Thank you!!
J-F
Some other things to try would be bypass caps of the plate of each tube to ground, such as a .0001 mfd, and if the cathodes are above ground potential, (they have a resistor between the chassis ground and the cathode) you can try adding small value caps there too- But this will cut into some high end definition.
Are the inputs bypassed? And do they feed through .1mfd caps? in other words, between the input jack and grid of first stage, is there a cap to ground as well as between that jack and the grid? for both the pre amp and amp? You might also consider putting a 1 Meg resistor parallel to the caps to ground too. If the inputs remain floating, they will be the source for most of the noise.
Any tube can pick up hum, any grid can pick up hum from unshielded AC sources- usually heater string, which should be twisted around each other. Disc caps, stacked caps, similar designs can also pick up hum, so they need to be located in the chassis very carefully. They can also be quite noisy in and of themselves.
Spring loaded tube shields can help somewhat if there is only loose contact in the tube socket, but they do help damp vibrations in the room that can be picked up by a tube, or in some extreme cases like Ted Nugent's usual volume level at concerts, where the tube can get shook out the socket.
An open grid, or cold solder joint can also create the noise, or allow hum to be picked up. As can excessively long connections between gain stages, or unshielded signal carrying wires.
Also make sure that any connection to the chassis, if through a rivit, as on a tube connection- solder them to the chassis.
:Hi, I restored 2 identical mono amps.
:Pre-amp section: 1x 12AX7 (2x volt. amp) for RIAA Phono and Tape Head.
:Amp section: 6AV6, 6U8, 2x 6BQ5.
:The 2 amplifiers work well, without noise,no hum. I use them at present. But, the pre-amp section is really noisy (for the 2) with a little hum and much shhhh. The first amplifiers still has its shields wires, the second not (it was empty before the restoration). But same problem. I know that pre-amp + amp = several stages of amplifications... It is perhaps the problem... The noise is amplified. I badly reassembled the amplifiers? There is a shield on the 12AX7. Can I modify the circuit to decrease the noise?
:
:Thank you!!
:J-F
So, there is answers to your questions: no bypass caps on any plate of 12AX7EH in pre-amp and no caps on cathodes (all near 1V) too. Inputs not bypassed in pre amp and even in amp (after volume pot.). Ceramic phono jack is floating but Magnetic Phono not (these 2 are connected together, so always one floating). No disc caps only mica and «orange drop» caps in all circuit. I can try to apply your hints.
Thank you,
J-F
:Hi,
:Some of that noise is inherent in the tubes, you would really need to go through a large number to find quiet ones. Current manufacture 12AX7's are more prone to noise than older US made old stock tubes. 6U8's can be noisy too, and finding a sub might help.
:
:Some other things to try would be bypass caps of the plate of each tube to ground, such as a .0001 mfd, and if the cathodes are above ground potential, (they have a resistor between the chassis ground and the cathode) you can try adding small value caps there too- But this will cut into some high end definition.
:
:Are the inputs bypassed? And do they feed through .1mfd caps? in other words, between the input jack and grid of first stage, is there a cap to ground as well as between that jack and the grid? for both the pre amp and amp? You might also consider putting a 1 Meg resistor parallel to the caps to ground too. If the inputs remain floating, they will be the source for most of the noise.
:
:Any tube can pick up hum, any grid can pick up hum from unshielded AC sources- usually heater string, which should be twisted around each other. Disc caps, stacked caps, similar designs can also pick up hum, so they need to be located in the chassis very carefully. They can also be quite noisy in and of themselves.
:
:Spring loaded tube shields can help somewhat if there is only loose contact in the tube socket, but they do help damp vibrations in the room that can be picked up by a tube, or in some extreme cases like Ted Nugent's usual volume level at concerts, where the tube can get shook out the socket.
:
:An open grid, or cold solder joint can also create the noise, or allow hum to be picked up. As can excessively long connections between gain stages, or unshielded signal carrying wires.
:
:Also make sure that any connection to the chassis, if through a rivit, as on a tube connection- solder them to the chassis.
:
:
::Hi, I restored 2 identical mono amps.
::Pre-amp section: 1x 12AX7 (2x volt. amp) for RIAA Phono and Tape Head.
::Amp section: 6AV6, 6U8, 2x 6BQ5.
::The 2 amplifiers work well, without noise,no hum. I use them at present. But, the pre-amp section is really noisy (for the 2) with a little hum and much shhhh. The first amplifiers still has its shields wires, the second not (it was empty before the restoration). But same problem. I know that pre-amp + amp = several stages of amplifications... It is perhaps the problem... The noise is amplified. I badly reassembled the amplifiers? There is a shield on the 12AX7. Can I modify the circuit to decrease the noise?
::
::Thank you!!
::J-F
The mica caps might be looked at to see if they are close to the heater string or incoming power, as they can sometimes pick up noise- you can test this with a soldering gun tip held near the cap (a soldering iron will not usually work, but a tape head demagnetizer can be used also if a soldering gun is not available).
It wouldn't hurt to see if you can locate a passive component that might pick up some noise, or generate some by placing a soldering gun tip or tape head demagnetizer close to the suspect components when the amp is on, which can be difficult, as the magnetic field is intense, and may get picked up by any unshielded lead. And while you are there, just check each capacitor with a light tap of a screwdriver to see if any of the caps are microphonic (This is rare, but I had one once that took a little while to locate, and was just agravation until it was found.)
:Hi,
:I have new 12AX7 Electro-harmonix at present (in my pre-amps). But, I have used Valvo which tests good. I could use them to test. I have NOS 6U8A RCA and Sylvania. It's true that these tubes are microphonic for some. But the problem, is not in the amp section...
:
:So, there is answers to your questions: no bypass caps on any plate of 12AX7EH in pre-amp and no caps on cathodes (all near 1V) too. Inputs not bypassed in pre amp and even in amp (after volume pot.). Ceramic phono jack is floating but Magnetic Phono not (these 2 are connected together, so always one floating). No disc caps only mica and «orange drop» caps in all circuit. I can try to apply your hints.
:
:Thank you,
:J-F
:
:
:
::Hi,
::Some of that noise is inherent in the tubes, you would really need to go through a large number to find quiet ones. Current manufacture 12AX7's are more prone to noise than older US made old stock tubes. 6U8's can be noisy too, and finding a sub might help.
::
::Some other things to try would be bypass caps of the plate of each tube to ground, such as a .0001 mfd, and if the cathodes are above ground potential, (they have a resistor between the chassis ground and the cathode) you can try adding small value caps there too- But this will cut into some high end definition.
::
::Are the inputs bypassed? And do they feed through .1mfd caps? in other words, between the input jack and grid of first stage, is there a cap to ground as well as between that jack and the grid? for both the pre amp and amp? You might also consider putting a 1 Meg resistor parallel to the caps to ground too. If the inputs remain floating, they will be the source for most of the noise.
::
::Any tube can pick up hum, any grid can pick up hum from unshielded AC sources- usually heater string, which should be twisted around each other. Disc caps, stacked caps, similar designs can also pick up hum, so they need to be located in the chassis very carefully. They can also be quite noisy in and of themselves.
::
::Spring loaded tube shields can help somewhat if there is only loose contact in the tube socket, but they do help damp vibrations in the room that can be picked up by a tube, or in some extreme cases like Ted Nugent's usual volume level at concerts, where the tube can get shook out the socket.
::
::An open grid, or cold solder joint can also create the noise, or allow hum to be picked up. As can excessively long connections between gain stages, or unshielded signal carrying wires.
::
::Also make sure that any connection to the chassis, if through a rivit, as on a tube connection- solder them to the chassis.
::
::
:::Hi, I restored 2 identical mono amps.
:::Pre-amp section: 1x 12AX7 (2x volt. amp) for RIAA Phono and Tape Head.
:::Amp section: 6AV6, 6U8, 2x 6BQ5.
:::The 2 amplifiers work well, without noise,no hum. I use them at present. But, the pre-amp section is really noisy (for the 2) with a little hum and much shhhh. The first amplifiers still has its shields wires, the second not (it was empty before the restoration). But same problem. I know that pre-amp + amp = several stages of amplifications... It is perhaps the problem... The noise is amplified. I badly reassembled the amplifiers? There is a shield on the 12AX7. Can I modify the circuit to decrease the noise?
:::
:::Thank you!!
:::J-F