I am looking to put together a low gain preamp. I do want a little gain so a cathode follower is not going to work. One suggestion I came across was to use a 71a in a basic grounded cathode config (I think the original suggestion was to use a choke on the plate.)
However, another post suggested that miller capacitance could be a problem in this setup. Could someone give me a 5 line primer on what this means exactly and perhaps help me determine if this really is an issue.
Also, if anyone has other suggestion for a low gain preamp, I'd be interested in those too, but the 71a has me intrigued.
Thanks,
-d
However, another post suggested that miller capacitance could be a problem in this setup. Could someone give me a 5 line primer on what this means exactly and perhaps help me determine if this really is an issue.
Also, if anyone has other suggestion for a low gain preamp, I'd be interested in those too, but the 71a has me intrigued.
Thanks,
-d
The Miller Effect amplifies a triode's grid to plate capacitance by mu. The Miller capacitance is part of the load the upstream circuitry has to drive.
Search this site and other sites for information on a 12B4 based line stage. You will get everything you want, without the problem of a LARGE Miller capacitance.
Search this site and other sites for information on a 12B4 based line stage. You will get everything you want, without the problem of a LARGE Miller capacitance.
Hi Dave,
The 71A has a cgp of 7pF and a mu of slightly less than 4 which would result in a miller capacitance of worst case about 28pF - in most reasonable designs with a 100K pot or less this is not going to cause much of a problem. (Output loading will reduce the effective mu making the miller cap even smaller.)
The 71A is a great sounding tube but will probably be a little microphonic in this application.
I published an article in PFO a while back about my 26 based transformer coupled line stage. There is information you might find useful in your quest for a 71 based line stage. You can get to it easily from my site. (below)
It's definitely do-able and if you are up to the challenge why be derivative.
The 71A has a cgp of 7pF and a mu of slightly less than 4 which would result in a miller capacitance of worst case about 28pF - in most reasonable designs with a 100K pot or less this is not going to cause much of a problem. (Output loading will reduce the effective mu making the miller cap even smaller.)
The 71A is a great sounding tube but will probably be a little microphonic in this application.
I published an article in PFO a while back about my 26 based transformer coupled line stage. There is information you might find useful in your quest for a 71 based line stage. You can get to it easily from my site. (below)
It's definitely do-able and if you are up to the challenge why be derivative.
First, all the DHTs I have used sounded better than a 12b4, so although this is a nice tube and has many fans including me, for better sound why not go with a DHT as you suggest. I tried out a whole load of DHT tubes in a line stage, and the 71a came pretty far down the list. If you want low gain, you could try the 31 (mu 3.8). The 49 in triode (mu 4.7)is intriguing - didn't really set it up properly (lots of flying leads to a 4 pin socket) , very detailed but maybe a bit sharp. Frankly I prefer some of the other DHTs anyway to all the above, but they're all about mu=9.
Filament supply will have a huge effect on the sound. There was a thread here on 300b filament supplies which was a very interesting read. Basically the best sound is a current source of some description. the RonanReg, for instance was two voltage regs followed by a current source. A current mirror also sounds good. I tried common mode chokes at the end of current sources but the sound was worse. Interestingly CMCs improved the sound from plain voltage regs.
Filament supply will have a huge effect on the sound. There was a thread here on 300b filament supplies which was a very interesting read. Basically the best sound is a current source of some description. the RonanReg, for instance was two voltage regs followed by a current source. A current mirror also sounds good. I tried common mode chokes at the end of current sources but the sound was worse. Interestingly CMCs improved the sound from plain voltage regs.
Filament supply is critical to both the sound and the noise floor. A ronan type regulator would be an excellent choice, there are others particularly if you are willing to experiment.
I have used foil chokes with voltage regulators (not common mode) with good results (mainly for rfi reduction)
Good tubes to consider with transformer coupling which will give you generally less than 6dB of gain are the 30, 26, 12, 01A.
With chokes the 71A, 45, 6AS7GA are good choices.
I have used foil chokes with voltage regulators (not common mode) with good results (mainly for rfi reduction)
Good tubes to consider with transformer coupling which will give you generally less than 6dB of gain are the 30, 26, 12, 01A.
With chokes the 71A, 45, 6AS7GA are good choices.
Good tubes to consider with transformer coupling which will give you generally less than 6dB of gain are the 30, 26, 12, 01A.>>
Yes - I agree completely with that. 30 is a leaner sound and very clear, 26 is warmer, richer and more lush, 01a is clear and neutral, 12a is similar. There's a family resemblance, with the 26 rather different in sound - warmer if you like that. All good. I found the 30 less microphonic than the 31, though others report the opposite. All my 31s would ring, though it dodn't come through the speakers. The 30 seemed a bit less prone to this.
I did also try the 6B4G (Sovtek) and 2a3 (plain chinese) in my line stage, but preferred the smaller tubes. But in my opinion even the 2a3 was better than a 12b4 or other indirectly heated triodes. Going back to indirectly heated tubes seems right away to make the sound "thicker" - only way I can describe it, that last bit of clarity just goes (I tried a 30 compared with a 12b4 with a friend and we both agreed). Unless you have just switched back from a DHT you don't notice the slight cloudiness of indirectly heated tubes. And let's face it, how many people have even heard DHT small tubes in line stages. I was happy for years until I did. The first time I heard a 26 line stage it was a shock - it's quite possible my draw literally dropped.
Yes - I agree completely with that. 30 is a leaner sound and very clear, 26 is warmer, richer and more lush, 01a is clear and neutral, 12a is similar. There's a family resemblance, with the 26 rather different in sound - warmer if you like that. All good. I found the 30 less microphonic than the 31, though others report the opposite. All my 31s would ring, though it dodn't come through the speakers. The 30 seemed a bit less prone to this.
I did also try the 6B4G (Sovtek) and 2a3 (plain chinese) in my line stage, but preferred the smaller tubes. But in my opinion even the 2a3 was better than a 12b4 or other indirectly heated triodes. Going back to indirectly heated tubes seems right away to make the sound "thicker" - only way I can describe it, that last bit of clarity just goes (I tried a 30 compared with a 12b4 with a friend and we both agreed). Unless you have just switched back from a DHT you don't notice the slight cloudiness of indirectly heated tubes. And let's face it, how many people have even heard DHT small tubes in line stages. I was happy for years until I did. The first time I heard a 26 line stage it was a shock - it's quite possible my draw literally dropped.
Thanks for all the suggestions. I do tihink the 71 is worth trying, but some of the others may get a round as well. If microphonics are an issue, does anyone have a good strategy to reduce them?
Can you suggest a transformer? I have had trouble finding a transformer to use for a line stage.
Hmmm. I had a run in with the 6080/6as7 in a line stage a while back and found it to be a very noisy tube. I tried a ton of different tubes and different setups and could not get it quiet.
The 45 looks great too, but is a little too expensive for now.
kevinkr said:
Can you suggest a transformer? I have had trouble finding a transformer to use for a line stage.
Hmmm. I had a run in with the 6080/6as7 in a line stage a while back and found it to be a very noisy tube. I tried a ton of different tubes and different setups and could not get it quiet.
The 45 looks great too, but is a little too expensive for now.
To reduce microphonics do stuff like the following:
a) Enclose the tubes, either in the chassis or some kind of tube over them
b) Try O rings
c) Make the chassis as heavy and dead as possible - I got good results with mounting the sockets straight into 4mm alu plate. The chassis should not ring when tapped.
d) If you want to be extra careful, try something like this - worked for me. Mount the two tube sockets on a rectangular 6 mm teflon sheet. You will secure this with two holes drilled across the middle, and on the two ends put rubber under the teflon, using cut pieces of a normal stationary type eraser. Put a couple of solid bolts through the bottom of the chassis and tighten with nuts on the inside of the chassis, so the bolt ends are sticking up. These go through the holes in the teflon, but cut the holes bigger so you can put a rubber grommet in each, so the bolts don't touch the teflon. Put larger rubber grommets top and bottom of the teflon and finally bolt the lot down so the teflon goes down and is held at the two ends by the rubber pieces. The teflon is non-resonant - you could use some other material - and doesn't touch the chassis because of the rubber grommets.
e) Put the chassis on some kind of soft rubber feet or other dampening material like sponge.
a) Enclose the tubes, either in the chassis or some kind of tube over them
b) Try O rings
c) Make the chassis as heavy and dead as possible - I got good results with mounting the sockets straight into 4mm alu plate. The chassis should not ring when tapped.
d) If you want to be extra careful, try something like this - worked for me. Mount the two tube sockets on a rectangular 6 mm teflon sheet. You will secure this with two holes drilled across the middle, and on the two ends put rubber under the teflon, using cut pieces of a normal stationary type eraser. Put a couple of solid bolts through the bottom of the chassis and tighten with nuts on the inside of the chassis, so the bolt ends are sticking up. These go through the holes in the teflon, but cut the holes bigger so you can put a rubber grommet in each, so the bolts don't touch the teflon. Put larger rubber grommets top and bottom of the teflon and finally bolt the lot down so the teflon goes down and is held at the two ends by the rubber pieces. The teflon is non-resonant - you could use some other material - and doesn't touch the chassis because of the rubber grommets.
e) Put the chassis on some kind of soft rubber feet or other dampening material like sponge.
Hi Dave,
Both Magnequest and James make suitable transformers. I'm using vintage HA-133 which have gotten a bit ridiculous in cost lately.
01A/12A/26/30
15K:600 ohms will give you 4dB of gain with the 26 or 01 and a source Z around 300 ohms. See the table in my article for further details. Note that I don't normally terminate into 600 ohms, but more like 10K - 100K so effectively the limiting factor on gain is the mu divided by the transformer ratio.
It's important to make sure the transformer has relatively high primary inductance to assure low distortion at low frequencies. I'd say something like 150H (xl = 18.85K ohms @ 20Hz) as an absolute minimum, this is critical with dhts with an rp of about 8K as is the case with the 26 and 01, somewhat less critical with the 12A as it's rp is about 5K..
71A/45
A transformer can be used here too, in this case something on the order of 5K ought to suffice. Here you will need about 50H for good low end performance, and you will get right around unity gain. Choke loading seems like a good choice here and will give some gain as well. 100H chokes are quite practical at the currents needed here and a lot of the problems of using transformers just go away, at the expense of considerably higher source z, (basically rp) and increased noise/microphonics due to the missing attenuation from the plate to line transformer.
Pretty much everyone around here (Northern New England Tube Group or NNETG as we are known) has built a 12B4 except me, and I have a/b'd many of these extensively and just don't need to go down that road.
I have heard one 12B4 pre-amp I really liked and it used a high transconductance pentode as the upper device in a dissimilar mu follower confiuration. Quite unusual, and the best sounding 12B4 design I have heard to date.
Both Magnequest and James make suitable transformers. I'm using vintage HA-133 which have gotten a bit ridiculous in cost lately.
01A/12A/26/30
15K:600 ohms will give you 4dB of gain with the 26 or 01 and a source Z around 300 ohms. See the table in my article for further details. Note that I don't normally terminate into 600 ohms, but more like 10K - 100K so effectively the limiting factor on gain is the mu divided by the transformer ratio.
It's important to make sure the transformer has relatively high primary inductance to assure low distortion at low frequencies. I'd say something like 150H (xl = 18.85K ohms @ 20Hz) as an absolute minimum, this is critical with dhts with an rp of about 8K as is the case with the 26 and 01, somewhat less critical with the 12A as it's rp is about 5K..
71A/45
A transformer can be used here too, in this case something on the order of 5K ought to suffice. Here you will need about 50H for good low end performance, and you will get right around unity gain. Choke loading seems like a good choice here and will give some gain as well. 100H chokes are quite practical at the currents needed here and a lot of the problems of using transformers just go away, at the expense of considerably higher source z, (basically rp) and increased noise/microphonics due to the missing attenuation from the plate to line transformer.
Pretty much everyone around here (Northern New England Tube Group or NNETG as we are known) has built a 12B4 except me, and I have a/b'd many of these extensively and just don't need to go down that road.
I have heard one 12B4 pre-amp I really liked and it used a high transconductance pentode as the upper device in a dissimilar mu follower confiuration. Quite unusual, and the best sounding 12B4 design I have heard to date.
Dave,
I would strongly urge you to use a separate filament transformer when using DHTs. On my 1LE3 pre, which uses a ronan reg for LT, the amount of hum, ringing and hash that came through when using a common transformer was unbelievable.
Using a separate LT transformer has dropped the amount of garbage considerably, but it still quite audible - especially an amplified version of the high pitched buzzing sound that emanates periodically from transformers.
pm
I would strongly urge you to use a separate filament transformer when using DHTs. On my 1LE3 pre, which uses a ronan reg for LT, the amount of hum, ringing and hash that came through when using a common transformer was unbelievable.
Using a separate LT transformer has dropped the amount of garbage considerably, but it still quite audible - especially an amplified version of the high pitched buzzing sound that emanates periodically from transformers.
pm
More on microphonics -
I've had very good results with diff pairs in the line stage, which were mounted on a 4mm thick alu top plate - simple as that, just mass, no rubber damping.
I'm getting worse results with a SE setup in a normal type preamp case, 2U high with a 3mm alu bottom plate - seems to ring more, even when I put the tubes on a teflon sheet decoupled with rubber from the chassis. that did help, because it was worse before, but I'm going back to thinking you need mass. Like thick alu, granite, whatever. I'm starting to believe that the whole chassis must be quite dead - no ringing anywhere.
Otherwise the microphonics comes though the volume control, cables, all sorts of things. Anything that rings seems to set it off.
If anybody has got further than I have with this, do let me know. I was in the process of selling a line stage to a friend which we both agreed sounded great and I ended up putting it on a cushion and chucking another cushion on top of it (that silenced it!!). I took it back to the bench to solve the problem, so have to make some progress to make the sale. Andy
I've had very good results with diff pairs in the line stage, which were mounted on a 4mm thick alu top plate - simple as that, just mass, no rubber damping.
I'm getting worse results with a SE setup in a normal type preamp case, 2U high with a 3mm alu bottom plate - seems to ring more, even when I put the tubes on a teflon sheet decoupled with rubber from the chassis. that did help, because it was worse before, but I'm going back to thinking you need mass. Like thick alu, granite, whatever. I'm starting to believe that the whole chassis must be quite dead - no ringing anywhere.
Otherwise the microphonics comes though the volume control, cables, all sorts of things. Anything that rings seems to set it off.
If anybody has got further than I have with this, do let me know. I was in the process of selling a line stage to a friend which we both agreed sounded great and I ended up putting it on a cushion and chucking another cushion on top of it (that silenced it!!). I took it back to the bench to solve the problem, so have to make some progress to make the sale. Andy
My question re microphonics was actually meant to mean wheather there was anything one should do with a DHT as opposed to other tubes. In other contexts, to reduce microphonics, I have lined the inside of a chassis with dynamat which worked pretty well.
As for transformers, thoughts on these? http://jianshin.myweb.hinet.net/PRE_OUTPUT_TRANS.htm
As for transformers, thoughts on these? http://jianshin.myweb.hinet.net/PRE_OUTPUT_TRANS.htm
Hi Dave,
You probably want to look at the local James distributor's web site if you can find it. (I had it, but with the last crash some months ago lost all of my bookmarks.)
I assume you are talking about the 71A as a candidate here, in which case 5K is a good choice, however it is not clear to me whether these are SE or PP transformers. Unless building a PP pre you don't want a PP transformer as it is not designed to cope with the quiescent dc in an SE application.
Don't expect much voltage gain with a 71A and an output transformer as opposed to a choke. You will however get much lower output Z.
Heavy mass loaded chassis, possibly shock mounting the dhts, and power supply remotely located work well.
You probably want to look at the local James distributor's web site if you can find it. (I had it, but with the last crash some months ago lost all of my bookmarks.)
I assume you are talking about the 71A as a candidate here, in which case 5K is a good choice, however it is not clear to me whether these are SE or PP transformers. Unless building a PP pre you don't want a PP transformer as it is not designed to cope with the quiescent dc in an SE application.
Don't expect much voltage gain with a 71A and an output transformer as opposed to a choke. You will however get much lower output Z.
Heavy mass loaded chassis, possibly shock mounting the dhts, and power supply remotely located work well.
More on microphonics:
A friend just reminded me NEVER to go over the rated filament voltage, and in fact to stay 5 per cent at least under it. Reason being that DHTs can start to scream above their design filament voltage. I'll have to go back and check mine. He also thought it was possible that in diff pairs the difference in frequency of ringing might cancel out to some degree - certainly was less of a problem with diff pairs. All this is worth noting - you can have some nasty surprises with DHTs in line stages, like really crazy feedback so you have to quickly shut the amp off.
A friend just reminded me NEVER to go over the rated filament voltage, and in fact to stay 5 per cent at least under it. Reason being that DHTs can start to scream above their design filament voltage. I'll have to go back and check mine. He also thought it was possible that in diff pairs the difference in frequency of ringing might cancel out to some degree - certainly was less of a problem with diff pairs. All this is worth noting - you can have some nasty surprises with DHTs in line stages, like really crazy feedback so you have to quickly shut the amp off.
kevinkr said:
It's Doug by the way.
The plan is to try 71a with a plate load of either a ccs or a choke, or maybe just a resistor in the first go to see what happens. I am investigating transformers in case I decide I want to go that route with a different tube. And, for now, I have some edcor 10K:600 transformers that I can use to experiment. Just figured I'd do the reserach to figure out where to get thjem, how much they are, etc. so I can plan a bit.
Sherman said:For James transformers check out Euponia Audio, that's where I got mine!
I do have a pair of james transformers from euphonia for a headphone amp (still waiting for some parts to complete the assembly) but they don't seem to have the preamp type in stock. I have seen a few posts that James will begrudgingly send you some at a good price if you bug them enough, but otherwise I can't find any other distributors.
-d
EC8010 It must have been one of those days I guess, you are of course absolutely right Cag(1+mu)=Cmiller.. Ouch....
And Doug my apologies for calling you Dave, having been suitably chastened I won't do it again..
You might want to take a look at some of the Magnequest wares as I suspect by the time you factor in shipping by anything other than surface a pair of James transformers shipped from the Far East is going to be in the same price range. Mike's plate to line transformers are very good. (from experience)
And Doug my apologies for calling you Dave, having been suitably chastened I won't do it again..
You might want to take a look at some of the Magnequest wares as I suspect by the time you factor in shipping by anything other than surface a pair of James transformers shipped from the Far East is going to be in the same price range. Mike's plate to line transformers are very good. (from experience)
kevinkr said:
A lot of these tubes (30, 26, 01A at any rate) seem to have very similar operating points but different filament voltages (the datasheets all suggest 135V, a grid bias around 8-10V, and a plate current between 3 and 5.5mA). Is it reasonable to build an amplifier for one or another and (adjustng the filament supply) simply swap between them? Obviously I am getting ahead of myself here, but I am curious if there is some clever bit I am missing.
Doug
Well, since we’re getting picky about the definition of the Miller effect, the multiplier is really based on voltage gain, not mu, and it is 1-Av, where Av is the voltage gain (note that Av is negative for common-cathode stages). Only if the plate load is >>rp, AND if the cathode resistor is bypassed, will Av approach -mu. In those cases, 1-Av = 1+mu, as you said. Otherwise the magnitude of Av will be less than mu and would have to be calculated using the usual equations. For quick and dirty estimates of Miller capacitance, I agree that 1+mu is a conservative guess. I know that EC8010 and Kevinkr know this, but just to make sure the record is straight for others… And I’ve had nothing else to post this morning yet
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