Sometimes hum can come from the heater wiring. It should not matter but I have cured this a few times by making sure the heaters are wired in phase throughout.
I know you changed all the filter caps and bias cap. but even new ones can leak. I dont know who made the TAD caps or Mojo caps. I have found more than a few leaky electrolytics made in China etc. I have had good luck with the F&T brand.
Third gain stage (I think) hum and white noise
Moderators: pompeiisneaks, Colossal
Re: Third gain stage (I think) hum and white noise
Hey thanks for all of the info. As for the caps, I've read somewhere that the TAD's are made by F&T (I hope so). I have a 16uf of each brand and they appear to be the same size and shape.
Now some good news (sort of)...
I cleaned up the grounds by rewiring all ground wires going to the brass plate with #18 solid wire from the preamp cathode grounds, and the preamp filter cap to a crimped and soldered lug, nut and bolted through the chassis and brass plate near the bass channel input jacks with an 8-32 screw and keps nut. I left the volume pot ground wires to the brass plate and the midrange resistors to the back of the pots.
Should I run ground wires from the vol pots to the new "star-ground"?
I shined up the brass plate and where the chassis and brass plate meet and tightened up the jacks and pots.
The fillemnt leads look to all be going to the same pin to pin on the tube sockets and the transformer is center tapped, and soldered to a lug with the main H-T center tap wire to a transformer bolt. I ceaned-up the mating surfaces and tightened the bolts for the PT.
Well... The amp hums much less. Maybe tolerable.
The amp still has other hiss/sound and I'll change out some more resistors.
Thats all for now. Thanks again.
Now some good news (sort of)...
I cleaned up the grounds by rewiring all ground wires going to the brass plate with #18 solid wire from the preamp cathode grounds, and the preamp filter cap to a crimped and soldered lug, nut and bolted through the chassis and brass plate near the bass channel input jacks with an 8-32 screw and keps nut. I left the volume pot ground wires to the brass plate and the midrange resistors to the back of the pots.
Should I run ground wires from the vol pots to the new "star-ground"?
I shined up the brass plate and where the chassis and brass plate meet and tightened up the jacks and pots.
The fillemnt leads look to all be going to the same pin to pin on the tube sockets and the transformer is center tapped, and soldered to a lug with the main H-T center tap wire to a transformer bolt. I ceaned-up the mating surfaces and tightened the bolts for the PT.
Well... The amp hums much less. Maybe tolerable.
The amp still has other hiss/sound and I'll change out some more resistors.
Thats all for now. Thanks again.
Re: Third gain stage (I think) hum and white noise
I would not star ground your amp. The fender ground plane seems to work pretty well. They are not known for ground location induced hum.
Hiss and hum are two different issues. Hiss is amplified contact noise usually from resistors and the amplifier(tube) itself.
Common cures are to bypass the anode load resistor with small value capacitors ( 500pf, 250, 120pf) and to use lower noise resistors in the signal path.
Selecting the lowest noise preamp tubes helps as well.
Of course you can reduce the gain and or deemphasize the gain at those frequencies.
Hiss and hum are two different issues. Hiss is amplified contact noise usually from resistors and the amplifier(tube) itself.
Common cures are to bypass the anode load resistor with small value capacitors ( 500pf, 250, 120pf) and to use lower noise resistors in the signal path.
Selecting the lowest noise preamp tubes helps as well.
Of course you can reduce the gain and or deemphasize the gain at those frequencies.
Re: Third gain stage (I think) hum and white noise
That's disappointing news, since I already did the star ground thing. My solder iron wasn't doing the trick on the brass plate. It was good enough to un solder the wires on the plate, but soldering back on was a different story.
I saw some posts from Milkman recommending not to use the brass plate, so I figured you guys would give me a pat on the back for going to the trouble of doing the star ground thing.
Do you feel that it will make matters worse, or that it just shouldn't be necessary?
As for the the caps on the resistors, I'm shy to do that because most of the amps that I'm interested in do not use them.
Anyway, I won't be able to look at the amp again untill tomorrow.
Thanks,
Chris.
I saw some posts from Milkman recommending not to use the brass plate, so I figured you guys would give me a pat on the back for going to the trouble of doing the star ground thing.
Do you feel that it will make matters worse, or that it just shouldn't be necessary?
As for the the caps on the resistors, I'm shy to do that because most of the amps that I'm interested in do not use them.
Anyway, I won't be able to look at the amp again untill tomorrow.
Thanks,
Chris.
Re: Third gain stage (I think) hum and white noise
What you've done is probably fine. It's most important to keep the noisy, high-current grounds from the power amp as far away from the preamps and inputs as you can.
You can't really evaluate an amp's hum with the chassis out of the cabinet (there's a piece of screen or sheet aluminum in there to close off the open side). We are surrounded by 60Hz hum from the mains wiring and every grid wire in the amp wants to pick it up and amplify it.
You can't really evaluate an amp's hum with the chassis out of the cabinet (there's a piece of screen or sheet aluminum in there to close off the open side). We are surrounded by 60Hz hum from the mains wiring and every grid wire in the amp wants to pick it up and amplify it.
Re: Third gain stage (I think) hum and white noise
Latest update:
I decided to try to tweak the Bass channel. All of the resistors (new carbon comp)and caps on the board are replaced (ecept for the stock 330pf ceramic treble cap). I replaced the 1M and 68K input resistors with metal film, and I ran shielded wire to the grids (grounded at the pot and input jack) of V1.
No change in noise or hum.
I do get the scratchy sound from the volume pot even though I used (Radio Shack) contact cleaner/lube several times.
Change the pots next?
Another question:
I was checking voltages and I saw something like 55 volts on one of the phase inverter grids and 0v on the other. Is that normal?
Thanks,
Chris.
I decided to try to tweak the Bass channel. All of the resistors (new carbon comp)and caps on the board are replaced (ecept for the stock 330pf ceramic treble cap). I replaced the 1M and 68K input resistors with metal film, and I ran shielded wire to the grids (grounded at the pot and input jack) of V1.
No change in noise or hum.
I do get the scratchy sound from the volume pot even though I used (Radio Shack) contact cleaner/lube several times.
Change the pots next?
Another question:
I was checking voltages and I saw something like 55 volts on one of the phase inverter grids and 0v on the other. Is that normal?
Thanks,
Chris.
Re: Third gain stage (I think) hum and white noise
Scratchy pot can be leaky coupling cap. Check for dc Any noise on the tone control pots? Radio shack cleaner can be hazardous to the pot, I would use fader lube.
Re: Third gain stage (I think) hum and white noise
Thanks again for the replies and ideas,
I've been pretty busy with work and some family stuff, so the amp project got put to the side. I plan to order some parts including new pots, and some metal film resistors. I plan to change everything in the bass channel, and put in new B+ voltage dropping resistors. I want to see if I can at least gey one channel to operate quietly.
As far as leaking caps, I did measure something like .05vdc on theexisting treble cap (ceramic 330pf).
How much DC leaking would be considered acceptable?
Oh, and the tone pots dont seem to make noise while turning them, it seems like just the vol pots do (both channels).
When I get my parts and make the changes, I'll post my results (hopefully I'll have good news)
Thanks again,
Chris.
I've been pretty busy with work and some family stuff, so the amp project got put to the side. I plan to order some parts including new pots, and some metal film resistors. I plan to change everything in the bass channel, and put in new B+ voltage dropping resistors. I want to see if I can at least gey one channel to operate quietly.
As far as leaking caps, I did measure something like .05vdc on theexisting treble cap (ceramic 330pf).
How much DC leaking would be considered acceptable?
Oh, and the tone pots dont seem to make noise while turning them, it seems like just the vol pots do (both channels).
When I get my parts and make the changes, I'll post my results (hopefully I'll have good news)
Thanks again,
Chris.