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Ming-Da MC34-B

First I added another 22K resistor which reduced by half roughly, then I added a 100K resistor, then connected the wire to the 4 ohm terminal. Finally I removed it completely.
 
Battery bias is done. I measured 1.18V at the cathode before removing RC, and the charged NiMh batteries measured 1.44V out of circuit - the sound after installing was a darker background (increased downward dynamic range) with increased sibilance on vocals, cymbals etc. This has improved day by day and a week later, the batteries measure 1.35V in circuit and the sibilance is still there, but much decreased (perhaps I've become acclimatised to how it sounds?!!). Overall, it's an improvement on the standard bias setup. I'll try a pair of Nicad batteries to get closer to 1.2V - I'm thinking the higher voltage = current through the 12AX7 is outside the optimum range.

The Sovtek 6H30pi and the Winged C 61NP are very close; overall I prefer the 6H30 but on some recordings the extra gain required makes the background hash more noticeable on headphones, but still below the ambient threshold on the main system.
 
That's a nice write up there Dave. I'm using 12ax7 or 5751's in the input stage. The lower noise floor was amazing after I put the batteries in. I haven't notice any sibilance like you mentioned. What I'm considering doing is putting 2 AA's per side, so I can increase the available amperage. I can't give any impressions with the tubes your using. Let us know if the use of Nicad improves it.
 
I like it very much, a little less gain, but I think it's a little more dynamic. I'm using a Sylvania JAN right now.
 
I think the EL34s have been running too cold at 30 to 35mv across the cathode resistor. I measure plate voltage as 420v so with max plate dissipation of 25w, 80% of max bias current is 47.6ma - so 47.6mv across the cathode resistor (476mv across the 10ohm resistor indicated on the DMM) should be correct, and it sounds richer and more detailed at this bias point. Any comments about this method of determining bias?
I've gone back to the 6N1P - what was immediately impressive about the 6H30pi ultimately caused fatigue, and the 6N1P is more musical, equally detailed but less forward.
Thinking about trying the JJ KT77.
 
Nice thread DanTana and others. I would like to clarify the change from 6L6's to EL34's. Do I need to make any modificationd to hardware: resistors, connections and so on? I understand that bias will be adjusted of the potentiometers as adviced earlier but how is it with the wiring and components connected to the tubes?

Thanks a lot! I am just waiting for my new MC34-B, but I feel that the mods are quite soon on the schedule.
 
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I have tried it at one time, and although it wasn't one of the highly regarded versions of the tube, I wasn't impressed. I like the ECC99 or 12BH7 now that I'm using much better, but then you have to change the wiring for 12v heaters.
 
Thanks again. I have a large set of 6DJ8/E88CC tubes used in other devices. I put Tesla E88CC instead of 6N1 and Siemens E88CC for 6N2. The sound was much more better with Revel Performa F30 speakers; big floorstanders. Next I will put EL34 tubes for 6P3P.
 
Sounds pretty sweet. Does anyone have the schematic, so I can take a peek? I'm currently using some 6L6WXT+'s myself in a DIY tube amp and the ability to rebias to take EL34's or 6550's would be cool.
 
Triode mod

Sorry for the question discussed a lot earlier, but I wish to clarify the mod to triode mode in MC34-B. Shall I need to do anything else than to connect pins #3 and #4 by a 100ohm resistor in every four output tubes (6L6 or EL34)?
 
Manolito, you also need to remove the ultralinear wire that is connected to the pin and tape it up securely. Then solder the 100ohm resistor to pins 3-4. I used a 1/2 watt 5% carbon film resistor you can buy Radio Shack.
 
Thanks. Yes, that was also my doubt after reading quite many articles of the triode mode connections compared to ultralinear using pentodes, but I wanted to be sure of the matter.
 
Hello Dantana,
I have been following your experiences with the Ming Da MC34B with great interest as I have owned one for about 18 months and I was so pleased to have found your threads.
I have changed the preamp tubes to ECC83 (re-wired accordingly) and the main tubes to EL34.
The driver tubes are now ECC88. Today I snipped the NFB wires at the +ve speaker terminals and I was rewarded with an amazingly clear liquid sound.
However, a hum was then audible on both speakers, one louder than the other.
What can I do to prevent this? Should I carry out the Triode Conversion to the EL34 Tubes first or should I just reconnect the NFB with an additional High resistance?
e.g 1Mohm.
Thanks for your assistance.
 
I would recommend doing the triode conversion first. I think you will like it better although less power than Push-pull. If you still experience hum after that, something else is wrong. I didn't totally remove NFB until after I wired in triode. I believe PP sort of needs some NFB to cancel out distortion. I don't believe triode requires any NFB.
 
Thanks for your prompt reply and your helpful information. Reduced power should not be an issue as I seldom go beyond 11 'o' clock on the volume control (replacement ALPS Blue (50K)), however my speakers are Uni Q and rated at 6 ohm, will this impact on the load? Will move forward with the Triode conversion. After this, which mod would be most beneficial? the Battery Bias, Capacitor Upgrades or another in order to extract the most music out of this amazing amp. Thanks again.
 
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Context: MC34B driving a pair of Bohlender Graebner 28.1 planar magnetics after an active digital crossover at 250Hz - no passives between amp and driver. The BG is somewhat 'cool', and I've been loking for extra 'warmth' without losing detail. Things I've tried in the last 6 months:

6H30pi at input and/or phase splitter positions
Etched detail, closer to a SS sound. I've heard this tube sound good in other circuits, and I suspect it's not getting the heater current it needs. Back to the JJ ECC83S.

JJ KT77 replaced JJ EL34
Smoother midrange, extended top end.

EH 5751 replaced JJ ECC83S
Less gain so less hum, which is good - don't want to feed any more LF to the drivers than is necessary. Also more delicacy - something like the change from pentode to triode.

GE JAN 12AT7WC replaced EH 5751
Tried this after reading the El Cheapo thread on the Decware forum (thanks Eli and Jim ) - the basic idea is that PP amps cancel most of the 2nd harmonic, making the 3rd dominant. Perhaps a fundamental difference between SE and PP amps? The story goes that the 12AT7 has overall higher distortion than other 12A*7 types, but it is mostly the 2nd harmonic and after PP cancellation the distortion 'waterfall' sounds better. It worked for me.

Jensen Copper/Oil coupling capacitors replacing Solen/SCR MKP
Wow. This is the single largest improvement I've heard from all the tweaks. If you do nothing else, my recommendation is to find room to fit better coupling caps. The circuit is good enough that this is a reward.

David
 
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