Hi Hugh,
Would you mind sharing a little more, maybe a typical application or a link..I can't find much on the topic. Since the mu is variable does this mean the slope of the penthode are not spaced equally therefore not linear ? Can't you tell this is way over my head..
BR,
Eric
Eric,
The tube I suggest is 6ES8, a frame grid from the same family as the 6DJ8/ECC88. It is also used in a compander, the ES8, a famous component used in the recording industry.
Normally mu is fixed, and this improves the linearity of a tube over a large voltage output, within the Childs Law transfer function. If mu is variable, the linearity is smoothly curved, and this tends to produce harmonics according to throughput. It was originally designed to reduce mu at higher plate currents, making it useful as an automatic gain control, used in AM radio receivers 60 years back. Used as a cathode follower, it richens the sound - 'tube sound' - within limitations. If you use the tube as a line level CF, as I do in my GK1 (which I don't make these days) it varies the attenuation ratio, between 0.92 to 0.94, and people report a richer sound, more musical, which is very helpful using 'sterile' SS amplification following the preamp stage. You do need to modify the amazing linearities of tubes when you are dealing at line level; the 'distortion' levels are very low at 2Vpp output an any tube, so it's hard to hear you are listening to a tube. If you use a tube, you really need it to sound like a tube - not very good SS. As it happens, the Cathode Follower topology is almost completely transparent; ergo, I use a 6ES8. Most people like this sound, but a few (most technical people who see harmonic profile as distortion) do not and prefer a 6DJ8/ECC88/6922.
Behind all this thinking there is a world of reality. Most people prefer toast with butter, buffed Aluminium anodised, women with makeup...... we humans are very strange beasts..........
Cheers,
Hugh
The tube I suggest is 6ES8, a frame grid from the same family as the 6DJ8/ECC88. It is also used in a compander, the ES8, a famous component used in the recording industry.
Normally mu is fixed, and this improves the linearity of a tube over a large voltage output, within the Childs Law transfer function. If mu is variable, the linearity is smoothly curved, and this tends to produce harmonics according to throughput. It was originally designed to reduce mu at higher plate currents, making it useful as an automatic gain control, used in AM radio receivers 60 years back. Used as a cathode follower, it richens the sound - 'tube sound' - within limitations. If you use the tube as a line level CF, as I do in my GK1 (which I don't make these days) it varies the attenuation ratio, between 0.92 to 0.94, and people report a richer sound, more musical, which is very helpful using 'sterile' SS amplification following the preamp stage. You do need to modify the amazing linearities of tubes when you are dealing at line level; the 'distortion' levels are very low at 2Vpp output an any tube, so it's hard to hear you are listening to a tube. If you use a tube, you really need it to sound like a tube - not very good SS. As it happens, the Cathode Follower topology is almost completely transparent; ergo, I use a 6ES8. Most people like this sound, but a few (most technical people who see harmonic profile as distortion) do not and prefer a 6DJ8/ECC88/6922.
Behind all this thinking there is a world of reality. Most people prefer toast with butter, buffed Aluminium anodised, women with makeup...... we humans are very strange beasts..........
Cheers,
Hugh
Last edited:
Member
Joined 2009
Paid Member
I never tried this variable mu tube, it seems counter-intuitive to me ?
A triode is a transconductance device (just like transistors) and produces a change in current for a change in input voltage. This current produces a change in voltage across the load resistance. For a common cathode stage this load resistance is both the internal anode resistance and the external load resistor. Although the gm varies with current the internal anode resistance varies in a complementary fashion so that the voltage at the anode is fairly linear and the gain is fairly constant. Remember that mu = gm x internal anode resistance. When gm goes up with current, internal anode resistance falls and mu is roughly constant.
For a cathode follower there's very little impact on operation from the internal anode resistance and the gain no longer follows mu, it follows mu / (mu + 1) and the impact of a variable mu tube will be very small unless you operate at a very bendy place in the curves. There are very few variable mu tubes with high gm, a feature you want for a CF to have a low output impedance so variable mu tubes are not convenient here.
You can achieve varying levels of distortion results without a variable mu tube by simply choosing your operating point and your load resistance appropriately. The circuit I've used has the nice property that the distortion profile is dominated by 2nd and low order of decreasing magnitude which is the traditional 'tube sound' and can be readily adjusted through a simple resistor value.
A triode is a transconductance device (just like transistors) and produces a change in current for a change in input voltage. This current produces a change in voltage across the load resistance. For a common cathode stage this load resistance is both the internal anode resistance and the external load resistor. Although the gm varies with current the internal anode resistance varies in a complementary fashion so that the voltage at the anode is fairly linear and the gain is fairly constant. Remember that mu = gm x internal anode resistance. When gm goes up with current, internal anode resistance falls and mu is roughly constant.
For a cathode follower there's very little impact on operation from the internal anode resistance and the gain no longer follows mu, it follows mu / (mu + 1) and the impact of a variable mu tube will be very small unless you operate at a very bendy place in the curves. There are very few variable mu tubes with high gm, a feature you want for a CF to have a low output impedance so variable mu tubes are not convenient here.
You can achieve varying levels of distortion results without a variable mu tube by simply choosing your operating point and your load resistance appropriately. The circuit I've used has the nice property that the distortion profile is dominated by 2nd and low order of decreasing magnitude which is the traditional 'tube sound' and can be readily adjusted through a simple resistor value.
Member
Joined 2009
Paid Member
Some related plots...
The 6DJ8/ECC88 curves show how mu is mostly constant because the gm (S) and internal resistance (rp) compensate for each other.
If mu were constant the usual triode curves would show a set of equally spaced plate curves, but for 6ES8 the curves start leaning over quite early so mu is clearly not constant....
The 6DJ8/ECC88 curves show how mu is mostly constant because the gm (S) and internal resistance (rp) compensate for each other.
If mu were constant the usual triode curves would show a set of equally spaced plate curves, but for 6ES8 the curves start leaning over quite early so mu is clearly not constant....
Attachments
Member
Joined 2009
Paid Member
Convinced it was an earthing issue I have made lots of experiments and measurements. To no avail.
Finally I tried moving the pcb around and lo and behold I have magnetic pick-up from the power trafo. The height of the pcb makes all the difference. When it is low down I get pick up and when it is high up I get pick up but the spikes change phase. When the board hovers so that it is half-way up the height of the power trafo the offending noise appears to abate. If I slip in a 1/8" piece of Al plate between the trafo and the pcb I can also reduce the pick-up (eddy losses).
I have some room to play with so I will raise the pcb and introduce some metallic shielding and see if I can rescue this!
Finally I tried moving the pcb around and lo and behold I have magnetic pick-up from the power trafo. The height of the pcb makes all the difference. When it is low down I get pick up and when it is high up I get pick up but the spikes change phase. When the board hovers so that it is half-way up the height of the power trafo the offending noise appears to abate. If I slip in a 1/8" piece of Al plate between the trafo and the pcb I can also reduce the pick-up (eddy losses).
I have some room to play with so I will raise the pcb and introduce some metallic shielding and see if I can rescue this!
Member
Joined 2009
Paid Member
Hi Andrew - the bottom panel - all panels - are Al. There is no iron except the power transformer core.
It seems that the most likely explanation is magnetic emissions from the power transformer since the effect is clearly dependent on how I position the pcb wrt to the transformer.
I'd like to understand better what elements on the pcb are picking up the fields. It's clearly the back half of the pcb.
It seems that the most likely explanation is magnetic emissions from the power transformer since the effect is clearly dependent on how I position the pcb wrt to the transformer.
I'd like to understand better what elements on the pcb are picking up the fields. It's clearly the back half of the pcb.
You might try Mumetal, magnetic shielding material. Works really well - I have seen it used to bring down fields effectively to almost nil.
Ultraperm 80 Metal Shield; MuMetal Mu Metal Alloy Shielding Sheet Audio HiFi USA | eBay
https://www.amazon.com/MuMETAL-Magnetic-Shielding-Thick-Sheet/dp/B017ABLYRO/
Have you got a big tin can, i.e. a 1gallon oil can made of tin plated steel?
I have not implemented but thought of doing.
Cutting a long spiral from the can, the width to match the height of the toroid.
It will have a join where the can strip is formed into a round.
If this is long enough you could wrap three turns around your toroid.
I have not implemented but thought of doing.
Cutting a long spiral from the can, the width to match the height of the toroid.
It will have a join where the can strip is formed into a round.
If this is long enough you could wrap three turns around your toroid.
Member
Joined 2009
Paid Member
I have access to some mu-metal, mostly cobalt. But simply elevating the pcb achieves the desired results without it. Unfortunately, there isn't the headroom in my chassis for this because of the height of the ECC88. I could just raise up just the back-end of the board so the thing is on a tilt and that nulls out the magnetic interference without lifting up the tube too much. I may try this just to see if it gives me an acceptable result.
There are still some other things to sort out - I'm not happy with the noise level from the buffer stage and need to figure out the source of that.
And I'm not yet sure the power amp has enough turn-on delay of the output relay to suit the warm up period of the tube.
I may also decide to investigate the use of a pair of Nuvistors in place of the tube I have now. I have a few Nuvistors lying around that would plug right in and would heat up faster and not be very tall. They also have a bit more of that variable mu thing going on that Hugh likes.
But I have to go on a business trip for a week so there will be a hiatus in progress.
There are still some other things to sort out - I'm not happy with the noise level from the buffer stage and need to figure out the source of that.
And I'm not yet sure the power amp has enough turn-on delay of the output relay to suit the warm up period of the tube.
I may also decide to investigate the use of a pair of Nuvistors in place of the tube I have now. I have a few Nuvistors lying around that would plug right in and would heat up faster and not be very tall. They also have a bit more of that variable mu thing going on that Hugh likes.
But I have to go on a business trip for a week so there will be a hiatus in progress.
Member
Joined 2009
Paid Member
Well I'm back at the project, slowly at least.
I've decided to re-spin the pre-amp board. It taught me a lot, not least that I need to make some improvements. I want a mute function on the output of the tube buffer. And the phono needs to be less noisy due to magnetic interference - one option maybe to leave it off and put it in an external box. The power supply and power amp are good and finished as far as I'm concerned at this point.
I've decided to re-spin the pre-amp board. It taught me a lot, not least that I need to make some improvements. I want a mute function on the output of the tube buffer. And the phono needs to be less noisy due to magnetic interference - one option maybe to leave it off and put it in an external box. The power supply and power amp are good and finished as far as I'm concerned at this point.
Last edited:
Member
Joined 2009
Paid Member
A quick update:
How time flies. I have abandoned the idea of squeezing everything into one box. Integrated amplifier maybe, but more in spirit than in practice. There were too many compromises. I had so little space the phono was picking up magnetic hum from the power transformer. I could not use the dual power supply board properly as I could fit only one transformer into the case. Everything was getting just too frustrating and the fun was not there.
So I'm making it a 3-chassis integrated amp
The pre-amp is going into a separate box, there's a thread somewhere in the relevant pre-amp forum. It's being re-designed to improve it - making the 2nd attempt my best effort.
The power supply is also going into a separate dedicated PSU box. I've just ordered the metal-work. The transformers are on-hand already. It will be a true dual-mono set-up and I'll be re-using the power supply pcb already built in this thread.
How time flies. I have abandoned the idea of squeezing everything into one box. Integrated amplifier maybe, but more in spirit than in practice. There were too many compromises. I had so little space the phono was picking up magnetic hum from the power transformer. I could not use the dual power supply board properly as I could fit only one transformer into the case. Everything was getting just too frustrating and the fun was not there.
So I'm making it a 3-chassis integrated amp
The pre-amp is going into a separate box, there's a thread somewhere in the relevant pre-amp forum. It's being re-designed to improve it - making the 2nd attempt my best effort.
The power supply is also going into a separate dedicated PSU box. I've just ordered the metal-work. The transformers are on-hand already. It will be a true dual-mono set-up and I'll be re-using the power supply pcb already built in this thread.
Bigun
Most all of my audio builds have the supply in a separate box from the circuits either completely or partially. The trafo and bridges are usually the big troublemakers but the caps can be too when they are very large. Have you considered putting the line stage and other circuits in the same box and powering it from a separate one?
Most all of my audio builds have the supply in a separate box from the circuits either completely or partially. The trafo and bridges are usually the big troublemakers but the caps can be too when they are very large. Have you considered putting the line stage and other circuits in the same box and powering it from a separate one?
Member
Joined 2009
Paid Member
I've started work on putting the power supply in it's own box, a line stage in yet a separate box and then I will be able to use these boxes with a selection of my power amplifiers. This will allow me to keep the mains related wiring safely away from everything else and I can fiddle around with my different amplifier designs to suit my mood.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.