RH807 with a pentode driver?
- Thread starter dnewma04
- Start date
Well, now that someone's dug this old nugget up again...
Dave, if you're still looking for a Loktal pentode driver, I would now suggest maybe looking for some 7AD7 or 7AK7 tubes. 300K plate impedance and 9500u transconductance on the 7AD7, and 11K (!) plate impedance and 5500u transconductance for the 7AK7.
I haven't found gain figures for those, but boy, they at least have DRIVE...
Regards,
Gordon.
Dave, if you're still looking for a Loktal pentode driver, I would now suggest maybe looking for some 7AD7 or 7AK7 tubes. 300K plate impedance and 9500u transconductance on the 7AD7, and 11K (!) plate impedance and 5500u transconductance for the 7AK7.
I haven't found gain figures for those, but boy, they at least have DRIVE...
Regards,
Gordon.
dnewma04
The Healer
Interesting tubes, Gordon. I hadn't found those in my travels. I do have a quad of loctal drivers with lots of drive (C3g). I have the outputs ready to go, I will buy an Antek power transformer and some nice chokes to run a choke input PSU. The only thing I haven't had is time.
Gordon, did I PM or email you about some Ohm drivers that I need to have rebuilt?
Gordon, did I PM or email you about some Ohm drivers that I need to have rebuilt?
I'm not that good at flirting, so that would obviously be a style to avoid...
When it comes to the hair blow thing... I must admit being bald: I would not recommend imitating that one either.
But when it comes to tubes and unusual solutions...
About the "Schade issue":
I have finally found the time to read the text "Beam Power Tubes" by Schade. Maybe there is some other text as well, but with all due respect for Mr. Schade's accomplishments and his wonderful text giving an unprecedented insight in beam power tubes (but it's the buzzword of their time, is it not? no wonder they could give such insights...) -- I must say that I do not see a direct correlation between the RH amps and his proposed voltage feedback.
Among other reasons, because there are no caps in the feedback loop of RH amps... and it is not voltage feedback... but the calculations he makes are very interesting.
Maybe there is some other text by Schade showing anode to anode feedback? Otherwise, the only anode to anode feedback I' ve seen so far is as in "RCA Hi-Fi amplifiers", but it's for push-pull, and a different approach definitely.
I have finally found the time to read the text "Beam Power Tubes" by Schade. Maybe there is some other text as well, but with all due respect for Mr. Schade's accomplishments and his wonderful text giving an unprecedented insight in beam power tubes (but it's the buzzword of their time, is it not? no wonder they could give such insights...) -- I must say that I do not see a direct correlation between the RH amps and his proposed voltage feedback.
Among other reasons, because there are no caps in the feedback loop of RH amps... and it is not voltage feedback... but the calculations he makes are very interesting.
Maybe there is some other text by Schade showing anode to anode feedback? Otherwise, the only anode to anode feedback I' ve seen so far is as in "RCA Hi-Fi amplifiers", but it's for push-pull, and a different approach definitely.
thefragger
Certified Crazy.
rage
Well-Known Member
I refer those who are interested in the origin of "Schade Feedback" to look here, at Schade's original article from March 1938:
http://www.pmillett.com/tubebooks/tubedata/beam_power_tubes.pdf
I refer you to pages 176-180. In that, it talks about EXACTLY this sort of plate-to-grid feedback, and gives the equations for calculating reduction of distortion by its usage in various ratios...
The topology looks different, with a transformer-coupled circuit in the last of his three diagrams- but other than the insertion of coupling capacitors (AC shorts, in essence- just there to block DC) and coupling transformers (the same principles would apply to resistance-coupled circuits, as transformer coupled, naturally), it's exactly the same methodology, as we've seen used in many places, including here with the RH-type amp circuits, Micah's (Bricktop's) modification to the Heathkit W4 posted here on this site, and many other occasions...
Regards,
Gordon.
http://www.pmillett.com/tubebooks/tubedata/beam_power_tubes.pdf
I refer you to pages 176-180. In that, it talks about EXACTLY this sort of plate-to-grid feedback, and gives the equations for calculating reduction of distortion by its usage in various ratios...
The topology looks different, with a transformer-coupled circuit in the last of his three diagrams- but other than the insertion of coupling capacitors (AC shorts, in essence- just there to block DC) and coupling transformers (the same principles would apply to resistance-coupled circuits, as transformer coupled, naturally), it's exactly the same methodology, as we've seen used in many places, including here with the RH-type amp circuits, Micah's (Bricktop's) modification to the Heathkit W4 posted here on this site, and many other occasions...
Regards,
Gordon.
What is being called "anode-to-anode feedback", in essence, IS still a form of anode-to-grid feedback. Note the direct connection to the output tube grid in the RH-type circuits, once you go through the output coupling capacitor (only there to block DC, in essence). There is still a path, straight from the feedback line, to the output tube grid.
As for resistor placement- all that is happening, is that the resistor in the feedback loop, the plate impedance of the driver tube, and the plate resistor in the driver circuit, are forming a voltage-divider for the feedback (in essence, the plate impedance of the driver tube and the plate resistor are in parallel, as far as the voltage-dividing effect on the feedback resistor are concerned)- that is, changing the ratio from unity, to some fraction less than unity. That technique of voltage-dividing the feedback is directly discussed in Schade's article I posted the link for, above. It's exactly what Schade was advocating, only done in a resistance-coupled circuit rather than his own example using a transformer-coupled circuit...
Regards,
Gordon.
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Kegger
R.I.P. 1/12/1966 - 6/1/2017
Douglas/Pakprotector an Pete Millet use the same basic schade ideas as well in there E-Linear circuits.
(just for another reference if anyone wants to see more about the use of plate to plate/grid feedback)
Yes it is generally connected plate to plate, but as you point out Gordon so you don't have to "add in"
another cap as this way it's already there, but yes in essence it's Plate to Grid feedback or you would
get no feedback if this wasn't the case.
I've also seen this referenced as "partial or shunt feedback" by some of the guy's who work on the old
table top radios as apparently it was used quite often there as well in some schade type form there to.
That's all just to say it's been around for a while an has many practical uses and ways to implement it.
(I like any usage of feedback that gets around going through the output tranny an looping back front)
Short path feedback is certainly what I prefer to run, that is when it's practical to implement anyway.
(just for another reference if anyone wants to see more about the use of plate to plate/grid feedback)
Yes it is generally connected plate to plate, but as you point out Gordon so you don't have to "add in"
another cap as this way it's already there, but yes in essence it's Plate to Grid feedback or you would
get no feedback if this wasn't the case.
I've also seen this referenced as "partial or shunt feedback" by some of the guy's who work on the old
table top radios as apparently it was used quite often there as well in some schade type form there to.
That's all just to say it's been around for a while an has many practical uses and ways to implement it.
(I like any usage of feedback that gets around going through the output tranny an looping back front)
Short path feedback is certainly what I prefer to run, that is when it's practical to implement anyway.
If you ask me, there is a considerable difference -- but sometimes I wonder whether people are asking me at all: most prefer their own conclusions, educated or not.
Schade was advocating voltage feedback, that is what the caps are doing in his circuits. In order to simulate that kind of feedback, you would have to add a cap in series with the Rfb resistor in RH amps. The driver does actually draw current through that resistor, you know
Even more so in the 2nd generation RH amps, where all the current for the driver tube is drawn through resistor Rfb (which in that case is as well the anode resistor).The coupling cap is a different story, as it allows for proper biasing of the output tube (this is not a direct coupled amp). Therefore it should be excluded from the feedback path in the sense that the feedback is not between the output tube and the output tube, but between the output tube and the driver tube! I hope this is simple enough as to be seen and understood by anyone, no matter the preparation or training?!
You know, I am an economist and was not trained in electronics (self taught). I imagine people who studied electronics should know this stuff better than I do (which unfortunately sometimes is not true).
Nevertheless, this does nothing to deduct from the fact that I did not know about the applications of others when I came to that idea, therefore it is mine. I never implied I invented the feedback: it would be like inventing hot water -- someone must have gotten to that in all those years!
What I am trying to say is that there is no "evidence" that Schade applied (or described) the same feedback type as used in RH amps. In case he did not, we cannot call it "Schade feedback" or attribute it to Schade...
How did it go, in the end?
Trying to be more "on-topic", how did it go, eventually? This was about building an RH807, about some "not so good results obtained" - probably applying a different driver tube :nono: ? But I have the impression that in the end the amp was not built at all?!
This leads me to recall how some time ago a friend was given some 1622s and 1619s and contemplated building a new RH807 using some of these tubes. I pushed him towards the 1619, which is basically an 807 with direct heating, on octal socket. In the end, the only problem he had was very loud hum, until he switched to heating the tubes with DC... the overall result was very interesting once the hum problem was overcome. Of course, the driver tube remained the ECC81.
I am contemplating a slightly different alternative to that, redesigning it for 307A tubes (306A being virtually extinct) -- which is something I promised several people a long time ago. By now, I do have some 307A tubes, but am still trying to find some sockets and caps... anyone interested in building an RH307A (and yeah, the driver tube will most probably remain ECC81/12AT7 :thmbsp?
Trying to be more "on-topic", how did it go, eventually? This was about building an RH807, about some "not so good results obtained" - probably applying a different driver tube :nono: ? But I have the impression that in the end the amp was not built at all?!
This leads me to recall how some time ago a friend was given some 1622s and 1619s and contemplated building a new RH807 using some of these tubes. I pushed him towards the 1619, which is basically an 807 with direct heating, on octal socket. In the end, the only problem he had was very loud hum, until he switched to heating the tubes with DC... the overall result was very interesting once the hum problem was overcome. Of course, the driver tube remained the ECC81.
I am contemplating a slightly different alternative to that, redesigning it for 307A tubes (306A being virtually extinct) -- which is something I promised several people a long time ago. By now, I do have some 307A tubes, but am still trying to find some sockets and caps... anyone interested in building an RH307A (and yeah, the driver tube will most probably remain ECC81/12AT7 :thmbsp
?
Schade was advocating voltage feedback, that is what the caps are doing in his circuits. In order to simulate that kind of feedback, you would have to add a cap in series with the Rfb resistor in RH amps. The driver does actually draw current through that resistor, you know
I will grant this, readily enough. There is a loading effect on the driver tube, that's for sure. In essence, it's like a combination of a Schade feedback, with current feedback/"bootstrapping", a la Stu Hegeman. It is, however, the exact same thing that was done in several RCA amps in the 1950s, as well as a couple of organ amps from that same era, that I've seen, though the brand and model escapes my memory right now. They also had a direct resistor connection between the plate of the output tube, to the plate of the driver tube...
The cap carries ALL the AC signal (what we care about, in terms of sound output- bias, as alluded to above, is only a "housekeeping function", in essence, required to set the idle condition of the output tube, and has no influence on the actual signal, once that condition is set) to the output tube, in essence. So, if there is feedback to the anode of the driver tube, that feedback ALSO impresses upon the grid of the output tube. At least that part of the effect, IS anode-grid feedback...
It's a VARIATION on the principle, but the fact remains- the first person to promote this principle, was Schade... and Hegeman, IIRC, as I mentioned above...
Regards,
Gordon.
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Who is Stu Hegeman? (I beg your pardon, but I really do not know...) What is worse, not knowing who Schade and Hegeman are/were -- or being negated the originality of the idea because you did not know that someone had already come to that? Or maybe, the worst is that while none of the (unknown) "inventors" had the complete solution you came up with, you are still negated the originality of the solution?
Yes, of course, otherwise it would not be a feedback loop.
What I was trying to say is that Schade's feedback was exclusively voltage and basically shown as the tube looping itself (no additional elements where shown, apart from a transformer secondary, which is not an active element). He actually wrote explanations about unity and sub-unity feedback, concentrating on the output tube around which the feedback loop was drawn... Therefore, his theory misses the driver tube althogether: and I am not all that confident that it is his own theory, actually - feedback was already known and invented, and he is thankful throughout the text to "others" who have helped him whith their insight (paraphrase).
Frankly, while it is nice being in such good company (Schade, RCA, etc.) one still wishes to be recognized for one's own contribution. The question here should not be "who did it first on whichever of the elements" but whether there was someone who did EXACTLY THE SAME (or all of it at once in some form) before... and I am not aware of such a case.
I sometimes get confused with pictorial representation... does that stand for "amuzing oneself like when you watch a good movie" or there is some better explanation for the overdeveloped smiley?
Kegger
R.I.P. 1/12/1966 - 6/1/2017
I would suggest readup on the E-Linear circuits an the developments there.
(started out exactly as your latest version but using a pentode for driver)
Both Pakprotector and Pete Millet were building this long ago and give all the
credit to (Schade) for the feedback type to explore further upon his findings.
There are many designers who come up with things only to search the net to
find it's been done before, happens all the time there isn't anything new under
the sun that hasn't been done before, just different ways to "implement" them.
(started out exactly as your latest version but using a pentode for driver)
Both Pakprotector and Pete Millet were building this long ago and give all the
credit to (Schade) for the feedback type to explore further upon his findings.
There are many designers who come up with things only to search the net to
find it's been done before, happens all the time there isn't anything new under
the sun that hasn't been done before, just different ways to "implement" them.
Where can I find that?
How long ago... before 2000... before 1950 (hehehe - but I'm serious here)?
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Go googling for E-Linear..
You should find plenty on it..
All of Pakprotector here on AK and Bandersnatch on Diyaudio his amps use this circuit type.
(and he's been refining the circuit for quite some time as well)
Pete wrote a paper for Audioexpress on his E-Linear SE KT88 amp, using a D3A for an driver.
(I believe him an Doug worked on it at about the same time before putting out the design)
You should find plenty on it..
All of Pakprotector here on AK and Bandersnatch on Diyaudio his amps use this circuit type.
(and he's been refining the circuit for quite some time as well)
Pete wrote a paper for Audioexpress on his E-Linear SE KT88 amp, using a D3A for an driver.
(I believe him an Doug worked on it at about the same time before putting out the design)