Hi
I hope someone can help me work out noise issues that I’m having with a Mojotone Bassman head build.
I have built quite a lot of solid state recording equipment but valves are very new to me. I built a 5E3 and it sounds great, unfortunately the Bassman has me stumped.
I have completely removed the bass preamp from the circuit. I want to get the normal channel working before bringing the bass circuit back in.
I have also fully checked continuity between all components and wiring measured and checked resistor values and visually checked capacitor values and can't find an issue.
The amplifier works but I have two noise issues.
First Noise Issue
The normal channel passes audio, but I have lots buzz does not sound like mains hum, but more the noise you get from guitar pickups. However when I turn the volume fully up the noise disappears.
Second Noise Issue
When I chopstick touch the grid wires on the preamp tube pin 2 & 7 I get even more buzz. However if I touch the Chassis this buzz from touching the grid wires disappears.
If I remove the preamp tube I get same noise when chopstick touching on Pin 2 Grid of the phase inverter. Again if I touch the chassis it goes.
Phase inverter Voltages
1. 297v
2. 43.5v
3. 72v
Heaters 6.5v
6. 292v
7. 44.8v
8. 73.2v
Pre amp voltages
1. 280v
2. 1mV
3. 2.2v
Heaters 6.5v
6. 281v
7. 0v
8. 2.2v
Any ideas of what to try or measure next would be very much appreciated.
I hope someone can help me work out noise issues that I’m having with a Mojotone Bassman head build.
I have built quite a lot of solid state recording equipment but valves are very new to me. I built a 5E3 and it sounds great, unfortunately the Bassman has me stumped.
I have completely removed the bass preamp from the circuit. I want to get the normal channel working before bringing the bass circuit back in.
I have also fully checked continuity between all components and wiring measured and checked resistor values and visually checked capacitor values and can't find an issue.
The amplifier works but I have two noise issues.
First Noise Issue
The normal channel passes audio, but I have lots buzz does not sound like mains hum, but more the noise you get from guitar pickups. However when I turn the volume fully up the noise disappears.
Second Noise Issue
When I chopstick touch the grid wires on the preamp tube pin 2 & 7 I get even more buzz. However if I touch the Chassis this buzz from touching the grid wires disappears.
If I remove the preamp tube I get same noise when chopstick touching on Pin 2 Grid of the phase inverter. Again if I touch the chassis it goes.
Phase inverter Voltages
1. 297v
2. 43.5v
3. 72v
Heaters 6.5v
6. 292v
7. 44.8v
8. 73.2v
Pre amp voltages
1. 280v
2. 1mV
3. 2.2v
Heaters 6.5v
6. 281v
7. 0v
8. 2.2v
Any ideas of what to try or measure next would be very much appreciated.
Thanks for the link I'm now studying this. It offers a lot of information and different things to try.
It states that I should run PT CT to the reservoir caps. I also have 2 x 100 ohm resistor CT for the heaters. I read on one website that you should only use one or the other. However I got so much mains hum unless both were connected.
Would take the artificial center tap to the reservoir caps?
Thanks for you help.
It states that I should run PT CT to the reservoir caps. I also have 2 x 100 ohm resistor CT for the heaters. I read on one website that you should only use one or the other. However I got so much mains hum unless both were connected.
Would take the artificial center tap to the reservoir caps?
Thanks for you help.
The centre tap for the B+ winding goes to the reservoir cap(s) - it's important that this connection is not 'shared' with any other 'ground path'. It is also useful to keep this connection close to, or even twisted with (if that is possible) the wire that connects the rectifiers to the other end of the reservoir cap(s).
The web site was probably referring to a centre tap in the PT heater winding - you should either use that or the 'artificial centre tap with 100 ohm resistors' but not both.
The ground connection for the artificial centre tap carries negligible current to ground - so it is not too important where it is connected to ground. There should probably be only one audio ground connection to the chassis - so it could be 'star' connected to that point.
The web site was probably referring to a centre tap in the PT heater winding - you should either use that or the 'artificial centre tap with 100 ohm resistors' but not both.
The ground connection for the artificial centre tap carries negligible current to ground - so it is not too important where it is connected to ground. There should probably be only one audio ground connection to the chassis - so it could be 'star' connected to that point.
Last edited:
Thanks for your input Malcolm, the knowledge and time you have shared with me is appreciated. My problem was relating to dirty ground from the center tap that now has it own connection directly to my "star ground"
I've now split the amp into sections and then bused those to the star ground. My separate sections are as follows CT, voltage rails, pre amp 1, pre amp 2, Pre caps, reservoir caps and taken these to star ground, located close to the filter caps.
This grounding scheme from a electronics point of view is by far the most sensible. I'm unsure why there is so much information saying star grounds should not be used. If done correctly they work well and will offer the least noise.
Do you isolate input Jacks, mine are currently lifted off the chassis? Will experiment with how this will effect noise.
Do you run the speaker output to the star ground or just rely on the chassis?
I believe the power supply at the moment is "insufficient" and will later measure the ripple. I accidentally hooked the center tap to the wrong negative reservoir cap, which ment only one cap was in the circuit and thus doubling the capacitance. The amp was silent, and correcting the error introduced more mains hum.
I'll definitely change to the AA165 power supply layout which gives V1 its own supply. I'll probably double the 20uf preamp caps to 40uf and see if I can get away with boosting the 20uf on the B+ without creating to much in rush for the GZ34.
I've now split the amp into sections and then bused those to the star ground. My separate sections are as follows CT, voltage rails, pre amp 1, pre amp 2, Pre caps, reservoir caps and taken these to star ground, located close to the filter caps.
This grounding scheme from a electronics point of view is by far the most sensible. I'm unsure why there is so much information saying star grounds should not be used. If done correctly they work well and will offer the least noise.
Do you isolate input Jacks, mine are currently lifted off the chassis? Will experiment with how this will effect noise.
Do you run the speaker output to the star ground or just rely on the chassis?
I believe the power supply at the moment is "insufficient" and will later measure the ripple. I accidentally hooked the center tap to the wrong negative reservoir cap, which ment only one cap was in the circuit and thus doubling the capacitance. The amp was silent, and correcting the error introduced more mains hum.
I'll definitely change to the AA165 power supply layout which gives V1 its own supply. I'll probably double the 20uf preamp caps to 40uf and see if I can get away with boosting the 20uf on the B+ without creating to much in rush for the GZ34.
Glad to hear its working.
If the amp includes global negative feedback, I think the speaker ground should be to the stage where the feedback is introduced.
Otherwise, it can be to the chassis (it's just a voltage reference and will have negligible current to ground). This ground could also be regarded as a safety ground as far as the speaker wiring is concerned.
I think it's best to isolate the body of the jacks from the chassis (or use the plastic jacks) to avoid the possibility of a 'ground loop'.
If the amp includes global negative feedback, I think the speaker ground should be to the stage where the feedback is introduced.
Otherwise, it can be to the chassis (it's just a voltage reference and will have negligible current to ground). This ground could also be regarded as a safety ground as far as the speaker wiring is concerned.
I think it's best to isolate the body of the jacks from the chassis (or use the plastic jacks) to avoid the possibility of a 'ground loop'.
I have done it that way and it can work well with the grounding arrangement you now have. If you use a shielded wire up to the point where input to wire for preamp 1 goes to grid and run a wire from the shield (at the grid end) to "preamp 1" ground point. Have seen that done on commercial amps. Just don't ground that shielded wire to the chassis at the input jack, use an insulating jack.
Hi Guys
A star ground is easy to comprehend but is completely the wrong way to go in an audio amp. It is the best way to guarantee that the amp will be noisy since you are tying all of the noisey and quiet currents together so they cannot do anything other than mingle. These should be separated using the Galactic ground method shown in TUT3. This is universally apllicable to any type of amp of any technology.
The CT for the PT should go to the negative end of the main filter cap. A second wire from that cap should tie to the ground buss. This isolates the rectifier noise which is the noisiest current in the amp.
The input jack is NOT where you want all grounds to go. This is the tiniest and most sensitive current so why assure its contamination by adding other currents to it? Ideally all of the jacks are insulated from the chassis and a wired ground as above is used. Then the ground buss is tied to the chassis by a single wire from about the middle of the buss. This one wire and the AC third-wire safety ground are the only things tied to the chassis.
The chassis should not be thought of as "ground" rather as a "shield".
The heater faux-CT should tie to a DC-standoff, comprised of a voltage divider with a filtered tap point. This should set the heaters to about 60Vdc or so. Typical values are 56k to ground, 330k to the screen node and a 10uF cap. Note that the heaters themselves still only see the 6Vac they need. This reduces heater-induced hum by 30dB.
Most kit layouts for hand-wired amps from the various providers are heavily compromised as far as grounding and wiring goes. Multi-section caps are often used, and as TUT3 explains, these further compromise the attainment of quiet operation. Most of the layouts follow what fender did back in the day. This was "quiet enough" considering that fender sold single-coil pickup guitars that picked up more noise than the amp produced, so the amp's noise level was relatively unimportant.
There is no reason why a tube amp should buzz, hum or hiss. In fact,m even a high-gain tube preamp can be made dead quiet.
Have fun
A star ground is easy to comprehend but is completely the wrong way to go in an audio amp. It is the best way to guarantee that the amp will be noisy since you are tying all of the noisey and quiet currents together so they cannot do anything other than mingle. These should be separated using the Galactic ground method shown in TUT3. This is universally apllicable to any type of amp of any technology.
The CT for the PT should go to the negative end of the main filter cap. A second wire from that cap should tie to the ground buss. This isolates the rectifier noise which is the noisiest current in the amp.
The input jack is NOT where you want all grounds to go. This is the tiniest and most sensitive current so why assure its contamination by adding other currents to it? Ideally all of the jacks are insulated from the chassis and a wired ground as above is used. Then the ground buss is tied to the chassis by a single wire from about the middle of the buss. This one wire and the AC third-wire safety ground are the only things tied to the chassis.
The chassis should not be thought of as "ground" rather as a "shield".
The heater faux-CT should tie to a DC-standoff, comprised of a voltage divider with a filtered tap point. This should set the heaters to about 60Vdc or so. Typical values are 56k to ground, 330k to the screen node and a 10uF cap. Note that the heaters themselves still only see the 6Vac they need. This reduces heater-induced hum by 30dB.
Most kit layouts for hand-wired amps from the various providers are heavily compromised as far as grounding and wiring goes. Multi-section caps are often used, and as TUT3 explains, these further compromise the attainment of quiet operation. Most of the layouts follow what fender did back in the day. This was "quiet enough" considering that fender sold single-coil pickup guitars that picked up more noise than the amp produced, so the amp's noise level was relatively unimportant.
There is no reason why a tube amp should buzz, hum or hiss. In fact,m even a high-gain tube preamp can be made dead quiet.
Have fun
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.