RH 84 Variation: Direct Coupled Pentode

[rowuk]

There are indeed multiple paths, ands we have to learn by the errors of our own experiments, but there is no benefit in starting out on the path with an approach which is going to result in a low power and high distortion design - that seems peverse to me

Shoog

Maybe the perversion is not reality, rather your only perception of it. I do not understand your vehemence. Most 2 stage SE tube solutions have low power - not an issue with (non full range) high efficiency speakers. The distortion in this design appears to be much lower than designs with comparable tube complements. I remember even seeing a site where the RH84 was measured by someone not associated with AK. The low distortion was confirmed by a real life amp.

I of course would be really interested in a design that you are USING yourself that has solved all of the problems that you criticize here. I am 57 years old and working on an "end of life" system. I'll give RH a chance after not being satisfied with SS, PP-UL and SET solutions.

 

[H E Pennypacker]

At the risk of going OT what was it that you did not like about SET?

 

[Shoog]

I have never said I don't like SET, I have said that a well designed PP can do anything a well designed SET can do - but more efficiently and generally with better bass. For me there is no special magic to SET amps.

I am currently listening to a SET with Schade feedback in my bedroom system and it sounds fine.

What people are failing to understand here is the objective of Schade feedback is to allow a pentode to sound like a triode whilst maintaining its overall pentode efficiency. This has not been achieved when we make the driver triode the limiting factor, all we have achieved is an amplifier of similar efficiency to a traditional SET. Thats right - why not just build the plain zero feedback SET in the first place if a low efficiency amplifier is acceptable ? My answer is (and always has been) do it right and get the efficiency of a pentode with the sound of a triode - how ? by using a beefy pentode driver stage.

shoog

 

[Shoog]

Simply looking at the calculation of the feedback factor of a Schaded output stage will show you why a triode driver will add to the overall distortion - it varies the feedback ration with the triodes output signal. Thats not good.

Shoog

 

[mach1]

P

Have done so many times.

BTW the RH84 acxtually WORKS. I would love to hear a well implemented example against the stuff you post here.
A

 

[Shoog]

Have done so many times.

BTW the RH84 acxtually WORKS. I would love to hear a well implemented example against the stuff you post here.
A

Build Peter Millets Mighty Midget and compare it and you will understand what a real SE Schade amp should look like. There are good example's out there and I make no claims other than the evidence of comparing a good design with a poor design. Peter publishes his actual measurements on his actual circuit and it all stacks up.

Shoog

 

[smoking-amp]

Keep in mind that the RH does not make any attempt to control the linearity of the driver.

Shoog is correct as to keeping the driver linear. To prevent the shunt load from agravating the tube's already high 2nd harmonic (from loading) via further feedback modulation (changing Rp affecting the Fdbk divider) would require a triode driver of sufficiently low Rp that it could have been used for the SET output in the 1st place. (a series R added after the low Rp to hold the Fdbk constant)

The RH is deliberately making high 2nd H in the driver. And it probably even has a phase reversal in the 2nd H as the signal increases in amplitude. (Driver 2nd H prevails at low amplitude, output 2nd H prevails at high amplitude)

 

[mach1]

Whatever you say . Build one ( preferably with a Sylvania gold brand 6201 driver) and judge for yourself.

 

[M Gregg]

I like my RH PPE El84/EF86,

its just greeeeeeatttt...

The more hours its been run for the more I like it..<<so there you GOOOOO!


But the measurements! EEK..well what are those then???

Its sill GREAT!

You can squeeze it into a smallish box as well and it works when you turn it on honest..
LMAO...its playing once in a life time by Talking heads as I type this..

Regards
M. Gregg

 

[M Gregg]

However,

I have a problem...I must try the universal version so I can use my Chinese KT66 and GE6l6 and just have some fun..perhaps I can make it shake the roof with a bit of added current and OP tubes.. SE PP...

But a PP (push me pull you)would do the job<<<maybe but its not as much fun!

Regards
M. Gregg

 

[Shoog]

However,

I have a problem...I must try the universal version so I can use my Chinese KT66 and GE6l6 and just have some fun..perhaps I can make it shake the roof with a bit of added current and OP tubes.. SE PP...

But a PP (push me pull you)would do the job<<<maybe but its not as much fun!

Regards
M. Gregg

Try the Baby Huey if you want to see what a PP Schaded amp can do. The version I heard sounded very nice. It uses a triode driver - but applies various tricks to minimise its negative effects, and PP will intrinsically limit the distortion inherent in a triode driver. It also doesn't claim to be anything other than a relatively high distortion design.

Shoog

 

[tinitus]

Keep in mind that the RH does not make any attempt to control the linearity of the driver.

Shoog is correct as to keeping the driver linear. To prevent the shunt load from agravating the tube's already high 2nd harmonic (from loading) via further feedback modulation (changing Rp affecting the Fdbk divider) would require a triode driver of sufficiently low Rp that it could have been used for the SET output in the 1st place. (a series R added after the low Rp to hold the Fdbk constant)

The RH is deliberately making high 2nd H in the driver. And it probably even has a phase reversal in the 2nd H as the signal increases in amplitude. (Driver 2nd H prevails at low amplitude, output 2nd H prevails at high amplitude)

ah! ok

not that I can claim to understand it, but I guess it makes sense

if the result ia a 'full sound with exstra volume', I have no doubt it might be convenient to some people, and probably much depending on what speakers they have

 

[SY]

I've removed a lot of personal attacks and characterizations. If it's repeated, no mercy- you're in read only mode for a good stretch of time.

 

[rowuk]

Well, my RH307A Super has been finished for much more than 6 months, so I feel that I can say something about it.

First of all, distortion? What distortion. I listen to mostly classical music, much of the time HQ recordings (analog and digital) of the orchestras that I play with. I am intimately familiar with live sound and the RH amp is definitely not in the way. It has none of the euphonics of the DECWARE EL84 SET, or the last EL34 PP amp that I still have kicking around. The RH amp has BASS. Currently it is driving a 3 way high efficiency speaker at appropriate symphonic levels. Voices have all of the characteristic roughness and smoothness. Instruments have the correct size and space around them. Detail is excellent, and above all, after 6 months there is NOTHING that I want to change - except maybe to lose a bit of gain.

Even if the vehemence shown against these amps may have a bit of technical merit (I have my doubts in real life audio installations), I do not understand the crusade, but to be honest, no longer care. The amp works beyond my expectations and a much lower powered RH amp is in the works for the horn and UHF ribbon to turn the amplification into a dedicated single ended multi amp scheme.

RH307A Super Fane Colossus12MB Neodymium Midwoofer (80-ca. 1k)
RH84 or similar 2-3 Watt for the midrange horn (ca. 1K- ca. 10K)
RH-Spud? 1-2 Watt for the UHF(+10K)

The crossovers will be integrated into the amps passively.

Thanks to the naysayers for really sparking my interest. What a great place DIY-Audio is - inspiration from all sides!

 

[nigel pearson]

I noticed in No 57 some of my graphs. I abandoned the RH design and went for pentode input after seeing a Michael Koster ( spelling ) FET input amp with 5881. It gave about 2% THD with Jean Hiraga style reducing harmonics. I used many samples of the choosen pentode of which some were officially worn out. Distortion trends the same albeit 50% higher with the rejects. All the pentodes could reach the desired current but not book current for the rejects. Ideal new life for them as some came from Chipping Norton Studios.

My final design about 1 % at 8 watts and 0.2% all second at 1 watt . My preferred valve EL34 in the end. EL34 in mine needs to slightly clip to do 8 watts.

Did anyone try EF184 in a RH design?

I apologies to Mr K if my graphs do not meet the spec he implies. I am fairly sure I got everything right.

 

[DAK808]

However,

I have a problem...I must try the universal version so I can use my Chinese KT66 and GE6l6 and just have some fun..perhaps I can make it shake the roof with a bit of added current and OP tubes.. SE PP...

But a PP (push me pull you)would do the job<<<maybe but its not as much fun!

Regards
M. Gregg

Hi I just finished a RH PPEL34 amp which operates in mostly class A. It uses an input transformer for the phase splitter duty. The amp is based on the rh universal design with some changes to accommodate fixed bias of the output tubes. I am very pleased with the results but i am still fine tuning the amp with different types of resistors. for example, I found that using metal film resistors in the - voltage bias circuit produced too much sibilance. Switching to carbon comps helped a lot. The amp sounds great!
If you had no trouble building your rh amp a PP version is the same except accommodating the IT is a bit more work in terms of layout. Also the IT is expensive. Mine were Lundahl LL1544A, if memory serves. They are 90 bucks a pop. cheers.