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Old 28th December 2012, 01:56 AM   #1
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Default RH 84 Variation: Direct Coupled Pentode

I just tried plate-grid "Schade" feedback with an all pentode direct SE design of mine. I had been using a feedback loop from the plate of the output tube to the cathode of the driver. I find I prefer this version.
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Old 28th December 2012, 08:01 AM   #2
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Why give credit to that rubbish design the RH - yours is substantially better. However it would be better if your driver were running at least 5mA.
I use direct coupling from a pentode in my PP designs. but I avoid the big cathode resistor by using an input transformer and a negative rail.

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Old 28th December 2012, 12:58 PM   #3
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Originally Posted by Shoog View Post
Why give credit to that rubbish design the RH (...)

Shoog
Harsh words, considering the fact that undisputed designs are almost nonexistent. Nature of the trade I guess – if one follows it long enough, one can witness circuit designers coming up with 'my precious' only to, with time, feel disenchanted and move onto something else... Consumed by the road to and not the destination.

Of all RH84 critics it's you and revintage that seemingly sound as if it were something personal.

Harsh manner, considering the fact that Kitic (wrong or right) is not able to respond...
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Old 28th December 2012, 01:05 PM   #4
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Harsh manner, considering the fact that Kitic (wrong or right) is not able to respond...
Why is he not able to respond?
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Old 28th December 2012, 01:19 PM   #5
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Originally Posted by mctavish View Post
Harsh manner, considering the fact that Kitic (wrong or right) is not able to respond...

Maybe because he has been banned from every discussion board he took part in due to his very disagreeable personality.

The designer here has ditched the distortion generating triode driver in favour of the pentode - this is what Kitic should have done in the first place and why honoring Kitic for a superior design is the wrong thing to do.

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Old 28th December 2012, 02:53 PM   #6
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I referred to the RH design because it seems to have a fan base, and because I think the basic idea of a SE EL84 with local feedback is sound, but the design could be improved. I could as easily called it a Mullard 3-3 variation with Schade feedback. I consider this design an improvement over the Mullard since I don't starve the driver like it did.

Ultimately, I intend for this to become a direct coupled E-linear amp with the load and feedback resistors replaced by a single resistor connected to the screen tap on the output transformer.
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Old 28th December 2012, 05:27 PM   #7
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I have the load and feedback resistor serving the dual function in my designs. Makes it mighty difficult to draw enough current through the driver whilst maintaining a decent load value.

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Old 1st July 2013, 07:46 PM   #8
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Originally Posted by Shoog View Post
Why give credit to that rubbish design the RH - yours is substantially better. However it would be better if your driver were running at least 5mA.
I use direct coupling from a pentode in my PP designs. but I avoid the big cathode resistor by using an input transformer and a negative rail.

Shoog
Would you care to explain why you consider it rubbish?

Paul, you are drawing too little current through the driver. The B+ value is not shown on the schematic, but if the output tube draws 40mA this means just 40V under the cathode of the output tube and mot more than 50V on the anode of the driver. It is lowish for the 6SJ7, and while the design is on the right path, it needs a lot of optimizing. Higher voltages, Rk of at least 2k for the output tube. If you insist on using a pentode driver, than use the second grid to control current draw and voltage drops.

And, thumbs up for crediting where credit is due, Paul - otherwise you end-up being a copycat.
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Old 1st July 2013, 08:23 PM   #9
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Originally Posted by Alpha Kentaury View Post
Would you care to explain why you consider it rubbish?

Paul, you are drawing too little current through the driver. The B+ value is not shown on the schematic, but if the output tube draws 40mA this means just 40V under the cathode of the output tube and mot more than 50V on the anode of the driver. It is lowish for the 6SJ7, and while the design is on the right path, it needs a lot of optimizing. Higher voltages, Rk of at least 2k for the output tube. If you insist on using a pentode driver, than use the second grid to control current draw and voltage drops.

And, thumbs up for crediting where credit is due, Paul - otherwise you end-up being a copycat.
Because its a distortion generator.

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Old 1st July 2013, 09:06 PM   #10
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Would you care to explain why you consider it rubbish?
I think "rubbish" is maybe a bit too harsh. I'd put the design in the "quick 'n' dirty category. Good enough for an AM ham rig plate modulator, or a gee-tah amp. High end, it ain't. Here's the original design: RH-807. There's a lot wrong with this. One is using a difficult final as a SE. The 807 likes to make nasty sounding, high order harmonics. It does require the addition of local NFB to help tame it, but also needs gNFB to really take the edge off. It's even worse in SE as you don't have the cancellation of even order harmonics in a SE. (I did a design that uses 807 finals, but that was PP, included both local and global NFB, fixed bias, and active screen regulation. The type can give excellent sonic performance, but you have to work at it.)

The thing also uses an unbypassed, 10K series dropping resistor for the screen supply. This is hideous. Running pents as pents requires screen voltage regulation. (You can get away with it for small signal voltage amplification in low level stages since the plate current variations are much smaller than what you see with large signal finals.) At the very least the screen needs a stiff voltage divider with a large enough bypass capacitor. The 807 has screen voltages close to what you can get from a series string of VR tubes, and that's what he should have used at the very least. Active regulation is better still. What you have here is absolutely the worst possible screen supply.

The RH-807 is an FX box, not an amp. Maybe suitable as a guitar practice amp, but that's about it.

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Why give credit to that rubbish design the RH - yours is substantially better. However it would be better if your driver were running at least 5mA.
I use direct coupling from a pentode in my PP designs. but I avoid the big cathode resistor by using an input transformer and a negative rail.
I figure ~3 -- 5mA for the driver is about right. The other possibility is to ditch the singleton pentode, substitute a triode/pentode like the 6KE8 and use the triode as a cathode follower grid driver. He could also go with the "E-Linear" design that uses an OPT with an Ultralinear tap-off. The main problem here is that DC connection in the lNFB loop that puts limits on the feedback resistor. Maybe make that an AC only connection for greater flexibility in selecting plate resistors for the driver? (E-Linear would accomplish the same thing.)
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