levels if fixed biased will put a 6CA7 very close to class b operation.

6CA7 are an option if the transformer filament winding has sufficient reserve to run them safely, but not without the precautions mentioned above.

I would take a look at the JJ 6L6GC as one choice as well as Philips/Sylvania ECG 7581A (they sound quite good in Citation V and several other amps I have tried imho) and are a drop in for 6L6GC. Another obvious choice would be one of the several repro KT66 floating around out there.
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Old 24th February 2006, 06:31 PM   #7
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hello


Thanks for your posts

one of my Sovtek 6L6's is flashing to the music, i.e the electrons dim and brighten in time to bass and drum beats etc. the other three dont do it and it stops when i turn the sound down. Ive only just noticed this last night so not sure how long its been doing it. Is the tube going to fail or is this natural?

Regards

Jack.
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Old 24th February 2006, 06:54 PM   #8
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Hi Jack,
That tube might doesn't sound healthy. Replace that channel's outputs.

I have had very good luck with the Electroharmonix 6L6EH. It looks very close to some 7581A's I have (another excellent choice). You may see 7581A also branded with KT-66, they are very close to the best of my knowledge. I hear JJ is another good brand, I haven't tried them.

-Chris
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Old 24th February 2006, 08:54 PM   #9
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Default My 6CA7 mod IS SAFE

Quote:
Originally posted by kevinkr
I don't recommend substituting 6CA7/EL34 for 6L6 without some other changes to the amplifier and some good scope work to make sure the amplifier is still stable.

Kevin,

I have had a MC34B purchased new for three years. For the past two years I have logged hundreds of hours with a set of 2nd generation, vintage good Mullard 6CA7's. I have the quiescent cathode current set at 35 mA per valve by adjustment of G1 bias available as a separate control for each output valve. I too was concerned about the extra filament current needs of these tubes and watched power transformer heating carefully. As it is set up my amp has been left running for as long as 24 hours with nothing but safe mild temperature rise. As tube amps go I consider this one to be a cool runner. Note that I did specify to leave the tube cover off to help the transformer enclosure cool.

A wattmeter check shows exactly the same consumption as the nameplate rating, so I must have ended up lower on my cathode current than stock with the 6L6's to maintain the same draw on the power xfmer.

With my Mullard 6CA7's and cathode current settings of 35 mA this amp now outputs 28 wartts rms per channel, both channels driven and sounds fantastic. It put out a measured 18 watts/ch with the SINO 6L6's out of the box.

I would not make a recommendation unless I had tried it myself with tested results. This works very well on this amp and I feel confident to recommend it.

That said, try any other brand 6CA7, particularly the newly manufactured junk and take your chances.
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Old 24th February 2006, 09:04 PM   #10
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I purcased a quad of brand new Sovtek 6L6WXT's to try in the MC34B.
Two of the four tubes failed in less than 50 hours. Garbage!!!! Am I impressed? NO! The sound was not as good as I get from the Mullard 6CA7's either.

I bought a quad of brand new fat bottle ElectroHARMonix 6CA7's. Two had glowing grid two segments that ran a bright yellow orange. Garbage!!!!

I am still trying to obtain one good set of ElectroHARMonix gold grid 300B's. In the first set one was fine and one does a lightshow every turn on from partial Grid cathode short. So far the one sent to replace it has the same problem. Garbage!!!!
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