Marantz 8B Project

I have a request...
Could one of you take a picture of each original external cap so I can print the original label on a sticker I will put on my grey cans.

Thanks in advance
 
I have a request...
Could one of you take a picture of each original external cap so I can print the original label on a sticker I will put on my grey cans.

Thanks in advance

Let me get you some clear pics, not easy to do since some of the lettering on one of the caps in my amp is somewhat faded.

In the meantime, take a look at VAC's gray cans. I like the Marantz sticker here better...

8b-0.jpg
 
Hi,

Here is a picture of:


* Original can
* Replacement can on my 8b as supplied by VAC
* Innards of my 8b with critical film caps replaced as supplied by VAC

If I were to replace coupling caps today, I might very well go with the Russian K40y as noted earlier in this thread.

Good Luck,

Kcin
 

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Thanks a lot guys. I am very lucky to have you.
I will try to print a sticker like VAC reissue.

Here is where I am :
IMGP5542.jpg


I am done with heaters wiring and I am done with the AC grounding. I am also done with the Power Supply (except for the choke I will install only at the end).

Regarding the grounding I have a question for you.
The AC one is done and is very clever it is done on the 40/40/20 cap ground pin and from there to a single point on the frame and to pin 1 of the EL34, pin 9 of the 6CG7 and center pin of the 6BH6.
My question regarding the grounding of the input and output. This one is done on the first lug of the 7 lugs strip you can see below the 6BH6 sockets (when I mean first lug I mean the one the closest to the speaker terminal strip. On mine this was stopping here but I can see on pictures of other 8Bs there is a wire between this lug and the third one and then this lug is linked to a grounding lug which is on the bolt next to this lug (I can see that on VAC reissue picture).
Could you tell me if I am right ? Could you tell me if this is the case on the original one (Crooner ?), or can you tell if this first lug is the end of the input grounding or if there is a link to somewhere? In fact could you make pictures of this part.

Many thanks for your valuable input.
 
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I zoomed in on my high resolution pics. Is this the area you are trying to visualize?

Cedus: do you use 240V AC in your country? If so, you may be able to rewire your power transformer from 120 to 240V. Check your serial number. Units # 6606 and up have split transformer primaries and can be converted to 240V.
 
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Cedus: do you use 240V AC in your country? If so, you may be able to rewire your power transformer from 120 to 240V. Check your serial number. Units # 6606 and up have split transformer primaries and can be converted to 240V.

I use 230V AC in my country and I knew about the split transformer but my 8B is well before the # 6606. This means I have to go through a 230-110 transformer.

Thank you both for the pictures.
Kcin I see your red lines and this is exactly what I had on mine. My question is : Is the first lug on the top of your picture (the ones where the caps are attached to) wired to anywhere else and more important is this ground wired by anyway to the AC ground (on the ground lug of the 40/40/20 cap)
 
No,

The first lug common to all caps is not grounded anywhere else. No it is not tied to the AC ground anywhere. The ground of the main filter cap goes to a ground lug underneath that is held by the rightmost male cage pin with a screw.

BTW I use a replacement 3prong conductor for mains and the ground is bolted to the stud of the power transformer with a toothed ring lug no hum issues whatsoever and it is safe.

I don't think I've opened up my 8b as much in 10 years!...Good work so far.

Kcin
 
Great! Your help is much appreciated !
Next wiring session wednesday night ! I am not very satisfied with the look of my wiring but I hope this will look better with everything done.

I am very happy with the Teflon Silver wire I bought on the bay. It is very flexible and very easy to solder.
 
You know what !
Tonight I am done. Finished ! Last solder done. Everything soldered. It is unbelievable how long it took me to solder everything. Tens of hours. But now the joy should start.

In fact I have still the power cable to solder but I want to check again and again my soldering and wiring to be sure I could start the test procedure.

The test will take place next friday. I will give a visit to one of my friend who starts to be famous in France with his amps. Go to his site you will see some of his creation : http://pagesperso-orange.fr/boutailh-tan/ampli/ampli.htm
This guy has all the necessary frequency generator and other stuff to test and setup my 8B.

So more news during the week-end.

But I would like to give you my concern about one point.
I am in the way of thinking there is a wiring mistake in the majority of the 8Bs I see pictures on the internet. I have to recheck everything before stating that but I would like your help to give me your opinion.
Next post for expressing my concern
 
The mistake regards this part of the schematic :
marantz8Bpb1.jpg


And more especially the 1M and the 12K resistor you see in the schematic. How it is wired in all the 8Bs I saw pictures of. Even mine was wired like that :
marantz8Bpb2.jpg


To summarize on this picture :
Pin A is linked to : Pin 7 of the 6CG7, 12K resistor,
Pin B is linked to : the 0.1uF and 0.33uF cap through the white wire, 12K resistor, 1M resistor
Pin C is linked to : Pin 2 of the 6CG7, 1M resistor

Ok now let's compare this to the upper shematic. Obviously there is a problem.
The 1M resistor should be in fact between the Pin A and Pin C otherwise this is not at all the schematic but something else.
When I found that I went to check in all the photo I had and everywhere (this should more than 6 original 8Bs it was wired like that !!! I scratched my head for tens of minutes trying to find if I missed a wire or something but after checking and rechecking I could not find any solution. And the light came the solution was to find a VAC Marantz 8B reissue to see how it was wired and I had one in stock :

marantz8Bpb3.jpg


And... what do we see... the 1M is wired correctly between pin A and C.
So what's wrong have the original 8Bs got a wiring mistake nobody never saw except the people in VAC.
I have no explanation for the moment and I would like to have your opinion on the matter... please...


I hope this did not disturb too much your christmas time. I found that today and I do not know what to think !!!
 
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Hi Cedus,
Looks like you have your project all wrapped up, just in time for the new year! :thmbsp:

Excellent and very informative couple of posts. Yes, it is puzzling that Marantz wired those parts different from the schematic. However, it is possible that the factory made small production changes due to parasitic oscillations and such. Deviations from the actual schematic are not uncommon.

But it does appear VAC followed the schematic to the letter.

Again, great job and please let us know how it sounds!
 
The mistake regards this part of the schematic :
marantz8Bpb1.jpg


And more especially the 1M and the 12K resistor you see in the schematic. How it is wired in all the 8Bs I saw pictures of. Even mine was wired like that :
marantz8Bpb2.jpg


To summarize on this picture :
Pin A is linked to : Pin 7 of the 6CG7, 12K resistor,
Pin B is linked to : the 0.1uF and 0.33uF cap through the white wire, 12K resistor, 1M resistor
Pin C is linked to : Pin 2 of the 6CG7, 1M resistor

Ok now let's compare this to the upper shematic. Obviously there is a problem.
The 1M resistor should be in fact between the Pin A and Pin C otherwise this is not at all the schematic but something else.
When I found that I went to check in all the photo I had and everywhere (this should more than 6 original 8Bs it was wired like that !!! I scratched my head for tens of minutes trying to find if I missed a wire or something but after checking and rechecking I could not find any solution. And the light came the solution was to find a VAC Marantz 8B reissue to see how it was wired and I had one in stock :

marantz8Bpb3.jpg


And... what do we see... the 1M is wired correctly between pin A and C.
So what's wrong have the original 8Bs got a wiring mistake nobody never saw except the people in VAC.
I have no explanation for the moment and I would like to have your opinion on the matter... please...


I hope this did not disturb too much your christmas time. I found that today and I do not know what to think !!!

A very interesting "discovery" you've made. Being curious, I checked my 8B and it too is wired as in your picture and not like the schematic. I find it really strange that so many of these amps could have been produced with a wiring error like this. What's even more interesting is that there appears to be no service bulletin regarding this discrepancy. I'd be interested to know if there are any original 8Bs wired as per the schematic in the field. All of which got me to thinking...could the schematic be in error. Mac's "official" schematic for the MC240 has a significant and uncorrected error in the bias supply circuit so it's not unprecedented.

A close look at the circuit in question provides further evidence for the possibility of a schematic error.

The 8B uses a fairly conventional, direct coupled, long tailed pair phase inverter but with a couple of interesting twists. Typically, C6A (0.33uF) would be tied directly to the junction of pin 7 and R7A (1M). This allows g1 of both sections of the 6CG7 (pins 2&7) to be biased equally at ~175VDC by way of R7A but places pin 7 at AC ground to allow the second section to operate in grounded grid mode which in turn produces a signal at its plate 180 deg from the signal in the first section i.e. inverted phase. However, in the Marantz circuit we find that C6A doesn't tie directly to pin 7 but rather has R5A (12K or 18K) betw C6A and pin 7 (g1 sect 2). The reason for this is that Marantz uses cap coupled high freq inverse feedback from the plates of the output tubes coupled into g1 (pins 2&7) of the phase inverter. These are C9A and C10A (1.5pF). If R7A wasn't present, the feedback signal from the output tube plate would be shorted to ground thru C6A thus negating the inverse feedback and unbalancing the inverter at high frequencies.

The difference betw the schematic and the production units is where R7A is tied into the circuit. In the schematic, R7A ties in directly betw pin 2 and pin 7. In the production units, R7A ties in betw pin 2 and the junction of R5A, C6A and C5A. In either case, pin 7 (g1, sect 2) will receive proper DC bias. However, in the schematic version the AC signal is forced to pass thru R7A on its way to ground. This is the equivalent of adding 12 or 18K ohm ESR to C6A (parallel with C5A) thus reducing the already less-than-perfect balance of this type of phase inverter. Yes, Marantz did provide for phase inverter balance adjustment but they were obviously going for as much inherent balance as practical in the basic circuit.

That Marantz was concerned with inherent AC and DC balance of the phase inverter is evidenced by the presence of C5A. This cap couples a fraction of DC power supply disturbances into pin 7 (g1 sect 2) that would normally be filtered out by C6A. This helps balance out the disturbances inherently coupled into pin 2 (g1 sect 1). IOW, the typical implementation of the direct coupled LTP is unbalanced WRT the power supply and Marantz's circuit attempts to correct this situation. FWIW Mac did similar in the MC30 and possibly other amps so Marantz wasn't alone in their concern regarding inverter balance.

Overall conclusions: 1. Both the schematic and the production circuits should work well and provide similar results.
2. The production circuit appears to provide slightly better inherent PI balance. Based on this alone I suspect that the schematic is in error and the production units are correct. My 8B will remain as originally manufactured.

OTOH, it would be nice to hear from the designer or someone familiar with the designer's thoughts on this amp. I’m not sure the repro 8Bs represent this…

Comments/critique welcome!
 
Hey Steve that's one heckuva post! :thmbsp:

I suspected this had to do with the special compensated feedback network used in the 8B.

I believe some manufacturers of the golden years actually introduced small errors in their schematics to disuade others from replicating their circuits. This could well be the case with the Marantz.

I do know they were quite secretive about certain proprietary circuits. The 10B tuner for instance. They didn't publish an schematic until late 1965. No service manual was ever available for it.

I have the original service manual for the 8B, published by Marantz in the late 1960s (Woodside NY address). It has very little information about the feedback network. There are no instructions on how to adjust feedback for best bandwidth and minimal parasitic oscillation, for instance.

This would be mandatory in case an output transformer was replaced.

As for the VAC reissue, I wonder if the sound would improve by replicating the "error" of the original 8B. Food for thought....
 
Hey Steve that's one heckuva post! :thmbsp:

I suspected this had to do with the special compensated feedback network used in the 8B.

I believe some manufacturers of the golden years actually introduced small errors in their schematics to disuade others from replicating their circuits. This could well be the case with the Marantz.

I do know they were quite secretive about certain proprietary circuits. The 10B tuner for instance. They didn't publish an schematic until late 1965. No service manual was ever available for it.

I have the original service manual for the 8B, published by Marantz in the late 1960s (Woodside NY address). It has very little information about the feedback network. There are no instructions on how to adjust feedback for best bandwidth and minimal parasitic oscillation, for instance.

This would be mandatory in case an output transformer was replaced.

As for the VAC reissue, I wonder if the sound would improve by replicating the "error" of the original 8B. Food for thought....

Thanks! I hope it all makes sense.

As for Marantz's secretiveness, I agree, there isn't much really good documentation for most of their equipment. I used to think it was a result of, shall we say, "unfocused" business practices. Lately I'm more inclined to beileve it was a result of competitive pressures on a relatively small (at the time) business unable/unwilling to defend patents in court (there goes that savage capitalism again:D).

As for replacing an output transformer, I have no idea what parameters Marantz was optimising with the multiple feedback windings and take-off points. Nor have I ever heard of anyone claiming they knew other than broad generalities such as optimum transient performance and unconditional stability. So it would appear that if an OPT ever required replacing, proper optimization of all those adjustments and replacement of the amp specific part values woud be an extremely tedious exercise in trial and error. And this assumes we know what the desired outcome of proper adjustment really is.

As regards VAC reproductions, I doubt that anyone could hear the differences betw the two versions if not told of them, I believe it's more a case of technical excellence than practical excellence. Most of the reviews of either amp verison have been quite positive. I've read comments by those who've heard both and they say that there are differences but they're subtle and one isn't necssarily better than the other.

At the end of the day I'd like to think that Marantz would not produce prducts with obvious errors in the circuitry w/o a service bulletin of some sort esp considering the perfectionist image they were projecting at the time. OTOH, I'd also like to think that Kevin Hayes and co. were complete in their research when they reproduced the 8B such that schematic errors weren't propagated. For the time being I'm siding with the Marantz of old...but I could be dissuaded by a voice from the past (or designated rep).
 
Thanks a lot Steve for your precious answers. I will keep with the wiring I had in my 8B (meaning not the schematic one).

I know our work next friday will be tough as the service manual is quite light considering the setup of the 8B. As I had to change all the components perhaps I will have to tweak as well some values. For the moment I think I am going to setup the balance with the pots reachable from inside and the bias and leave it like that without touching the variable caps and others. At least I have taken exactly the value I had in my amp which differ for a couple of resistor with the schematic (in the complex feedback network).

Here is a picture of my wiring (without the choke for a better view):
IMGP2170.JPG


What to say... I am a little bit ashame with the result. It just looks ugly and very disappointing considering the amount of time I spent on this amp. I will never make a leaving with building amps... but I do not intend to ;)
At least I hope everything will be working properly.

As you can see I had to isolate the russian PIO with heatshrink tubing because they are conductive and as the space is limited I did not want them to touch anything wrong.

Rewiring was made using the followings :
Riken Ohms resistor for most of them and Kiwame for the value I could not find (the only problem is those kiwame look awful)
Silver wire PTFE isolated
Russian PIO caps and silver mica caps
Caps for the PS :
CE Man. :80/40/30/20uF 525V and 2 100uF F&T german cap for the voltage doubler. The small CE Man. external cap is not used and is there regarding to its size as cosmestical only.
The only components I kept from the original was :
Variable caps 170-780 pF
The two little 6.8uH inductors parallel with the 3.3Ohms resistors in the feedback network (I was not able to source new ones and they look fine to me).

I cross my fingers and I hope will be working next friday. I can tell you I am quite stressed about it.

Anyway have a nice party tonight and talk to you next year ;)
 
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