This sounds dumb, but....need help identifying a resistor value.

[Ghostplanet]

I have been trouble shooting a noisy Sovtek Mig 50H. I have narrowed the problem down to bad power supply caps. Also there are 2 resistors that are burnt. I'm not sure what the value of the resistors are, and some pics I have found of others people's amps only have one resistors in between the caps where mine has 2. Here is a pic of my amp, the burnt resistors are between the 2 power supply caps on the right. I have also attached a schematic of the amp for reference. Thanks for the help identifying the crispy resistors. Also I am thinking about using F & T caps to replace the bad Illinois caps that are currently in the amp. Will a better quality cap improve the sound? Thanks.

[Jazz P Bass]

On the schematic, which resistor is it?

The resistors look like they are in parallel (lower overall resistance).

As to power supply capacitors affecting the sound:
Brand new caps, whatever brand, will do what they are supposed to do and that is smooth out the rectified Vdc.
If they are doing there job, there will be no improvement in the 'sound' from one brand to the other.

Keep in mind that not all caps are the same.
There are specs like heat rating, ripple rating, length of use before failure.
In a tube amp, I would think that the heat rating is the most important as caps tend to 'dry out' when overexposed to heat.

[Jazz P Bass]

On a closer look at the pic, there appear to be multiple paralleled resistors.
If the white sockets are the output tubes, they may be the screen resistors.
Also, there appears to be a resistor to ground from one pin. (pin 8, bias sense?)

Me thinks some one was monkeying with the amp.
Maybe not somuch in a bad way.

What is odd is the fact that the resistors all look like the factory type.
It could be that it was modded in the East European block.

[Tom Phillips]

I think it's a good bet that the burnt resistors are R47 which is listed as a 470 Ohm / 5 Watt resistor on the schematic.
Even though they are burnt it is likely that they still work. I suggest that you measure them with your Ohm meter and report back so we can get some confirmation that they are R47. The value will likely have drifted but the measurement is still useful information. Please do that and report back.

Another important task will be to determine why the resistors burnt. Something down line from that point in the circuit was drawing too much current. If that isn't fixed then replacement resistors will burn too.

[MarkusBass]

What is odd is the fact that the resistors all look like the factory type.
It could be that it was modded in the East European block.

I'm not sure what you mean by "factory type". All the resistors (also 0.5W) are Russian type MLT. Also the smaller capacitors are Russian. Only the caps in the power supply (on the left) are not Russian. Actually, the whole board looks like manufactured in Russia (a copy ???). If this is a copy, it is not bad. It's funny that non-Russian capacitors have failed :-).
I may have such resistors like the one you need (2W MLT) but you should use rather correct resistors - maybe 5W resistor is required in this case.

EDIT: the resistor should be most likely 470/5W so you may have 2x1k/2W. The resistors looks overheated but there is a chance that they are not burnt. Measure the voltage drop on the resistors and report it back here.

Mark

[Ghostplanet]

I think it's a good bet that the burnt resistors are R47 which is listed as a 470 Ohm / 5 Watt resistor on the schematic.
Even though they are burnt it is likely that they still work. I suggest that you measure them with your Ohm meter and report back so we can get some confirmation that they are R47. The value will likely have drifted but the measurement is still useful information. Please do that and report back.

Another important task will be to determine why the resistors burnt. Something down line from that point in the circuit was drawing too much current. If that isn't fixed then replacement resistors will burn too.

I will measure them and report back on the readings. I was thinking they are the 470 ohm as well. Would the bad power filter cap cause the resistor to burn up? I checked again and noticed white residue on the + side of the caps.

[Ghostplanet]

I'm not sure what you mean by "factory type". All the resistors (also 0.5W) are Russian type MLT. Also the smaller capacitors are Russian. Only the caps in the power supply (on the left) are not Russian. Actually, the whole board looks like manufactured in Russia (a copy ???). If this is a copy, it is not bad. It's funny that non-Russian capacitors have failed :-).
I may have such resistors like the one you need (2W MLT) but you should use rather correct resistors - maybe 5W resistor is required in this case.

EDIT: the resistor should be most likely 470/5W so you may have 2x1k/2W. The resistors looks overheated but there is a chance that they are not burnt. Measure the voltage drop on the resistors and report it back here.

Mark

I will check the values and report back ASAP. Thank you all for the help. I am new to forum, and everyone has been great. I can't wait to get my amp back in order. It has such a killer tone.

[Tom Phillips]

...Would the bad power filter cap cause the resistor to burn up? I checked again and noticed white residue on the + side of the caps.

That is one of the possible causes but may not be the primary or the only cause.The F&T caps will be an upgrade in the sense that they have a better life rating than the ICs. That gives me more confidence in their overall quality. However, as Jazz already pointed out I wouldn't count on much (or any) sound performance difference between new working ICs and new working F&Ts.

After you get the amp working and measure the voltage drop across R47 you can calculate the power dissipation in the resistor. I would expect it to be well under 1 watt. Put your DVM probes on each side of the resistor. Be careful because this must be done with the amp turned on. Take a DC voltage reading. The power dissipated in the resistor is equal to the voltage squared divided by the actual resistance of the resistor.

[Ghostplanet]

So I checked the burnt resistors and it read ~480 ohms. Looks like our suspicions were corrects. When I replace it can I use single 5 watt 470 ohm resistor? Thanks.

[MarkusBass]

So they are not burnt - they are overheated only. This can be caused by another problem in the amp. Just to be sure, I would measure voltage drop on the resistor (when the amp is turned on). If the voltage drop is not too high, I would replace the resistors with one 470 Ohms/5W resistor. If the voltage drop is high, you have to find out what is causing it.

Mark

[Ghostplanet]

So they are not burnt - they are overheated only. This can be caused by another problem in the amp. Just to be sure, I would measure voltage drop on the resistor (when the amp is turned on). If the voltage drop is not too high, I would replace the resistors with one 470 Ohms/5W resistor. If the voltage drop is high, you have to find out what is causing it.

Mark

How do I check the voltage drop?

[MarkusBass]

Switch multimeter to DC Volts range. Connect the first lead of the multimeter to one end of the resistor and the second lead to the other end. Switch on the amp and read the voltage on the multimeter.

Mark

[Tom Phillips]

How do I check the voltage drop?

See post #8

[Ghostplanet]

See post #8

Got it. I will check it when I get home from work. Thanks.

[J M Fahey]

Will a better quality cap improve the sound?

Not a chance.

But better ones will probably improve reliability, which is a good thing.