Friends,
I'm not expecting miracles but I am looking for areas of pursuit regarding an issue with a Fender 5E4 circuit I made. There is background hiss but only with low frequencies. If I turn the treble all the way down they diminish. I switched out the speaker and the symptoms remain. Voltages are where they should be. Also, this issue is intermittent though lately it has become pretty consistant. The low frequencies themselves aren't distorted just carrying this extra noise along. High frequencies are clear. Any ideas would be appreciated.
Hiss with low frequency
Moderators: pompeiisneaks, Colossal
Re: Hiss with low frequency
It sounds parasitic, so lead dress or coupling cap placement could be the culprit. If it happens mostly when the amp is being overdriven, it might be an issue with the cathodyne inverter Merlin B discusses. See here:http://www.freewebs.com/valvewizard/cathodyne.html
Re: Hiss with low frequency
Are you using a 12AY7 for V1 and a 12AX7 for V2 and V3?
Do you have screen resistors on the power tubes?
Do you have screen resistors on the power tubes?
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Hiss with low frequency
I have 12ay7 in V1 and V2 and 12 ax7 in V3. I do have the power tubes with 1.5k on the grids but I cathode biased them. 500 ohms on the cathode.
Re: Hiss with low frequency
Do you have other tubes on hand that you can try?
If so try different tubes in the preamp.
Has this amp ever operated properly or has it always had problems since you built it?
If so try different tubes in the preamp.
Has this amp ever operated properly or has it always had problems since you built it?
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Hiss with low frequency
yes it worked great for about 6 months.
Re: Hiss with low frequency
tubes would be the first thing I would look at. it's possible the tubes are good but noisey.
If the amp worked for any period of time. it's most likely not lead dressing unless the amp has taken a fall or forceful impact. at which point a quick chopsticking should isolate the problem.
It's also possible one of your electroyltic caps was not 100% when new. and over time it's taken a quick turn for the worst. Measure the caps for leakage.
also tighten all screws nuts bolts and grounds as they may have come lose from the vibration of the speaker. a lose ground could also contribute to the ground hiss.
if you used carbon comp resitors it's possible that one of them is noisey. carbon comp are more stable and metal film though debatable as to tone altering they are the quietest and most stable.
while you are in the amp just for kicks and giggles spray the pots with caig moving fader lubricant and exercise the pots.
If the amp worked for any period of time. it's most likely not lead dressing unless the amp has taken a fall or forceful impact. at which point a quick chopsticking should isolate the problem.
It's also possible one of your electroyltic caps was not 100% when new. and over time it's taken a quick turn for the worst. Measure the caps for leakage.
also tighten all screws nuts bolts and grounds as they may have come lose from the vibration of the speaker. a lose ground could also contribute to the ground hiss.
if you used carbon comp resitors it's possible that one of them is noisey. carbon comp are more stable and metal film though debatable as to tone altering they are the quietest and most stable.
while you are in the amp just for kicks and giggles spray the pots with caig moving fader lubricant and exercise the pots.
My Daughter Build Stone Henge
Re: Hiss with low frequency
I went in and poked around with a chopstick. I did change a layout issue as I found a signal power suppyl coming awfully close to each other. I also changed som resistors in the power supply as the preamp stage voltages were a little high. So now it works great again but I'm not sure what fixed it. I thank all of you for your insights as they were helpful in guiding my search.