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.
Thanks.
UL & CFB output stage with differential input and driver stages, a lot could be learned here, a classic design!
Thanks Man
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.
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.
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!
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!
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.)
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.)
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.
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.
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.
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.
I'm not sure if Patrick Turner even bothered with the definitions, in any case, his write-up can be found here.
Hi,
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.
Ciao, 😉
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.
Ciao, 😉
Hi there Frank!
Long time .....
This is the problem. I have several 6AC7s (brand of one: National Electronics - Made in E Germany), all being pentodes, i.e. with clearly visible G3. Then again I have once seen an EL34 with beam forming electrodes visible; no connection to pin 1. Then you will know that there are many tubes marked "EL34/6CA7" - all pentodes; but that agrees with what you said.
I do read that mainly Sylvania and JJ marketed the beam 6CA7s. But then others say that after the Mullard patent expired a number of manufacturers jumped in and made 6CA7 pentodes, exactly like the EL34. Then I read that at that time Mullard also rebranded their EL34 as 6CA7! Who knows what happened with "re-branding" over the years ....
Not to go into this further. It would appear that the only way to be certain is to actually look inside the tube - the suppressor or beam forming electrodes are easy to see. One wires for a G3-connection at pin 1 and forget about it, it would appear.
There are also EL34s with more sturdy plate construction, probably to woo the band/guitar market. There are types with large cooling fins on G1 and others with smaller ones. What a business ....
We went somewhat off-topic but much information came out. My thanks.
Long time .....
This is the problem. I have several 6AC7s (brand of one: National Electronics - Made in E Germany), all being pentodes, i.e. with clearly visible G3. Then again I have once seen an EL34 with beam forming electrodes visible; no connection to pin 1. Then you will know that there are many tubes marked "EL34/6CA7" - all pentodes; but that agrees with what you said.
I do read that mainly Sylvania and JJ marketed the beam 6CA7s. But then others say that after the Mullard patent expired a number of manufacturers jumped in and made 6CA7 pentodes, exactly like the EL34. Then I read that at that time Mullard also rebranded their EL34 as 6CA7! Who knows what happened with "re-branding" over the years ....
Not to go into this further. It would appear that the only way to be certain is to actually look inside the tube - the suppressor or beam forming electrodes are easy to see. One wires for a G3-connection at pin 1 and forget about it, it would appear.
There are also EL34s with more sturdy plate construction, probably to woo the band/guitar market. There are types with large cooling fins on G1 and others with smaller ones. What a business ....
We went somewhat off-topic but much information came out. My thanks.
Hi there, Johan,
Good to hear from you as well.
Valves from National (Richardson) can be just about anything really, from Eastern European manufacture to USSR to their own local US manufacture. Anything goes for as long as it will work.
Anyway, as you notice it's a bit of a mess but basically and I assumed that's was what you were asking, the EL34 is a penthode the 6CA7 is a "Beam Penthode". That's how it was.
Later on, with the advent of the KT series of valve (the famous KT66, 88, 77 and derivatives) and the on going race for ever more power some manufacturers changed their EL34 penthodes into to tertrodes making in essence a new tube without bothering to give it a new name.
As you said it's no biggie, you just need to be aware of it hence my mention of the caveat in my previous post.
I'll see if I can find anything more specific about the history of the EL34 on Jogi's Roehrenbude and post it here for later reference.
Take care, 😉
EDIT:
Here it is:
http://www.jogis-roehrenbude.de/
Scroll down to "Roehren Geschichtliches" . Only in German but worth a visit.
Good to hear from you as well.
Valves from National (Richardson) can be just about anything really, from Eastern European manufacture to USSR to their own local US manufacture. Anything goes for as long as it will work.
Anyway, as you notice it's a bit of a mess but basically and I assumed that's was what you were asking, the EL34 is a penthode the 6CA7 is a "Beam Penthode". That's how it was.
Later on, with the advent of the KT series of valve (the famous KT66, 88, 77 and derivatives) and the on going race for ever more power some manufacturers changed their EL34 penthodes into to tertrodes making in essence a new tube without bothering to give it a new name.
As you said it's no biggie, you just need to be aware of it hence my mention of the caveat in my previous post.
I'll see if I can find anything more specific about the history of the EL34 on Jogi's Roehrenbude and post it here for later reference.
Take care, 😉
EDIT:
Here it is:
http://www.jogis-roehrenbude.de/
Scroll down to "Roehren Geschichtliches" . Only in German but worth a visit.
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This amp will not go away!
Reopening the thread for further elucidation for those still having patience; apology to others
Many moons have passed. Referring to the power supply schematic above; specifically to the four diodes in the lower right hand side (wish folks would annotate!).
I have recently picked up some flack for stating earlier that these were zener diodes at all! [That was because of what I found in two models encountered for refurbishing; the diodes themselves were 1N4760. They looked like having been factory fitted. But a parts list indicates only 'semiconductors', and the question marks with all diodes do not help either - particularly not in a manufacturer parts list! Must one come to the conclusion that the given schematics were not original factory drawn artworks but compiled elsewhere?]
It would now appear that those were not zener diodes in another model appearing, and which was consequently easier to adjust (not by me, by another technically proficient person).
The purpose of this long epistle then is to please have someone with experience confirm what is/was going on. Were those diodes in fact not zeners originally but ordinary rectifiers as indicated on the schematic - there perhaps to clamp certain points to certain potentials (although that again makes no sense either)? Can any member kindly confirm what exactly the numbers were? (The models refurbished by me have long gone; no others available for checking.)
[To avoid misunderstanding: I am not referring to the two diode bridges leftmost on the schematic; they are clearly rectifiers for the h.t. and bias voltages respectively.]
I am not trying to pull importance out of perspective, but there is also vague reference around to 'old' and 'new' models of the MQ-70.
Can someone please put this to rest? 😀
Reopening the thread for further elucidation for those still having patience; apology to others
Many moons have passed. Referring to the power supply schematic above; specifically to the four diodes in the lower right hand side (wish folks would annotate!).
I have recently picked up some flack for stating earlier that these were zener diodes at all! [That was because of what I found in two models encountered for refurbishing; the diodes themselves were 1N4760. They looked like having been factory fitted. But a parts list indicates only 'semiconductors', and the question marks with all diodes do not help either - particularly not in a manufacturer parts list! Must one come to the conclusion that the given schematics were not original factory drawn artworks but compiled elsewhere?]
It would now appear that those were not zener diodes in another model appearing, and which was consequently easier to adjust (not by me, by another technically proficient person).
The purpose of this long epistle then is to please have someone with experience confirm what is/was going on. Were those diodes in fact not zeners originally but ordinary rectifiers as indicated on the schematic - there perhaps to clamp certain points to certain potentials (although that again makes no sense either)? Can any member kindly confirm what exactly the numbers were? (The models refurbished by me have long gone; no others available for checking.)
[To avoid misunderstanding: I am not referring to the two diode bridges leftmost on the schematic; they are clearly rectifiers for the h.t. and bias voltages respectively.]
I am not trying to pull importance out of perspective, but there is also vague reference around to 'old' and 'new' models of the MQ-70.
Can someone please put this to rest? 😀
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My guess is that all 4 are ordinary Si diodes and that the whole purpose is to create
a negative bias to the 12ax7 cathodes.
What is the voltage on the 12ax7 cathodes ?
A similar idea was in the dynaco PAS-4 , i have a replica board here but that i sometime
will populate, but i think i will use a LED instead of this somewhat complicated design.
a negative bias to the 12ax7 cathodes.
What is the voltage on the 12ax7 cathodes ?
A similar idea was in the dynaco PAS-4 , i have a replica board here but that i sometime
will populate, but i think i will use a LED instead of this somewhat complicated design.