Dear Sirs,
I am using a line stage consisting in a pot followed by a 12AU7(ECC82) in a cathode-follower configuration.
I have been told that substituting my present Philips Miniwatt for a mil spec tube like the 5814A, others things remaining unchanged, I will get a more powerful and defined bass.
I like very much resolution and detail in thelower part of the audio bandwidth.
Thank you very much indeed.
Kind regards,
beppe
I am using a line stage consisting in a pot followed by a 12AU7(ECC82) in a cathode-follower configuration.
I have been told that substituting my present Philips Miniwatt for a mil spec tube like the 5814A, others things remaining unchanged, I will get a more powerful and defined bass.
I like very much resolution and detail in thelower part of the audio bandwidth.
Thank you very much indeed.
Kind regards,
beppe
Going to a mil-spec tube is unlikely to make your bass more powerful. That said, the 12AU7 is not a particularly great choice in a cathode follower- the transconductance is pretty meagre. You might consider a higher gm tube like the classic ECC88 family or one of the Russian high-slope triodes.
It will fit pin-for-pin in your current preamp. You'll need to change the heater voltage from 12V to 6V, which isn't too hard. Depending on how the CF is biased, you might need to change a resistor value, but maybe not- do you have a schematic of your current unit?
SY said:
Thank you so much again Sir.
First of all I have to state that I am a very unexperienced DIYer.
And I am a little scared about working with the unit on inside to check values and voltages.
What interests me very much is the possibility of powering the ecc88 with low voltages.
do you have a schematic of your current unit?
Unfortunately not Sir.
It is an old British Micro Croft preamp 1st series.
Established that a ecc88 buffer would be much better than a ecc82 buffer I would like take that route.
My amps are a though load, at about 5 kohm of input impedance.
But with a 0,5V signal I can get max power.
So gain in the buffer stage is not needed.
So to sum up all my ramblings, the question is which could be the best tube for a basic cathode-follower design.
If a kit of a very basic buffer based on a single ecc88 (or other better tube for buffer) is available I would prefer to buy it of course and not proceeding with mods on my current preamp-
Starting from a kit more chances to build it right I believe.
Thank you very much indeed.
Kind regards,
beppe
Wow, no wonder you don't have much bass! Are those amplifiers really 5K input impedance?
I think the ECC88 is going to be a little hard pressed to drive such a low load impedance, but it will do a much better job at it than the 12AU7A.
One wonders at how much additional current the power supply in the croft can provide. I'd be careful about increasing current in the CF by more than a few mA..
You will also need to increase the value of the output coupling caps or if possible increase the input impedance of your amplifier to at least 20K if not higher.
I think the ECC88 is going to be a little hard pressed to drive such a low load impedance, but it will do a much better job at it than the 12AU7A.
One wonders at how much additional current the power supply in the croft can provide. I'd be careful about increasing current in the CF by more than a few mA..
You will also need to increase the value of the output coupling caps or if possible increase the input impedance of your amplifier to at least 20K if not higher.
I have to disagree...
The 12AU7 in follower configuration is roughly 400 ohms output impedance.. give or take depending on operating conditions of course... I have designed many 12AU7 followers in Hi-End applications with good sound and good measured results....
Yes, the ECC88 makes a mucho better follower in the 80 ohms range... But the 12AU7 still can get the job done...
Chris
The 12AU7 in follower configuration is roughly 400 ohms output impedance.. give or take depending on operating conditions of course... I have designed many 12AU7 followers in Hi-End applications with good sound and good measured results....
Yes, the ECC88 makes a mucho better follower in the 80 ohms range... But the 12AU7 still can get the job done...
Chris
cerrem said:
Thank you very much indeed Chris for the valuable advice.
Do you think that all 12AU7s are equal or for instance the mil spec 5814A can give a more robust output ?
Anyway your following words Yes, the ECC88 makes a mucho better follower in the 80 ohms range make me think that a ECC88 would be a wiser choice considering that this buffer must drive a 5K load (!).
Thank you very much again and kind regards,
beppe
The 5814A and the 12AU7A are the same tube...they come off the same factory run .....The difference is that the 5814A tubes usually measure stronger than 12AU7A spec and are seperated and sorted.... They are the cream of the crop 12AU7A tubes... SO this means the gm will be higher and the plate resistance a bit lower which makes for better low frequency reponse compared to a plain 12AU7A... The spec sheet for the 5814A read exactly like a 12AU7A but that is just for satisfying MIL requirements...in reality they measured better than the data sheet...
Chris
Chris
cerrem said:
Dear Mr Chris, thank you very much again for the very kind and precious advice.
They are the cream of the crop 12AU7A tubes...in reality they measured better than the data sheet...
Assuming that the stage being that of a commercial unit is optimized, my idea would be to try, as last attempt, a nice 5814A (suggestion of any particular brands ?).
Then if the sound will be still not convincing I would start with a DIY project about a ECC88 cathode-follower.
Thank you very much again.
Kind regards,
beppe
Dear Sirs,
an old review of the preamp in question:
http://www.eminentaudio.co.uk/Frontier/CrftReviews/Reviews/Micro2.htm
In the specifications at the end of the page I see 500 ohm output impedance , maybe just too much for a 5 K load.
Anyway, before throwing it away, I would strongly like to try a 5814A tube in the buffer position in place of the 12AU7.
I am optimistic.
Thank you very much to all of your Sirs for the extremely kind and precious advices.
Kind regards,
beppe
an old review of the preamp in question:
http://www.eminentaudio.co.uk/Frontier/CrftReviews/Reviews/Micro2.htm
In the specifications at the end of the page I see 500 ohm output impedance , maybe just too much for a 5 K load.
Anyway, before throwing it away, I would strongly like to try a 5814A tube in the buffer position in place of the 12AU7.
I am optimistic.
Thank you very much to all of your Sirs for the extremely kind and precious advices.
Kind regards,
beppe
Regardless of source impedance the 12AU7A is not going to do that great a job driving a 5K load impedance, although in this case the power amplifier's high input sensitivity of 0.5Vrms will help. I would expect very mediocre linearity at best.
For an application like this the 5687 would be much better, but the much higher filament and plate currents required would probably make it a non-starter in the Croft line stage. You could however build something using a 5687 to replace the Croft..
Your low frequency corner at 3.2Hz seems low enough, but even here a lower corner can't hurt in terms of phase shift, and could improve the subjective bass quality of the pre-amplifier. I'd try increasing the capacitance further and seeing whether or not there is an audible improvement.
For an application like this the 5687 would be much better, but the much higher filament and plate currents required would probably make it a non-starter in the Croft line stage. You could however build something using a 5687 to replace the Croft..
Your low frequency corner at 3.2Hz seems low enough, but even here a lower corner can't hurt in terms of phase shift, and could improve the subjective bass quality of the pre-amplifier. I'd try increasing the capacitance further and seeing whether or not there is an audible improvement.
Quoting Mr Kevin
Regardless of source impedance the 12AU7A is not going to do that great a job driving a 5K load impedance,
although in this case the power amplifier's high input sensitivity of 0.5Vrms will help.
I would expect very mediocre linearity at best
Thank you very much Sir for your very useful advice.
It seems to me a game lost in advance.
I am thinking to going directly with a ecc88 buffer instead.
From what I read a much more powerful driver.
After all these amps are a very tough load.
For an application like this the 5687 would be much better, but the much higher filament and plate currents required would probably make it a non-starter in the Croft line stage.
You could however build something using a 5687 to replace the Croft..
Thank you for your advice.
But I would like to stay with a much more common tube, like ecc88.
By the way I see that the last preamps from Audio Research are getting extremely favourable review.
They use a 6h30 tube, a stronger version of the ecc88.
Do you know it ?
Maybe one of this used as a buffer could drive even my amps.
Your low frequency corner at 3.2Hz seems low enough, but even here a lower corner can't hurt in terms of phase shift, and could improve the subjective bass quality of the pre-amplifier
I'd try increasing the capacitance further and seeing whether or not there is an audible improvement
Thank you very much again but it seems to me that this ecc82 is not the right tube for my purpose.
I am getting weak bass even in the 100 Hz region.
The piano is weak, drums are weak without the right impact.
I am sorry because all in all the sound is nicely natural and not fatiguing. But also boring and not emotionally involving.
Thank you very much again.
Kind regards,
beppe
Regardless of source impedance the 12AU7A is not going to do that great a job driving a 5K load impedance,
although in this case the power amplifier's high input sensitivity of 0.5Vrms will help.
I would expect very mediocre linearity at best
Thank you very much Sir for your very useful advice.
It seems to me a game lost in advance.
I am thinking to going directly with a ecc88 buffer instead.
From what I read a much more powerful driver.
After all these amps are a very tough load.
For an application like this the 5687 would be much better, but the much higher filament and plate currents required would probably make it a non-starter in the Croft line stage.
You could however build something using a 5687 to replace the Croft..
Thank you for your advice.
But I would like to stay with a much more common tube, like ecc88.
By the way I see that the last preamps from Audio Research are getting extremely favourable review.
They use a 6h30 tube, a stronger version of the ecc88.
Do you know it ?
Maybe one of this used as a buffer could drive even my amps.
Your low frequency corner at 3.2Hz seems low enough, but even here a lower corner can't hurt in terms of phase shift, and could improve the subjective bass quality of the pre-amplifier
I'd try increasing the capacitance further and seeing whether or not there is an audible improvement
Thank you very much again but it seems to me that this ecc82 is not the right tube for my purpose.
I am getting weak bass even in the 100 Hz region.
The piano is weak, drums are weak without the right impact.
I am sorry because all in all the sound is nicely natural and not fatiguing. But also boring and not emotionally involving.
Thank you very much again.
Kind regards,
beppe
A single ECC88 as cathode follower will drive a 5k load, and do it far better than a 12AU7-oid. But the distortion performance will not be optimal. A 10k load would be kinder. Hmmmm, what to do, what to do...
Here's what: if you parallel two section of the ECC88, it's the same as running a single section with a 10k load. The distortion performance will be pretty good, too- with a 10k plate load, 8mA current and 2.5VRMS out, an ECC88 shows about 0.2% THD as a grounded cathode amplifier, dominated by second harmonic. So the distortion of the same tube and same operating conditions as a cathode follower will be divided by the gain; in this case, gain is about 25, so the distortion of a cathode follower with that load will be 0.2%/25 = 0.008%. Not bad at all!
The main precaution to doing this is that each section of the ECC88 will need to have its own grid-stopper resistor- compared with 12AU7, the ECC88 has a greater tendency to oscillate, but it's easy to handle if you know this going in. With separate 1k resistors wired right at the grid pins (old fashioned carbon composition do very well in this spot), you'll probably have no trouble at all.
Here's what: if you parallel two section of the ECC88, it's the same as running a single section with a 10k load. The distortion performance will be pretty good, too- with a 10k plate load, 8mA current and 2.5VRMS out, an ECC88 shows about 0.2% THD as a grounded cathode amplifier, dominated by second harmonic. So the distortion of the same tube and same operating conditions as a cathode follower will be divided by the gain; in this case, gain is about 25, so the distortion of a cathode follower with that load will be 0.2%/25 = 0.008%. Not bad at all!
The main precaution to doing this is that each section of the ECC88 will need to have its own grid-stopper resistor- compared with 12AU7, the ECC88 has a greater tendency to oscillate, but it's easy to handle if you know this going in. With separate 1k resistors wired right at the grid pins (old fashioned carbon composition do very well in this spot), you'll probably have no trouble at all.
SY said:
Thank you very much Sir for the kind and valuable advice.
So the best solution would be parallelling two section of a ECC88 per channel .
I understand that 5K are indeed a very tough load for a tube without recurring to expensive coupling transformers.
What is your opinion of the 6H30 tube used as cathode follower ?
Or better, what would be in your opinion the ultimate cathode-follower (if such tube exists of course) ?
Thank you very much again.
Kind regards,
beppe
I've never used a 6H30 so I can't comment.
My "ultimate" CF would not be much different- I'd put my attention toward raising the load impedance.
My "ultimate" CF would not be much different- I'd put my attention toward raising the load impedance.
SY said:
Thank you very much again.
I have no question left.
By the way I will try to get the schematic at least of the input stage of the amps in order to check if it is possible to raise that 5K very low value.
A design choice that I do not understand completely.
It makes the matching with a preamp a quite difficult task.
Thank you very much indeed.
Kind regards,
beppe
Fet source follower
If you want very low O/P Z, Try an IRF710/IRF720. It is good for up to 100Khz bandwidth and you can use the same power supply (up to 300 volts) with little mods. O/P Z about 25 ohms. Fets are the next best thing to tubes. I have a preamp that uses 12AU7 as an amp/cathode follower and it works very well. The only downfall is that it is a little noisy (shot noise) which I think it is inherent in these tubes since they are switching tubes. I would like to substitute to an ecc88 to see if the noise level reduces. One job
for me to do on a cold winter's day.
Regards
Billy...
If you want very low O/P Z, Try an IRF710/IRF720. It is good for up to 100Khz bandwidth and you can use the same power supply (up to 300 volts) with little mods. O/P Z about 25 ohms. Fets are the next best thing to tubes. I have a preamp that uses 12AU7 as an amp/cathode follower and it works very well. The only downfall is that it is a little noisy (shot noise) which I think it is inherent in these tubes since they are switching tubes. I would like to substitute to an ecc88 to see if the noise level reduces. One job
for me to do on a cold winter's day.
Regards
Billy...
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