Driving the EL34s from a cathode follower is a good idea if you want some class AB2 power and avoid blocking distortion. But in this case both of those are cancelled by using coupling capacitors. So these is no advantage gained.
Yes, adding individual cathode resistors to the 6CG7 is necessary to avoid canceling the necessary inverted signal from the previous stage. But you would need to choose them carefully. I'd start with 20K each and go from there. And returning through that negative current source would have to be trimmed accordingly. The pair are mostly balanced through R28.
Also that schematic has other problems in the way things are connected to the negative voltage supply. R75 and R76 are wired such that the supply could be shorted out if one or both controls were set too near the end of their rotation. And God only knows what R78 and C24 are there for. I wonder if the person was stoned when they drew that up.
I must be getting tired (time to go to bed).
The 6DJ8 is almost-long-tailed with a single common 36.8k to ground for both cathodes. The one 6DJ8 grid is driven by the signal, the other 6DJ8 grid is driven by very high frequency negative feedback; a few pF to 22k; but that same grid does not get any significant signal (signal to 1 Meg Ohm to a 0.68uF cap to ground).
That looks like a working phase splitter to me.
The 6CG7 circuit needs to be fixed. OK.
I re-edited this part of the post, I made a mistake, sorry.
Just my sleepy opinions.
Please correct me if I am wrong.
Thanks!
The 6DJ8 is almost-long-tailed with a single common 36.8k to ground for both cathodes. The one 6DJ8 grid is driven by the signal, the other 6DJ8 grid is driven by very high frequency negative feedback; a few pF to 22k; but that same grid does not get any significant signal (signal to 1 Meg Ohm to a 0.68uF cap to ground).
That looks like a working phase splitter to me.
The 6CG7 circuit needs to be fixed. OK.
I re-edited this part of the post, I made a mistake, sorry.
Just my sleepy opinions.
Please correct me if I am wrong.
Thanks!
Last edited:
Regarding using of the 10M45S as constant current source (CCS), I believe the idea was used to replace the 38.3K cathode resistor of V3 6DJ8. The 6CG7 shall be the same as the original version as cathode following driving the EL34's.
Hey everyone thanks very much for all your help I really appreciate it. After looking at the Model 9 schematic I've decided to abandon using that schematic, I don't want to use the voltage doubler power supply, the phase shift nonsense at V1 or the convoluted feedback mechanism. The amp is unnecessarily complicated and compromised by the level of technology at the time. Instead I'm going to use Pete's Uniamp schematic. I've built it using 300b's and AD1's for other people and it sounds great. Besides I think the real star of the 8b and model 9 is not the schematic it is the EL34. I love the sound of the EL34! So thanks again for all the help, Kevin
I fully concur! As the high gm EL34's really aren't that difficult to drive, the 6CG7 cathode followers simply are superfluous here. Remember that Jim Marshall drives two capacitively coupled pairs of EL34's with a ECC83 LTP! Apart from that, what would that trimpot in the 6CG7 grid circuitry do?
Best regards!
The 6CG7 grid trimpot will adjust (balance) the individual current through each triode section since the source follower can only set total combined cathode current. Remember that a small change of grid voltage has a lot more effect then a small change of plate voltage. All this is useless here because there is no signal at the 6CG7's cathode. They are shorted together and the push-pull (differential) signal is cancelled out leaving only whatever minuscule imbalance there is.
I thought about building a Uniamp with my Model 9 transformers but still want to explore the possibility of a differential 9. I have the tertiary feedback winding on the output transformer secondary. If I draw feedback form each end tap of that winding and then cross them to the cathodes of the first two preamp tubes would global feedback be possible?
Also from the Marantz manual the test mode switch allows an AC balance adjustment without external input, (As far as I can tell.) where is that AC signal generated in the amp? I ask because as a differentail amp balance is assured.
Also from the Marantz manual the test mode switch allows an AC balance adjustment without external input, (As far as I can tell.) where is that AC signal generated in the amp? I ask because as a differentail amp balance is assured.
Also, most of the amplifier I understand but there are parts that I simply can't see the purpose of? And I would like to simplify the amp keeping only the switch to set the bias, I have 0-100ma meters.
If you were to build a true fully differential amplifier, which the Marantz 9 is not, and successfully connected the output transformer tertiary winding back to the input stage cathodes, that would be global feedback. But that is an extreme version and seldom ever done. Balanced feedback to the driver stage is much more common.
The internal AC signal in the Marantz 9 that's used for testing is derived from the power transformer bias winding. It's picked off through a voltage divider (R78, R79 & C24) and sent through SR9-4R to the cathode of V1b. (Original Marantz 9 schematic)
Thanks HollowState, Being an ultralinear amp it already has some feedback. I admit those multiple switches are a bit hard for me to understand. I want to simplify the amp while preserving as much of it's original quality and low distortion. So the normal route is to apply feedback to the 6CG7's? Doing away with the phase reversal and the phase splitter deletes valves 1a and 1b. So it becomes a three stage amp which is much more like differential schematics I'm familar with. Thanks again, Kevin
Are C17 and C18 feedback? And what is the purpose of the neon bulb? That switch thing again. The meter circuit is picking up the current level from the cathode, through the meter to ground via R58? Also, am I right that the amp is in ultralinear not triode mode in the drawing? And does anyone know what the purpose of the TC-6 chokes are? And why only on valves 6 & 8?
C17 & 18 are technically feedback but are there to tweek the square wave overshoot response. These will be dependent on the OPT used and may not be needed.
The NE2 was explained in post 12. It's there to limit voltage between the 6DJ8 elements during startup.
The meter circuit is operating as you describe. It measures voltage across the resistors R45 to 48 which is interpreted as tube current.
Yes, switch ST2T is set for UL operation.
The inductors TC-6, which are very small, are probably there to further improve the square wave response. Why only 2, maybe all that's needed, and again depend largely on the OPT used.
The NE2 was explained in post 12. It's there to limit voltage between the 6DJ8 elements during startup.
The meter circuit is operating as you describe. It measures voltage across the resistors R45 to 48 which is interpreted as tube current.
Yes, switch ST2T is set for UL operation.
The inductors TC-6, which are very small, are probably there to further improve the square wave response. Why only 2, maybe all that's needed, and again depend largely on the OPT used.
Post 12 made me feel like an idiot so I blanked on it. Am I correct in that the "driver Stage " are the 6CG7s? Thanks again HollowState.
You can substitute that NE2 by a LED (cathode to pins 3 and 8, anode to pin 2). It will clamp the grid voltage to it's forward voltage during warmup instead of ~75 V with the neon.
Best regards!
Best regards!
Admittedly, an NE2 with a firing voltage of 90V, and an ionized voltage of about 60V to 75V does not protect a 6DJ8 grid very well, especially under certain hot start conditions.
However, be careful of using an LED to replace it.
What is the capacitance of the LED at zero volts?
What is the reverse breakdown voltage of the LED?
What is the leakage resistance when the grid of the 6DJ8 swings negative with signal?
And, is the resistance of the LED linear for the complete range of signals to the 6DJ8 grid?
However, be careful of using an LED to replace it.
What is the capacitance of the LED at zero volts?
What is the reverse breakdown voltage of the LED?
What is the leakage resistance when the grid of the 6DJ8 swings negative with signal?
And, is the resistance of the LED linear for the complete range of signals to the 6DJ8 grid?
Never had any issues with LED's as the clamping device, neither in hifi nor in geetaar amp service. I don't know the juction capacitance or the reverse resistance values, but breakdown voltage is about 5 V, which allows for 10 Vpp signal input level. Should be enough. But of course we could use a chain of two or three 1N4148's instead.
Best regards!
Best regards!
Still, a device that could be damaged by less than 10 Volts isn't really a good choice to protect a vacuum valve's grid (presumably safe at the NE2's striking voltage). But this is just a tiny, tiny bit of the particular*ness* of the Marantz model 9. The same particular*ness applies to the question about C17 and C18 (1.5pF! WTF) or discussions of other contemporary amplifiers like the Citation II. Without intimate knowledge of the very fine details of circuit design, including especially details of the output transformer, we can't say anything real or meaningful about the "why" of the design.
To a greater or lesser extent this is generally true of the current trend of "computer modelling". Sounds like a great idea, produces numbers (and who could disagree with numbers?), easy and often good enough. But the "good enough" part is less true as the model becomes more less two dimensional; for example, a transformer. An accurate model of a transformer could be made, to any necessary level of accuracy, but even a decent approximation is very complex. Wrapped into a multi-stage amplifier with feedback around several stages and the transformer (and with indeterminent loading, like a loudspeaker) and we have a confidence problem.
Folk making random changes for random reasons to a complicated and very particular* design before even trying to understand the original work are wasting valuable time. Should be doing the work first.
*Jill Conner Browne quotes her daddy telling her to "Be particular". In the South it's pronounced without the first "r".
All good fortune,
Chris
To a greater or lesser extent this is generally true of the current trend of "computer modelling". Sounds like a great idea, produces numbers (and who could disagree with numbers?), easy and often good enough. But the "good enough" part is less true as the model becomes more less two dimensional; for example, a transformer. An accurate model of a transformer could be made, to any necessary level of accuracy, but even a decent approximation is very complex. Wrapped into a multi-stage amplifier with feedback around several stages and the transformer (and with indeterminent loading, like a loudspeaker) and we have a confidence problem.
Folk making random changes for random reasons to a complicated and very particular* design before even trying to understand the original work are wasting valuable time. Should be doing the work first.
*Jill Conner Browne quotes her daddy telling her to "Be particular". In the South it's pronounced without the first "r".
All good fortune,
Chris