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Luxman MB3045 6550 mod different from usual ones

david winter

david winter

Classical music lover.
Hi,

I'm trying to bring a pair of MB3045 back to life. They are both equiped with Sovtek 6550 wich I will replace by NOS Sylvania.
I found a few mods inside, and they seem to differ from what I read:
- Around the 3x47µF/450V capacitor, I see 12K-68K-150K instead of 12K-100R-150K in the original schematic. Resistors seem original but I could be wrong.
- Around the 3x47µF/350V capacitor, I see 37.5K-22K-4K7 instead of 33K-33K-4K7. The 37.5K is made of 4x150K in parallel. The 22K and 4K7 seem to be original (gray type).
- Sceens of 6550 tubes are connected through a 27 ohm resistor to the diode bridge and 12K resistor of the 3x47µF electrolytic.
- Both 1K grid resistors are still in place.

As is, with a pair of new Sylvania 6550, I can set the bias to 50mV as suggested but don't know how to adjust the DC as it affects the bias. I guess it's due to the nature of the actual mod.

My concern is that I read two mods:
1) Add a 220 ohm between screen/plate of 6550 and replace 1K grid resistor by 4K7. This was found on a french forum.
2) On original Luxman conversion guide, I see the 1K/4K7 change, the 220 ohms going to an unused pin of a mold terminal, two other resistor changes on the power supply section, and finally two changes on the circuit boad (one 27pF capacitor added over 39K R126, and capacitor C110 removed).

Therefore, what should be done ? Should I leave the actual changes untouched or is there something better to do ?

I'm posting a few close-ups just in case. Any help greatly appreciated.

Thanks in advance.

MB3045_1.jpg


MB3045_2.jpg


MB3045_3.jpg
 
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can you post the schematics so we have a little better idea of what exactly those resistors are doing?
 
seems like the link to the mod isn't working, at least for me.

ok so the second schematic shows the KT88 tubes in pentode mode, 220 ohm resistor feeding the screen from the plate supply. I've seen the mod instructions before and I recall it had options for both triode mode and pentode (they called it ultralinear I think, which isn't true). The triode mode would have a resistor between the screen and plate terminal on the bottom of the socket. The other setup feeds it from the power supply side of the output transformer.

the resistors in the power supply you mentioned, those are R305, R306, R307 ? Looking at the voltages, 68K seems to make more sense. To get from 370 to 160v across a a100 ohm resistor would involve a lot more current flow than a 12ax7 and a 150k bleeder resistor could possibly manage. I'm guessing the 100 ohm value on the schematic is just a mistake.

The other set of resistors may have been changed to get the correct bias adjustment range. Not sure on that.
 
Oops, I fixed the link, sorry.
So far as I believe, the mod in those blocks seem to have worked for some time, so maybe it is safer to leave them as is.
However I don't know what exact bias / DC settings should be applied. Maybe put the DC in middle position and adjust the bias for 50mV ?
 
I'm going to watch as gadget helps out on this one. He got my amps up and running after I did the mod and the choke was not connected to the output tubes because this step was not written down on the mods pages.

The bias and DC balance are the adjustments on this amp. The balance will affect the bias and this is why the manual says to go back and forth a few times to get the final setting of these two adjustments.
 
Thanks. I don't know what to do:keep the (different) mod as is or apply the original one.
Right now the screens directly go to the diode bridge output through the 27 ohms resistor, so even before B1. I find this strange.
One the circuit boad there is a 47pF capacitor accross R126 so either this was added for the mod, or is originally installed. I didn't check the solders to see any difference.
After that, the resistors on the power supply section are as photographed.
What's interesting for me is to know your opinion: should I keep the actual setup or apply the original mod. In the later case, I'll need to know any steps not shown in the original document, like the choke that you describe.
I think it would be useful to write down the complete mod document for those who are interested, and put it into the database.
 
The instructions don't actually say to connect those two screen resistors to a power source, so the amp doesn't work with the steps as-written.

The missing step is basically this
7A connect a wire from the 220R resistor at mold terminal A to the 220R at mold terminal B
7B connect a wire from mold terminal B to point B3 in the power supply.


don't know physically where B3 is, looks like the schematic implies some have fuses and some do not, so I guess it varies depending when your specific amp was made. It might be a fuse holder, it might be the C304 + terminal or you could land it on the choke since thats probably easier. Just confirm which terminal goes to where on the choke if you run it to that point. I've been inside of exactly two of these amps, so I won't pretend to have in-depth experience with them.



On yours the 220R is going to pin 6, which is fine to use as a tie point, or you can move it to the terminal strip. It really doesn't make any difference. If these were mine I'd probably leave the resistors where they are and just move the voltage supply to point B3.
 
The resistor going to pin 6 is not a 220 but a 27 ohm resistor. I got mixed as well but it's clearly marked 27R. The wiring is very similar to steps 7A/7B but instead of going to B3, the wire goes before the 12K resistor, so straight at the diode bridge output and the first electrolytic (C305). I don't know the purpose of doing this, but according to the schematic the screen voltage must be significantly higher than with the usual mod. No idea how much the 27 ohms resistor will drop, but I guess we could easily have close to 400V there (if not more as our mains are mostly at 230V instead of 220V).
 
The value isn't hugely critical but 100 ohms or more tends to work better for screen protection. I'd probably make that a 220 ohm like the Lux docs suggest. The current flow isn't enough to drop much voltage though. Moving where the screen is fed in the power supply will do more to fix that.
 
Ok I'll follow your suggestion, in any case it makes a lot more sense to move the screen power to where it's supposed to be.
Another question though: how reliable are those three big electrolytics ? Is it usualy safe to keep them or should they be replaced ?
Thanks again.
 
Its a 30-odd year old amplifier. I doubt those caps are more or less reliable than any other old capacitor. @Blue Shadow is the only person I know of who owns a pair. I *think* his has original filter caps still but I won't swear to that. I know something went south on the circuit board underneath and needed some repair. I think it was the large grey coupling caps that went leaky and caused the 10 ohm cathode resistors to cook. I see some damage to one of those 10 ohm resistors on yours as well.
 
Indeed but they are fine. The problem was an internal short in a 6550 which caused excessive current draw (the ampmeter would raise close to the amp before I shut it down).
I've put a NOS Sylvania 6550 pair in place since and the amp draws something like 400-450mA idle.
As for the capacitors, I'm surprised there isn't a replacement list somewhere. The problem is more around finding multiple capacitors of the same size than the values / voltages themselves.
 
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I swapped the large grey caps in one because the 2W 68kΩ resistors were dropping about 2.25W. Burn board. Repaired that, did a thread.

I see the bias and balance resistors look pretty serious. These are 10Ω 3W in stock form but recommended to lower the wattage to prevent problems from killing a transformer. 1W is recommended, IIRC. I have some replacements to install.

The can caps are original in my amps, but these amps were only worked hard early in life, then took a couple decades off. Looking at stuffing them but haven't found the parts I can use, yet.
 
After applying the official mod I found that both 6240 tubes are weak.
I plan to replace then by NOS Sylvania 6CG7 supposed to be equivalents, but after comparing their specs I found that the 6240 outputs 7mA under 250V plate and -5V grid whereas the 6CG7 outputs 9mA at 250V plate and -8V grid.
Testing the 6CG7 at -5V grid makes it draw around 14mA so twice what the 6240 outputs.
Is the 6CG7 a safe direct replacement (unless there is a mod) ? Or is there a better reference to use ?

Thanks.
 
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