Class A isn't a question of how the gain devices are hooked up, it's whether the signal is present at all times in all devices. This devolves into a question of bias current (influenced in your case by the voltage offset between the signal grid and the cathode).
In general, it's not a good idea to simply run the tube harder without careful consideration of the circuit. The two main things to watch out for are:
1) The plate dissipation. A 6550 has a plate dissipation of about 35W (plus or minus a few depending on the version of the tube and who manufactured it). This is the amount of heat the tube can dissipate before it turns cherry red and slips sideways into the next dimension.
2) The power supply. For monetary reasons, most people don't put any more power supply into a circuit than they have to. Since biasing a circuit to class A requires much more current from the power supply, the rail may collapse.
There are other, more subtle points to consider, such as the change in load to the output transformer (not that big a problem, but be aware of it), the cathode current limits, possible changes to the feedback, etc. These aren't really limiting factors, as they can generally be fiddled, but changing the operating class of an amplifier is not something to be done lightly.
Do you have schematics for the amp? If so, begin there. Determine how close the 6550s are to their limits now before going any further.
Grey
In general, it's not a good idea to simply run the tube harder without careful consideration of the circuit. The two main things to watch out for are:
1) The plate dissipation. A 6550 has a plate dissipation of about 35W (plus or minus a few depending on the version of the tube and who manufactured it). This is the amount of heat the tube can dissipate before it turns cherry red and slips sideways into the next dimension.
2) The power supply. For monetary reasons, most people don't put any more power supply into a circuit than they have to. Since biasing a circuit to class A requires much more current from the power supply, the rail may collapse.
There are other, more subtle points to consider, such as the change in load to the output transformer (not that big a problem, but be aware of it), the cathode current limits, possible changes to the feedback, etc. These aren't really limiting factors, as they can generally be fiddled, but changing the operating class of an amplifier is not something to be done lightly.
Do you have schematics for the amp? If so, begin there. Determine how close the 6550s are to their limits now before going any further.
Grey
Grey:
I kinda figured a look would have to be taken at the load line of the tube. However I am not comfortable enough with tube circuits YET to give any advice. That is soon to change as I am about to delve into one of Gabevees SE designs very shortly. I have been unmoved by the printed spec for sometime now and I have to see what the fuss is all about.
From what I understand, SE tube amps love simple and efficient speaker circuits and I am considering a Lowther driver in the Medallion II cabinet. Do you have a favorite design of choice I might look at?
I kinda figured a look would have to be taken at the load line of the tube. However I am not comfortable enough with tube circuits YET to give any advice. That is soon to change as I am about to delve into one of Gabevees SE designs very shortly. I have been unmoved by the printed spec for sometime now and I have to see what the fuss is all about.
From what I understand, SE tube amps love simple and efficient speaker circuits and I am considering a Lowther driver in the Medallion II cabinet. Do you have a favorite design of choice I might look at?
thank you folks !!!
these tubes are in a mcintosh mc 275 !
the cathode from the 2 output tubes are tied to a winding
from the output transformer (local feedback) & center tapped to
earth. (class ab1) mcintosh specs.
they are biased at -57 volts ( tried -50 volts having good results.
(just for curiosity i am trying different types of designs like)
1= placed a 50 ohm resistor on the center tap of the cathodes
windings to earth (called self bias) but tubes runs cooler !
on the scope i dont see the switching distortion in this mode.
they are already running in triode mode (can be swithced to tetrode mode by a flip of a switch).
2= the most aggresive change was eliminating the negative bias voltage & intead of 50 ohm resistor i placed a 350 ohm resistor
(cathode bias) sounds good but i think that i only have 15 watts
from 75 watts & runs really hot (sounds really good but no power)
now i am playing around with negative feedback or not.
the thing is that an amplifier on tetrode mode is very efficient
but low on information (resolution) !
on triode mode is less efficient but with a lot of definition.
with no feedback you dont have the dynamics (slamming effect)
butis more detailed
i am trying to do it by a flip of a switch too ! (complicated)
because now i completly removed the first tube & making a jumper between grid & plate & a 200 kohm resitior to earh(ground)
any information will be appreciated !!!
p.s. this dosnt mean that mcintosh are bad amps .
they are great sounding amps. but me like all elct. engineer
want more !
thank you richt
these tubes are in a mcintosh mc 275 !
the cathode from the 2 output tubes are tied to a winding
from the output transformer (local feedback) & center tapped to
earth. (class ab1) mcintosh specs.
they are biased at -57 volts ( tried -50 volts having good results.
(just for curiosity i am trying different types of designs like)
1= placed a 50 ohm resistor on the center tap of the cathodes
windings to earth (called self bias) but tubes runs cooler !
on the scope i dont see the switching distortion in this mode.
they are already running in triode mode (can be swithced to tetrode mode by a flip of a switch).
2= the most aggresive change was eliminating the negative bias voltage & intead of 50 ohm resistor i placed a 350 ohm resistor
(cathode bias) sounds good but i think that i only have 15 watts
from 75 watts & runs really hot (sounds really good but no power)
now i am playing around with negative feedback or not.
the thing is that an amplifier on tetrode mode is very efficient
but low on information (resolution) !
on triode mode is less efficient but with a lot of definition.
with no feedback you dont have the dynamics (slamming effect)
butis more detailed
i am trying to do it by a flip of a switch too ! (complicated)
because now i completly removed the first tube & making a jumper between grid & plate & a 200 kohm resitior to earh(ground)
any information will be appreciated !!!
p.s. this dosnt mean that mcintosh are bad amps .
they are great sounding amps. but me like all elct. engineer
want more !
thank you richt
Skippy,
I haven't messed with SE tube amps yet. The speakers I'm using aren't efficient enough to benefit from low power like that. I have some rather vague ideas for a speaker that may or may not pan out, but it's going to be a while before I get around to that. Until then, the smaller power amps aren't going to do me much good. I'll get there.
richt,
You would go and pick one of the most complicated tube amps there ever was...
<i>Sigh.</i>
I'm at work--don't have all my schematics with me--but as I recall, the MC-275 used the arrangement that Frank came up with where the output transformer had about sebenty-lebben taps on it, going every which a way. Oi! This is not going to be easy.
Honestly, I'd recommend building something from scratch rather than tear into a perfectly good Mac. (Pixie, you wanna hear <i>good</i> McIntosh, go find a MC-275 amp in good condition [not all of them are]. That was back when McIntosh was a serious contender...and I mean <i>serious.</i> Yes, it's tubes. You'll live though the experience. Note that McIntosh is back into tube design again.)
1) Yes, if you're cathode/self biasing (resistor under the cathode) the tube in addition to fixed bias (separate DC voltage applied to the grid) you'll get less bias, not more, unless you lessen the amount of fixed bias to compensate. You'll be moving further towards class B operation.
Keep in mind that if you start running 6550s in triode mode, you're going to <i>completely</i> change the characteristics of the tube. It won't mate with the ouput transformer as efficiently.
2) You're into two different things, here. One is the value of the resistor you use to bias the tube(s), meaning whether you've kept the same bias point, gone cooler (less bias) or hotter (more). I can't tell from here. The other point is that an un-bypassed cathode resistor gives you a truck-load of negative feedback, so your gain goes down the tubes (ahem...). Takes a heap of voltage swing from the driver stage to overcome that much NFB, and when it falls short, you simply don't get the power out that you're accustomed to. Try bypassing the cathode resistor with a couple hundred uF electrolytic and see if you don't get back most of the gain that you lost.
The most important suggestion I can make is to keep an eye on your rail voltage. Make sure you don't unload the rear end so much that the rail pops up and exceeds the voltage ratings of the main caps. Conversely, don't increase the bias so much that you draw too much current from the power supply and blow the power transformer. You will not have an easy time replacing those transformers if you destroy one. If you burn up a tube, those can be replaced, but it'll be easier on your wallet if you don't exceed the ratings in the first place.
Good luck.
Grey
I haven't messed with SE tube amps yet. The speakers I'm using aren't efficient enough to benefit from low power like that. I have some rather vague ideas for a speaker that may or may not pan out, but it's going to be a while before I get around to that. Until then, the smaller power amps aren't going to do me much good. I'll get there.
richt,
You would go and pick one of the most complicated tube amps there ever was...
<i>Sigh.</i>
I'm at work--don't have all my schematics with me--but as I recall, the MC-275 used the arrangement that Frank came up with where the output transformer had about sebenty-lebben taps on it, going every which a way. Oi! This is not going to be easy.
Honestly, I'd recommend building something from scratch rather than tear into a perfectly good Mac. (Pixie, you wanna hear <i>good</i> McIntosh, go find a MC-275 amp in good condition [not all of them are]. That was back when McIntosh was a serious contender...and I mean <i>serious.</i> Yes, it's tubes. You'll live though the experience. Note that McIntosh is back into tube design again.)
1) Yes, if you're cathode/self biasing (resistor under the cathode) the tube in addition to fixed bias (separate DC voltage applied to the grid) you'll get less bias, not more, unless you lessen the amount of fixed bias to compensate. You'll be moving further towards class B operation.
Keep in mind that if you start running 6550s in triode mode, you're going to <i>completely</i> change the characteristics of the tube. It won't mate with the ouput transformer as efficiently.
2) You're into two different things, here. One is the value of the resistor you use to bias the tube(s), meaning whether you've kept the same bias point, gone cooler (less bias) or hotter (more). I can't tell from here. The other point is that an un-bypassed cathode resistor gives you a truck-load of negative feedback, so your gain goes down the tubes (ahem...). Takes a heap of voltage swing from the driver stage to overcome that much NFB, and when it falls short, you simply don't get the power out that you're accustomed to. Try bypassing the cathode resistor with a couple hundred uF electrolytic and see if you don't get back most of the gain that you lost.
The most important suggestion I can make is to keep an eye on your rail voltage. Make sure you don't unload the rear end so much that the rail pops up and exceeds the voltage ratings of the main caps. Conversely, don't increase the bias so much that you draw too much current from the power supply and blow the power transformer. You will not have an easy time replacing those transformers if you destroy one. If you burn up a tube, those can be replaced, but it'll be easier on your wallet if you don't exceed the ratings in the first place.
Good luck.
Grey
Hello Grey, skippy, richt,
Grey,
it always is the 1st Watt of an amp that counts. With a speaker of 94dB/W/m eff. you should get tears of joy and goosebumps
Agreed, (sigh)
Here I got lost, Grey, maybe we both are meaning the same.
If a tube runs in fixed bias mode, the cathode/filament is grounded and the grid is pulled via a grid resisitor of some 10 or 100 KOhm or a grid choke to a negative voltage (called bias) referring to ground.
If a tube runs in self-biasing mode, the grid is pulled to ground via a grid resistor (or grid choke) of about 3 to 5 times the grid resistor in fixed bias mode and the quiescent current flowing through the cathode resistor (a few 100 Ohms) lifts the cathode/filament to a positive voltage referring to ground. The bias voltage referring to the cathode however still is negative.
If the tube has a combination of both, the voltage across the cathode resistor and the fixed bias negative voltage in reference to the cathode have to be added, overlayed. What let me get lost is that "less bias" means "less negative bias voltage" to most of my buddies and me. If I add fixed bias to self-bias, I get more bias and the resulting quiescent current is reduced. You say, the tube is pushed more to class B operation, to me this means the tube's operation point sits in the knee of the transfer characteristics. Yes, Grey, that fits, agreed.
From own experience, I would not see it as dramatic.
A pentode usually is run in near-to-short-circuit operation (load resistance is small compared to plate resistance, say, 10 to 15 %) and the load resistance has to be within close close tolerance in order to yield the maximum output power specified for a given THD: leaving this narrow optimum lewads to dramatic THD increase.
A Triode OTOH runs best with the opposite, the load being 2.3 to 3 times as big *** the tube's plate resistance. at this load the output power for a specified THD is maximum. But the maximum is not near as narrow. A 2A3 e.g. having 800 Ohm Rp is mostily used with an OPT's primary impeadance of 2-3 kOhm but also recommended with 3 or 3.5 kOhm.
Not a rule but a pleasant coincidence: most pentodes or tetrodes love a certain load impedance and happen to love the same load impedance wired as triode. With an EL34 and a KT66, I have used the same OPT for pentode and triode mode, no problems, power loss, deteriotated sonics. richt wrote his McIntosh
already is triode-wired so this would indicate the same.
I would feel comfortable to use the same trannie and just rewire the output tube. One other thing, Operation slightly moved to AB1 position should not deterioate sonics (provided there is a lil'bit of negative loop feedback) and the tubes live much longer. In a PP amp, I would not force the tubes into pure class A.
Exception: a purely differential output stage with a common CCS in the cathode.
Technically no objection, Grey. If you tell my which electrolytic to buy and where, I mean, an electro not messing up the sonics, I kiss your feet. (and a huge foil cap is as problematic simply due to its mere size; huge caps never sing).
This cap is a coupling cap in the ground circuit. In a PP output stage with a common cathode resistor (which is not a bad idea at all, provided way enough wattage !!) it becomes less dramatical as the coupling cap is a common mode cap then, but still clearly (messy) audible.
Better: hunt more gain in the input and driver stage and live with the un-bypassed cathode resistor. Much better. Maybe get rid of the nice but gainless concertina phase splitter (dunno if the amp uses one)
Even better (as you suggested, Grey): leave the amp as it is and build a new one from the scratch. Better for the long-term blood pressure
Good advice, very good advice!
And, ... sorry for the smart-assing, I thought, if I misunderstood you, Grey, then others may join me ... and better some of this than a fried amp.
Yawn !!!, ... need some sleep
Grey,
it always is the 1st Watt of an amp that counts. With a speaker of 94dB/W/m eff. you should get tears of joy and goosebumps
Agreed, (sigh)
Here I got lost, Grey, maybe we both are meaning the same.
If a tube runs in fixed bias mode, the cathode/filament is grounded and the grid is pulled via a grid resisitor of some 10 or 100 KOhm or a grid choke to a negative voltage (called bias) referring to ground.
If a tube runs in self-biasing mode, the grid is pulled to ground via a grid resistor (or grid choke) of about 3 to 5 times the grid resistor in fixed bias mode and the quiescent current flowing through the cathode resistor (a few 100 Ohms) lifts the cathode/filament to a positive voltage referring to ground. The bias voltage referring to the cathode however still is negative.
If the tube has a combination of both, the voltage across the cathode resistor and the fixed bias negative voltage in reference to the cathode have to be added, overlayed. What let me get lost is that "less bias" means "less negative bias voltage" to most of my buddies and me. If I add fixed bias to self-bias, I get more bias and the resulting quiescent current is reduced. You say, the tube is pushed more to class B operation, to me this means the tube's operation point sits in the knee of the transfer characteristics. Yes, Grey, that fits, agreed.
From own experience, I would not see it as dramatic.
A pentode usually is run in near-to-short-circuit operation (load resistance is small compared to plate resistance, say, 10 to 15 %) and the load resistance has to be within close close tolerance in order to yield the maximum output power specified for a given THD: leaving this narrow optimum lewads to dramatic THD increase.
A Triode OTOH runs best with the opposite, the load being 2.3 to 3 times as big *** the tube's plate resistance. at this load the output power for a specified THD is maximum. But the maximum is not near as narrow. A 2A3 e.g. having 800 Ohm Rp is mostily used with an OPT's primary impeadance of 2-3 kOhm but also recommended with 3 or 3.5 kOhm.
Not a rule but a pleasant coincidence: most pentodes or tetrodes love a certain load impedance and happen to love the same load impedance wired as triode. With an EL34 and a KT66, I have used the same OPT for pentode and triode mode, no problems, power loss, deteriotated sonics. richt wrote his McIntosh
already is triode-wired so this would indicate the same.
I would feel comfortable to use the same trannie and just rewire the output tube. One other thing, Operation slightly moved to AB1 position should not deterioate sonics (provided there is a lil'bit of negative loop feedback) and the tubes live much longer. In a PP amp, I would not force the tubes into pure class A.
Exception: a purely differential output stage with a common CCS in the cathode.
Technically no objection, Grey. If you tell my which electrolytic to buy and where, I mean, an electro not messing up the sonics, I kiss your feet.
(and a huge foil cap is as problematic simply due to its mere size; huge caps never sing).This cap is a coupling cap in the ground circuit. In a PP output stage with a common cathode resistor (which is not a bad idea at all, provided way enough wattage !!) it becomes less dramatical as the coupling cap is a common mode cap then, but still clearly (messy) audible.
Better: hunt more gain in the input and driver stage and live with the un-bypassed cathode resistor. Much better. Maybe get rid of the nice but gainless concertina phase splitter (dunno if the amp uses one)
Even better (as you suggested, Grey): leave the amp as it is and build a new one from the scratch. Better for the long-term blood pressure
Good advice, very good advice!
And, ... sorry for the smart-assing, I thought, if I misunderstood you, Grey, then others may join me ... and better some of this than a fried amp.
Yawn !!!, ... need some sleep
Bernhard is clearly trying to give me a run for my money in length of posts...
--I don't have any objection to efficient speakers, per se, just the use of horns which tends to lead to compromises that I'm not willing to make. Add that to the fact that I'm currently playing with line sources--which are rarely so efficient--and anything less than 30 to 50W won't fly for me. Yet. We'll see what happens later when I get time to play with speakers again.
--Bias, to a solid state person, nearly always means current. Bias, to a tube person, can mean either the negative voltage on the grid, relative to the cathode, or the current that flows through the tube, essentially the same way that a solid state person would look at things. Unfortunately, the two uses mean opposite things: less bias voltage (a smaller negative number, that is) at the grid means more bias current through the tube, conversely more bias voltage (a larger, more negative number) leads to less bias current. Sometimes I even confuse myself if I get to rushing headlong through these things. If I was unclear above, I apologize. If the original fixed bias (approximately negative 50-odd volts applied at the grid) was left alone, and a cathode resistor was added, then the combination of the two would lead to a net increase (a larger negative number...confused yet?) in bias voltage, which would lead to a decrease in (bias) current through the tube. It is possible to mate the two biasing schemes, but it requires a deft hand to achieve optimal results.
How's that? Did I manage to say it more clearly?
--The McIntosh MC-275 wasn't wired for triode use at the factory. It will have been modified for that somewhere along the line. My point here--triode vs. pentode--is that the McIntosh output concept, with all the extra taps coming off of the primary of the output transformer, lead to a particular output design. One I wouldn't tinker with lightly. Give me something like a Conrad Johnson (Ultralinear output) or an Audio Research (I believe they usually use straight plate-to-plate outputs), and I'll fiddle the thing until the cows come home. An old Dynaco...fire up the soldering iron, let's get wicked. But a Mac...uh unh. That amp walks a rather sharply defined knife edge, and I'd take it or leave it unless I was intending to use another output transformer. At which point, you no longer have a Mac.
--You don't need to kiss my feet. I wasn't suggesting that he live with an electrolytic bypass, just try it as an experiment to see if the gain came back. Not having a schematic handy, I don't remember how they handled the cathodes. If they're wired together (i.e. differential) then it wouldn't do much good. If the cathodes are separate, then it will make a huge difference in gain.
An electrolytic bypassed with a decent film cap is always a possibility, I suppose, but I prefer to go all film when I can afford the (admittedly) steep price.
It's more likely that folks will have an electrolytic in their junkbox than a large film cap. Solen has metalized polypropylene caps up to, I think, 330 uF (250V), somewhat smaller values at 400V or 630V. That would be the cap I would use in that application, perhaps bypassed with an MIT or Hovland or something. But...the chances of most people being able to pull one of those out of a hat are slim.
Note that using cathode bias will decrease available power anyway, since it will eat about 50 volts of the potential voltage swing. Fixed bias is a lot more efficient in that respect. If I'm remembering the front end of those amps, I don't think anyone will be able to find much in the way of unrealized gain given the design. A whole new front end would probably be required, which leads us back to the possibility of building an entirely new amp.
--Yeah, it'd be a pity to destroy a classic amp like that. There aren't that many of them hanging around. I wouldn't mind getting my hands on one...but would do no more than update the parts.
Grey
--I don't have any objection to efficient speakers, per se, just the use of horns which tends to lead to compromises that I'm not willing to make. Add that to the fact that I'm currently playing with line sources--which are rarely so efficient--and anything less than 30 to 50W won't fly for me. Yet. We'll see what happens later when I get time to play with speakers again.
--Bias, to a solid state person, nearly always means current. Bias, to a tube person, can mean either the negative voltage on the grid, relative to the cathode, or the current that flows through the tube, essentially the same way that a solid state person would look at things. Unfortunately, the two uses mean opposite things: less bias voltage (a smaller negative number, that is) at the grid means more bias current through the tube, conversely more bias voltage (a larger, more negative number) leads to less bias current. Sometimes I even confuse myself if I get to rushing headlong through these things. If I was unclear above, I apologize. If the original fixed bias (approximately negative 50-odd volts applied at the grid) was left alone, and a cathode resistor was added, then the combination of the two would lead to a net increase (a larger negative number...confused yet?) in bias voltage, which would lead to a decrease in (bias) current through the tube. It is possible to mate the two biasing schemes, but it requires a deft hand to achieve optimal results.
How's that? Did I manage to say it more clearly?
--The McIntosh MC-275 wasn't wired for triode use at the factory. It will have been modified for that somewhere along the line. My point here--triode vs. pentode--is that the McIntosh output concept, with all the extra taps coming off of the primary of the output transformer, lead to a particular output design. One I wouldn't tinker with lightly. Give me something like a Conrad Johnson (Ultralinear output) or an Audio Research (I believe they usually use straight plate-to-plate outputs), and I'll fiddle the thing until the cows come home. An old Dynaco...fire up the soldering iron, let's get wicked. But a Mac...uh unh. That amp walks a rather sharply defined knife edge, and I'd take it or leave it unless I was intending to use another output transformer. At which point, you no longer have a Mac.
--You don't need to kiss my feet. I wasn't suggesting that he live with an electrolytic bypass, just try it as an experiment to see if the gain came back. Not having a schematic handy, I don't remember how they handled the cathodes. If they're wired together (i.e. differential) then it wouldn't do much good. If the cathodes are separate, then it will make a huge difference in gain.
An electrolytic bypassed with a decent film cap is always a possibility, I suppose, but I prefer to go all film when I can afford the (admittedly) steep price.
It's more likely that folks will have an electrolytic in their junkbox than a large film cap. Solen has metalized polypropylene caps up to, I think, 330 uF (250V), somewhat smaller values at 400V or 630V. That would be the cap I would use in that application, perhaps bypassed with an MIT or Hovland or something. But...the chances of most people being able to pull one of those out of a hat are slim.
Note that using cathode bias will decrease available power anyway, since it will eat about 50 volts of the potential voltage swing. Fixed bias is a lot more efficient in that respect. If I'm remembering the front end of those amps, I don't think anyone will be able to find much in the way of unrealized gain given the design. A whole new front end would probably be required, which leads us back to the possibility of building an entirely new amp.
--Yeah, it'd be a pity to destroy a classic amp like that. There aren't that many of them hanging around. I wouldn't mind getting my hands on one...but would do no more than update the parts.
Grey
thank you very much folks for all of your replies.
i have mcintosh amps since 4 years ago.
they are a great sounding amp & solid as a brick wall, if
it says 75 watts ,you feel like you have a 150 watts amp !
the mcintosh topology is very simple ! you have to think that
this amp was made for multipurpose use like 70 volts/
600 ohms out (for p.a. systems) & lab reference amp.
thats why you see more winding in the output transformer than usual. i consider that transformer one of the best sounding
availabe today ! that why i choose to mod mines (i have 3 of them)
remember this ++++++++++++++++++++++++++++++++
in mcintosh designes there are 3 negative feedback !!!!!!!!!!!
1 is located between the 2 cathodes of output tube
(as the same design in audio research vt-100)
2 is located between the 12 bh7 driver & the grid of the 2 ouput
tubes ,plates are fed with another winding from out transf.
it was easy to eliminate this tube (12az7) & bypass it.
this circuit when was working creates the effect that the speakers
will eat you alive !!! (some kind of slamming effect)
very good for rap & dj.s (kidding)
3rd is the global feedback (as usual in all amps) output sample feed on the first tube.
(i designes a very simple circuit that witha flip of a switch you
have negative feedback or not without altering gain)
this is useful for a delicate piece of music you want to really feel !
as i told ithe first post i placed another switch to make it triode
or tetrode.
the wierd thing is depending of the music you want to hear
you can combine these 2 switches.
for very low levels of music i prefeer triode with no feedback !
the best choice but remember that you have only 40 % of power.
(audio research from a good friend , iwired it triode mode &
no global feedback obtaining grat results !) 6 months ago with no complains.
i am happy with my amps but i want to hear it class a on the
outputs.
i dont find good info for that. i only have a lot of power amp schematics that i studied a lot
thank you folks
schematica available upon request (fax only)
richt
i have mcintosh amps since 4 years ago.
they are a great sounding amp & solid as a brick wall, if
it says 75 watts ,you feel like you have a 150 watts amp !
the mcintosh topology is very simple ! you have to think that
this amp was made for multipurpose use like 70 volts/
600 ohms out (for p.a. systems) & lab reference amp.
thats why you see more winding in the output transformer than usual. i consider that transformer one of the best sounding
availabe today ! that why i choose to mod mines (i have 3 of them)
remember this ++++++++++++++++++++++++++++++++
in mcintosh designes there are 3 negative feedback !!!!!!!!!!!
1 is located between the 2 cathodes of output tube
(as the same design in audio research vt-100)
2 is located between the 12 bh7 driver & the grid of the 2 ouput
tubes ,plates are fed with another winding from out transf.
it was easy to eliminate this tube (12az7) & bypass it.
this circuit when was working creates the effect that the speakers
will eat you alive !!! (some kind of slamming effect)
very good for rap & dj.s (kidding)
3rd is the global feedback (as usual in all amps) output sample feed on the first tube.
(i designes a very simple circuit that witha flip of a switch you
have negative feedback or not without altering gain)
this is useful for a delicate piece of music you want to really feel !
as i told ithe first post i placed another switch to make it triode
or tetrode.
the wierd thing is depending of the music you want to hear
you can combine these 2 switches.
for very low levels of music i prefeer triode with no feedback !
the best choice but remember that you have only 40 % of power.
(audio research from a good friend , iwired it triode mode &
no global feedback obtaining grat results !) 6 months ago with no complains.
i am happy with my amps but i want to hear it class a on the
outputs.
i dont find good info for that. i only have a lot of power amp schematics that i studied a lot
thank you folks
schematica available upon request (fax only)
richt
[supergrin] just slips out ... not intended
but it certainly will provoke me to post lengthy again
I am fervently awaiting my Fertin FLB20EX drivers; they are meant to run in an open baffle and they yield 97dB/W/m. Enough for a 45 SET to satisfy with it. The open baffle speaker also is big but nothing compared to a horn.
I was pondering about horn speakers for quite a while but I gave up. I heard extraordinary horn systems; They had in common that the voice coils were vertically aligned to avoid phase mess. Which means that the tweeter horn has 7 cm, the midrange about 45 cm and the bass horn 2 meters or more. I do not have the space for that. However, Tom Danley's Unity horn sounds very nice and is an intelligent approach to the phase thing. Of course, it needs a closed or vented enclosure for the bass.
Using simple tube amps all over and even trying to make them behave like XOs, I refuse to repair the phase mess with a digital XO most horn users are using nowadays.
Another issue are compression distortions. I heard three horn speakers so far not having much of it and few long-term listening fatigue is to be expected: The Unity horn, a German horn speaker from Visaton with a vented bass enclosure and the best, Peter Bahnsen's Maximator, a 3way tractrix horn. The other horns were exciting on the 1st run but fatiguing on the long run ..compression distortion.
Interestingly, the German horn speaker as well as the Bahnsen horn were most pleasant; both are intentionally reduced in efficiency. Hence less compression distortion.
OTOH, the Unity is not and with consequences, but its compression tweeter can be used as weapon substitute with a SET 71A yielding 700mW. See the guys raving about the 71A at the SET Asylum.
L'Audiophile, Paris, once suggested a line source called Totem. It was using 8 samples of the Fostex FE103Sigma; efficiency was in the 100 dB/W/m range and a buddy reported bleeding ears and shocking dynamics with a SET 300B.
Yes, great! Learned new stuff.
Warning, yours truly is a non-native speaker, has English as 2nd hobby
and prefers it to his mother tongue when describing technical stuff and causal relations. English is waaay tauter than German. I use to write my tech specs and my technical notes in English and feel depressed about the result when I have to do it in German ROTFLMAO! added to the repertoire
Agreed. When I read richt's post about his Mac, I get the impression he is atleast as good informed about the amp as we two together, Grey
. However, I do not think a normal PP amp has to run in pure class A. From own experience with an EL34 PP amp a friend of mine designed decreasing the bias to a class A operationg point does not lead to improved sonics, but certainly to improved tube wear. Richt, you are your own bad luck's blacksmith
do what you cannot avoid phew, was hoping you would say that
Sorry for assuming you would do. Sorry also for pecking on the differential issue, got another lesson from my friend Manfred this evening about self-bias, fixed bias and self-bias with common cathode resistor which is right in between concerning SOA. Safest operation is with self-bias over two separated cathode resistors.
However, sonically the differential operation with common cathode resistor is very good; if the resisitor is replaced by a CCS, it is PP heaven.
Hell, how could I get Manfred to post here, this would be tube university. The odd thing: he loves this forum and reads it, just generally loathes to post.
Thanks, had forgotten that.
Caps: electros are a NoNo in my book except for the heater, as far as tube amps are concerned. The friend who designed the EL34 PP mentioned above found out that in tube amps electrolytics are responsible for the sonic changes during warm-up. Once the electros were gone, no warm-up changes anymore, the amp was on top 2 minutes after switch-on.
Foil caps: try out Siemens/Epcos MKV. And ... don't make me responsible afterwards for consequences on your budget
They ain' cheap, they are high-tech. But not insanely priced either. I hope the MIT are better than the Hovlands although reports from buddies tell they are not; cannot tell haven't tried them out myself. But I have A/B-ed the Hovland against the MKV. Forgetabout the Hovland. It is a question of quality, not of taste. (Bernhard, stop crusading!!)Bernhard,
--You're trying to make me thirsty. Maximator is also the name of a tasty doppelbock, though not my favorite. (I prefer Spaten Optimator.)
--With the exception of my subs, all segments of my current speakers are single-driver line sources. I won't take up space here with all that; it's described elsewhere.
--Your English is better than my German...don't worry about it.
--I've been thinking about rebuilding my tube amps ever since I finished the first pair of Alephs. There are two reasons for this. One is that I've rethought some of the basic building blocks that I used the first time around. The other is that the Alephs are nipping at the heels of my tube amps in sound quality...for about 1/4 the parts cost. The tube circuit wins out in detail retrieval and imaging, but only by a small margin. In all other categories, it's pretty evenly matched.
Hmmm.
Once I get a few other issues settled out, I'm going to tear the tubes down and see what I can do to widen the lead.
Grey
--You're trying to make me thirsty. Maximator is also the name of a tasty doppelbock, though not my favorite. (I prefer Spaten Optimator.)
--With the exception of my subs, all segments of my current speakers are single-driver line sources. I won't take up space here with all that; it's described elsewhere.
--Your English is better than my German...don't worry about it.
--I've been thinking about rebuilding my tube amps ever since I finished the first pair of Alephs. There are two reasons for this. One is that I've rethought some of the basic building blocks that I used the first time around. The other is that the Alephs are nipping at the heels of my tube amps in sound quality...for about 1/4 the parts cost. The tube circuit wins out in detail retrieval and imaging, but only by a small margin. In all other categories, it's pretty evenly matched.
Hmmm.
Once I get a few other issues settled out, I'm going to tear the tubes down and see what I can do to widen the lead.
Grey
Cannot refuse commenting that
I was brewer before I became engineer and so I feel somehow qualified to crusade around ... AFAIR the Maximator is from Augustiner Bräu. The Spaten-Optimator has more in common with most bad doppelbock beers and so does the Augustiner. Few breweries resist the temptation to brew a thick, syrup-like fluid with faint rests of carbon dioxide in it and tasting like malt cough-drops. What a pity, market restrictions usual for large quantity lager beers don't apply here (DON'T ever change the taste the customer expects!!!) so the brewmaster could produce a real fancy stuff.
Best doppelbocks come from small unknown breweries. Two exceptions Paulaner (Salvator) and Löwenbrau (Triumphator) do not stick to the cough drop syrup, their dopplebocks are lean and fresh and worth to be called a beer. I pretty much prefer the Löwenbräu Triumphator.
And then there is the fancy Weihnachtsbock from Mahrsbräu Bamberg.
Mahrsbräu's brewmaster makes one lot each Xmas, not more.
It is a bock beer tasting like the finest Pilsener beer you can dream of.
With this beer the brewmaster doe not look on the money; it is meant as a Xmas gift to his customers. He avoids anything usually done to sqeeze the last penny out the raw materials. And he looks for usual brewing mistakes, uses a special procedure, only hops used is flavour hops etc.
..... Result is a heavy but light-tasting bright beerwhich has about the quality of a good Bordeaux wine; just about the same body; you take a tiny sip and have your mouth full. Fantastic! This brewmaster is proving what quality level is possible with a beer!
Thank you! As my father uses to pray: sloppy thinking causes sloppy speaking and inverse!
Salvator I've had, and like. In fact, I find it rather similar in taste to Optimator, which leads me to wonder if there are 'export' versions of those brews that are different from the domestic versions. Or perhaps they suffer in shipment.
Note that it's impossible to get doppelbocks here in SC. The Southern Baptists believe them to be bottled sin (along with barley wines, Belgian trippels etc.). Anything above 6% (I believe that's the limit) is illegal. Yes, you can buy wine, port, sherry, Scotch...you expect logic from these people? At any rate, I have to pick up higher alcohol beers on trips to other states, and the selection is catch as catch can.
And now we've managed to shift far enough from the topic that even I am feeling guilty.
Grey
Note that it's impossible to get doppelbocks here in SC. The Southern Baptists believe them to be bottled sin (along with barley wines, Belgian trippels etc.). Anything above 6% (I believe that's the limit) is illegal. Yes, you can buy wine, port, sherry, Scotch...you expect logic from these people? At any rate, I have to pick up higher alcohol beers on trips to other states, and the selection is catch as catch can.
And now we've managed to shift far enough from the topic that even I am feeling guilty.
Grey
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