Power supply filtering....too much?

[DudeNiceAmp]

I have a Carvin MTS3200. It had a terrible hum when I got it. It is an easy amplifier to work on for the most part. I did the standard cap replacement and swapped tubes for some different sets of "good used" tubes I have laying around. The hum never did go away. So I started to clip some caps in across my caps that I just replaced.

An important thing to note is C56 and C51 were not installed in this rev. For whatever reason the PCB has spots for it, but its obvious that from factory they were not installed. So naturally I installed them and the hum reduced. It did not reduce to an acceptable volume. I then clipped a larger value into C56 and the hum is gone. I admit power supply design is an area that I lack. I need to read Merlins book on this. It was not available for many years and now that it is I have not gotten around to it. I know there are limits to using certain size caps in this position when using a tube rectifier, but what do I need to consider here since this is solid state. I have a cap size of 220uf in c56 currently (only because I am out of all other values). This along with the 22's on C51 and C52 equate to 264uf total in this position. 

schematic here: https://el34world.com/charts/Schematics/files/Carvin/Carvin_mts3200.pdf

[tubenit]

My understanding is that with a solid state rectifier that you should be fine with the 220uf cap there if it's at least a 500v cap.


You could also try a 100uf/500v and see if that is sufficient for reducing hum to your liking also.


With respect, Tubenit

[Merlin]

SS diodes have very high ripple current ratings; so high we usually forget about them. They should be fine with 264uF. But such excessive hum makes me wonder if you just have rather mismatched output tube pairs?

[DudeNiceAmp]

SS diodes have very high ripple current ratings; so high we usually forget about them. They should be fine with 264uF. But such excessive hum makes me wonder if you just have rather mismatched output tube pairs?

I was wondering if I was masking a problem.
I am making an assumption in this case as I have tried a few sets of tubes, however I know that assumption could be the problem.

[Merlin]

Maybe the PI tube has mismatched triodes?

[acheld]

SS diodes have very high ripple current ratings; so high we usually forget about them. They should be fine with 264uF. But such excessive hum makes me wonder if you just have rather mismatched output tube pairs?

I was wondering if I was masking a problem.
I am making an assumption in this case as I have tried a few sets of tubes, however I know that assumption could be the problem.

I think is unlikely that you're masking a problem.  If the hum goes away, there you go!  One thing to mention:  it is easy to mis-identify 120 Hz hum with 60 Hz (or 100 with 50).  Always best to be sure what your humming frequency is.   Hmmm?

[HotBluePlates]

... such excessive hum makes me wonder if you just have rather mismatched output tube pairs?

You may find it interesting to see this output tube pair I tried once in a Super Reverb; the recording below was done right after snapping the photo.




https://soundcloud.com/hotblueplates/mismatched-tubes

[DudeNiceAmp]

it is easy to mis-identify 120 Hz hum with 60 Hz (or 100 with 50).  Always best to be sure what your humming frequency is.   Hmmm?

That’s true. I always match the frequency with sound clip.

[stratomaster]

A spectrum analyzer or FFT phone app is incredibly useful for this.  You'll have proof positive of the frequency and amplitude of the noise you're experiencing.  Simply fire up the app and place the mic near the speaker.

I've also had interesting results like 180 Hz noise on the reverb return in a Princeton.  Not to the level it would be objectionable--but audible and detectable as a strong component of the (very low) noise floor (this was after a number of adjustments to reduce the 60 and 120 Hz components, increased capacitance, moving reverb to a later node, elevating heater CT).