7193 (2c22) preamp buzz/hum

[alexg]

I built a 7193 (21c22) preamp using plate chokes.

It is very quiet and sounds great. After using it for about 40 minutes to 1 hour, it will develop a buzzing/humming sound present in both the tweeters and the woofer, it is quite loud, can be heard from 1.5 meters from an 87db speaker. The buzz/hum is present on both channels.

The 7193 has two "horns" for the anode and grid on top of the tube envelope and I just used ordinary teflon insulated wires on it. The PSU is a CLCLC regulated using an 0D3.

I suspect the hum/buzz is coming from the PSU since it is present in both channels.

BTW, am using an SE KT88 amp using Mikael Abdellah's circuit.

Any suggestion/thoughts would be appreciated.

Thanks.

 

[kevinkr]

Do you have an oscilloscope or access to one?
Problem could be due to a variety of causes, possibly the pre-amp is marginally stable and after that elapsed time the transconductance becomes sufficiently high or some other parameter changes causing the 2C22 to oscillate.

I would not be surprised to find that it is oscillating at VHF and the buzz you are hearing is the audible manifestation of that. Try an AM/FM portable radio or even tv set near the pre-amp and see what if anything you pick up.

Small resistors right at the plates may help (10 - 100 ohm and you may wrap a few turns of wire around them and make low q chokes as well)

Grid stopper resistors present? How about ceramic bypass caps from the filament pins right to chassis. (0.01uF will do nicely)

Don't dismiss the supply either as a possible cause, but I think it pretty unlikely - your plate chokes should provide a pretty high psrr.

 

[alexg]

Do you have an oscilloscope or access to one?
Problem could be due to a variety of causes, possibly the pre-amp is marginally stable and after that elapsed time the transconductance becomes sufficiently high or some other parameter changes causing the 2C22 to oscillate.

I would not be surprised to find that it is oscillating at VHF and the buzz you are hearing is the audible manifestation of that. Try an AM/FM portable radio or even tv set near the pre-amp and see what if anything you pick up.

Small resistors right at the plates may help (10 - 100 ohm and you may wrap a few turns of wire around them and make low q chokes as well)

Grid stopper resistors present? How about ceramic bypass caps from the filament pins right to chassis. (0.01uF will do nicely)

Don't dismiss the supply either as a possible cause, but I think it pretty unlikely - your plate chokes should provide a pretty high psrr.

Unfortunately, I do not have access to oscilloscope. Grid stopper is present (5K).

Yesterday afternoon, I was using the preamp for more than two hours and the buzz/hum was not present.

Since the buzz/hum was bugging me so much, I worked on the preamp last night. I replaced the grid wire with a shielded coaxial wire, the shielding grounded inside the preamp and the other end floating near the grid connection at the top of the tube. (I did this because everytime I touched the grid wires, I get serious hum). After replacing the wires, when I touch it no hum.

I also re-soldered all the ground connection (when it is buzzing/humming, when I touch the chassis, the buzz/hum disappears, might be ground loop problem).

I also took out the 470uf electrolytic on my PSU, I just left the 3 5uf+25uf motor run capacitors (according to PSUD simulation, the ripple just increased from 180uV to 1.1mV, which should not be significant). This opened the top and tightened the bass.

I have been using the preamp since last night, 10 hours and the buzz/hum has not come back yet. If I fixed the problem, the only thing that would bug me now is which of the things I did fixed it.

 

[arnoldc]

I would speculate - bad solder. a common mistake is using the solder to fuse the parts together. a good practice is to hook the component leads to each other or to the terminal lugs.

the amps and preamps that we made, are dropped 2 feet above ground on a carpet and tested again. all of them still work and will survive DHL or FedEx even if one was physically twisted a bit (I don't know how these courier guys can do that) when it arrived in California.

I'd like to hear that preamp, when you're not busy let me know when I can drive to Laguna

 

[alexg]

I would speculate - bad solder. a common mistake is using the solder to fuse the parts together. a good practice is to hook the component leads to each other or to the terminal lugs.

the amps and preamps that we made, are dropped 2 feet above ground on a carpet and tested again. all of them still work and will survive DHL or FedEx even if one was physically twisted a bit (I don't know how these courier guys can do that) when it arrived in California.

I'd like to hear that preamp, when you're not busy let me know when I can drive to Laguna

Thanks Arnold.

I used to solder like you have described, but now I learned to mechanically couple the parts if possible and then solder.

I was using a very thick (around 2mm diameter) copper wire as my main ground point for star grounding. I think my 30w soldering iron cannot heat it up adequately for a good solder joint, so I borrowed our company's Weller soldering gun and it seemed to be perfect for soldering the ground wires.

My system is powered up now for almost 12 hours and the hum/buzz has not come back yet.

Thanks.

 

[Mikewong]

Would you care to share your schematic?
BTW, I do have a handfull of these tubes which I would like to
build some preamps for my own use.

 

[alexg]

Would you care to share your schematic?
BTW, I do have a handfull of these tubes which I would like to
build some preamps for my own use.

Here is the link to the designer's page, scroll to the bottom of the page for the circuit and psu schematic.

http://www.the-planet.org/linestage.html