Hi
Went ahead and tried the resistors to ground as suggested. Each one when connected, made the hum louder?
Also, did a new set of measurements on the entire amp, and the DC voltages (in blue) are all VERY close to what the schematic is calling for. Thats good I guess.
I measured AC leakage from the power supply (those are in green) and there is 3.6VAC coming out of the EZ81. That drops to .91VAC after the first filter cap, then to 20mVAC and finally to 10mVAC after the final cap. At the 8ohm speaker terminal, there is 8.4mVAC (measured across an 8ohm dummy load)
As the speakers are 102db efficient, the hum is audible, albeit barely, from my listening position (8' from the speaker).
I turned the amp upside down and while powered up and connected to speakers, poked about with a wooded screwdriver and found noting that changed the tone/volume of the hum.
Could it be the rectifier tube that is causing the hum seeing as how there is AC leaking from it, or is that amount of AC to be expected?
Thanks again,
Per
Went ahead and tried the resistors to ground as suggested. Each one when connected, made the hum louder?
Also, did a new set of measurements on the entire amp, and the DC voltages (in blue) are all VERY close to what the schematic is calling for. Thats good I guess.
I measured AC leakage from the power supply (those are in green) and there is 3.6VAC coming out of the EZ81. That drops to .91VAC after the first filter cap, then to 20mVAC and finally to 10mVAC after the final cap. At the 8ohm speaker terminal, there is 8.4mVAC (measured across an 8ohm dummy load)
As the speakers are 102db efficient, the hum is audible, albeit barely, from my listening position (8' from the speaker).
I turned the amp upside down and while powered up and connected to speakers, poked about with a wooded screwdriver and found noting that changed the tone/volume of the hum.
Could it be the rectifier tube that is causing the hum seeing as how there is AC leaking from it, or is that amount of AC to be expected?
Thanks again,
Per
Your hum got louder when you connected the 1k resistors from the g1 grids to ground?
It sounds like you have a ground loop on your ground wiring.
The worst contributor to power supply ground loop is from the B+ center tap to the first filter cap; and next the second filter cap. Those has the highest transient currents, and also have the most high frequency noise too. Keep the length of those 2 loops very short. Only after those connections, you can connect the 2nd cap negative to the amplifier's central ground point.
When you use the 1k resistors to ground, that attenuates the signals to g1 of the driver stage, and then to g1 of the output stage.
But any ground loop hum that is on the ground return wires of those cathode circuits will cause hum just as if it was on the grids.
Why are you switching the ground connection of the RCA input connector? A switch is just another Ohmic connection, where any internal currents, or external currents from the signal source can develop a voltage (ground loop hum).
The input ground loop, includes the RCA input connectors (hopefully insulated from the chassis where they mount).
The RCA input connector should be connected by a wire directly to the input return parts this way:
It should be hard wired to the 275k resistor returns, and from there to the returns of the volume control, 1meg, and 330 Ohm cathode resistors. From there, now you can connect that to the central ground point.
The input pentode has lots of gain. That means that any hum, or noise that appears on g1 of the pentode is amplified a lot.
You have input connectors, switches, resistors, volume control, coupling caps, resistors before you ever get to g1.
Lots of opportunity for all those parts and wiring to pick up any hum and noise from any other wiring and parts near that circuitry.
0.91V on the filter cap that runs the output tubes screens is a lot of hum. If the tubes are not well matched, that can result in hum at the output. Are those screen resistors 39k or 3.9k? A bypass cap on those screens might reduce the hum from the output.
And if the output tube plate currents are not well matched, that 0.91V B+ ripple will make hum at the output too.
If the 180 Ohm resistors are exactly matched, then the 8.8V and 9.1V means that the screen current plus plate current of one tube is 1.7mA different than the other tube.
Finding what parts of the circuit is causing the hum can be very difficult.
Were you able to measure the amount of hum there is at the output transformer secondary?
What do you mean by "rectifier tube is leaking AC"?
It sounds like you have a ground loop on your ground wiring.
The worst contributor to power supply ground loop is from the B+ center tap to the first filter cap; and next the second filter cap. Those has the highest transient currents, and also have the most high frequency noise too. Keep the length of those 2 loops very short. Only after those connections, you can connect the 2nd cap negative to the amplifier's central ground point.
When you use the 1k resistors to ground, that attenuates the signals to g1 of the driver stage, and then to g1 of the output stage.
But any ground loop hum that is on the ground return wires of those cathode circuits will cause hum just as if it was on the grids.
Why are you switching the ground connection of the RCA input connector? A switch is just another Ohmic connection, where any internal currents, or external currents from the signal source can develop a voltage (ground loop hum).
The input ground loop, includes the RCA input connectors (hopefully insulated from the chassis where they mount).
The RCA input connector should be connected by a wire directly to the input return parts this way:
It should be hard wired to the 275k resistor returns, and from there to the returns of the volume control, 1meg, and 330 Ohm cathode resistors. From there, now you can connect that to the central ground point.
The input pentode has lots of gain. That means that any hum, or noise that appears on g1 of the pentode is amplified a lot.
You have input connectors, switches, resistors, volume control, coupling caps, resistors before you ever get to g1.
Lots of opportunity for all those parts and wiring to pick up any hum and noise from any other wiring and parts near that circuitry.
0.91V on the filter cap that runs the output tubes screens is a lot of hum. If the tubes are not well matched, that can result in hum at the output. Are those screen resistors 39k or 3.9k? A bypass cap on those screens might reduce the hum from the output.
And if the output tube plate currents are not well matched, that 0.91V B+ ripple will make hum at the output too.
If the 180 Ohm resistors are exactly matched, then the 8.8V and 9.1V means that the screen current plus plate current of one tube is 1.7mA different than the other tube.
Finding what parts of the circuit is causing the hum can be very difficult.
Were you able to measure the amount of hum there is at the output transformer secondary?
What do you mean by "rectifier tube is leaking AC"?
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Great!
Loads of info, thanks!
I think I understand everything you suggest, and I'll work my way through making the changes.
The hum on the OPT at the secondary is 8.2mVAC (At least that is what I'm measuring there - I'm assuming this steady voltage I'm reading is the hum?)
Thanks again,
Per
Loads of info, thanks!
I think I understand everything you suggest, and I'll work my way through making the changes.
The hum on the OPT at the secondary is 8.2mVAC (At least that is what I'm measuring there - I'm assuming this steady voltage I'm reading is the hum?)
Thanks again,
Per
Morning.
Made the changes as per your suggestions, and I rewired the ground connections to a star ground. Re routed a bunch of wires to help with that. The hum was reduced ever so slightly, but still there.
Could it be the AC on the heaters causing the hum? Or my heater wire set up? I’ve attached a photo of the wires for the heaters. Tried to keep them neatly twisted and away from source wires and have tried to have them cross signal wires at right angles and with some space between them.
When I referenced leakage, I was wondering about how much AC is typical to see on the B+ after the AC is rectified. Right out of the tube, there is 3.6VAC. Not sure if this is a high amount? I’ve attached a detail of that from the schematic. Curious if the rectifier tube is not working as it should? It was tested on a Hickok tester and measured well within spec. But I know the older testers do not necessarily tell the whole story.
Other than the annoying 120hz hum, the amp is super quiet and is sounding quite nice. I’ve attached a photo of a measurement of the hum I did w my phone, very definitely a 120hz problem.
Other than tearing it down completely and redoIng the wiring, curious what else can be done.
Thanks!
Per
Made the changes as per your suggestions, and I rewired the ground connections to a star ground. Re routed a bunch of wires to help with that. The hum was reduced ever so slightly, but still there.
Could it be the AC on the heaters causing the hum? Or my heater wire set up? I’ve attached a photo of the wires for the heaters. Tried to keep them neatly twisted and away from source wires and have tried to have them cross signal wires at right angles and with some space between them.
When I referenced leakage, I was wondering about how much AC is typical to see on the B+ after the AC is rectified. Right out of the tube, there is 3.6VAC. Not sure if this is a high amount? I’ve attached a detail of that from the schematic. Curious if the rectifier tube is not working as it should? It was tested on a Hickok tester and measured well within spec. But I know the older testers do not necessarily tell the whole story.
Other than the annoying 120hz hum, the amp is super quiet and is sounding quite nice. I’ve attached a photo of a measurement of the hum I did w my phone, very definitely a 120hz problem.
Other than tearing it down completely and redoIng the wiring, curious what else can be done.
Thanks!
Per
I just realized, the EZ81 shares the 6.3V filament supply with all the other tubes.
Perhaps there is lots of leakage from the EZ81 cathode to its filament.
All of the amplifier stages have cathodes that are either partially un-bypassed, or are fully un-bypassed.
Any leakage from the rectifier filament to cathode, can impress that onto the amplifier stages filaments to cathodes, that might cause hum.
So,
Try paralleling 1k Ohms across the 10k resistors, R34 and R35, (the 'virtual center tap' to ground resistors).
If that improves the hum, you might then try using 100 Ohm resistors to reduce it further.
Perhaps there is lots of leakage from the EZ81 cathode to its filament.
All of the amplifier stages have cathodes that are either partially un-bypassed, or are fully un-bypassed.
Any leakage from the rectifier filament to cathode, can impress that onto the amplifier stages filaments to cathodes, that might cause hum.
So,
Try paralleling 1k Ohms across the 10k resistors, R34 and R35, (the 'virtual center tap' to ground resistors).
If that improves the hum, you might then try using 100 Ohm resistors to reduce it further.
Hi.
Thanks! Sorry for tardy reply. Was with the kids and then visiting a friend after. I just checked in to post a question relating to this...
I realized that I have not wired the filament wires as they suggest in the schematic. This is how they have the tube order from Heathkit.
EZ81 > 7199 > EL84 > GND
I did it this way (I know, I know...) This puts the 7199's at the ground point. Not so good I suspect as it puts the very sensitive 7199's at the end of it?
EZ81 > EL84 > 7199 > GND
I'm thinking I should re-route them so they are like the original??
Could this possible help, or would this make it worse?
7199 > EL84 > EZ81 > GND
Or does this not matter at all?
I will do the test you suggest on one channel and see if there is any change in the hum.
Thanks again for your help!
Per
Thanks! Sorry for tardy reply. Was with the kids and then visiting a friend after. I just checked in to post a question relating to this...
I realized that I have not wired the filament wires as they suggest in the schematic. This is how they have the tube order from Heathkit.
EZ81 > 7199 > EL84 > GND
I did it this way (I know, I know...) This puts the 7199's at the ground point. Not so good I suspect as it puts the very sensitive 7199's at the end of it?
EZ81 > EL84 > 7199 > GND
I'm thinking I should re-route them so they are like the original??
Could this possible help, or would this make it worse?
7199 > EL84 > EZ81 > GND
Or does this not matter at all?
I will do the test you suggest on one channel and see if there is any change in the hum.
Thanks again for your help!
Per
Why are the "virtual center tap" resistors each 10K Ohms?
Why not 100 Ohms each?
Was Heathkit afraid that the EZ81 filament to cathode is potentially 'leaky'?
Were they afraid that they would eventually short . . . Wow!
That would destroy all the other tube's filament to cathode interfaces.
Afraid . . . Then increase the expense of the power transformer and provide a 5V rectifier winding, and a larger 5V rectifier tube.
Many of the EZ81 6.3V rectifiers work very well, for years, as evidenced by many threads on this forum.
But . . . you are having a hum problem; and possibly it is caused by the EZ81 filament to cathode leakage, and such leakage will produce voltage between ground and the other tube filaments.
It should be noted that your hum is 120Hz. 120Hz is the same as the ripple frequency that is on the EZ81 rectifier cathode.
60Hz power mains that are full wave rectified, are 120Hz.
Why not 100 Ohms each?
Was Heathkit afraid that the EZ81 filament to cathode is potentially 'leaky'?
Were they afraid that they would eventually short . . . Wow!
That would destroy all the other tube's filament to cathode interfaces.
Afraid . . . Then increase the expense of the power transformer and provide a 5V rectifier winding, and a larger 5V rectifier tube.
Many of the EZ81 6.3V rectifiers work very well, for years, as evidenced by many threads on this forum.
But . . . you are having a hum problem; and possibly it is caused by the EZ81 filament to cathode leakage, and such leakage will produce voltage between ground and the other tube filaments.
It should be noted that your hum is 120Hz. 120Hz is the same as the ripple frequency that is on the EZ81 rectifier cathode.
60Hz power mains that are full wave rectified, are 120Hz.
Right?
Thanks for that! The 120hz is a pretty clear tone 🙂
The tube was tested, but could it be faulty somehow and actually causing this? From the looks of things, the EZ81 is original as are the 7199's. The EL84's are new JJ's.
This was a tiny little amp, and it was STUFFED full of stuff. From what I could find online about this amp, and there is not much, it was not sold as a kit. And it was available for one year only, again, from what I could find.
I'll check with a lower resistors as you suggest, but should I re direct the heater wires, or does that not matter at this point?
Per
Thanks for that! The 120hz is a pretty clear tone 🙂
The tube was tested, but could it be faulty somehow and actually causing this? From the looks of things, the EZ81 is original as are the 7199's. The EL84's are new JJ's.
This was a tiny little amp, and it was STUFFED full of stuff. From what I could find online about this amp, and there is not much, it was not sold as a kit. And it was available for one year only, again, from what I could find.
I'll check with a lower resistors as you suggest, but should I re direct the heater wires, or does that not matter at this point?
Per
The kits all may have had hum problems.
Heater wires that run along grid, and grid circuitry may be a problem, but those wires are twisted.
But I suspect either the EZ81 leakage, 120Hz (even with a good EZ81) may be the cause of hum.
A B+ ground loop can also be a problem, and it is 120Hz.
B+ filtering that is not sufficient can also be a problem.
Heater wires that run along grid, and grid circuitry may be a problem, but those wires are twisted.
But I suspect either the EZ81 leakage, 120Hz (even with a good EZ81) may be the cause of hum.
A B+ ground loop can also be a problem, and it is 120Hz.
B+ filtering that is not sufficient can also be a problem.
Did the 1K parallel test and no luck sadly.
The amp was so noisy when I received it that I didn't really listen to the details what kinds of noise it was making, could've very well have been there from the get go - meaning the tube could very well have been bad when it arrived. SS rectifier could be an option I suppose, but would rather keep the tube there.
I got some more research to do.
Google, here I come.
Thanks again!
Per
The amp was so noisy when I received it that I didn't really listen to the details what kinds of noise it was making, could've very well have been there from the get go - meaning the tube could very well have been bad when it arrived. SS rectifier could be an option I suppose, but would rather keep the tube there.
I got some more research to do.
Google, here I come.
Thanks again!
Per
Ok.
Re did the heater wires to match the original schematic, no difference.
Is there any value in pulling all the tubes and reinserting them one at a time, starting with the EX81, then 7199's and finally EL84's to see if the hum shows up at one of the stages?
Thanks
Per
Re did the heater wires to match the original schematic, no difference.
Is there any value in pulling all the tubes and reinserting them one at a time, starting with the EX81, then 7199's and finally EL84's to see if the hum shows up at one of the stages?
Thanks
Per