Hi, I tried a 3.3uF capacitor in my 5f6a, in place of the 220uF cap. Now I'm getting a large increase in hum which is present with nothing plugged in ...
... I tried a 10uF cap and the same resistor (and moved some V1 wiring around a bit to protect the grids), and now the hum is gone! Mystery whether it was the cap ...
Yes, the larger cap was masking/eliminating hum. More in a moment...
+1 tubenit
This cap have nothing to do with hum .
It does, but not in the way you might realize at first.
Schoolie: you might have had some lead dress issues, however, doesn't 220-250uF seem like a rediculously large value for a cathode bypass? If you ignore the resistance looking into the cathode of the first stage, 220uF and 820Ω have a -3dB point below 1Hz. Yes, the Bassman was originally intended to be a bass amp, but isn't that overkill?
The low E on a guitar is around 80Hz, so the electric bass has a low E around 40Hz. That puts 60Hz heater hum right in the range intended to be amplified. The sure way to keep the hum out is to use d.c. heater power, but that's expensive and un-tweed-Fenderlike.
You might have meticulous heater wiring and lead dress, yet still get hum in the first tube stage. Why? There is a leakage current from the heater to the cathode through the heater insulation inside the cathode sleeve. This insulation needs to keep the heater electrically isolated from the cathode, yet in intimate thermal contact in order to heat the cathode and cause the tube to have proper emission.
Not all companies and all tube samples have excellent heater-to-cathode insulation and construction, and therefore exhibit varying degrees of heater-to-cathode leakage. The leakage current (which amounts to a 60Hz current) then induces hum directly into that tube stage. If the stage is V1, then it is amplified by the rest of the amplifier's gain.
Fender's first solution to this problem was to use a 12AY7. If you look at the early
data sheets, the biggest selling point of a 12AY7 is the claims of low noise and hum and special design for low-level amplifier stages. Since gain is between that of the 12AU7 and 12AT7, a 12AT7 could have been just as easily used for the input stage instead, except for the claimed low hum characteristic.
To hedge the bet of the choice of input tube type, Fender then used an over-sized cathode bypass cap to ensure low hum.
You could look at the heater, heater-cathode insulation, and cathode as a capacitor (two conductors separated by an insulator). You could also look at the heater-to-cathode leakage as a cap that may pass some amount of hum from heater to cathode. If hum is passed, you'd like to short that to ground to prevent it from mixing with your intended amplified signal.
You can make a voltage divider with a pair of resistors, but you can also make a voltage divider that works only with a.c. by using a pair of capacitors. If you want the divided signal to be very small, the grounded cap should have a very low impedance at the frequency of interest, which happens when that cap is very large. We can imagine the capacitance from heater-to-cathode is small (couple-pF range), so an enormous 220-250uF cap would make a good-quality short circuit, and make the cap-based voltage divider reduce hum to the minimum amount.
So you'll see that the big cap is there to reduce hum due to what could be called a "tube defect". The problem is all tubes exhibit the defect, but the really good ones have such a minimal amount of leakage as to not cause any noticeable hum. It is possible, though, for the leakage to worsen over time, and it varies a lot with even individual tubes from a manufacturer.
So, if you're an amp builder it's much cheaper to include the large cap to ensure you kill any possible hum from leakage, and this choice costs less than making a d.c. heater supply. You'll also see that if you are unlucky and have a tube that hums due to leakage, and reduce the value of that bypass cap, that you reduce its ability to eliminate the hum due to leakage. It can be maddening to figure out the cause if you swap tubes, and the hum disappears simply because the new tube has less leakage.
Topic: Lower V1 bypass cap value increases hum? (Read 8018 times)