I just picked up a few 6c33c-b tubes. As I only need a watt or 2
I was wondering if the white cathode follower would be any good
here with the bias set at 250ma. And what would the output impedience be without feedback?
Thanks for all the feedback
Woody
I was wondering if the white cathode follower would be any good
here with the bias set at 250ma. And what would the output impedience be without feedback?
Thanks for all the feedback
Woody
Hi,
You probably mean global feedback?
The WCF is 100% local feedback loop already, I guesstimate a pair of 6C33-Cs as a WCF to have a Zout in the order of 10 to 15 ohm depending on operating conditions.
Add a global loop of say 18-20 db, I guesstimate once agin here,
and you should arrive at a Zout of about .1 ohm for a 8 ohm load.
With same quantity of output tube you can build a higher power OTL in a PP configuration without the need for a coupling cap at the output as well.
Mind you, this could be done with SE configuration as well but it'll be trickier to get rid of the cap and I can't guarantee it will be failsafe either.
Cheers, 😉
You probably mean global feedback?
The WCF is 100% local feedback loop already, I guesstimate a pair of 6C33-Cs as a WCF to have a Zout in the order of 10 to 15 ohm depending on operating conditions.
Add a global loop of say 18-20 db, I guesstimate once agin here,
and you should arrive at a Zout of about .1 ohm for a 8 ohm load.
With same quantity of output tube you can build a higher power OTL in a PP configuration without the need for a coupling cap at the output as well.
Mind you, this could be done with SE configuration as well but it'll be trickier to get rid of the cap and I can't guarantee it will be failsafe either.
Cheers, 😉
Hi,
Hard to tell by just looking at it but I quite like the differential input.
I do have some small reservation about the 12BH7A SRPP stage to drive 6C33-Cs.
It would be nice to see how it would sound with regulated input and driver stages.
The bias is clamped by zeners, IMHO if you regulate one side you better do the rest as well but that's just my view on things.
In my own amps I use a pair of 12BH7As as well and I'm considering changing it to the ECC99 which is a better valve.
Actually with some further work the Ciuffoli OTL can be turned into a very good OTL; it would be nice to get rid of the E182CC (it's really becoming hard to come by) and use individual bias for all output tube for example.
BTW, I'm not an expert on anything...Just an amateur like most of us.😱
Cheers,😉
Hard to tell by just looking at it but I quite like the differential input.
I do have some small reservation about the 12BH7A SRPP stage to drive 6C33-Cs.
It would be nice to see how it would sound with regulated input and driver stages.
The bias is clamped by zeners, IMHO if you regulate one side you better do the rest as well but that's just my view on things.
In my own amps I use a pair of 12BH7As as well and I'm considering changing it to the ECC99 which is a better valve.
Actually with some further work the Ciuffoli OTL can be turned into a very good OTL; it would be nice to get rid of the E182CC (it's really becoming hard to come by) and use individual bias for all output tube for example.
BTW, I'm not an expert on anything...Just an amateur like most of us.😱
Cheers,😉
Hi,
I use a 1986 Croft Series II that I modded, Multicap RTX coupling caps, better PS caps, silver wiring, Vishay and Holco resistors etc.
For me this amp does everything I ask from it in that it's extremely fast and ultimately transparent to what it's being fed.
It's detailed without throwing detail in your lap, you can tell the mic capsules apart if you want to and it will tell you what the recording venue sounds like.
Another thing I really like about it is that it's always absolutely dead quiet, no hum, no buzzes no hissing and apart from the valves everything runs really cool.
IOW, it's the chameleon I always looked for in an amplifier.
Here's Ken Kesslers' assessment of the monster and while mine sports a huge 1.5 KVA EI transformer, the layout of the valves is identical to the one in the picture.
The speakers I use with these are modded SD Acoustics SD1s which I bought together with the amps at the 1987 Penta Show from Steen Doessing and Glenn Croft.
While the rest of the system evolved over the past years, the amps and speakers still serve as a reference.
If you want a more forgiving OTL then the Futtermans, Aitos may be more to your liking. To me these sound too sweet, mid-hall, to be true of the recording but with a little work you can make them just like mine anyway....
For anything called OTL the trick is to get the PS right, all else will follow...A bit of an overstatement perhaps but there definetely is a relationship between the two.
So, there you have my inner soul laid bare...
CROFT SERIES II OTL.
Cheers,😉
I use a 1986 Croft Series II that I modded, Multicap RTX coupling caps, better PS caps, silver wiring, Vishay and Holco resistors etc.
For me this amp does everything I ask from it in that it's extremely fast and ultimately transparent to what it's being fed.
It's detailed without throwing detail in your lap, you can tell the mic capsules apart if you want to and it will tell you what the recording venue sounds like.
Another thing I really like about it is that it's always absolutely dead quiet, no hum, no buzzes no hissing and apart from the valves everything runs really cool.
IOW, it's the chameleon I always looked for in an amplifier.
Here's Ken Kesslers' assessment of the monster and while mine sports a huge 1.5 KVA EI transformer, the layout of the valves is identical to the one in the picture.
The speakers I use with these are modded SD Acoustics SD1s which I bought together with the amps at the 1987 Penta Show from Steen Doessing and Glenn Croft.
While the rest of the system evolved over the past years, the amps and speakers still serve as a reference.
If you want a more forgiving OTL then the Futtermans, Aitos may be more to your liking. To me these sound too sweet, mid-hall, to be true of the recording but with a little work you can make them just like mine anyway....
For anything called OTL the trick is to get the PS right, all else will follow...A bit of an overstatement perhaps but there definetely is a relationship between the two.
So, there you have my inner soul laid bare...
CROFT SERIES II OTL.
Cheers,😉
Frank,
You certainly got our attention with the Crofts...
Since you mention the importance of PS and possible regulation of the driver stages of the amp (like Rozenblit's design), would you suggest a way to improve on Ciuffoli in this respect?😕
You certainly got our attention with the Crofts...
Since you mention the importance of PS and possible regulation of the driver stages of the amp (like Rozenblit's design), would you suggest a way to improve on Ciuffoli in this respect?😕
Hi,
OA1 I don't know, but in this case OA2/150C2 is a glowdischarge tube much like a zener diode.
Cheers,😉
OA1 I don't know, but in this case OA2/150C2 is a glowdischarge tube much like a zener diode.
Cheers,😉
Hi -
This might sound a bit strange, since I've designed my own DC coupled OTL amplifier (and I mean really DC coupled input to the output - it'll put out a DC voltage indefinitely into a load if DC is presented at the input - its photo is on page 10 of the DIYaudio projects section) - I've never heard or even seen another OTL besides the ones I've designed and built, so my comments on OTL sound apply only to my own design, which by the way, has a very high damping factor.
It is extremely neutral across the whole audio range (the best I've heard from any amp) very incisive sounding and capable of super inner detail and great micro- to macro- dynamics but without any noticeable intrinsic grain and will throw super deep and wide soundstages if they're on the source material, or sound hazy, rather indistinct and flat as a wall with most digitally sourced material (from CD or LP), with maybe the slightest touch of euphony and quite dynamic sounding - if the output stage is underbiased (in terms of idle current), even a bit overly dynamic. The reason for that appears to be that the underbiased 6AS7's voltage gain increases slightly as they draw more current on waveform peaks. If I made their grid bias less negative, I could eliminate this characteristic as well as reducing full power distortion, but I consciously opted for a bit less heat and longer tube life.
One thing interesting about this OTL is that, into a resistive load, it clips just like a transistor amp as seen on a scope, but due to its high voltage swing capability, it sounds much cleaner into real world loads when driven into mild clipping than its 50-60 watts a channel would indicate. In fact, presented with a 16 ohm or over load, it sounds power-wise more like 200 solid state watts a side.
I don't know how sensitive the Croft really is to missing loads and inputs, etc., but I've always pretty much turned my OTL on and off like any other amp with or without a source or load. If there's no output load, it will momentarily swing 100V upon turnon (apparently dc feedthrough through a resistor from the driver stage before the output stage biases up), but that is reduced to less than a volt into anything under 50 ohms and there's no thumps on either turn on or turn off. One good thing about the dc loop feedback is that it doubles as a speaker protection circuit. If there's an output stage tube short, for instance, the feedback correction signal will blow either or both output rail fuses in its effort to compensate which has worked flawlessly at protecting speakers connected to the OTL for sixteen years so far.
Oops. I wanted to type a little more about the sound of my OTL. It's all triode nature is readily evident in its sound with a distinctness between inner voices through the midband that resembles that of my recapped Lux MB3045's that are also all triode and fully balanced & something that doesn't come across with my Stereo 70, although it also has all polypropylene capacitors in the signal path, as does the modified Lux and my DC coupled OTL. The OTL, unlike the Lux, maintains these sonic characteristics straight across the entire audible frequency range with a higher level of overall tonal neutrality, in addition.
Hi,
That's exactly how it works in the Croft as well.
It doesn't seem to care whether or not something is connected to either end and unlike most OTLs I've seen there's not even a load resistor across the speaker terminals.
In my case, as bias is derived from the main rail, everything in the outputstage ramps up simultaneously and that hasn't caused any problem so far.
What I did notice over the years of use is that when they haven't seen any service for a couple of months they tend to pop a fuse or two before_ I presume_ the big caps are reformed again.
Other than that I find them as reliable as I can possibly wish for.
In fact looking back at some service I did on big name tube amps in the past I can safely say they're very user friendly and pretty foolproof.
Cheers,😉
P.S. Do you use Russian 6AS7Gs or 6080s in the amp?
That's exactly how it works in the Croft as well.
It doesn't seem to care whether or not something is connected to either end and unlike most OTLs I've seen there's not even a load resistor across the speaker terminals.
In my case, as bias is derived from the main rail, everything in the outputstage ramps up simultaneously and that hasn't caused any problem so far.
What I did notice over the years of use is that when they haven't seen any service for a couple of months they tend to pop a fuse or two before_ I presume_ the big caps are reformed again.
Other than that I find them as reliable as I can possibly wish for.
In fact looking back at some service I did on big name tube amps in the past I can safely say they're very user friendly and pretty foolproof.
Cheers,😉
P.S. Do you use Russian 6AS7Gs or 6080s in the amp?
Hi, Fdegrove -
That's good to hear about your Croft. I actually have an odd mix of Russian 6AS7G's along with some RCA NOS in this amp. Not that they sound the same as each other. To me the Russian tubes sound a little 'cool' and provide a bit less power but never had problems with arcover. The RCAs sound a little 'fatter' with a little more power output but occasionally one would arc over once when first put in the amp, taking out a rail fuse. Then it would be ok afterwards. In a way, the Russian and RCA 6AS7G's sort of complement each other sonically in this amp. But when I took the OTL off the shelf a few months ago after it was sitting idle for a couple years, I slapped in mostly fresh RCAs to replace a few tubes that had become mechanically damaged during my last move, powered it up and it played with no problems. For some reason, perhaps because I have the supply rails set a little high, between 160-170V, 6080's haven't seemed to work out well with this amp. As I recall, the ones I've tried kept arcing over which is strange because I've had almost no similar problems with the 6AS7Gs which I thought had the same basic internal structure
That's good to hear about your Croft. I actually have an odd mix of Russian 6AS7G's along with some RCA NOS in this amp. Not that they sound the same as each other. To me the Russian tubes sound a little 'cool' and provide a bit less power but never had problems with arcover. The RCAs sound a little 'fatter' with a little more power output but occasionally one would arc over once when first put in the amp, taking out a rail fuse. Then it would be ok afterwards. In a way, the Russian and RCA 6AS7G's sort of complement each other sonically in this amp. But when I took the OTL off the shelf a few months ago after it was sitting idle for a couple years, I slapped in mostly fresh RCAs to replace a few tubes that had become mechanically damaged during my last move, powered it up and it played with no problems. For some reason, perhaps because I have the supply rails set a little high, between 160-170V, 6080's haven't seemed to work out well with this amp. As I recall, the ones I've tried kept arcing over which is strange because I've had almost no similar problems with the 6AS7Gs which I thought had the same basic internal structure
OTLs and the amp-speaker interface...
One of the main problems with OTL amps, and this applies to all the OTL topologies I'm familiar with, is the need to parallel several sections of output tubes in order to obtain any sginificant output power. This in turn leads to potential difficulty in driving the grids of these parallel output devices due to Miller capacitances. Back in the golden days of tube hi-fi, it wasn't uncommon to see OTL amps driving speakers with impeadances of more than 100ohms, which tended to mitigate the problems noted above. The Philips design posted to this thread, which was designed to drive an 800-1000ohm speaker is a perfect example. Not only does the higher impeadance eliminate the need for paralleling several output devices, it also means that in those designs that require an output cap, the output cap can be much smaller, making it much easier/affordable to use a higher quality cap on the output.
Of course, in today's market we can't buy commercial speakers with more than 16ohms impeadance, and there are very few that are more than 8ohms. Even if we DIY, drivers of more than 16ohm impeadance are basically non-existent unless we dive into the vintage market. However, that doesn't necessarily mean we can't build high impeadance speakers, we just have to be creative.
So, how do we build high impeadance speakers? Well, one simple approach is a line array. A line array built with 16 8ohm drivers will have an impeadance of 128ohms, a significant step up from the traditional 8ohm speaker. Of course, if it's a multi-way line array, one would have to use multiple high frequency drivers as well in order to maintain the higher impeadance across the entire frequency range. Once one goes beyond about 150ohms speaker impeadance, off the shelf inductors of large enough values to build crossovers become an issue, but that's not too much of a problem as they can easily be put in series, and the DCR of the inductors becomes trivial compared to the high impeadance of the speaker system. The caps used in the crossover get smaller at the system impeadance is raised, which makes that part of the crossover even easier.
So, has anyone tried this combination? I've got (in storage right now) a pair of full-range open baffle line arrays with 18 4" 8ohm drivers/side, which would make 144ohms with all 18 in series. My current plan is to build a small, simple SE OTL with a single 6336B section per channel, which should be able to drive 3-4 watts into 144ohms, which should be plenty for these speakers...
Will let you know how things turn out...
Peace
One of the main problems with OTL amps, and this applies to all the OTL topologies I'm familiar with, is the need to parallel several sections of output tubes in order to obtain any sginificant output power. This in turn leads to potential difficulty in driving the grids of these parallel output devices due to Miller capacitances. Back in the golden days of tube hi-fi, it wasn't uncommon to see OTL amps driving speakers with impeadances of more than 100ohms, which tended to mitigate the problems noted above. The Philips design posted to this thread, which was designed to drive an 800-1000ohm speaker is a perfect example. Not only does the higher impeadance eliminate the need for paralleling several output devices, it also means that in those designs that require an output cap, the output cap can be much smaller, making it much easier/affordable to use a higher quality cap on the output.
Of course, in today's market we can't buy commercial speakers with more than 16ohms impeadance, and there are very few that are more than 8ohms. Even if we DIY, drivers of more than 16ohm impeadance are basically non-existent unless we dive into the vintage market. However, that doesn't necessarily mean we can't build high impeadance speakers, we just have to be creative.
So, how do we build high impeadance speakers? Well, one simple approach is a line array. A line array built with 16 8ohm drivers will have an impeadance of 128ohms, a significant step up from the traditional 8ohm speaker. Of course, if it's a multi-way line array, one would have to use multiple high frequency drivers as well in order to maintain the higher impeadance across the entire frequency range. Once one goes beyond about 150ohms speaker impeadance, off the shelf inductors of large enough values to build crossovers become an issue, but that's not too much of a problem as they can easily be put in series, and the DCR of the inductors becomes trivial compared to the high impeadance of the speaker system. The caps used in the crossover get smaller at the system impeadance is raised, which makes that part of the crossover even easier.
So, has anyone tried this combination? I've got (in storage right now) a pair of full-range open baffle line arrays with 18 4" 8ohm drivers/side, which would make 144ohms with all 18 in series. My current plan is to build a small, simple SE OTL with a single 6336B section per channel, which should be able to drive 3-4 watts into 144ohms, which should be plenty for these speakers...
Will let you know how things turn out...
Peace
Hi,
The only comparison I've done so far was between the 6080 and US made 6AS7G (mainly RCA, Chatham) and I completely agree with your findings.
That's exactly like mine but that never caused any concern with the 6080s. However I do find it's wise to use tubes from the same manufacturer to prevent runaway due to too high dispersion.
For the past five years I've been using TFKs that were made by GE in the Eighties. These seem to be very good and I like their ceramic basing which seems to reduce stray capacitance a little and keeps them just that little bit cooler.
Experience convinced me of the contrary, they don't sound the same and don't behave in the same way either.
They are obviously interchangeable for the intended service but as far as audio use goes I'd like to find out more about the inner details.
Hi Roscoe,
Apart from the high asking price, I can't say my past experience with the 6336A/B was positive.
With the advent of the much cheaper 6C33-C the 6336s have mostly been forgotten about and maybe some bargains could be picked up though.
Have you considered using the 6C33-C or even the smaller 6C41-C for your project?
Cheers,😉
The only comparison I've done so far was between the 6080 and US made 6AS7G (mainly RCA, Chatham) and I completely agree with your findings.
That's exactly like mine but that never caused any concern with the 6080s. However I do find it's wise to use tubes from the same manufacturer to prevent runaway due to too high dispersion.
For the past five years I've been using TFKs that were made by GE in the Eighties. These seem to be very good and I like their ceramic basing which seems to reduce stray capacitance a little and keeps them just that little bit cooler.
Experience convinced me of the contrary, they don't sound the same and don't behave in the same way either.
They are obviously interchangeable for the intended service but as far as audio use goes I'd like to find out more about the inner details.
Hi Roscoe,
Apart from the high asking price, I can't say my past experience with the 6336A/B was positive.
With the advent of the much cheaper 6C33-C the 6336s have mostly been forgotten about and maybe some bargains could be picked up though.
Have you considered using the 6C33-C or even the smaller 6C41-C for your project?
Cheers,😉
Ciuffoli 8.2 OCL
Frank,
If you dont mind I have two question on my project of the Ciuffoli OTL OCL:
1. How does one measure the 200ma which is supposedly the optimum current for the 6c33cs? where do i have to place the ammeter probes?🙁
2. What changes do I need to implement on the circuit to get rid of the e182cc which is a scarce tube and one cannot find it easily nowadays. Perhaps the ptimum replacement would be the 12au7 or other. Do I need to change the values of any resistors?
(i owe you two bits of free legal advice!!)😀
Frank,
If you dont mind I have two question on my project of the Ciuffoli OTL OCL:
1. How does one measure the 200ma which is supposedly the optimum current for the 6c33cs? where do i have to place the ammeter probes?🙁
2. What changes do I need to implement on the circuit to get rid of the e182cc which is a scarce tube and one cannot find it easily nowadays. Perhaps the ptimum replacement would be the 12au7 or other. Do I need to change the values of any resistors?
(i owe you two bits of free legal advice!!)😀
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