Thoughts on hiss in bass frequencies

[sdorer]

Friends,
I know this isn't much to go on, but I built a 5e4 amp (yes it is a 5E3 with a cathode follower in the tone stack). I'm getting background hiss with low frequencies. Not distortion mind you just hiss along with the lower frequencies. Sort of like the low notes bring along a friends you don't like but sound fine in and of themselves. The high end is nice and chimey. I learned a lot with this one though as I biased the power tubes like Leo did instead of my flakey old cathode bias self and it does sound nice this way. Could this be a speaker issue as it is a second hand speaker from a Mesa Boogie amp. Any insight is appreciated even if it is just a list of usual suspects. Thanks for your insight as always.

[Colossal]

Out of curiosity, have you checked the cathode voltage on the follower? Usually I elevate heaters to DC if I'm going to run a follower especially if the B+ voltage the follower is operating at is high. I think the heater-to-cathode voltage limit for a 12AX7 is 100V.

[martin manning]

For Sovtek (AKA EH) Vh-k max is 100V, others (JJ, e.g.) are 180V. In the good old days most were 200V.

[sdorer]

The cathode follower voltage is right around 140 like the schematic indicates. Is this too high for todays tubes? Would the solution be to lower the voltage here, I'm not sure what you mean by raising heater to dc? I apologize for my ignorance.

[Colossal]

The cathode follower voltage is right around 140 like the schematic indicates. Is this too high for todays tubes? Would the solution be to lower the voltage here, I'm not sure what you mean by raising heater to dc? I apologize for my ignorance.

My instinct is that the hiss is caused by the high cathode voltage. As Martin mentions, new tubes might not be as tolerant of this as good old tubes. By elevating to DC, this means that normally, your heaters run at 6.3VAC, right? So by elevating them to DC, you are floating DC "on top of" the 6.3VAC. You can do this by making another B+ node that is set to say 50VDC. Start by taking a lead from your B+2 node (screen high voltage supply), drop this high voltage across a 180k2W resistor, add a 10-100uF 150VDC electrolytic cap, add a 47k to ground (forming the 180k/47k divider with the cap in the middle). This should give you roughly 50-80VDC as a ballpark guess. You then attach the center tap of your heater winding directly to this voltage divider (the center of your voltage divider). This will "float" DC across all of your tube filaments elevating them to whatever the DC voltage is at the B+ node you just made. The heaters will "find" AC ground via the electrolytic capacitor. I generally do this with most builds and find it makes for very quiet and buzz free amps. If your power transformer does not have a center-tapped heater filament supply, you can make one via a pair of 100R resistors. You then attach your DC source to the CENTER of this pair, thus creating an 'artificial' center tap. Alternatively, you can use a 500R 2W pot. You would connect each leg of the filament supply to the 1 and 3 legs of the pot and your DC source to the wiper (2). This gives you an adjustable balance pot for your heater supply.

Elevating your cathode follower filaments to DC will offset the cathode voltage to take the stress off the tube. I do this with every Marshall build which employs a cathode follower.

Hope this helps. Don't worry about the ignorance; that's how we learn. The best thing, before you go through this hassle of adding DC elevation, is to bypass the cathode follower and see if this lowers the hiss way down. If it does, then it is reasonable to assume that the CF is causing this issue.

[sdorer]

You're not going to believe this but right after your first response I went to Merlin's website and read about raising heater voltage, then you come along and give me the values to the schematic he explained. I went ahead and did this. However I think the problem was caused by the cathode follower coming off the wrong part of the power supply. I had it coming of the preceding filter cap than the schematic had indicated. I moved it down to the correct part of the filter chain. It seems to have resolved now and I learned something elese as well. Thanks a million for your insight

[Colossal]

Right on! Glad you got it sorted out.

[bluesfendermanblues]

Ditto that

Funny thing about these small single ended amps... I had more issues building a champ, than a tweed deluxe. If you don't use the exact 5f1 lead dress (and shielded grid wires) you get a lot of oscillation issues. Amps with a temper for sure.