After sarching around for some time, just gathering info, I've decided to do something tubewise for myself.
I have been reparing /servicing tube eqpt. for many years, so I don't expect any serious problems from a strictly practical point of view. For the time being, I'm not in for a lot of experiments. I want to make something with a decent result, for starters, and as "therapy" from other chores. The experiments will have to come later, when I have more time. I've decided for something around 40-60 W or so.. don't need much more.....
I found this one, which looked interesting
http://www.jogis-roehrenbude.de/Leserbriefe/Roessler-Amp/Bauanleitung.htm
which is closely related to this one
http://www.jogis-roehrenbude.de/Verstaerker/GU50-Grommes/GU50.htm
I think I'll stick to EL/KT tubes tho' - mainly for reasons of easy supply.
But then there's a lot of them out there, ---lots of various topologies. As for OPTs, Lundahl and Hammond are within easy reach..
The output part is fairly easy, but what about the input / splitter part?? Concertina, CCS etc. etc.
Any thoughts about better/worse, characteristics...??
My mains experiences with tube amps, are mostly applications where linearity and distortion are not major factors....
I have been reparing /servicing tube eqpt. for many years, so I don't expect any serious problems from a strictly practical point of view. For the time being, I'm not in for a lot of experiments. I want to make something with a decent result, for starters, and as "therapy" from other chores. The experiments will have to come later, when I have more time. I've decided for something around 40-60 W or so.. don't need much more.....
I found this one, which looked interesting
http://www.jogis-roehrenbude.de/Leserbriefe/Roessler-Amp/Bauanleitung.htm
which is closely related to this one
http://www.jogis-roehrenbude.de/Verstaerker/GU50-Grommes/GU50.htm
I think I'll stick to EL/KT tubes tho' - mainly for reasons of easy supply.
But then there's a lot of them out there, ---lots of various topologies. As for OPTs, Lundahl and Hammond are within easy reach..
The output part is fairly easy, but what about the input / splitter part?? Concertina, CCS etc. etc.
Any thoughts about better/worse, characteristics...??
My mains experiences with tube amps, are mostly applications where linearity and distortion are not major factors....
It's an adaption of a good 1960 circuit, but the 6SN7 is not a great candidate for cascoding. These days we can do better, especially regarding the balance of the phase splitter.
For a KT-series tube, you'll need on the order of 30-40VRMS drive, which can be best achieved by cascaded constant-current-tail diff amps, or something like a Williamson.
If you're adventurous and open-minded, try using a well-wound 1: 1+1 input transformer to get the signal balanced right from the get-go, then use diff amps the rest of the way through.
For a KT-series tube, you'll need on the order of 30-40VRMS drive, which can be best achieved by cascaded constant-current-tail diff amps, or something like a Williamson.
If you're adventurous and open-minded, try using a well-wound 1: 1+1 input transformer to get the signal balanced right from the get-go, then use diff amps the rest of the way through.
Just one thing bothering me in the given GU50 amplifier circuit: Surely R12 and R13 cannot be only 22K? That will give a highly unsatisfactory load to the phase inverter. I would believe that to be a misprint for 220K, but such a value would somewhat exceed the maximum specified for fixed bias operation of the power tubes.
Perhaps phase splitter anode loads of 47K/56K working into 100K grid resistors? Mmmmmm...
Perhaps phase splitter anode loads of 47K/56K working into 100K grid resistors? Mmmmmm...
This GU50 is evidently some russian / east German pentode type, which I will not use, mainly for supply reasons, - even if it might be good. I will most certainly stick to some El or KT, - or something very close.
Edit: BTW- the referred german article says tteh GU50 is russian copy of af a "Wehrmacht" type LS50, also known as a former East German tube SRS 552......
Even if there are a few variations, like straight pentode, UL, triode strap etc, the actual output stage is fairly straight forward. Some issues about choosing the correct anode impedance load, perhaps.
The difficulties seems to be in choosing the input/splitter circuit, and this is where I seek the advice. A few months back, I felt like going for a classical Dynaco Mk-III, which I know from a rather famous scandinavian PA setup from way-back-when, but surely something must have happened in the tube world in 40 years???
Any possible links to proven circuits that are reputably better than the Mk-III ??
Edit: BTW- the referred german article says tteh GU50 is russian copy of af a "Wehrmacht" type LS50, also known as a former East German tube SRS 552......
Even if there are a few variations, like straight pentode, UL, triode strap etc, the actual output stage is fairly straight forward. Some issues about choosing the correct anode impedance load, perhaps.
The difficulties seems to be in choosing the input/splitter circuit, and this is where I seek the advice. A few months back, I felt like going for a classical Dynaco Mk-III, which I know from a rather famous scandinavian PA setup from way-back-when, but surely something must have happened in the tube world in 40 years???
Any possible links to proven circuits that are reputably better than the Mk-III ??
Johan Potgieter said:Ray,
So what is that doing in the reference of the Grommes 260 or whatever? And I cannot even blame it on my knowledge of German!
I am now retiring to bed (it is about 2:00 in the am here) and shall take a stiff brandy to try drown my inferiority feelings.
Yours liquorly.
Somehow I think the Grommes topology inspired the GU amp. And I kind'a liked the looks of the Grommes ..- I see alu and rack mount boxes every day, and I am totally fed up with them!!
Mahogany and brass will be my choice of materials when I start building!!
Come to think of it---Does anybody know anything about these Grommes amps???
Yours liquorly.....
( I just found some Scotch in my innermost cabinet corner
)AuroraB,
Just before drowning the sorrows inflicted by Ray, I cannot now access a particular circuit, but I have been using a pentode input directly coupled to a Schmitt (long-tailed pair), much like the given circuit, for years (decades) now. As you said, there are no particular outstanding circuits, but I found that very reliable and capable of low distortion, though you mentioned that that was not too important.
I hesitate to tout my own circuits around, but could put something on here in the morning that I have used. But others will probably oblige with references; there are many very suitable circuits in this category.
Regards.
Just before drowning the sorrows inflicted by Ray, I cannot now access a particular circuit, but I have been using a pentode input directly coupled to a Schmitt (long-tailed pair), much like the given circuit, for years (decades) now. As you said, there are no particular outstanding circuits, but I found that very reliable and capable of low distortion, though you mentioned that that was not too important.
I hesitate to tout my own circuits around, but could put something on here in the morning that I have used. But others will probably oblige with references; there are many very suitable circuits in this category.
Regards.
Johan..
Maybe I've left myself open to rather severe misunderstandings_
Distortion and linearity, quality of sound is of course of MAJOR importance to me!!
What I meant, was that my previous experience with tube equipment, were in applications where these factors are of minor importance, - like speech and background music distribution, tube transmitters, instrumentation gear etc. etc.
I've built SS amps for over 30 years, but never done any audio with tubes.
Maybe I've left myself open to rather severe misunderstandings_
Distortion and linearity, quality of sound is of course of MAJOR importance to me!!
What I meant, was that my previous experience with tube equipment, were in applications where these factors are of minor importance, - like speech and background music distribution, tube transmitters, instrumentation gear etc. etc.
I've built SS amps for over 30 years, but never done any audio with tubes.
Another tack is to "clone" the Harman/Kardon Citation II circuit.
I own a Cit. 2 and it sounds GOOD. Costly HIGH performance O/P trafos are essential. The Plitron PAT4142 might be up to the task.
I own a Cit. 2 and it sounds GOOD. Costly HIGH performance O/P trafos are essential. The Plitron PAT4142 might be up to the task.
The Citation II interests me too, but I think it might be a "hard act to follow". Even H-K never built another amp on that design and neither did any other manufacturer AFAIK.
I think SY's idea of an all-diff amp is good. I have one like that (first stage is LTP splitter) and it's totally hum-free (I call it my Humphrey model). However, it uses triode-strapped EL34s, which don't need much NFB. It might be a bit of a reach to get enough open loop voltage gain to be able to apply sufficient NFB for pentode-mode. Modeling with LTSpice shows it can be done, though, achieving a damping factor of 10 and sensitivity of 750mv RMS for 50 watts output. That's with three NFB loops: one local balanced loop, one 2-stage cross-coupled balanced loop and one global loop.
Of course, if I put a transformer splitter on the front as SY says, than I wouldn't have to use the first stage to split the phase and I'd get twice the voltage gain, making it much easier. I don't have such a tranny, although I believe there is a very good Lundahl model for that purpose. I suppose a concertina splitter could be added on the front instead of a tranny to achieve the same thing, but that would be a real shame because I would lose all that lovely PSRR I get now.
I think SY's idea of an all-diff amp is good. I have one like that (first stage is LTP splitter) and it's totally hum-free (I call it my Humphrey model). However, it uses triode-strapped EL34s, which don't need much NFB. It might be a bit of a reach to get enough open loop voltage gain to be able to apply sufficient NFB for pentode-mode. Modeling with LTSpice shows it can be done, though, achieving a damping factor of 10 and sensitivity of 750mv RMS for 50 watts output. That's with three NFB loops: one local balanced loop, one 2-stage cross-coupled balanced loop and one global loop.
Of course, if I put a transformer splitter on the front as SY says, than I wouldn't have to use the first stage to split the phase and I'd get twice the voltage gain, making it much easier. I don't have such a tranny, although I believe there is a very good Lundahl model for that purpose. I suppose a concertina splitter could be added on the front instead of a tranny to achieve the same thing, but that would be a real shame because I would lose all that lovely PSRR I get now.
Something like Jensen 11P1 or the Cinemag equivalent can also be dragooned into service as an input phase splitter (with concomitant CMR) by putting two matched resistors in series across the secondary and grounding their junction.
If we cascade 6SL7 and 6SN7 diff amps (an old and honored pairing), we can get a driver stage gain of 400 pretty easily. That leaves us roughly 20dB of excess gain which we can use for feedback. As you point out, Ray, that can be apportioned into several loops.
If we cascade 6SL7 and 6SN7 diff amps (an old and honored pairing), we can get a driver stage gain of 400 pretty easily. That leaves us roughly 20dB of excess gain which we can use for feedback. As you point out, Ray, that can be apportioned into several loops.
What bothers me about an all-diff amp is that it might be too much of a good thing. The 6SL7 cascaded with 6SN7 that you mention is what I use. It's a very easy design to get working and, as I said before, is very quiet. However, all the harmonics are odd - no second, no fourth, etc., but plenty of third, fifth, seventh, ninth, etc. (talking about pentode OP tubes here).
I suspect a bit of even order harmonic distortion in the mix might sound more natural - some 'sugar to sweeten the bitter medicine' of odd harmonics. There may be something to be said, therefore, for a single (unbalanced) voltage amp at the front end, as in Mullard and Williamson for instance.
I suspect a bit of even order harmonic distortion in the mix might sound more natural - some 'sugar to sweeten the bitter medicine' of odd harmonics. There may be something to be said, therefore, for a single (unbalanced) voltage amp at the front end, as in Mullard and Williamson for instance.
The PP-1C Svein_B referred to, is one likely candidate
http://www.vacuumstate.com/schematics/pp-1c_s.gif
I also found this, one by Pete Millet - has the extra iron as input splitter, but also has switchable UL-triode mode ( How useful is that really?? I have seen it a few other places too...)
http://www.pmillett.com/push-pull_kt88_class_a_amp_with_universal_driver_pcb.htm
This one, by Claus Byrith, is on the Lundahl web. Very well explained and a thoroughly described project
http://www.lundahl.se/claus_b.html
This one, the TS 5000 by Rene Cariou, looks alot like the Byrith, but also shows that the Byrith can be given some more "soup" !!
http://home.tele2.fr/boutailh-tan/ampli/ampli.htm
And of course, the main item from the opening post:
http://www.jogis-roehrenbude.de/Leserbriefe/Roessler-Amp/Bauanleitung.htm
AARGHHH - the agony of choice......
Anyone care to comment ?????
http://www.vacuumstate.com/schematics/pp-1c_s.gif
I also found this, one by Pete Millet - has the extra iron as input splitter, but also has switchable UL-triode mode ( How useful is that really?? I have seen it a few other places too...)
http://www.pmillett.com/push-pull_kt88_class_a_amp_with_universal_driver_pcb.htm
This one, by Claus Byrith, is on the Lundahl web. Very well explained and a thoroughly described project
http://www.lundahl.se/claus_b.html
This one, the TS 5000 by Rene Cariou, looks alot like the Byrith, but also shows that the Byrith can be given some more "soup" !!
http://home.tele2.fr/boutailh-tan/ampli/ampli.htm
And of course, the main item from the opening post:
http://www.jogis-roehrenbude.de/Leserbriefe/Roessler-Amp/Bauanleitung.htm
AARGHHH - the agony of choice......
Anyone care to comment ?????
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