MC30 bias question

[phantomboxer]

Hi guys.
On a MC30 with a 15-35-80 can cap, I replaced the selenium diode with a silicone one. There was a 470 ohm resistor between diode and PT winding. What resistor value do you suggest?
Thanks

 

[Bob the Tech]

Schematics I've got show 3.3k resistor in series with the Se rectifier. I'm not sure why you've got a 470 in there, send a picture?

I had good luck with a 9.1k replacement, and I think I got that recommendation from a Terry DeWick post. Bias and performance were good with that value.

 

[ramjet]

Hi guys.
On a MC30 with a 15-35-80 can cap, I replaced the selenium diode with a silicone one. There was a 470 ohm resistor between diode and PT winding. What resistor value do you suggest?
Thanks

Look here: https://audiokarma.org/forums/index.php?threads/mcintosh-hints-and-kinks.87157/

 

[phantomboxer]

Since I already replaced the diode, I can't send a picture of the original setting, only the components that I took out. I replaced the 470 ohm resistor with a trimmer set to 9.1K, since I had no resistor of that value at hand. Voltage after the diode is - 339V, and bias voltage is -33V. I will probably have to replace a few other resistors as well, starting with the 220k between pin 5 and 8 on the output tubes.

 

[Bob the Tech]

If the rest of the unit is assembled per the schematics and working correctly you should not need to change anything else. The modified dropping resistor should drop enough added voltage to compensate for the Se to Si change, the voltage after the diode and the rest of the voltages should fall into line.

9.1k should get you close, perhaps you’ll need to adjust it down a bit, but if you can’t get. It close to the schematic you may have something else that’s not working right …

 

[phantomboxer]

It's been a while...
Regarding the MC-30 S/N 16580 (earlier version, 40-80-20 can cap) and S/N 5E631 (later version, 35-80-15 can cap):
If you look at the schematic for the S/N 5E631, there is no resistor at all between diode cathode and power transformer (PT).
Also, the PT is wired different then the earlier version, and has a different part number.
Unfortunately, the only schematics that I have found are hardly readable, they also have no voltages mentioned, not on the caps and neither on the tubes.

However, the PT on the S/N 16580 is p/n M-152B, and so is the PT on the S/N 5E631. Possibly someone changed it during the last 65 years, probably it was put there already in the factory. I got both working but the output on S/N 5E631 is slightly lower than on S/N 16580 and it lacks a little bit of bass.

So, PCB boards PT and OT (output transformer) on both amps are identical, the can caps are indeed different. And why was there a 470 ohm resistor between diode and PT, instead of a 3.3K???

Initially, replacing the selenium diode with a silicone one, I also replaced the 3.3K resistor (earlier version) with 9.1K, and did the same on the S/N 5E631: 470 ohm to 9.1K. (Well, since I had no 9.1K resistor ond hand, I used an "old style" trimmer. It burned once while dialing it down, then I replaced it with another one set to 4.7K - wild guess, but no difference in performance. It burned again after a few times of testing so I used a real 4.7K resistor, 1 watt I think.

The wires on my S/N 5E631 had a green coloration on some solder joints, possibly indicating that some wires had corroded over time. Since I am also replacing the chassis on both amps, I am replacing also the wires. Painstaking process....

Any thoughts?

 

[prww]

The grid voltage on the 6L6 finals calls for -45 VDC. -33 is not enough and will cause you problems. Too much current draw. When I replaced the selenium on my pair, I removed a 3.3 k, and replaced with a 9.1k to compensate for lower resistance of the replacement diode The supply for this came from one of the 5U4 rectifier plates. The schematic I have shows the 3.3k in series with the selenium diode. I find no 470 ohm in my schematics bias circuit. My transformers are M-152B. Somewhere I read that a separate bias winding was used for later models. If that was the case here, your transformers should have a different number, and the standard "use a 9.1K" should not be applied here. I don't know what value resistance would apply in this situation. You need -45VDC on those grids. Where the supply comes from should not change the bias needs of the 6L6's. Paul.

 

[dcgillespie]

The 470Ω resistor in series with the bias rectifier made its appearance in the original A-116 (no suffix) amplifier, which used a similar B+ supply and output stage, but significantly different front end and driver section. The power transformer in that design also employed a separate winding for the bias voltage. Apparently, somebody along the way lifted that value and used it in the repair/restoration of your amplifier??

Dave

 

[phantomboxer]

It is possible. But I have no clue.

 

[prww]

The 470Ω resistor in series with the bias rectifier made its appearance in the original A-116 (no suffix) amplifier, which used a similar B+ supply and output stage, but significantly different front end and driver section. The power transformer in that design also employed a separate winding for the bias voltage. Apparently, somebody along the way lifted that value and used it in the repair/restoration of your amplifier??

Dave

Thanks for clearing this up. For this time, and the next time this issue comes up. Paul.

 

[phantomboxer]

Hello Fellas.
It's been a while since my last post. Now I had some time to get back at the pair of MC30's.
Now I can set bias to between -44.4 and -44.8 V. The issue was that R26 820K and R27/29 both 220K since they had increased in value on both amps. Also replaced R18 220K and R22 220K
There are 2 resistors R15 27K and R17 3K according to the schematic. On both actual amps, R17 is 30K. And on one amp (S/N 5E631), R15 has increased to a higher value than R17, to approx. 36K.
This 5E631 amp actually has a little less output (both with gain to max.) than the other, also the lower frequencies sound a little flat. I have not replaced those resistors because I don't have any on hand for now.
Could that be the reason for the different output?

 

[prww]

I believe that in order to achieve identical outputs, both in watts and sound, you must have identical components in both amps. Some components will make more or less difference, depending on what and where they are. Make sure your power supply B+ is also close to each other. Paul.

 

[phantomboxer]

Hi guys.
Already replaced both chassis for the new ones. It's not too hard, a lot of planning and sketching involved and then painstaking work (burnt fingers included).
I've been messing around with both amps. Particularly the one that sound weaker. But I still cannot make them sound the same, and before replacing more resistors where most still are within specs and by the way not so easily available where I live, I would like some advice from the masters...
I'm aware that voltages on the can cap and diode should be according to the schematics (best case scenario) or at least be the same.

Here are the measured volts on each tube:

V1
Good sounding amp:
pin1: 124v
pin2: 0v
pin3: 1.19v
Weak sounding amp:
pin1: 113v
pin2: 0v
pin3: 1.06v

V2
Good amp:
pin1: 264v
pin2: 124v
pin3: 131v
pin6: 259v
pin7: 101v
pin8: 130v
Weak amp:
pin1: 263v
pin2: 111v
pin3: 118v
pin6: 256v
pin7: 91v
pin8: 118v

V3
Good amp:
pin1: 349.5v
pin2: 0v
pin3: 16v
pin6: 342v
pin7: 0v
pin8: 16v
Weak amp:
pin1: 351v
pin2: 0v
pin3: 17.4v
pin6: 337v
pin7: 0v
pin8: 17.4v

V4
Good amp:
pin1: 433v
pin2: -44.0v
pin3: -44.8v
pin6: 424v
pin7: -44.2v
pin8: -44.7v
Weak amp:
pin1: 426v
pin2: -44.0v
pin3: -44.6v
pin6: 427v
pin7: -43.7v
pin8: -44.5v

V5, V6
Good amp:
pin3,4: 433v
pin5: -44.8v
pin8: 0.345v
Weak amp:
pin3,4: 426v
pin5: -44.6v
pin8: 0.334v

V7 pin 8
good amp: 439v
weak amp: 440v

Diode (anode)
good amp: -340v
weak amp: -348v

Can cap, good amp:
22uf: 345v
100uf: 417v
47uf: 429v
Can cap, weak amp:
22uf: 361v
100uf: 432v
47uf: 434v

I'm counting on you. Please...

 

[prww]

Have you tried swapping tubes between units to see if the weak follows the tubes? Paul.

 

[phantomboxer]

Yes I have. Doesn't change anything, it definitely is the circuit.
The 1614's have not much juice left, by the way.

 

[Bob the Tech]

Those bias voltages track quite well, I'd say. Have you swapped rectifier tubes? or just the amplifier tubes? The rectifier tubes and their output will determine a lot of what you get from the power supply. There's always going to be some variation.

When you say sounds weaker, what do you mean? Lower output power? Or a difference in the tones? Do you have the ability to generate signals of different frequencies, and measure signal levels with a scope? You might move away from worrying about bias and start looking at whether you are getting proper flat frequency response from both units as that may be the cause of what you're hearing.

 

[prww]

Obvious, but I have to ask. Have you swapped left and right inputs. Maybe the issue is at your source, not the MC-30's. Paul.

 

[Bob the Tech]

Obvious, but I have to ask. Have you swapped left and right inputs. Maybe the issue is at your source, not the MC-30's. Paul.

Or outputs - change speakers and see if it tracks the speaker?

Or - since you moved from original to new chassis - did you connect the output transformers correctly? If you're on the wrong transformer tap it will definitely impact what you are hearing at the output.

 

[phantomboxer]

I think I swapped all tubes. first the preamp tubes, including rectifier (I think...). Then the power tubes.
By "sounds weaker", I mean slightly less output power and different tone. The "good sounding" one is more punchier in the bass.
I do have an 'olde' QEE sound generator with fixed frequencies (30, 100, 700 Hz, 3, 10, 15KHz). But I do not have a scope.

Yes I have swapped left and right inputs. And left and right outputs.

Since performance of both amps has not changed before and after I swapped the chassis, I would think I connected every transformer wire in the correct place. I marked each wire with a piece of tape and a number, and wrote down where each wire connects to.

 

[ramjet]

I think I swapped all tubes. first the preamp tubes, including rectifier (I think...). Then the power tubes.
By "sounds weaker", I mean slightly less output power and different tone. The "good sounding" one is more punchier in the bass.
I do have an 'olde' QEE sound generator with fixed frequencies (30, 100, 700 Hz, 3, 10, 15KHz). But I do not have a scope.

Yes I have swapped left and right inputs. And left and right outputs.

Since performance of both amps has not changed before and after I swapped the chassis, I would think I connected every transformer wire in the correct place. I marked each wire with a piece of tape and a number, and wrote down where each wire connects to.

Please share the schematic you are using. Regarding your previous post about the values of R15 and R17; did you make any changes to either amplifier on these resistor values?