I think this is just a language problem....
From Anatech:
Cathode followers are much better than trying to drive the cable and following circuit with a high impedance plate. Also your eq network for the phono stage.
Same thought... new words:
CF is a low impedance output and good for driving the capacitance of cables and first grid in power amp.
The other choice (not so good) is plate output. Capacitance from cable and first grid increases plate load... increase distortion.
So.... CF is best choice.
Sorry, I do not know how to use the quote feature correctly.
From Anatech:
Cathode followers are much better than trying to drive the cable and following circuit with a high impedance plate. Also your eq network for the phono stage.
Same thought... new words:
CF is a low impedance output and good for driving the capacitance of cables and first grid in power amp.
The other choice (not so good) is plate output. Capacitance from cable and first grid increases plate load... increase distortion.
So.... CF is best choice.
Sorry, I do not know how to use the quote feature correctly.
Oh yes Poobah, I understand you very well! 😉
It is clear to me that I should mantain CF output for those reasons. It wasn't just clear to me what Chris said about driving the phono eq stage with the CF 😕
But...! Stop! There are some news. I've got another tube failing I think, another Siemens, one of the two I considered good. It has started to make a big hum in the left channel (those left-channel only faliures make me think
) and the usual cracking/frying. Same as usual, with an higher than normal cathode voltage (so increased plate current) of 9V instead of 7.8V. Replacing the tube with the last one has resolved the noise...
...but it cannot be that my preamp stress tubes. I run the heaters at 11.8V series connected, the heaters PT center tap goes to the elevated reference, the left wire goes to pin 9 and the right wire goes to pin 4 and 5. The heathers are referenced at a clean 39.6V, 320V on the plates, 7.8V cathode with no signal (it means 0.78mA Iq). That's 0.25W per section, it shouldn't hurt the tubes.
Another possibility is that the tubes were abused and stressed: I found them at a local electronics high school that was cleaning out the old electronics lab. Althought I didn't find PTs, OTs, sockets, etc, there is the possibility that those tubes have been used by hundred of students in their project.
Let's hope that this last tube will last... I hate the idea of going out, in a musical instruments shop and buy a new Chinese 12AT7 for 20 euros when 4 Siemens failed to me...
I'm still open to suggestions about my preamp.
It is clear to me that I should mantain CF output for those reasons. It wasn't just clear to me what Chris said about driving the phono eq stage with the CF 😕
But...! Stop! There are some news. I've got another tube failing I think, another Siemens, one of the two I considered good. It has started to make a big hum in the left channel (those left-channel only faliures make me think
) and the usual cracking/frying. Same as usual, with an higher than normal cathode voltage (so increased plate current) of 9V instead of 7.8V. Replacing the tube with the last one has resolved the noise......but it cannot be that my preamp stress tubes. I run the heaters at 11.8V series connected, the heaters PT center tap goes to the elevated reference, the left wire goes to pin 9 and the right wire goes to pin 4 and 5. The heathers are referenced at a clean 39.6V, 320V on the plates, 7.8V cathode with no signal (it means 0.78mA Iq). That's 0.25W per section, it shouldn't hurt the tubes.
Another possibility is that the tubes were abused and stressed: I found them at a local electronics high school that was cleaning out the old electronics lab. Althought I didn't find PTs, OTs, sockets, etc, there is the possibility that those tubes have been used by hundred of students in their project.
Let's hope that this last tube will last... I hate the idea of going out, in a musical instruments shop and buy a new Chinese 12AT7 for 20 euros when 4 Siemens failed to me...
I'm still open to suggestions about my preamp.
I think your are correct about the abused tubes... but still measure again and check! Something is strange... 3 tubes????
Yes it's strange... when I'll got my scope (it still needs a strange voltage regulator, so it's still in the shop) I'll search for strange things...
I'm starting now to draw a schematic for the ideas I have in mind. It will not include the psu since I'm searching for options about that.
I'm starting now to draw a schematic for the ideas I have in mind. It will not include the psu since I'm searching for options about that.
Here is the line amp section. Note that between the Phono Stage and the Line Amp there's a selector switch (to select if the line amp input accepts signal from the line input or the phono amp output), a 100K potentiometer (I will see if I can made a stepped attenuator).
Now the problem is: I want the lowest possible output impedance (and good sound of course 😀 ), which one is better to archieve this, ECC81, 82 or 83? Since the Zout from what I've read is related to 1/mu of the tube, an ECC83 here would perform best... but I'm not an expert
Any suggestions are welcome!!!!
No psu yet. If someone has something to suggest... I repost my ideas:
- I have a 117V no CT power tranny.
- I'd like a SS regulated B+, no chokes. I've got many big hi-volt ex-smps caps.
Heather supply is far more easy to treat, it will be rectified and regulated with a LM317 I think.
Now the problem is: I want the lowest possible output impedance (and good sound of course 😀 ), which one is better to archieve this, ECC81, 82 or 83? Since the Zout from what I've read is related to 1/mu of the tube, an ECC83 here would perform best... but I'm not an expert
Any suggestions are welcome!!!!
No psu yet. If someone has something to suggest... I repost my ideas:
- I have a 117V no CT power tranny.
- I'd like a SS regulated B+, no chokes. I've got many big hi-volt ex-smps caps.
Heather supply is far more easy to treat, it will be rectified and regulated with a LM317 I think.
Attachments
That line amp HAS to go! This is a classic Bad Cathode Follower circuit.
How are your heaters referenced to ground?
How are your heaters referenced to ground?
Could you explain a little bit more SY? That line amp is the CF that even you agreed that was good 
My heaters in the prototype are referenced to 39V, from a resistive voltage divider from the B+, I already mentioned that in one of my last posts. But in the definitive version of the preamp I'm planning to go on DC filaments as already said.
My heaters in the prototype are referenced to 39V, from a resistive voltage divider from the B+, I already mentioned that in one of my last posts. But in the definitive version of the preamp I'm planning to go on DC filaments as already said.
Hi Giaime,
The main point I was trying to make is that the output, and the circuit driving something like a feedback RIAA network should both be low impedance. A tape output also qualifies. A CF is one way of achieving this. Other methods will employ either feedback around the circuit or a transformer.
An easy test Giaime is to put a CF in circuit and listen to it in and out. You can see any difference it may make.
SY, what circuit do you prefer as the final stage? Just wondering as I'm planning on building something soon (I hope).
-Chris
The main point I was trying to make is that the output, and the circuit driving something like a feedback RIAA network should both be low impedance. A tape output also qualifies. A CF is one way of achieving this. Other methods will employ either feedback around the circuit or a transformer.
An easy test Giaime is to put a CF in circuit and listen to it in and out. You can see any difference it may make.
SY, what circuit do you prefer as the final stage? Just wondering as I'm planning on building something soon (I hope).
-Chris
Even with DC heaters, you want to reference them to ground or (equivalently) an elevated DC potential.
Regarding the CF design, by merely sticking a 10K resistor to ground, you doom the stage to run at a ridiculously low current and a challenging load. You need to either bias the cathode up by using a voltage divider at the grid or use a negative rail. Current source loads are a positive.
My own conception of the Perfect Cathode Follower Linestage has been posted here before. Do a search for "Heretical."
Regarding the CF design, by merely sticking a 10K resistor to ground, you doom the stage to run at a ridiculously low current and a challenging load. You need to either bias the cathode up by using a voltage divider at the grid or use a negative rail. Current source loads are a positive.
My own conception of the Perfect Cathode Follower Linestage has been posted here before. Do a search for "Heretical."
Thanks SY, many thanks: I remembered that thread, I'm starting to studing it now (college permitting), the main concepts I'm going to implement in my preamp are the CCS on the cathode of the ECC81.
Still my question: you designed it for an ECC88: what is the best tube for that application? Do I really have to buy a new tube or I will be happy with the ECC81/82/83? Still no one suggested me what is the best tube for CF. As already said, I read that the CF Zout is related to 1/mu of the tube: so ECC83 would perform better here.
Just waiting for suggestions
Still my question: you designed it for an ECC88: what is the best tube for that application? Do I really have to buy a new tube or I will be happy with the ECC81/82/83? Still no one suggested me what is the best tube for CF. As already said, I read that the CF Zout is related to 1/mu of the tube: so ECC83 would perform better here.
Just waiting for suggestions
No, CF output impedance goes as 1/gm, so the ECC83 is a poor choice. ECC81 is better, ECC88 is yet better.
I just read this thread. I have been fighting the snap crackle pop noise in my 300Beast amplifier for a couple of years. Unlike yours it comes and goes whanever it pleases. Always the left channel, usually when you turn the amp on, and it goes away after an hour or so of playing. The noise appeared slowly about a year after the amp was built. Like you I have changed many things, and have thus learned what is not causing the noise.
I have changed tubes (phase splitter - 5751, and driver 6CG7 tubes) so many times that I am convinced that it is not a tube problem. I discovered that if you change the tubes around, the noise may go away for a while, but it comes back. It may go away for a few days, or a week, but it comes back. If I swap the tubes between the two channels, the noise goes away, but it will come back in a few days, in the left channel. I have tried at least 20 different tubes of many different brands, and even different types. The amp may be silent for a while, but the noise always returns to the left channel.
I have changed the volume control, and ALL of the capacitors in the amp that are not common to both channels. The noise went away for a week, then came back in the left channel. I am convinced that it is a resistor breaking down, but I have not changed them yet. They are all PC board mounted, so this is not trivial. It could also be a tube socket.
Some of the smaller resistors were never intended to operate at vacuum tube voltage levels. I used what I had when I built this amp, and some are of unknown origin. Resistors have a maximum voltage rating, some are pretty low, like 50 volts.
I have changed tubes (phase splitter - 5751, and driver 6CG7 tubes) so many times that I am convinced that it is not a tube problem. I discovered that if you change the tubes around, the noise may go away for a while, but it comes back. It may go away for a few days, or a week, but it comes back. If I swap the tubes between the two channels, the noise goes away, but it will come back in a few days, in the left channel. I have tried at least 20 different tubes of many different brands, and even different types. The amp may be silent for a while, but the noise always returns to the left channel.
I have changed the volume control, and ALL of the capacitors in the amp that are not common to both channels. The noise went away for a week, then came back in the left channel. I am convinced that it is a resistor breaking down, but I have not changed them yet. They are all PC board mounted, so this is not trivial. It could also be a tube socket.
Some of the smaller resistors were never intended to operate at vacuum tube voltage levels. I used what I had when I built this amp, and some are of unknown origin. Resistors have a maximum voltage rating, some are pretty low, like 50 volts.
Hi tubelab,
Most people don't even read the specs on resistors. That's why I recommend 1W minimum for tube amps. Just for the voltage rating.
I think you're on the right track thinking about the socket. Too bad there are so many wires and components connected to it.
-Chris
Most people don't even read the specs on resistors. That's why I recommend 1W minimum for tube amps. Just for the voltage rating.
I think you're on the right track thinking about the socket. Too bad there are so many wires and components connected to it.
-Chris
Too many times for me; it is the things that DONT show up on a schematic, like sockets, that make things hard.
Actually my sockets have no wires or parts soldered to them, they are soldered into a PC board. A PC board that I made in my kitchen, that has been soldered on too much already. I may just redesign the whole driver board, since I have learned much in the 5 years since I built the amp.
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