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Old 28th December 2013, 03:14 PM   #1
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Default Schematic please: Luxman MQ-70?

Anybody on this astute body who can point to/supply me with a schematic of the Luxman MQ70 amplifier, please? It seems to be rather absent from the usuall sources.

Thanks.
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Old 26th February 2014, 02:07 AM   #2
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Quote:
Originally Posted by Johan Potgieter View Post
Anybody on this astute body who can point to/supply me with a schematic of the Luxman MQ70 amplifier, please? It seems to be rather absent from the usuall sources.

Thanks.
Repeat. Nothing, nobody - nowhere??
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Old 26th February 2014, 06:43 AM   #3
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UL & CFB output stage with differential input and driver stages, a lot could be learned here, a classic design!

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Old 26th February 2014, 11:19 PM   #4
Ejam is offline Ejam  Australia
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Thanks to Jazbo8, being looking for this schematic for ages and had given up. Then one day there it is! Any chance of the voltages? B1 looks to be around 460 - 480V. Just noticed that the output stage is ultralinear as well, so much for memmo's "super triode" invention. Again thanks.
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Old 26th February 2014, 11:26 PM   #5
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Thank ye kindly, Jazbo8!

Conforms mostly to the model in my possession, with some changes. Look like factory mounted parts, so I wonder if there was more than one MQ70 version?

Discussion if of interest:

My power transformer is designated S 2372. The primary is 220V, thus causing some problems here where the mains is 230- 240VAC. I also wish they would have indicated that the diode chain in the 12AX7 'tail' circuit circuit are 56V zener diodes - not shown as such. The one indicated as '1kV 0,5A' is a puzzle; the number on the diode is T86. I find that to be either a step-recovery type or varicap! (Obviously not the case.) Not sure what it is doing there.

I have one worry: The 12AX7 plate voltages seem to be some 125V, making the 6AQ8 exceed its specified heater-cathode voltage of 90V. While in doubt, I elevated the relevant heater winding c.t. to +70V. I also have a (factory-fitted) 20K balancing pot at the junction of the 12AX7 150K plate load resistors.

One could improve the high power distortion performance somewhat by installing the 1K screen resistors indicated in the original Mullard discussion on the EL34 (6CA7). There is some opinion that the 6CA7 is not a direct equivalent to the EL34 - that is another story though. My model has EL34s in - probably replacements.

Thanks again, Jazbo8!
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Old 27th February 2014, 12:24 AM   #6
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Ejam replied while I was typing.

Somewhat OT (but it is my thread!)

To refresh: Regarding the output topology, it is sometimes not recognised that cathode feedback in the case of pentodes is a form of UL. (The cathode winding forms part or whole of the screen - B+ part of the OPT primary.) The equivalent taps (fraction of primary signal voltage) in the case of the Luxman are at 47% for screen, with only a 6,7% part of that winding in the cathode circuit as additional G1 feedback. In e.g. the Quad II the whole screen - B+ portion is placed on the cathode side. (This results in the convenience of being able to use a lower screen voltage.) Placing the full screen-% of the winding in the cathode circuit results in a very large signal voltage required as input to the power tubes. (In the Quad II it was found that 20% U.L. taps rather than the more common 43%, gave optimum performance for tubes of the KT66 - 6L6 types. In fact, Quad II used an even lower ratio to ease the driver demand.)
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Old 27th February 2014, 06:42 AM   #7
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Glad to be of some help, I found that for Japanese schematics, yahoo.co.jp is a better search engine than google.

Ejam - Here are some voltage readings taken by a local Japanese user, obviously your voltages will be very different given the much higher mains voltage, but perhaps these could be used as references as the amp was originally designed for use in Japan:

Output stage B+ = 502V
Output tube Bias voltages: 41.7, 42.7, 39.5, 40.4V or ~ -40V as specified.
First stage plate voltages: 147.8, 146.2, 145.6, 145.8V (well regulated)
Driver stage plate voltages: 352, 362, 363, 351V.

Unfortunately, I can not find any reference for the Zener string used nor the part number.

Johan - WRT to the CFB (or KNF on the schematic) wouldn't it be in addition to the screen feedback instead of a form of UL? Since my understanding is that UL is a specific connection per Hafler and Keores, while the CFB is sometimes called the Acoustical connection after QUAD/Walker. I believe both types of connections were trademarked by Arco and QUAD respectively... Anyway, Patrick Turner has written pretty extensively on both connections on his site in case you are interested.
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Old 27th February 2014, 08:55 PM   #8
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Quote:
Originally Posted by jazbo8 View Post
Johan - WRT to the CFB (or KNF on the schematic) wouldn't it be in addition to the screen feedback instead of a form of UL? Since my understanding is that UL is a specific connection per Hafler and Keores, while the CFB is sometimes called the Acoustical connection after QUAD/Walker. I believe both types of connections were trademarked by Arco and QUAD respectively...
Jazbo8,

The short reply is that it is both. Actually, one cannot have CFB (in a pentode) without having some form of UL (unless in an unwieldy circuit the screen is bypassed to the cathode blah blah).

I apologise for not submitting a diagram, and for full explanation in favour of those not familiar with UL, perhaps boring to others. If one draws a pentode tube on paper, and a long vertical inductor next to it, and connect the top with the plate and the bottom with the cathode,

then, if one connects the screen to the plate, one has triode operation. When connecting G2 to the bottom (cathode), that gives the equivalent diagram for a pentode.

If now one attaches g2 to a tap on the inductor, one has the equivalent diagram for a UL stage (the h.t. being a short to a.c.). The portion between the cathode and the G2 tap carries cathode current. If one reverts that portion to the cathode circuit, the situation is still the same: UL operation. (One can envisage putting in the h.t. supply at a break in the inductor just below the G2-connection.)

Yes, there is also now cathode voltage feedback, but one thing at a time! Point is that any part of the OPT common to G2 and the plate, reverted 'down' to the cathode circuit, is still a form of UL (unless the intital connection was for triode; in that case there is only cathode feedback). In addition to this then, the cathode part of the inductor also gives (outside) voltage NFB, with its particular advantage. One needs to differentiate between external cathode NFB where the tube behaves as a pentode or triode with external feedback, and the UL mode, where the pentode itself (internally) has a change in characteristics, even though these things occur simultaneously. It is no longer internally either a pentode or triode apart from outside feedback. That is the difference.

As said, in the instance where the full G2-cathode signal is so high that the required grid input signal will need be overly large, part of the 'winding' between G2 and B+ (or common in the equivalent diagram) can be reverted to the cathode side instead of the whole 'winding' as in Quad II. This is done in the MQ70; only a small portion of the G2 - plate winding is in the cathode circuit, the rest remaining between common and G2 as a g2 tap.

So yes, there are two actions from the viewpoint of calculating say THD, although one cannot occur without the other. I could not quite find the Turner UL analysis; this aspect is also covered in a.o. a 'Wireless World' article: "Amplifiers and Superlatives" by Williamson and Walker about 1952. (The intitial 'UL', then called distributed load operation, was in fact patented by Blumlein as early as 1928, but found little practical application as there were no power pentodes those days.)

I am simply trying to point out that any cathode OPT feedback in a pentode/tetrode is a form of UL. As you say, much has been written about this.

Again, grateful for your participation.
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Old 28th February 2014, 04:24 AM   #9
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Johan - thanks for the clarification, I understand that you can not have CFB without effecting the screen as well - it is a "two-fer" - since the tube has both gm and gmg2, so it does have "UL" action. Our view differs not on CFB's electrical property but rather on semantics - to me, UL was a term used by Hafler & Keroes to market their transformers and amplifiers, but it was not a electrical definition per se, and many preferred/still prefer to call it a distributed load connection (after Blumlein). Or may be I should get my head checked...

I'm not sure if Patrick Turner even bothered with the definitions, in any case, his write-up can be found here.
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Old 28th February 2014, 07:21 PM   #10
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Hi,

Quote:
There is some opinion that the 6CA7 is not a direct equivalent to the EL34 -
Well, it's not a matter of opinion but a matter of fact.
Whilst in almost all cases a 6CA7 will work great in an amp designed for EL34s, the other way around fitting a EL34 in a design using 6CA7s may not necessarilly work.
The 6CA7 is in fact a tetrode. The European version of a tetrode EL34 would be the KT77.

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