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Ran Smoke Test Last Night on MC275

rkic

rkic

Active Member
No tubes --- put in 1 amp fast blow fuse and brought it up slowly on my variac. Took awhile to get oriented on hot and ground, DC and AC, (I'm new at this) but finally got it together and found a lot of pins way high on DC. Some within tolerance but a lot not... Will survey power supply voltages next.
As long as I don't burn something out, blow something up, or electrocute myself I'll be having more fun tonight.
Too bad work takes up so much of the day!!!
 
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Sounds pretty normal to me!

If you check out the current draw on just the 4 output tubes alone, that will be enough to bring down the B+ to normal. You have gotten past the 1st important stage, it fires up, not blows up!

In the event you want to see what the B+ should look like without the output tubes in it, find out the current draw the outputs would draw and figure out the resistor value that would approximate that current, install it accross the B+ where it would go to the output tranny's and then measure it again.

Be sure to figure out what wattage resistor is needed to handle that current, might be as high as 100 watts or more. The MC-275 experts could help out better in this regard than me as I don't have the schematic.

Hope this helps.
Ron
 
If yours is like mine...

then you should expect for the voltages to operate a bit higher than the schematic calls for. Ron's right, you're ok until you load down the amp with tubes and get current flowing through the grids. Even then, expect the voltages to be a bit higher due to all the new P/S components.

Sounds like you're on track!

Cheers,

David
 
Here are some of the voltages I read...
Please comment

(It's jumbled after submitting - the first item is the pin number, the secone number is the schematic voltage and the third number is my reading)

Point Schematic V Reading Volts
Rectifier 453 490
12AX7
PIN 1 100 176
PIN 3 .95 0
PIN 6 100 175
PIN 8 .95 0
12AU7a
PIN 1 240 427
PIN 2 100 174
PIN 3 106 0
PIN 6 240 428
PIN 7 100 138
PIN 8 106 0
12AU7b
PIN 1 240 428
PIN 2 100 172
PIN 3 106 0
PIN 6 240 427
PIN 7 100 40
PIN 8 106 0
12BH7a
PIN 1 365 440
PIN 2 22 40
PIN 3 40 0
PIN 6 365 441
PIN 7 22 39
PIN 8 40 0
12BH7b
PIN 1 365 440
PIN 2 22 39
PIN 3 40 0
PIN 6 365 420
PIN 7 22 39
PIN 8 40 0
12AZ7a
PIN 1 205 221
PIN 2 -57 -54
PIN 3 -57 -148
PIN 6 205 210
PIN 7 -57 -53
PIN 8 -57 -148
12AZ7b
PIN 1 205 210
PIN 2 -57 -53
PIN 3 -57 -149
PIN 6 205 220
PIN 7 -57 -53
PIN 8 -57 -148
KT88a
PIN 3 450 449
PIN 4 450 448
PIN 5 -57 -150
PIN 8 .6 0
KT88b
PIN 3 450 447
PIN 4 450 448
PIN 5 -57 -149
PIN 8 .6 0
KT88c
PIN 3 450 448
PIN 4 450 448
PIN 5 -57 -157
PIN 8 .6 0
KT88d
PIN 3 450 448
PIN 4 450 448
PIN 5 -57 -157
PIN 8 .6 0
 
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12AU7's voltages looks scary!

All the others look like I would expect with no current draw on the power supply from the tubes. I don't know the schematic, can anyone else comment on the 12AU7 voltages being SO high?
Ron
 
That seems perfectly reasonable to me. WIth the tubes out of the chassis you have the 450 volt supply through a 51k resistor going to the plate of the 12AU7. Without a tube in there, the only thing to draw any current through that resistor is a 2.2 Meg resistor. I always thought the +450 supply would run a bit higher than 450.

You are on target so far.
 
Thanks for the info....

I was hoping someone would come up with a answer to that voltage scenario.
Ron
 
- bias on the outputs way to high. Check the bias circuit, do not run until this is fixed.

Ron-C
 
Are you sure that one would not have -150 on the grid of the outputs with no tubes installed? Even if the voltage was too negative, the tubes would be in cutoff and not conduct at all.
 
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I looked at the schematic again and I cannot for the life of me figure out what would keep the negative voltage for the grids from going to -150 with no tubes installed. Am I missing something or is there something wrong with my schematic?
 
I checked the schematic against the unit itself last night. Pin 5 on each out put tube is fed through an 18K 1 Watt resistor from the single rectifier SR5. Voltage at SR5 reads -173 (per schematic should read -150 but this is just barely within the 15% tolerance). SR5 also supplies (plate?) voltage to each half of the 12AZ7 pins 3 and 8. This voltage should also be -57 but I read
-148 here as well. SR5 also supplies .6V to pin 8 of the output tubes - I read 0 Volts.
No tubes installed with these readings. All 18K resistors measure within spec.
I did change the selenium rectifier out for an IXYS FRED - that may explain the additional 23Volts above the 150V per the schematic at SR5. Should I install a resistor right after SR5 to lower the voltage?
I sure don't want to risk burning out the output transformers and the output tubes so I will proceed slowly with caution.

No one has commented on the voltage readings on Pins 3 and 8 of each 12AU7. Per the schematic it should read 106V but I get 0 Volts on each pin.

Thanks for all of your comments and ideas and keep them coming!

PS I'm still on the road but because I have this project and this forum it makes it more bearable.
 
I would not expect any voltages to be even close without the tubes in place you would have to do a circuit analysis for each point to find the unloaded voltage.
Do not add an extra resistor to the - bias at this time, a little higher will make things a bit safer during check out, after all is running and healthy you may want to add a larger series resistor to the - bias feed to tweak it a bit lower. I would do this while monitoring the 115V feed current, output power, and distortion. To much will limit power and to little will run the tubes to hard, up distortion, and make the amp run hot. Before tweaking make sure you have a good set of outputs installed, I have seen a person set bias to compensate for weak tubes - ended up cooking the power transformer when they put in new outputs without a re-check.
 
Here is how I interpret things. This may get a little lengthy so bear with me.

You circuit description seems much different that the schematic I have.

According to my copy, the negative supply rectifier is labeled D5. In addition there are a total of four 18k resistors in the bias supply for the tubes.

That .8 volts is not supplied by D5, as the capacitors C20 and C21 block that negative DC.

The schematic I am using is at www.berners.ch.

A text summary of the bias circuit is like this:

The -150 line is connected to one end of R54 , R55, R29 and R30. The other end of these resistors are connected to a cap (12uf@250) and to one end of R28, R31, R53, and R56. The other end of R28, R30, R53 and R56 are connected to Pin 5 of the output tubes and to
the cathodes of the 12AZ7.

Now, if the amp matches the schematic at berners, then things are ok. If not, then all bets are off.


Assuming that the amp matches the schematic, here is my take on the whole bias circuit.

WRT to the negative voltage supply for the output tubes, I believe the voltage that you are reading is correct and proper for a chassis without tubes in it. Since there is nothing to draw current through the resistors R53, R54, R55 R56, and R28, R29, R30 and R31, the voltage is going to be more negative than what would be there with tubes installed. Grids draw current. If that negative voltage was very low, (closer to 0 volts) then I would be concerned. A grid with 0 volts on it would turn those outputs into big expensive orange light bulbs.


Since you changed the rectifier from a selenium, you may have to play with things to get the bias voltage just right. My guess is that the values of R54, R55, R29, and R30 will have to be increased a bit. You are still good to go.

What is the upshot of having a bias voltage that is more negative than "normal"? The tube will then be placed deeper or closer into cutoff. While the tubes will last longer, there will likely be an increase in distortion and a lack of power.


WRT the lack of voltages on 3 and 8 of the 12AU7, there is 0 volts there because the tube is not in the circuit. If you look at the schematic, those pins relate to the cathode of the tube and the only connection they have is a single resistor to ground. Since conduction of the tube is required to create the measured voltage on the schematic, that is normal. In other words, the source of that voltage is the tube itself. No tube - No voltage. Again, you are good to go.

Overall summary?

You are good to go and to move on to the next step provided that the schematic and the amp match.
 
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If you put all the tubes in less the output tubes and bring the variac up to 117 volts, I think that you will find that you will have about -60 to -62 volts on the output grids. Remember that the output stage is DC coupled from the cathode follower (12AZ7) and there is positive voltage on the cathode of the 12AZ7 which must be added to the bias voltage to get the true bias voltage on the output grids. The bias can be trimed by inserting a small resister in series with the transformer lead and the rectifier. I would do this with the output tubes out!

The voltages around the 12AU7 are about normal for the "long tail" forced ballance phase inverter.

Frank
 
OK, I checked my unloaded MC275 and got -167 bias volts. I always fire the amps up with the small tubes in, no outputs.
This is what I would do then you can measure all voltages.
Looking at your MC275's earlier photos you may have a very late production unit which has the caps wired as yours. Your amp may also have had the power supply caps changes as the Mallory logo looks later than the amps I have looked into.
The Solen Caps in your amp, are they polarized electrolytic or Poly caps of the same 10 uf value? If you used Poly caps you will need to replace with electrolytic, 12 uf is fine and available.
Remember the amp has DC and AC on the components. Are you using a factory schematic? It has the AC, DC and 'resistance in ohms unit off' charts at the end. All you need is an ohm meter to make these measurements.

Ron-C
 
Ron-C Thank you for going to the trouble to check. I won't be able to do a small tube test until later in the week - still on the road with no tubes.
Rich - thanks for pointing out the obvious on why there is no voltage on the 12AU7 pins - after reading your explanation and then looking at the schematic all I could say was "Doh!".
Spkrman, Terry, and Frank - your comments are well received...
Thanks guys for the help.
I'll post my next adventure when I do a small tubes test.
 
Originally posted by ron-c
The Solen Caps in your amp, are they polarized electrolytic or Poly caps of the same 10 uf value? If you used Poly caps you will need to replace with electrolytic, 12 uf is fine and available.


Ron-C
Click to expand...

Ron-C, I got to ask the question... what is the problem with Solen Poly caps? The rated voltages are well above what they replaced. Please elucidate.
 
Rkic,

You can not substitute a non polarized poly cap for a polarized electrolytic. None of those are in the signal path anyhow. Your AC voltages will be wrong with the poly caps. Respect the original design of the amp and use modern parts but not parts of different value or type.

Thanks,
Ron-C
 
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