Earlier this month I posted a few pics of a sweep tube SEP I designed.
There were one or two things about it's design that I felt were not quite right so it has been revised to great effect.
It was first built with Alexandar Kitic's philosophies about SEPs in mind but I found it lacking.
I redid it using more established practices and the improvement in sound quality was staggering.
It really ia a great sounding bit of kit now, fast, full blooded and powerful.
I've had a few email enquiries from people on here but don't have the time to deal with them so I decided to post the
circuit in the main forum where those interested can discuss it, point and laugh, talk about compatible alternative Russian tubes...whatever.
If the thread sinks without trace then so be it, but at least those interested will have the latest schematic to work from, and play with as they see fit
I'm very happy with it but as always there's bound to be room for improvement.
Here it is then:
Steve
There were one or two things about it's design that I felt were not quite right so it has been revised to great effect.
It was first built with Alexandar Kitic's philosophies about SEPs in mind but I found it lacking.
I redid it using more established practices and the improvement in sound quality was staggering.
It really ia a great sounding bit of kit now, fast, full blooded and powerful.
I've had a few email enquiries from people on here but don't have the time to deal with them so I decided to post the
circuit in the main forum where those interested can discuss it, point and laugh, talk about compatible alternative Russian tubes...whatever.
If the thread sinks without trace then so be it, but at least those interested will have the latest schematic to work from, and play with as they see fit
I'm very happy with it but as always there's bound to be room for improvement.
Here it is then:
An externally hosted image should be here but it was not working when we last tested it.
Steve
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Hmmmm
There is a major flaw on that design.
The Pentode driver stage has an unbypassed cathode resistor and thus strong feedback which increases its output resistance. So, the output resistance is completely determinded by the 47K resistor.
The power stage is directly feedbacked to its grid. This parallel derived and parallel applied feedback reduces the input resistance of the PL509.
Because of that overloading, lots of distortion is produced. I couldn't quite believe that myself, but switched to a parallel-derived-series-applied (here)and it sounded much better. Measurement was just with a simple soundcard, but distortion improvement was clearly visible.
Don't get me wrong, but you're using the feedback in exactly the wrong way
You're increasing the output resistance of the driver and decreasing the input resistance of the power stage.
regards, simon
There is a major flaw on that design.
The Pentode driver stage has an unbypassed cathode resistor and thus strong feedback which increases its output resistance. So, the output resistance is completely determinded by the 47K resistor.
The power stage is directly feedbacked to its grid. This parallel derived and parallel applied feedback reduces the input resistance of the PL509.
Because of that overloading, lots of distortion is produced. I couldn't quite believe that myself, but switched to a parallel-derived-series-applied (here)and it sounded much better. Measurement was just with a simple soundcard, but distortion improvement was clearly visible.
Don't get me wrong, but you're using the feedback in exactly the wrong way
You're increasing the output resistance of the driver and decreasing the input resistance of the power stage.
regards, simon
Hmmmm
There is a major flaw on that design.
The Pentode driver stage has an unbypassed cathode resistor and thus strong feedback which increases its output resistance. So, the output resistance is completely determinded by the 47K resistor.
The power stage is directly feedbacked to its grid. This parallel derived and parallel applied feedback reduces the input resistance of the PL509.
Because of that overloading, lots of distortion is produced. I couldn't quite believe that myself, but switched to a parallel-derived-series-applied (here)and it sounded much better. Measurement was just with a simple soundcard, but distortion improvement was clearly visible.
Don't get me wrong, but you're using the feedback in exactly the wrong way
You're increasing the output resistance of the driver and decreasing the input resistance of the power stage.
regards, simon
Yep That's how it's supposed to work I know what I'm doing
There is no "major flaw"
It's working exactly as intended
I don't intend to spend time arguing to justify the design philosophy.
Siffice it to say that it is the whole amp and how the two stages interact
with each other that determines the final sound quality
This can't be judged on how individual stages work in isolation.
Steve
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The bass is absolutely superb from this amplifier, powerful detailed, tight and clean throughout the range.
Huge soundstage with plenty of depth and width and image location.
It'll drive a wide range of speakers but as with most tube designs the more senitive the speaker the better.
Mine drives an open baffle setup.
Treble is sweet, clean and detailed... lovely cymbal sound.
Plenty of presence in the midrange without being too forward.
Hardness is not an issue with this amp.
It has two local feedback loops
The output stage has the venerable Schade plate-to-grid feedback scheme
and the input stage has an unbypassed cathode resistor. (on purpose
)
But you don't see such a configuration on any good commercial amplifiers and that has a reason.
The 6AU6 sees two anode resistors in parallel. The 47k and the Rfb. Since the rfb is in the feedback loop, the driver sees this resistor reduced by the factor of µ of the PL509 ! This kind of feedback only works well when used in very small amounts. Your 6AU6 sees a very low dynamical anode resistance which causes more distortion than the feedback should compensate. Just read any book about feedback, it's the same whether tube or solid state.
Of course you can analyze each stage and with the laws of feedback the tube's parameters changes.
I'm sure this amp produces music and works, but I just wanted to point out that there are better ways.
Yes Simon and this argument has been done to death in this forum quite a few times, but the truth remains, Schade feedback or if you prefer, the anode follower configuration, when used with unbypassed pentodes (not triodes!) in the front end
produces arguably one of the finest sounds it is possible to get from a single ended pentode amplifier.
You're absolutely correct in what you say, it is well known that Schade feedback schemes if not implemented properly will cause a huge increase in second harmonic distortion from the driver stage. The value of the feedback resistor is critical to the distortion performance of these amps, but when done right, there is little to better them in the SEP universe.
If I seemed a little terse it was the grinning green smiley that was annoying about your post, easily taken the wrong way given the statement that preceded it.
There is a discussion going on right now about pentode drivers for the Kitic RH84 SEP using EL84s, you might want to look at.
You might not agree with what they are discussing, but plenty of people like the sound produced by pentode driven anode follower power amps
so it is a perfectly viable method that has been around since the Thirties. and incidentally I've read plenty of stuff on feedback, tried plenty of schemes and to my ears this one sounds best.
YMMV and only IMHO of course.
Steve
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Ah, I didn't know that this configuration had such a mass appeal
I've heard the name "Schade" a few times but didn't really pay attention about what has been discussed there.
Since that time I saw the distortion spectrum I wasn't really interested anymore.
Maybe I should give it another try. Thanks for your good explanation.
Regards, Simon
I've heard the name "Schade" a few times but didn't really pay attention about what has been discussed there.
Since that time I saw the distortion spectrum I wasn't really interested anymore.
Maybe I should give it another try. Thanks for your good explanation.
Regards, Simon
Thanks Steve, I feel 2 mono blocks coming on. This is exactly the sort of thing I've been contemplating. I have a very small stereo amp, which has EF86 driving 6CU6 (I *think*) producing a watt or two, which I adore. Uses similar topology without the exotic supply.
Hi Doz,
A bit of additional info.
The mains transformers were these:
Valve Transformer VT1042V
and the chokes were from the same source:
Valve Transformer Choke VT1133
All UK made stuff and excellent value for money.
I know Mike personally, he's a nice guy, who sells good stuff.
Steve
A bit of additional info.
The mains transformers were these:
Valve Transformer VT1042V
and the chokes were from the same source:
Valve Transformer Choke VT1133
All UK made stuff and excellent value for money.
I know Mike personally, he's a nice guy, who sells good stuff.
Steve
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Ah, I didn't know that this configuration had such a mass appeal
I've heard the name "Schade" a few times but didn't really pay attention about what has been discussed there.
Since that time I saw the distortion spectrum I wasn't really interested anymore.
Maybe I should give it another try. Thanks for your good explanation.
Regards, Simon
Cheers Simon.
The bass is absolutely superb from this amplifier, powerful detailed, tight and clean throughout the range.
Huge soundstage with plenty of depth and width and image location.
It'll drive a wide range of speakers but as with most tube designs the more senitive the speaker the better.
Mine drives an open baffle setup.
Treble is sweet, clean and detailed... lovely cymbal sound.
Plenty of presence in the midrange without being too forward.
Hardness is not an issue with this amp.
It has two local feedback loops
The output stage has the venerable Schade plate-to-grid feedback scheme
and the input stage has an unbypassed cathode resistor. (on purpose
That’s just cheating,
You never told us about the feedback.
Set your trap for unsuspecting souls..LOL
Hiding it with Rfb <<stealth resistors now is it!
Anyway where is the grommet?<<<<don't tell me it's in plain sight stealth mode as well!
Regards
M. Gregg
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Well with single-ended pentodes, you have a simple choice....feedback or bleeding ears
Steve
Hi Wavebourn
I would have loved to have built these amps with 2K5 OPTs so I could have run a higher HT, around 450V @ around 80mA giving 30W plate dissipation would have been my preferred voltage with 150V on the screen but due to budget restrictions, I had to use iron I had to hand. The only expense with these amps were the two aluminum chassis at £10 each and an extra mains transformer.
It works very well despite it not quite being the amp I would have liked.
I am lending my RH84 to a friend over the Summer and after I get it back may be able to use its James 6123HS TXs on the 2K5 tap,providing I don't exceed their 90mA DC rating.
but then the whole thing would need a redesign.
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Excellent!
I discussed the possibilities of using Schade feedback in an PL519 SEP amp with forum member reVintage a couple of years ago but that project was never built. Good to see that the concept works well IRL.
I have some large Hammond OPTs (1,9k PP and 1,25k SE) and a large box full of PL519s. Together with a huge surplus 1kVA 220VAC + 33VAC + 9VAC toroid I can see some interesting solutions to drive the low and midbass sections of my 4-way horn system...
(I bought all those parts to build a 4 channel cathode follower amp but Schaded pentodes may or may not be a better way to go)
I discussed the possibilities of using Schade feedback in an PL519 SEP amp with forum member reVintage a couple of years ago but that project was never built. Good to see that the concept works well IRL.
I have some large Hammond OPTs (1,9k PP and 1,25k SE) and a large box full of PL519s. Together with a huge surplus 1kVA 220VAC + 33VAC + 9VAC toroid I can see some interesting solutions to drive the low and midbass sections of my 4-way horn system...
(I bought all those parts to build a 4 channel cathode follower amp but Schaded pentodes may or may not be a better way to go)
OK
here's an alternative feedback scheme that works well in this amp.
The secondary of the output transformer has been included in a feedback loop that goes via the cathode bypass cap of the PL509.
The cathode resistor is left grounded so that DC offset is kept out of the speaker voice coils.
Disadvantages might be that a resonant circuit could be set up between the bypass cap and the inductance of the secondary winding,
producing a response peak in the audio band. My amps seem fine, but it's worth checking out.
Also if a valve gets broken whilst the amp is switched on, the HT could blow straight through the bypass cap, causing high DC voltage
to appear across the speaker terminals, if the cathode happens to be crushed against the plate structure. Extremely unlikely but best pointed out just in case.
Whether Schade or cathode feedback is used, the amps sound terrific.
The Schade is a tad sharper up top than the cathode feedback. Whether this matters or not depends on the sound balance of your speakers.
here's an alternative feedback scheme that works well in this amp.
An externally hosted image should be here but it was not working when we last tested it.
The secondary of the output transformer has been included in a feedback loop that goes via the cathode bypass cap of the PL509.
The cathode resistor is left grounded so that DC offset is kept out of the speaker voice coils.
Disadvantages might be that a resonant circuit could be set up between the bypass cap and the inductance of the secondary winding,
producing a response peak in the audio band. My amps seem fine, but it's worth checking out.
Also if a valve gets broken whilst the amp is switched on, the HT could blow straight through the bypass cap, causing high DC voltage
to appear across the speaker terminals, if the cathode happens to be crushed against the plate structure. Extremely unlikely but best pointed out just in case.
Whether Schade or cathode feedback is used, the amps sound terrific.
The Schade is a tad sharper up top than the cathode feedback. Whether this matters or not depends on the sound balance of your speakers.
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