Frank,
The current bleeder is 1M, probably a bit high. I will try a smaller resistor and see how things go.
The power-up sequence is somewhat interesting with the VR's...
I first get a large, but slow rise in DC through the PS, until the VR's regisiter about 250, then they strike, and settle to 192 VDC (OB3 and 0C3). Then the current begins to rise on the plate to the valueset by the CCS. I have not observed the plate voltage before and after the VR's strike, and I'm wondering what their behavior may be?
Anyway, I'll give the smaller bleeder and mute switch a try.
Is this type of scenario normal?
Thanks,
Bryan
The current bleeder is 1M, probably a bit high. I will try a smaller resistor and see how things go.
The power-up sequence is somewhat interesting with the VR's...
I first get a large, but slow rise in DC through the PS, until the VR's regisiter about 250, then they strike, and settle to 192 VDC (OB3 and 0C3). Then the current begins to rise on the plate to the valueset by the CCS. I have not observed the plate voltage before and after the VR's strike, and I'm wondering what their behavior may be?
Anyway, I'll give the smaller bleeder and mute switch a try.
Is this type of scenario normal?
Thanks,
Bryan
Hey Guys,
All I can say is OH MY GOD!
This little beast sounds GREAT!
Last night, after a family dinner, I sat down for my first stereo listening session. I was absolutley AMAZED at how good the preamp sounded. Now, it could have been the 3 margaritas at dinner adding to my perceived euphoria, but I was thoroughly impressed.
Even with many low cost items (cheap pot, cheap/thin wire, cheap RCA's, etc etc), the sound quality is far better than my previous preamp. Humm is VERY low, not measured yet, but at normal listening levels, your ear needs to be a the speaker to hear it. The bass is punchy, and tight. The highs seem a little rolled off, but this could have been due to my choice of listening material.
I'm currently running the 12B4A's at 27mA, -10.5V.
The only problem that I notice is a little high frequency noise, or hiss coming from the speakers. Any suggestions as to potential causes of this noise?
I'll be listening for a while, experimenting with different opperating points for a few weeks. Any suggestions/favorites?
Pics of the proto to follow soon.
I'll try to draw up the current schematic abnd post it this weekend.
Thanks for everyones help.
Bryan
All I can say is OH MY GOD!
This little beast sounds GREAT!
Last night, after a family dinner, I sat down for my first stereo listening session. I was absolutley AMAZED at how good the preamp sounded. Now, it could have been the 3 margaritas at dinner adding to my perceived euphoria, but I was thoroughly impressed.
Even with many low cost items (cheap pot, cheap/thin wire, cheap RCA's, etc etc), the sound quality is far better than my previous preamp. Humm is VERY low, not measured yet, but at normal listening levels, your ear needs to be a the speaker to hear it. The bass is punchy, and tight. The highs seem a little rolled off, but this could have been due to my choice of listening material.
I'm currently running the 12B4A's at 27mA, -10.5V.
The only problem that I notice is a little high frequency noise, or hiss coming from the speakers. Any suggestions as to potential causes of this noise?
I'll be listening for a while, experimenting with different opperating points for a few weeks. Any suggestions/favorites?
Pics of the proto to follow soon.
I'll try to draw up the current schematic abnd post it this weekend.
Thanks for everyones help.
Bryan
...quite possibly.
When I touch any of the wires going to the tube, you can hear the sound from the speakers, that is the mechanical vibration from the tube is transmitted through the audio signal.
Currently there is no dampening, and the tubes are horizontal, so maybe I'll tweak with the current setup to see if this alleviates any problems.... I may also try swapping a few tubes in search of a very quite pair.
Other suggestions?
Bryan
Bryan said:
Try using one of those shock-mounted/compliant tube sockets.... some sort of tube dampers might help as well.
A friend went thru some 1200 12B4s and his comment to me the other day was that the biggest problem with them was microphony.
dave
Hi,
No surprise there. Those TV and computer type tubes where never designed with low microphony in mind.
Tube dampers, anti-shock mount sockets and even suspended boards should help. Keeping it away from the direct soundfield and on a decent shelf should help too I hope.
Cheers,😉
P.S. Does anyone still use those sorbothane feet? They can be found cheap and are really effective if you put some weight on them. (3-6 kgs)
No surprise there. Those TV and computer type tubes where never designed with low microphony in mind.
Tube dampers, anti-shock mount sockets and even suspended boards should help. Keeping it away from the direct soundfield and on a decent shelf should help too I hope.
Cheers,😉
P.S. Does anyone still use those sorbothane feet? They can be found cheap and are really effective if you put some weight on them. (3-6 kgs)
Hey Guys,
I've just realized that my LM317T regulated heater supply is not grounded.
I've made this with a 12.6 VCT transformer, which has the CT grounded. The regulated 12.6 volts is from a schottky bridge-CRCRCRC-317T-C arrangement. Would it be wise to ground the 0 volt side of the supply?
Also, what are the factors one should look for when choosing the last cap for the regulator. Is their a limit to the capacitance one should use? I'm currently using a Panasonic TSHB, 10K. Is this overkill?
Thanks,
Bryan
I've just realized that my LM317T regulated heater supply is not grounded.
I've made this with a 12.6 VCT transformer, which has the CT grounded. The regulated 12.6 volts is from a schottky bridge-CRCRCRC-317T-C arrangement. Would it be wise to ground the 0 volt side of the supply?
Also, what are the factors one should look for when choosing the last cap for the regulator. Is their a limit to the capacitance one should use? I'm currently using a Panasonic TSHB, 10K. Is this overkill?
Thanks,
Bryan
Hi,
Wow...No need to go overboard here, the LM317 will reject most of the ripple already.
The 10.000µF behind the reg. is by no means necessary. I just use it because I had it.
Important to the reg is to have some ESR behind it or it may go into spontaneous HF oscillation.
This can be achieved by the cap or a small resistor in series.
Usually the minus is grounded on the supply but I don't consider it necessary in most cases.
Here's what I use for a trio of 12B4As ( incidentally):
Cheers,😉
Wow...No need to go overboard here, the LM317 will reject most of the ripple already.
The 10.000µF behind the reg. is by no means necessary. I just use it because I had it.
Important to the reg is to have some ESR behind it or it may go into spontaneous HF oscillation.
This can be achieved by the cap or a small resistor in series.
Usually the minus is grounded on the supply but I don't consider it necessary in most cases.
Here's what I use for a trio of 12B4As ( incidentally):
Cheers,😉
Attachments
my 12B4A lineamp
Hey,
Love that tube. Thought I would post a schematic of my 12B4A lineamp here. I think it is as built, except I moved the 20uF power supply cap to the heater DC. Very quiet humwise although I had a bit of a time while building it with the placement of the grounds. Output is 10Vrms on 2Vrms in with 2nd harmonic -41dB and 3rd >-70dB. Second is -55dB at 2Vrms in and out. Absolutely lovely sound and simple as a fella could want. Building a PP power amp with the 12B4A now.
Michael
Hey,
Love that tube. Thought I would post a schematic of my 12B4A lineamp here. I think it is as built, except I moved the 20uF power supply cap to the heater DC. Very quiet humwise although I had a bit of a time while building it with the placement of the grounds. Output is 10Vrms on 2Vrms in with 2nd harmonic -41dB and 3rd >-70dB. Second is -55dB at 2Vrms in and out. Absolutely lovely sound and simple as a fella could want. Building a PP power amp with the 12B4A now.
Michael
Attachments
Hi All,
I've been listening to the preamp, and I have a few questions...
Firstly, I still have a little noise in both channels, which seems to be a volume dependent hum. That is, if I increase the volume, the humm is increased. Any suggestions?
Secondly, I also have a noise in one channel. The noise soulds like a "scratching" sound, which is faint. It comes and goes, and is not steady. My plan is to go over all the solder joins, possibly correcting a dry joint? Any suggestions as to how to identify the culprit?
Thirdly, my heater supply, which is probably WAY overboard has me a little concerned. It is a CRCRCRC'LM317T-C-common mode choke supply. 4700uF-2Ohm for the filters, the last cap is 10K, and the choke is in the mH range. I'm wondering if the niose may be from the supply. When this supply is floated, the I have considderably more hum, and when grounded, this hum is decreased, but i think some of the background hum may be from the heaters???
Any other topologies which may be useful to try? Any suggestions as to hunt this down? Should I just make a simple DC supply and compare?
Again, any suggestions are greatly appreciated.
Thanks,
Bryan
I've been listening to the preamp, and I have a few questions...
Firstly, I still have a little noise in both channels, which seems to be a volume dependent hum. That is, if I increase the volume, the humm is increased. Any suggestions?
Secondly, I also have a noise in one channel. The noise soulds like a "scratching" sound, which is faint. It comes and goes, and is not steady. My plan is to go over all the solder joins, possibly correcting a dry joint? Any suggestions as to how to identify the culprit?
Thirdly, my heater supply, which is probably WAY overboard has me a little concerned. It is a CRCRCRC'LM317T-C-common mode choke supply. 4700uF-2Ohm for the filters, the last cap is 10K, and the choke is in the mH range. I'm wondering if the niose may be from the supply. When this supply is floated, the I have considderably more hum, and when grounded, this hum is decreased, but i think some of the background hum may be from the heaters???
Any other topologies which may be useful to try? Any suggestions as to hunt this down? Should I just make a simple DC supply and compare?
Again, any suggestions are greatly appreciated.
Thanks,
Bryan
Hi,
Re: Hum:
The hum you experience can come from a number of sources:
Insufficient shielding, magnetic coupling, a small groundloop, layout, etc.
I doubt it originates in the heater supply but I could be wrong...
Cheers,😉
Re: Hum:
The hum you experience can come from a number of sources:
Insufficient shielding, magnetic coupling, a small groundloop, layout, etc.
I doubt it originates in the heater supply but I could be wrong...
Cheers,😉
If it makes any sense.....
I tried a 100uF decoupling cap on the cathode resistors last night, and this seems to have dropped the noise a slight bit.
Still have the hum.
I'm wondering if I may need another LC stage in the PS?
I'll tweak with the layout and orientation of the iron, and grounds tonight to see if I can hunt it down....
All other thoughts are appreciated.
Thanks,
Bryan
I tried a 100uF decoupling cap on the cathode resistors last night, and this seems to have dropped the noise a slight bit.
Still have the hum.
I'm wondering if I may need another LC stage in the PS?
I'll tweak with the layout and orientation of the iron, and grounds tonight to see if I can hunt it down....
All other thoughts are appreciated.
Thanks,
Bryan
If the hum varies with volume setting, then it is not from a lack of B+ filtering, per se. Instead, you have noise entering within the signal circuit. Check your grounding scheme from input to output and also pay attention to the relationship between psu/heater and circuit wiring layout - if they must cross paths, try to make them cross at 90 degree angles.
Hey,
As the input is currently wired, I have the positive (signal) and ground coming from the input RCA to the Pot. From the pot, the positive (signal) and ground go to the tube, and ground buss associated with the tube components (grid leak resistor, cathode resistor, and output cap belleding resistor). Should I run the ground from the input RCA to ground seperately, isolating it from the gournd associated with the pot? As it is currently, the pot and the input share the same path to ground, from the pot.
Would this cause a loop which may be problematic?
The output goes from the coupling cap to RCA, and the ground is attached to the buss mentioned above.
Any thoughts?
Bryan
As the input is currently wired, I have the positive (signal) and ground coming from the input RCA to the Pot. From the pot, the positive (signal) and ground go to the tube, and ground buss associated with the tube components (grid leak resistor, cathode resistor, and output cap belleding resistor). Should I run the ground from the input RCA to ground seperately, isolating it from the gournd associated with the pot? As it is currently, the pot and the input share the same path to ground, from the pot.
Would this cause a loop which may be problematic?
The output goes from the coupling cap to RCA, and the ground is attached to the buss mentioned above.
Any thoughts?
Bryan
Will try tonight and report back!
Any thoughts on the "scratchy" noise described a few posts earlier?
Thanks,
Bryan
Any thoughts on the "scratchy" noise described a few posts earlier?
Thanks,
Bryan
Here's a nice write-up by Kurt Strain on grounding techniques:
http://www.sonic.net/~ktstrain/Groundtutor.htm
The scratchy sound could be a bad solder joint, or cap/resistor going bad.
If you tap your top plate, can you get the sound to occur? Also, you might try prodding your joints and components with a wooden probe(stick) to see if you can narrow down the offending area.
All the above is predicated on you already haven swapped tubes to make sure that the noise doesn't follow the tube.
http://www.sonic.net/~ktstrain/Groundtutor.htm
The scratchy sound could be a bad solder joint, or cap/resistor going bad.
If you tap your top plate, can you get the sound to occur? Also, you might try prodding your joints and components with a wooden probe(stick) to see if you can narrow down the offending area.
All the above is predicated on you already haven swapped tubes to make sure that the noise doesn't follow the tube.
Hey Guys,
I currently have the LM317T regulated 12.6 volt heater referenced to 0volts ground. Should I reference this to another voltage.....?
Thanks,
Bryan
I currently have the LM317T regulated 12.6 volt heater referenced to 0volts ground. Should I reference this to another voltage.....?
Thanks,
Bryan
No Love Guys 🙁
So, I'm thinking about the heater supply a little more...
What is the best way to ground the heater. It is a regulated 12.6 volt heater, with a CRC filter, a big 20K cap at the output with a common mode choke. I will try to draw up a cct diagram tomorrow.
As it is now, the ground comes off the last cap (20K) and goes to earth. Is this wright or wrong?
I still have some noise coming from the amp. It is heard in the tweeter, and sounds like little static discharges, which occur periodically. I also have some hiss which I have not been able to remove.
What is the best (proper) method to determine the source of this hiss and noise?
Thanks,
Bryan
So, I'm thinking about the heater supply a little more...
What is the best way to ground the heater. It is a regulated 12.6 volt heater, with a CRC filter, a big 20K cap at the output with a common mode choke. I will try to draw up a cct diagram tomorrow.
As it is now, the ground comes off the last cap (20K) and goes to earth. Is this wright or wrong?
I still have some noise coming from the amp. It is heard in the tweeter, and sounds like little static discharges, which occur periodically. I also have some hiss which I have not been able to remove.
What is the best (proper) method to determine the source of this hiss and noise?
Thanks,
Bryan
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