My first tube amp kit
- Thread starter Alan0354
- Start date
Hi Megasat16
I just added the two resistors on the filament of the EL34, that completes the modification, The 60Hz now down to 10mV peak to peak. Still high in my book. I unplugged the input tube, the 60Hz noise did not even get affected. So now, I am looking into the power tube section. The center tap of the output transformer has no noise, so it's generated in the tube section. I can see the plate of the EL34 has 200mV of noise looking very much the same as at the output, so I take that the signal just attenuated by the step down of the output transformer.
10mV peak to peak is not the end of the world as the signal is about 20V peak to peak. That is -66dB down. But it can be better. I am still working on this.
BTW, I think the "oscillation" ringing I complained yesterday is from vibration. I can tap the chassis and see the ringing. I just don't get use to tubes is susceptible to vibration unlike transistors. It is still hard for me to switch gear from SS amp to tube amp!!!! I just brought up my second SS amp last week, so my mind is still fill with transistors!!!!
I'll come back when I have more info.
Alan
I just added the two resistors on the filament of the EL34, that completes the modification, The 60Hz now down to 10mV peak to peak. Still high in my book. I unplugged the input tube, the 60Hz noise did not even get affected. So now, I am looking into the power tube section. The center tap of the output transformer has no noise, so it's generated in the tube section. I can see the plate of the EL34 has 200mV of noise looking very much the same as at the output, so I take that the signal just attenuated by the step down of the output transformer.
10mV peak to peak is not the end of the world as the signal is about 20V peak to peak. That is -66dB down. But it can be better. I am still working on this.
BTW, I think the "oscillation" ringing I complained yesterday is from vibration. I can tap the chassis and see the ringing. I just don't get use to tubes is susceptible to vibration unlike transistors. It is still hard for me to switch gear from SS amp to tube amp!!!! I just brought up my second SS amp last week, so my mind is still fill with transistors!!!!
I'll come back when I have more info.
Alan
I spent some time chasing the 60Hz, I might hit a dead end.
1) I removed the input tube, very little change, the noise is from the EL34 to output transformer.
2) I shorted the input of the EL34 to ground, just remove a little bit of hash, no major change in the amplitude.
3) I removed the EL34 so only the output transformer. The waveform change, but the amplitude almost the same. So this eliminates the power tube, only the transformer.
4) I measure the center tap ( to +B) with scope on AC, less than 3mV of noise. But when I look at the plate connection( without the tube) I see 100mV. So is the screen grid tap.
5) I remove the red wire ( center tap) from the +B and solder to the screen grid tap, noise goes away.
6) If put a thick aluminum plate ( 1/8" thick) between the power transformer and the output transformer on the other side of the chassis, the amplitude of the noise gone down 20%.
7) I moved the wires of the output transformer, and other wires I can move, no change.
From this, it tells me that the transformer is talking to each other. I think it's the power transformer talking to the output transformer. The reasons are:
1) The center tap of the output transformer is quiet, noise did not come from the power supply. I see over 100mV on both the plate tap and screen grid tap, so it got to be picking up within the output transformer.
2) I suspect the core leakage flux of the power transformer coupled to the core of the output transformer. So I put a thick metal plate between the core of the power transformer and the output transformer. It lowered by 20%. This is very important observation, that the field is couple on the other side away from the circuit. Because it's low frequency, skin depth is very deep, I don't even think 1/8" of aluminum is thick enough, but it tell me the story.
I don't think packing the transformer this close is a good idea. The choke is almost right on top of the output transformer inside the chassis. At least the power transformer is orthogonal to the output transformer axis, so the coupling should be much less. The axis of the choke and the output transformer are in line with each other to give maximum coupling.
One has to be very careful in the transformer placement, once it's wrong, it is impossible to fix like in this case. The two output transformer already has a thick can cover them already, but it's just not nearly thick enough. Problem with circuit this simple, you reach the end of the road really fast and there's not much you can do about it.
I am open for suggestion and opinion. Please let me know.
1) I removed the input tube, very little change, the noise is from the EL34 to output transformer.
2) I shorted the input of the EL34 to ground, just remove a little bit of hash, no major change in the amplitude.
3) I removed the EL34 so only the output transformer. The waveform change, but the amplitude almost the same. So this eliminates the power tube, only the transformer.
4) I measure the center tap ( to +B) with scope on AC, less than 3mV of noise. But when I look at the plate connection( without the tube) I see 100mV. So is the screen grid tap.
5) I remove the red wire ( center tap) from the +B and solder to the screen grid tap, noise goes away.
6) If put a thick aluminum plate ( 1/8" thick) between the power transformer and the output transformer on the other side of the chassis, the amplitude of the noise gone down 20%.
7) I moved the wires of the output transformer, and other wires I can move, no change.
From this, it tells me that the transformer is talking to each other. I think it's the power transformer talking to the output transformer. The reasons are:
1) The center tap of the output transformer is quiet, noise did not come from the power supply. I see over 100mV on both the plate tap and screen grid tap, so it got to be picking up within the output transformer.
2) I suspect the core leakage flux of the power transformer coupled to the core of the output transformer. So I put a thick metal plate between the core of the power transformer and the output transformer. It lowered by 20%. This is very important observation, that the field is couple on the other side away from the circuit. Because it's low frequency, skin depth is very deep, I don't even think 1/8" of aluminum is thick enough, but it tell me the story.
I don't think packing the transformer this close is a good idea. The choke is almost right on top of the output transformer inside the chassis. At least the power transformer is orthogonal to the output transformer axis, so the coupling should be much less. The axis of the choke and the output transformer are in line with each other to give maximum coupling.
One has to be very careful in the transformer placement, once it's wrong, it is impossible to fix like in this case. The two output transformer already has a thick can cover them already, but it's just not nearly thick enough. Problem with circuit this simple, you reach the end of the road really fast and there's not much you can do about it.
I am open for suggestion and opinion. Please let me know.
I tested the other channel, it is pretty much the same. The gain of both channels are 14, kind of low.
The square wave is not pretty, but it's rock solid, I don't know what I can do with that. The circuit is so simple that you don't have much to play around. I am going to go to speaker test tomorrow and see how it sounds. 10mV peak to peak of noise is not the end of the world, we'll see how loud it is. My bet is it's not going to be that bad.
The square wave is not pretty, but it's rock solid, I don't know what I can do with that. The circuit is so simple that you don't have much to play around. I am going to go to speaker test tomorrow and see how it sounds. 10mV peak to peak of noise is not the end of the world, we'll see how loud it is. My bet is it's not going to be that bad.
10mV peak to peak, is 3.8mV RMS
This is about 57dB less than 1W, so your 91dB/W speakers will give you about 33dB of hum at 1m
Not ideal, but about what one would expect from a cheap tube amp, and completely tolerable. If you have air conditioning you won't even notice it at all, in a silent room it will be there.
What really sounds horrible is buzz, as opposed to hum, even at very low levels its impossible to live with.
This is about 57dB less than 1W, so your 91dB/W speakers will give you about 33dB of hum at 1m
Not ideal, but about what one would expect from a cheap tube amp, and completely tolerable. If you have air conditioning you won't even notice it at all, in a silent room it will be there.
What really sounds horrible is buzz, as opposed to hum, even at very low levels its impossible to live with.
The noise floor of 10mVp-p is with the RCA input shorted or no signal applied? Is it at 8R or 4R tap?
I think 10mV won't be a problem with your 91dB speakers.
Are your transformers square? If so, drill holes may still align as you rotate them around. Transformers follow the rules of inductors. Rotate 90 degrees from each other in order to cancel L coupling between them.
Can you pull the P transformer away from the chassis temporarily and see if it helps the hum issues.
Also, put your ears very close to the transformer to hear if it has mechanical hum. If it does, it may be the cause of your noise. Mechanical hum can easily get into microphonic tubes.
I think 10mV won't be a problem with your 91dB speakers.
Are your transformers square? If so, drill holes may still align as you rotate them around. Transformers follow the rules of inductors. Rotate 90 degrees from each other in order to cancel L coupling between them.
Can you pull the P transformer away from the chassis temporarily and see if it helps the hum issues.
Also, put your ears very close to the transformer to hear if it has mechanical hum. If it does, it may be the cause of your noise. Mechanical hum can easily get into microphonic tubes.
I actually put it into my system. The 10mV is the quiet side, the other side is more, about 20mV. The quiet side really is not a problem in the system, I can hardly hear any hum even when I put my ears within 2 to 3 in to the woofer. The other side is a little louder, I can start hearing it in 12" from the woofer. All in all, it is really livable.
I listen to it for a little while, I cannot turn the volume too high as big boss is sleeping, you don't want to disturb her or bad things will happen!!! It's a little disappointing. Maybe it's not loud enough, the sound doesn't seems to come out, not enough dynamics. BUT I really need to have it run louder to tell tomorrow and have to have my other amp side by side to compare. Just I was hoping for more.
It's definitely loud enough, I am not going to turn it much louder than what I am listening just now, I just need to turn it up more for testing tomorrow. Either way, I think it's $250 well spent. I get to hear it in my system first handed. I know it's only a cheap amp and I can't expect too much. The question is would I get that much better if I sink $2000 building another one?
Everything is pending for the testing tomorrow. I think the only way is to separate the transformers more, but the chassis is too small, I cannot put the transformers much farther apart. It has to be built into a bigger chassis before I can do that.
I don't think I have problem with the ground loop noise. The wiring of my house is bad, Both my Acurus and Nakamichi hum like anything, this is still a lot quieter than those two. Those two are no dogs already particularly the Nakamichi PA-7. It was close to $2000 in the 80s. I think the 5ohm isolation between the earth ground and amp ground, also the 22ohm really helps.
I listen to it for a little while, I cannot turn the volume too high as big boss is sleeping, you don't want to disturb her or bad things will happen!!! It's a little disappointing. Maybe it's not loud enough, the sound doesn't seems to come out, not enough dynamics. BUT I really need to have it run louder to tell tomorrow and have to have my other amp side by side to compare. Just I was hoping for more.
It's definitely loud enough, I am not going to turn it much louder than what I am listening just now, I just need to turn it up more for testing tomorrow. Either way, I think it's $250 well spent. I get to hear it in my system first handed. I know it's only a cheap amp and I can't expect too much. The question is would I get that much better if I sink $2000 building another one?
Everything is pending for the testing tomorrow. I think the only way is to separate the transformers more, but the chassis is too small, I cannot put the transformers much farther apart. It has to be built into a bigger chassis before I can do that.
I don't think I have problem with the ground loop noise. The wiring of my house is bad, Both my Acurus and Nakamichi hum like anything, this is still a lot quieter than those two. Those two are no dogs already particularly the Nakamichi PA-7. It was close to $2000 in the 80s. I think the 5ohm isolation between the earth ground and amp ground, also the 22ohm really helps.
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The trick is to put a lot of pressure on the bit and take short bursts to chip the stainless steel away. When you drill at a constant high speed, the material gets hot and then work-hardens.
Amen brother!
Alan,
I know you're a big SS guy.
I see you're in the Bay area otherwise I'd invite you to listen (test) some better tube gear.
Nothing wrong with buying building.
Before you spend possibly a lot of money and time. Maybe you can find a local Aker to share a little time with you and see if its worthwhile to pursue this.
A tube amp will generally not test nearly as well as a SS amp, the overall presentation at your ears might surprise you.
I know you're a big SS guy.
I see you're in the Bay area otherwise I'd invite you to listen (test) some better tube gear.
Nothing wrong with buying building.
Before you spend possibly a lot of money and time. Maybe you can find a local Aker to share a little time with you and see if its worthwhile to pursue this.
A tube amp will generally not test nearly as well as a SS amp, the overall presentation at your ears might surprise you.
I tested it loud today, sadly it did not get better, it got worst as I can hear it clips. I know it's not fare to compare a cheap amp kit like this one to the SS amp that I went all out to design and built. I still have not do side by side, just plug it into the system and listen. From my leaking memory, it cannot even compare to my Acurus which is not an expensive amp. Sound just does not come out, it is really 2 dimensional. The sound is not full and rich. Maybe I expected too much after hearing people keep saying tubes are sweet, full all these years, that my expectation is very high and it's a big let down.
I might have to try with a lesser speaker cables, the cable I use is designed for very high damping factor of SS amp. It has very low series resistance, low series inductance but very high capacitance. Maybe the amp just cannot drive the cable. I just need to unplug my custom cable and leave the two pairs of 12 gauge monster cables to drive the speakers.
Other than that, it's the end of the road. It might not be the amp, maybe it's me that I like the SS amp. I just feel a simple circuit like this, there is only so much you can do to improve it.
I know people keep saying get better tubes, better transformers and all. But I don't think it's going to make that huge of an improvement.
I might have to try with a lesser speaker cables, the cable I use is designed for very high damping factor of SS amp. It has very low series resistance, low series inductance but very high capacitance. Maybe the amp just cannot drive the cable. I just need to unplug my custom cable and leave the two pairs of 12 gauge monster cables to drive the speakers.
Other than that, it's the end of the road. It might not be the amp, maybe it's me that I like the SS amp. I just feel a simple circuit like this, there is only so much you can do to improve it.
I know people keep saying get better tubes, better transformers and all. But I don't think it's going to make that huge of an improvement.
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cubdog
banging through drywall
Don't give up on tubes just yet. Hopefully a local AKer will give you a taste of what a good tube amp is capable of. Hell even my cheap Magnavox tube amps sound wonderful compared to my SS amps. I really think you need to expose yourself to other tube amps before throwing in the towel.
mroboto
Super Member
You can add 0.1uf 1000V Ceramic disc as well as 4.7uf 630V CBB metalized film capacitors for decoupling like I did to my two favorite amps
http://audiokarma.org/forums/index....84-el84-push-pull.711346/page-2#post-10686920
http://www.audiokarma.org/forums/index.php?threads/yaqin-ms-300c-upgrade.322511/page-2#post-10700768
And try some tube rolling...
http://audiokarma.org/forums/index....84-el84-push-pull.711346/page-2#post-10686920
http://www.audiokarma.org/forums/index.php?threads/yaqin-ms-300c-upgrade.322511/page-2#post-10700768
And try some tube rolling...
