AVForums South Africa

Audio/Video Hardware => Valves/Tubes => Topic started by: charles on December 01, 2019, 08:26:17 PM

Title: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on December 01, 2019, 08:26:17 PM
I obtained this Luxman MQ70 tube amplifier. It is in an excellent cosmetic condition.

The reissue one probably looks much better than the original one.
It has newly designed output transformers and a new circuit design. The physical layout of the amplifier differs from the old one. 
This is a very good designed amplifier and it looks very good.

When I switched it on, one of the power valves had a red anode.
The cause was a short circuit coupling capacitor.  A lesson, always check and measure the
voltage on the coupling capacitors. There was no damage to the power valve.
Set the bias, DC and AC balance.

I used it with a Luxman CL35 preamplifier and the combination sounds good.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Family_Dog on December 01, 2019, 08:59:02 PM
Well caught Charles, that could have caused nasty damage to the valve & OPT.


-F_D
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Agaton Sax on December 01, 2019, 09:02:16 PM
A brilliant little amp. I have never, ever heard Quad ESL 57s sound better than with the MQ 70. It can be somewhat reticent in the highs on some speakers but it has totally infectious  boppy sound that makes the big valve amps sound ponderous.

Only problem is that it uses the Lux only 6240G driver. I am not sure if there is any substitute for that valve but I think not.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: fredeb on December 01, 2019, 09:36:12 PM
Congrats Charles ! You always find the most amazing amps . :)
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on December 01, 2019, 11:08:45 PM
Thank you.

This new issue model uses ECC83 and ECC85 (amplifier/driver) valves.

The power valves are EL34's
 
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on December 02, 2019, 05:02:54 AM
Hi Charles!

Can you get a schematic for this amp?  I will check my files - might be a reason to take this to p.m.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on December 02, 2019, 08:20:17 AM
Hi Ampdog,

With the amplifier was an operating manual that describes the bias, AC an DC balance and other relevant operating procedures.

The schematic diagram is available on the Internet. I can forward the link or the diagram to you.

This amplifier has capacitor couplings between the stages,

I hope this will help you.

 
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: fredeb on December 02, 2019, 08:39:28 AM
I tried searching for the schematic , and only found schematics for other MQ models . Can you please  post the link Charles .

Sent from my SM-G935F using Tapatalk

Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Agaton Sax on December 02, 2019, 08:50:54 AM
(https://images.weserv.nl/?url=http://web1.kcn.jp/tube/MQ70main.jpg&w=800&t=fit)

(https://images.weserv.nl/?url=http://web1.kcn.jp/tube/MQ70power.jpg&w=800&t=fit)

Seems to be the one with 6AQ8 driver. Again I don't know if the 6240G is eq. It was a Luxman only tube built by NEC .
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: fredeb on December 02, 2019, 10:06:22 AM
Seems to be the one with 6AQ8 driver. Again I don't know if the 6240G is eq. It was a Luxman only tube built by NEC .

Thanks Agaton ! According to this audioasylum thread , 6FQ7/6CG7 is the direct replacement for 6240G .

https://www.audioasylum.com/cgi/t.mpl?f=tubes&m=196239 (https://www.audioasylum.com/cgi/t.mpl?f=tubes&m=196239)
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: El Sid on December 02, 2019, 01:53:39 PM
(https://images.weserv.nl/?url=http://web1.kcn.jp/tube/MQ70main.jpg&w=800&t=fit)


What the  is all that stuff on the output? A sort of band pass I suppose - RL passes below a certain frequency and attenuates frequencies above through the 3.3 Ohm R, and the RC shorts above some other frequency, but why not do all that before the OPT?
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: El Sid on December 02, 2019, 02:33:06 PM
What the  is all that stuff on the output? A sort of band pass I suppose - RL passes below a certain frequency and attenuates frequencies above through the 3.3 Ohm R, and the RC shorts above some other frequency, but why not do all that before the OPT?

Just to be clear, the engineers at Luxman obviously knew what they were doing. I want to find out - not querying the reason for it being there
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: handsome on December 02, 2019, 04:54:20 PM
The inductor/resistor combination aims to protect the amplifier from capacitive loads. At high frequencies a capacitor has a very low resistance: so at low frequencies the amplifier is connected to the speaker - the resistance of the inductor is very low - as the frequency increases so does the resistance of the inductor so “shielding” the amplifier from the load. The resistor in parallel both damps any resonance formed by the capacitance and inductance and provides a minimum load that the amplifier will see. I have never seen this kind of network used on a valve amplifier - it is standard on 99.9% of transistor amplifiers to avoid instabilty.

The resistor and capacitor network is called a Zobel network to protect the amplifier from inductive loads. At high frequencies an inductive load looks like a very high resistance, once this resistance gets high enough it is the same as if the amplifier output is open circuit. A Zobel aims to make the amplifier always “see” a resistive load at high frequencies. At high frequencies the capacitor’s resistance drops and the amplifier sees the 10R resistance as a load. At low frequencies the resistance of the capacitor is so high the amplifier ignores the zobel sees only the loudspeaker load.

Both networks are used to keep the amplifier stable. This design has regular global negative feedback, output stage cathode feedback and is ultra-linear (which is also feedback), thats 3 feedback networks, so this design is no doubt prone to instabilty.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: El Sid on December 02, 2019, 05:43:51 PM
The inductor/resistor combination aims to protect the amplifier from capacitive loads. At high frequencies a capacitor has a very low resistance: so at low frequencies the amplifier is connected to the speaker - the resistance of the inductor is very low - as the frequency increases so does the resistance of the inductor so “shielding” the amplifier from the load. The resistor in parallel both damps any resonance formed by the capacitance and inductance and provides a minimum load that the amplifier will see. I have never seen this kind of network used on a valve amplifier - it is standard on 99.9% of transistor amplifiers to avoid instabilty.

The resistor and capacitor network is called a Zobel network to protect the amplifier from inductive loads. At high frequencies an inductive load looks like a very high resistance, once this resistance gets high enough it is the same as if the amplifier output is open circuit. A Zobel aims to make the amplifier always “see” a resistive load at high frequencies. At high frequencies the capacitor’s resistance drops and the amplifier sees the 10R resistance as a load. At low frequencies the resistance of the capacitor is so high the amplifier ignores the zobel sees only the loudspeaker load.

Both networks are used to keep the amplifier stable. This design has regular global negative feedback, output stage cathode feedback and is ultra-linear (which is also feedback), thats 3 feedback networks, so this design is no doubt prone to instabilty.

Explained - thanks handsome!
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Agaton Sax on December 02, 2019, 06:13:59 PM
Non sequitur but we are busy building a 2 way high efficiency speaker. That is others are building an I am cheering. This particular set up uses a fairly complex crossover that included zobel neworks across the drivers. In the same breath some armchair pilots have been critisizing the crossover circuit  in the TAD monitors (2402 abn 2404)as it is textbook for 4 ohm  saying surely Technical Audio Devices (for it is their design) could have done better than assuming impedance is just 4 ohms across the board?

Well look at the effect of a Zobel network on driver impedance. Drivers here are TAD 1601. Blue without zobel,red with.

(https://live.staticflickr.com/65535/49158107988_02f34a9206_b.jpg) (https://flic.kr/p/2hTW9S1)zobel tad (https://flic.kr/p/2hTW9S1) by jdza (https://www.flickr.com/photos/jdza/), on Flickr
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: handsome on December 02, 2019, 07:08:51 PM
That shows exactly what a zobel network does!
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on December 02, 2019, 08:39:19 PM
Agaton Sax,

thank you very much for posting the circuit diagram, it is appreciated.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on December 03, 2019, 10:27:02 AM
I rather feared it was that one.

I would rather have discussed another amp in the Christmas month. :( :( :(

I had two of these units on my desk, and there are some points to be pointed out in order to understand what is going on.  (Charles, if the one you got passed my way earlier, you are going to find some mystifying alterations there. Before judging me - first look carefully at the schematic! Firstly you will find that the first two stages are d.c. coupled, dc.-linked by the three x 1Meg resistors in the coupling diagram. That transfers the d.c. on the first valve to the next stage. (The capacitors over two of the resistors have nothing to do with this.)

Furthermore in the power supply, it is not indicated that the three diodes respectively in parallel with three 150K resistors each, are zener diodes, each some 50V . (This may need some thinking. Some might say they are simply there as protection against voltages going the opposite way as result of failure of something, but that still makes no sense - eg. why split a 300K resistor with two diodes over them - when the split point goes nowhere?  But point is: I did find zeners there in the units I restored.

Briefly then, all this d.c. paraphernalia is expected to yield just the right voltage at C2 and corresponding points

(1) so that just the right voltages appear at the grids of next-stage ECC85, delivered there by a double triode of µ = 100,

(2) plus being within somewhere of 1V of each other to have the next stage ECC85 in its right working region - (remember it is direct-coupled by the 1Meg resistors mentioned)

(3)plus having the ECC85 anodes at quite equal voltages to deliver a rather large signal to the EL34 power valves. (Yes, there is a nondescript little 5K pot there to attempt the hopeless job of affording enough adjustment to set the anodes to remotely equal voltages for equal triode currents.

Does it matter that said anodes be at equal d.c. voltages?  Yes, plus the right voltages, because the output stage features partial cathode NFB (a la Quad II)  which requires a large signal amplitude on their grids; not much room for ECC85 anodes to sweep.

If it wasn't Christmas time I would have said that that the M70 is a prime example of a 'drawing-board design' without much regard for practical circumstances. From what I gathered from other forums, it was on the market for a limited time only.

So the circuit is fine - - for triode units of <1% tolerance, plus zener voltages also rather better defined than usual, and with superior temperature co-efficient - something high-voltage zeners definitely do not have.

Hopefully members can now appreciate the restorer's difficulty.  - and that which awaits friend Charles (unless he happens to have obtained one of the models previously restored by me).

Points of lesser importance disregarded - this should give enough food for thought/labour for folks interested.

To be kinder: As an audio amplifier this topology should deliver serious sound. The problem is not in the audio circuit; it is in misguided attempts to have d.c. stability and lack of capability to accommodate the usual spread in component tolerances.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on December 03, 2019, 01:31:21 PM
Ampdog which models did you serviced. There are more than one MQ70 amplifier.

Remember the circuit came of the internet and it could differ from the model I have.

 

Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on December 03, 2019, 03:20:47 PM
Charles,

Obviously!  The models that came my way (>1 year apart) were both the same, marked MQ70. They corresponded to the schematics shown here in earlier posts.

I was not aware of a second M70, neither could I find any reference to it on the internet. Although surprised that two different amplifiers will be marked the same number (as Doug Self said: 'Anything is possible these days ...'), I also did not pick up on different M70 models on other forums that I frequented at the time (Diy-audio, Audiokarma and Audioholics.)

I reread some of my commentaries then: The zener diodes in question are 1N4760A's of 68V.  Then another malady of this amplifier was pointed out by another member, viz. that the heater-cathode voltage of the ECC85 was exceeded: around 150V;  spec. +or -90V max.  This lead to noise and final demise of some ECC85s. (Other posts were offered; if of interest they can be googled.)

I'm am not here to bash any particular amplifier brand, just warning where warning is due. I have also not yet  located my file on this work; speaking off-the-cuff as it were. (I will rummage through my files when the temperature in Pretoria is more favourable for activity!)

If someone can enlighten us to the alternative MQ70, it should be of value. (Might be that the mentioned shortcomings were improved on in a later model?)
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on December 04, 2019, 11:06:20 AM
Follow:

I spoke to Charles. He was able to easily adjust his MQ70 to set values, which would not have been possible on the models I referred to. It is becoming clear that we have two models but with the same type number! (I really do not appreciate that from manufacturers  >:( )

Be that as it may, let me repeat that I have commented on units which came my way, (as in several years ago), not on Luxman products in general. I have failed to trace the 'new' model on internet, and repeat that it would be appreciated if someone could shed light on this. If Charles is in a festive mood, perhaps I could have a look 'under the skirt' of his model when next we come together (we live quite a distance from one another).

With this I will conclude my present comments until fresh information comes along, if anybody is still interested.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on January 08, 2020, 01:52:51 AM
Thank you.

This new issue model uses ECC83 and ECC85 (amplifier/driver) valves.

The power valves are EL34's

Charles,

Kind of waking up again to the MQ70 - any specific information on the 'other' MQ70 amplifier, like a schematic.? Or why do you say there were two models? I rummaged through my dusty notes (I do SO much like dust - yech!) but could not find more information on my previous encounter with said Luxmans (Luxmen?)  How possible is it that the unit now in your possession could be one of those I refurbished??  :D  (Hven't had time and energy to ask on other forums . . . . And with load shedding and electrical cable breaks/theft in our area
vying for the highest incidence of electricity interruptions . . .
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on January 08, 2020, 09:21:19 AM
Dear Ampdog, this set was probably not refurbish by you because you would not have used a mix of electrolytic capacitors and
normal non polarised capacitors for coupling capacitors to the EL34 valves. I have rectify it.

The set came with its owners manual and an alignment procedure.

Alignment procedure:
1. Alignment of power tube, bias current and DC balance.
2. Adjustment of DC balance for the primary-stage tube.
3. Adjustment of hum balance.
4. Adjustment of AC balance -in conjunction with a distortion meter
This procedure is probably not available on the internet.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on January 12, 2020, 02:21:14 AM
. . .  because you would not have used a mix of electrolytic capacitors and
normal non polarised capacitors for coupling capacitors to the EL34 valves.

You bet I would not have done that! How ?? - in series or do you mean in different places? Different values? (I have never seen values as low as 68nF in electrolytics)  - we can take that to PM or landline. [Nor would I have used a final coupling cap as low as 68nf, giving a pole (-3 dB roll-off frequency) at a high 24 Hz there - but let us leave matters at that.]  By the way, I notice Agaton Sax stating the schematic he provided is for the new Luxman. The ones I refurbished were then 'new' editions; it is the former (if any) ones which are then mysteries. (And yes, 'mine' did come with the mentioned prescripts plus schematic, although those are standard general procedures for valve amps.)

But as said . . . 

Happy for him if Agaton is pleased with his results.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on January 12, 2020, 05:09:21 AM
(https://images.weserv.nl/?url=http://web1.kcn.jp/tube/MQ70power.jpg&w=800&t=fit)

As a result of PMs and other discussions, the position regarding the power supply diodes needs to be reaffirmed.

There appears to be uncertainty regarding the power supply diodes -(the question marks on the diagrams do not help   >:( )

The 4 x diode bridge at top left, shown as 1KV 1A diodes appears to be in order, as does the bridge underneath, showing 1KV 0,5A ones.

However, as stated before, the four diodes shown at bottom right; three in parallel with three x 150K resistors and the fourth going to a 220K resistor, are not ordinary diodes but zeners (thus symbols incorrect). In both units which came my way, they were marked 1N4760A which Vishay records show as 68V zener diodes. All looked as having been factory installed so nobody's after-thoughts.

This makes quite a difference in trying to figure out how the circuit works and constitutes part of the ensuing adjustment problems which I referred to earlier.  (Again I wish/hope that Charles might help by having a look inside his amplifier and confirming.)

[I am afraid my popularity has not received a boost by referring to these negative matters. Yet again, I will try to be nice and not offer further comment.  But as they say . . . .  . . . . . . s. After I fixed what I found, there remained no reason for the amplifier not offering excellent performance.  Folks will recognise an U.L. power stage, part of the tapped G2 - B+ portion of the output being fed to the input as NFB.  After Quad II this was one of the earliest amplifiers to my knowledge to have done this, the other was a Bogen unit having been the first American amplifier to have used the Walker Quad II amplifier's mode of output circuitry. Since then many OPT manufacturers have followed suit with separate "G2 - B+" windings, to either provide for using a lower screen voltage or using cathode feedback as per Quad II.]                   

Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Agaton Sax on January 12, 2020, 08:01:41 AM
Quote
Happy for him if Agaton is pleased with his results.

Oh no. How am I now involved? I bought an MQ 70 new from Paul Bothners in maybe '84. It was really good sounding,much better than the other amps I had. It used a Luxman only valve as a driver the, 6240G-most emphatically not an ECC 85. It was a larger format tube. With the amp Bothner gave me a schematic . On it, in pencil, was written ECC 85 next to 6240G.

At that stage an ad appeared in The Absolute Sound for RAM tubes. This was selected tubes sold by Roger Modjeski,a well regarded academic and tube designer (Beveridge,Counterpoint SA4 as well as his own company).http://www.ramlabs-musicreference.com/bio.html (http://www.ramlabs-musicreference.com/bio.html) In the add was a picture of the 6240G,so I wrote to Modjeski. He stated that he had 2 6240G he would sell me and that there was no equivalent for the valve ,certainly not an ECC 85 . Did he lie? I do not know. I know squad about tubes.

From Audio Asylum:The 6240G plug in replacement is the 6FQ7/6CG7. andI understand the 6240G resulted from a higher voltage 12BH7, but can't verify

I do know the amp blew up under guarantee,was fixed by a most unpleasant  and dodgy person a few shops from Bothners and was never quite the same.He did tell me the output transformer blew but he fixed it. I may not be very clever but even I am not that stupid. I sold it years later to someone who obtained beautiful sounds from it using Quad Electrostatic speakers.I recently learned he gave that amp plus my ex Audio Research D90 to his gardener who said he could fix them. What are the chances that that is the same MQ 70 Charles ended up with? I mean ex Capetonian so called repair person plus gardener may result in what Charles has to deal with now?
 
I was looking for the schematic when I first read this thread,failed to find it and posted the circuit from DIY. However I still doubt the ECC 85 bit. The 6240G was used by Tim de Parvicini in the Luxmann 3045 as well so if someone is really interested better ask him?
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: hope on January 12, 2020, 08:39:22 AM
From my valve reference book(https://i.postimg.cc/sxV41y9x/20200112-083440.jpg) (https://postimg.cc/MMFQrCLJ)(https://i.postimg.cc/vHT5K00J/20200112-083429.jpg) (https://postimg.cc/CzyZBsT7)
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: hope on January 12, 2020, 08:41:55 AM
(https://i.postimg.cc/tCFnw8hm/20200112-083429.jpg) (https://postimg.cc/dLt0THN8)
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: hope on January 12, 2020, 09:41:46 AM
(https://i.postimg.cc/tCFnw8hm/20200112-083429.jpg) (https://postimg.cc/dLt0THN8)
Note 6CG7 equivalents
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on January 12, 2020, 05:18:32 PM
From Ampdog:

"And yes, 'mine' did come with the mentioned prescripts plus schematic, although those are standard general procedures for valve amps."

I had a good look at the MQ70 documents I have, call it a "service manual" part, that is different to what Ampdog is referring to.

There are no ECC 85 valves in the set according to this documentation.

There is a neat block diagram (description) of the set in the documentation that indicates:   The Driver Differential Amplifier use 6CG7 (6240G)
So this set use 6CG7/6240G valves and not ECC85 ones.

This is just as what Agaton Sax said of the amplifier he had.

I am going to remove the ECC85's in my set and replace it with the correct ones namely 6CG7/6FQ7. It would be difficult to find the 6240G ones.
After that I will again align the set according to the "service manual"manual. 

This amplifier came from an auction in Pretoria.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: hope on January 12, 2020, 06:27:12 PM
Aha!
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Agaton Sax on January 12, 2020, 06:42:47 PM
Charles

The MQ 70,ARC D 90, possibly Acoustat TNP pre amp, Tannoy Golds,Quad ESL and Linn Ekos Shure Ultra 500 were in storage in Gauteng for more than 15 years before ending up with the gardener.

 This is all I have left  of the documentation and I think is the block diagram you are referring to? Curiously the  pencilled ECC 85 bit seems to have disappeared. Maybe it was on the actual circuit diagram.

(https://live.staticflickr.com/65535/49373691356_4e67898b8d_k.jpg)

The elusive 6240G. I get cold just thinking of those anxiety filled times. The orange labelled bastids.



(https://www.hifido.co.jp/photo/14/119/11989/o.jpg)

deja dingus

(https://www.hifido.co.jp/photo/14/119/11989/p.jpg) (https://flic.kr/p/2idZ5hC)
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on January 12, 2020, 11:21:48 PM
Agaton Sax

My documentation is the same.

Mine is complete except for the original correct circuit diagram.

All I can say this amplifier sounds very good.
 
 
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on January 13, 2020, 07:37:30 AM
 Just a correction please:

"There is a neat block diagram (description) of the set in the documentation that indicates:   The Driver Differential Amplifier use 6GC7 (6240G)
So this set use 6GC7/6240G valves and not ECC85 ones."

it should read 6CG7 and not 6GC7


Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on January 14, 2020, 12:06:24 AM
Before we get too bogged down in minutiae:

One reads in various places that "the 6CG7/6FQ7 is a direct plug-in replacement for the 6240G . . ." - -
(direct plug-in replacement for me means exactly that only: The pin-puts are the same, plus the characteristics possibly similar . . .)

but then the schematic posted earlier shows the driver as 6AQ8 - which is the exact equivalent of the ECC85, not 6CG7  (American vs. Continental numbering system). Only, the 6AQ8/ECC85 has about 2,3 times the Gm of the 6CG7 at the same Ia; enough to make a noticeable difference to NFB stability, depending (apart from the former having a higher 'Vao  max.' rating.)

(Bottom line: Not simply a direct replacement-and-switch-on).

. . . and one is still hunting for confirmation of what exactly the elusive 6240G is supposed to be - and caring less by the minute.  (If of any earthly interest: for beefy high Va driver triodes I still prefer using E182CCs, despite some difficulty to get both triodes of a pair within narrow tolerance.)

[Hope, I am unfortunately unable to read the interesting information posted by you; cannot seem to get the brightness up.)

As said the MQ70s which came my way were happy with the ECC85 drivers with which they came (though I can no longer recall whether I affected minor NFB compensation capacitor changes).

If not mentioned: There was some interesting discussion of this amplifier on the internet several years ago.
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on January 14, 2020, 12:54:17 AM
and Charles and I Affected by the same spelling Effect . . .

Previous post: Should read "I cannot recall whether I effected minor adjustments  :blah:  I did not  make the time limit for modifications  :-[
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: hope on January 14, 2020, 06:20:43 AM

[Hope, I am unfortunately unable to read the interesting information posted by you; cannot seem to get the brightness up.)

Any better?(https://i.postimg.cc/VkMyVTR6/6CG7-1.jpg) (https://postimg.cc/0z8H6VhR)(https://i.postimg.cc/15k2wBQZ/6CG7-2.jpg) (https://postimg.cc/gnKgPvjM)karthik jayaram (https://celebsroll.com/karthik-jayaram-bio)(https://i.postimg.cc/xC8tWN2R/Book.jpg) (https://postimg.cc/jwVHf5Cw)
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Nidri on January 23, 2020, 08:01:56 PM
Still going for $2000 in good condition.

https://skyfiaudio.com/products/luxman-mq-70-stereo-tube-amplifier-220v?_pos=5&_sid=948b5094c&_ss=r
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on January 24, 2020, 05:36:46 AM
The reissue one probably looks much better than the original one.
It has newly designed output transformers and a new circuit design. The physical layout of the amplifier differs from the old one. 

Charles,
Here you mentioned two different amplifiers (newly designed output transformers and a new circuit design.) Can you explain or please give a source, since I do not seem to find anybody else globe-wise who knows about this?

Not to belabour so once again I will leave it there unless others are interested. (It requires no  more than 2nd year valve knowledge plus a set of proper Va/Ia graphs to show that the schematic shown earlier cannot be practically balanced with 'ordinary-spread' valves and zener voltages.)
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on January 27, 2020, 05:01:34 AM
This subject never seems to stop, does it :(

I am still receiving conflicting messages/questions - trying my best to keep peace. E.g. I have several times asked for elucidation on what the mysterious diodes are, particularly those which I found to be zeners, contrary to the symbols given - no information seems to be forthcoming.

But Charles:
In Post #27 you seem to be rather convinced that the MQ70 does not use ECC85 driver valves;

(https://images.weserv.nl/?url=http://web1.kcn.jp/tube/MQ70main.jpg&w=800&t=fit)

You then seem to dispute this schematic, indicating 6AQ8 (ECC85), as authentic?  Did you get any other schematic with your acquisitions or valves listed in your parts list?

(Other indications that this schematic is not issued by Luxman are the several question marks with the relevant diodes. Does Luxman not know what they are using? Such designations are most uncharacteristic for any official document.)   Further the vague designation "semiconductor" or similar description found in parts lists (under correction) confuses.

Thus one seems to be no closer to reality, judging from what is evident from several different models doing the rounds, and I would be rather careful to make categorical statements on what the MQ-70 actually was or should be  - if any one model did in fact exist.  (Again: claims to "this is the 'new' MQ70" . .  which is then which??) I have at least given diode numbers I found (1N4760), but nobody confirms/disagrees . . . .?)

A singular situation in my long years of amplifier experience; all this tends to make one wonder: What is actually 'official', if anything?
 :headbanger:   :popcorn:

(I am awaiting final replies from a few internationally known friends. I am not holding my breath - -  The sooner this can end the better; I think there are better amplifiers on the market.)

Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: charles on January 27, 2020, 12:23:25 PM
Refer to an earlier post:

I had a good look at the MQ70 documents I have, call it a "service manual" .

There are no ECC 85 valves in the set according to this documentation.

There is a neat block diagram (description) of the set in the documentation that indicates:   The Driver Differential Amplifier use 6CG7 (6240G)
So this set use 6CG7/6240G valves and not ECC85 ones.

In this set of original MQ70 documentation:

Complete alignment procedure and adjustment location diagram

Part List (with Luxman stock number) and description and a part layout diagram

Also the following valve list:

V1L & V1R = 12AX7
V2L & V2R = 6CG7
V3L & V3R = EL34
V4L & V4R = EL34 

This is a very good sounding amplifier
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on January 31, 2020, 02:26:09 AM
Time to conclude my contribution here:

I had little to no informative reaction to queries PM'd to other notable members of earlier contributions. Nobody offers any particular clarity regarding the 'mystery' diodes; in fact, surprise was expressed re further interest. I could also not retrace the previous thread (some 5-6 years ago).

Thus I  must suffice with own experience and memory. As said, all hail to whoever is happy with their MQ70s at present.  The amplifier iron at least is/was very good.

[Footnote: Regarding the driver valves, because of the design of the stage a number of medium-µ triodes will actually serve. Main requirement is a low Va(sat) value because of the extra signal amplitude required by the EL34 cathode feedback feature. In this respect the 6SG7  (equivalent of the old workhorse 6SN7)  is slightly better suited than the ECC85; the 12BH7 would be better still.  (Again the choice of a 'long' tail in the form of a cathode resistor of all of 10K in this stage , 'robbing' it of some sorely needed h.t., mystifies   . . . but enough.)]
Title: Re: LUXMAN MQ70 tube amplifier (new issue)
Post by: Ampdog on January 31, 2020, 06:31:56 AM
" In this respect the 6SG7  (equivalent of the old workhorse 6SN7) "

What is it with this thread and valve numbers!!

This time I mistyped. The '6SG7' above should of course have been '6CG7' !