RH807 with a pentode driver?

[Alex Kitic]

Go googling for E-Linear..
You should find plenty on it..

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)

While I have a lot of respect for Pete Millet, mostly because of the huge service he is giving the DIY audio (etc.) community by making his documentation available to everyone -- I must say that the amp does not look all that similar to the 2nd gen. RH88 (please check for yourself: http://www.pmillett.com/file_downloads/KT88_SCH.PDF )

Besides, it is dated somewhere in 2005. I believe I had actually wrote about the RH88 on another forum and sent the schematics of both the classic and the direct coupled version to some friends (also mentioned on the forum).

The RH84 and other amps are actually a lot older than that, approximately at the turn of the millenium, if I recall correctly

BTW, how old are you? Does age have anything to do with this?

 

[Kegger]

You and I are probably about the same age.

On both Petes an Douglas amps they started out taking plate
voltage and the plate resistor for the driver from the plates of
the output tubes, it later moved to the UL tap as it was found
they didn't need all of the feedback the plate of a output tube
provided so it moved, but same basic setup, later on Doug has
had output transformers made with multiple percentage UL type
taps made so the percentage of feedback can be "adjusted"

Look at how it is connected, the DC voltage source and the plate
load resistor are connected in the same way to form the feedback.
(and they are cap coupled to the outputs in the same fashion)

But if you can not see how the circuit is ran is not the same basic
setup as your second gen then there is nothing I can say to show.

And they were both working on it long before Pete published his, I
also believe Doug on Audio Asylum has info on it over there as well.
(you'll find fair bit of reference to E-Linear development on the net)

 

[Alex Kitic]

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)

I tried finding something about Barndersnatch, but he's got so many posts that I really do not have the time to crawl through all of that (no pun intended). I would really like to see one schematics illustrating his implementation of the anode to anode feedback -- mostly to be able to judge for myself whether it is or it is not the same approach (similar and the same are not more than look-a-likes)...

 

[Kegger]

As of now Doug/Bandersnatch/Pakprotector no longer uses the plate of the output
tube to get the DC voltage and AC feedback, he uses different percentage UL type
taps in the same fashion to use different ratios of feedback as his circuits "evolved".

 

[Alex Kitic]

You and I are probably about the same age.

I'm afraid I'm older

I could agree with you on taking "less feedback" from the UL tap etc. as being similar to my design... but the difference is quite pronounced actually, IMHO and I could envisage that if we were running for a patent the UL tap could easily be disregarded...

The differences are, mostly, the goal and the approach.
I tend to put things as simple as possible, and as straight-forward as possible. There are no UL taps, because I do not need them (if I need less feedback I usually calculate the value of the resistor...). There are no references to Schade, or RCA, or whoever, because I never heard of them or was not inspired by... or whatever.

Actually, I must admit that I have never built anything that was designed by someone else. One might even say "OK, let that crown tilt a little bit" as a colleague/professor has put it in nice words recently Maybe, if I knew of all these amps (if I could have, actually, known) -- I would have done something completely different

Anyway, due to the simplicity, my designs are quite simple for diyers to recreate, and while a lot is left to one's creativity -- a lot of insight has gone into making it work at its best without need for, or ease of modification.

 

[Kegger]

Anyway, due to the simplicity, my designs are quite simple for diyers to recreate, and while a lot is left to one's creativity -- a lot of insight has gone into making it work at its best without need for, or ease of modification.

I completely agree with this, and certainly like them for that. :thmbsp:

Mine are quite similar just using the output in UL an less feedback.
(It's just a setup that I prefer when the UL taps are available)

If I run SE amps with outputs that do not have UL taps, I run this
same basic design as your first gen RH amps, and it performs well.

 

[Kegger]

2 other driver tubes in a 9AJ pinout you might find interesting for your 12AT7 are the.
(just a slight change in the heater wiring, there just 6.3v tubes)

6AQ8/ECC85

6DT8

Both are basically 12AT7's but in the 9AJ pinout, I prefer the sound of the 6AQ8/ECC85.
(I build lots with 9AJ pinout tubes, 6BK7,6BQ7,6BZ7 Family tubes are quite useful as well)

 

[Alex Kitic]

Both are basically 12AT7's but in the 9AJ pinout, I prefer the sound of the 6AQ8/ECC85.

Yes, due to the different pinout they are not direct substitutes. Furthermore, they might be more difficult to find than members of the ECC81/12AT7 family.

You know, tube ordering is not so easy from Europe: beside the fact that UPS or others do not deliver in 24/48 hrs at low cost, in some countries the customs office is still very active when it comes to skinning the customers...

On the other hand, ECC81 is very popular and most DIYers have at least one at hand Not to mention the various NOS options like Telefunken or RCA or GE or... you name it - as well as the current production tubes.

I am sometimes inclined to considering the RH84 a "volksverstarker"... in the best meaning imaginable. On the other hand, the RH88 may be even simpler, but KT88s or 6550s are not as wide-spread as EL84 (although they can be found even as current production tubes).

 

[Kegger]

I hear yu, 12AT7's certainly are more widespread, I was just giving You a
couple alternatives if you wanted to try them out, and most of the ECC85
are the "quality" European brands, you may be able to get them near you.

Cheers!

 

[GordonW]

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?

Sorry... Stuart Hegeman was chief engineer for Harman Kardon, in the late 1950s to the end of the 1960s. Before that, he designed amps for Lafayette, and others. Among his other achievements, was his design of the HK Citation series, including the legendary Citation II power amp. He came up with some very interesting nested-feedback arrangements. Check out the HK Citation II schematic sometime- frankly, it took me a while to figure out exactly what the heck was going on in the triple-nested feedback loops. A little Schade, a little bootstrapping, and a little global, all "harmoniously" (haha, get it?) working together.

He was definitely one of the "thinker's engineers" of audio. He's kind of my hero, in audio design. He wasn't afraid to say something he did before didn't work well... he was quick with the truth, and didn't put up with nonsense...

I think that Hegeman at least came very close to the same type of feedback arrangements, in the RH-series amps. That said- I am a big believer in "standing on the shoulders of giants"... people that can make improvements in what's already there, by combining the best aspects of what has been done before, in ways that may not have been done before, are doing good work, IMHO. But, we also need to understand where this stuff originally started, too...

Regards,
Gordon.

 

[GordonW]

On the other hand, ECC81 is very popular and most DIYers have at least one at hand Not to mention the various NOS options like Telefunken or RCA or GE or... you name it - as well as the current production tubes.

Also, IIRC, there has been some evidence, that the particular distortion characteristics of the 12AT7/ECC81, is "complementary" to that of many of the popular beam-power and power pentode tubes. In fact, I remember reading somewhere (maybe posted by Bricktop here?) that the combination of the 12AT7 with these output tubes, can result in effective "distortion cancellation", resulting in lower overall harmonic distortion levels (before feedback) than the normal "sum" of distortions you would expect to get, when running the two tubes in series.

That's a neat trick! :thmbsp: If that's something that was originally deliberately done in the RH amp designs, then that's commendably clever, IMHO.

Regards,
Gordon.

 

[Alex Kitic]

In fact, I remember reading somewhere (maybe posted by Bricktop here?) that the combination of the 12AT7 with these output tubes, can result in effective "distortion cancellation", resulting in lower overall harmonic distortion levels (before feedback) than the normal "sum" of distortions you would expect to get, when running the two tubes in series.

That's a neat trick! :thmbsp: If that's something that was originally deliberately done in the RH amp designs, then that's commendably clever, IMHO.

Distortion cancellation is "the name of the game" if you ask me: that is why I am not that much worried about the distortion of the driver, as it is supposed to cancel (at least some of it) combining with the output tube.

If you do some research on the RH amps, most actually use the 12AT7 as driver, but the RH34 (EL34 output, obviously) uses the ECC83/12AX7 as driver... I just could not obtain the same result with the EL34 using the 12AT7 as driver (the EL34 being a pentode, if I am not mistaken about that).

Last but not least, I have designed long ago an SE amp without feedback for the 6080/6AS7G output tube. The driver was 5867, doing most of the work and leaving it's imprint on the sound. I adopted a highish OT impedance (5k) together with a combination of low current (approx. 50mA) and high voltage (approx. 250V) across the output tube... to cut it short, the distortions of the driver struggling to drive the output tube were effectively cancelled by the output tube struggling to give the last mW available

 

[elusive]

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?

Most people post that icon to show that they are sitting back watching the show. The show in this case is you trying to prove that a design feature of your amp is unique and has not been done before.

However, I think you will find that crucial elements of the design have been done before.

All of which is fine and well. I pass no judgment.

 

[thefragger]

So, did Dave get to experimenting with those funny German tubes?

 

[dnewma04]

Work always seems to get in the way of a good project.