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Old 7th December 2013, 05:14 AM   #1
mr2racer is offline mr2racer  United States
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Hey all,

I received a power transformer from Heyboer today. So this weekend I'm going to start an RH83 PPE SE amp. It should be good for about ten watts. Its a slight modification of Aleksandar's RH84 PPE amp.

Its a parallel SE design. I've made some slight modifications, Some at his suggestion. He built the original using 6AU6 and EL84. I will be using his circuit as designed for EF86. I recently received four 'Winged C' EF86 which I've tried in another amp to make sure they are not microphonic. Aleksandar suggested EF86 would be better than 6AU6. I'm using 6P15P/SV83 instead of EL84 hence RH83, (Because I have a bunch of them). He also suggested using LT1086 instead of 78L05 regulators for the cathodes of the outputs.

I've attached a picture of the chassis. To my knowledge this amp hasn't been built before so it should be interesting, at least for me!
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Old 7th December 2013, 02:46 PM   #2
artosalo is offline artosalo  Finland
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At best RH84 PPE is as good as an output amplifier from a lower priced tube radio from 1950's, nothing more.
Sorry if I was too frank, but this is just the fact.
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Old 7th December 2013, 04:22 PM   #3
mr2racer is offline mr2racer  United States
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What makes you say that? On what do you base this assertion? I'm not disagreeing I just want to know why?

And can you suggest a better design?

Last edited by mr2racer; 7th December 2013 at 04:36 PM.
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Old 8th December 2013, 05:23 AM   #4
artosalo is offline artosalo  Finland
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I am referring to this schematic: RH Amplifiers: RH84 PPE – "Parallel Pentode Edition"

Main flaw is the pentode connected single end output stage without sufficient NFB.
This sort of circuit has high output impedance and therefore the load impedance i.e. speaker, which is not a constant impedance, affects the frequency response and distortion. Also the damping of the speaker cone is low.

If pentode output stage is used for hifi amplifier it requires some amount of GNFB.
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Old 8th December 2013, 06:43 AM   #5
aardvarkash10 is offline aardvarkash10  New Zealand
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without questioning the overall intention, this:

"If pentode output stage is used for hifi amplifier it requires some amount of GNFB."

sounds like a generalisation to me.
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Old 8th December 2013, 07:14 AM   #6
Eli Duttman is offline Eli Duttman  United States
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Quote:
Originally Posted by aardvarkash10 View Post
without questioning the overall intention, this:

"If pentode output stage is used for hifi amplifier it requires some amount of GNFB."

sounds like a generalisation to me.

NFB of some kind is needed with a pentode O/P stage, but it need not be global. Without a substantial amount of NFB, damping factor will be wretched. FWIW, I'd use both a short, inner, loop and an outer, global, loop.

BTW, the screen grid in the 6П15П (6p15p) is fragile. That fragility can be dealt with, along with improving open loop linearity, by regulating g2 B+ with a VR150/0D3.
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Old 8th December 2013, 07:18 AM   #7
maxro is offline maxro  Canada
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Quote:
Originally Posted by artosalo View Post
This sort of circuit has high output impedance
Actually, the local shunt feedback lowers the output impedance of the output pentode(s) to equal or lower than triode strapping it.
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Old 8th December 2013, 08:35 AM   #8
planet10 is online now planet10  Canada
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Quote:
Originally Posted by Eli Duttman View Post
NFB of some kind is needed with a pentode O/P stage, but it need not be global. Without a substantial amount of NFB, damping factor will be wretched.
If one has a speaker that likes a current amplifier, then the "wretched" damping factor becomes a very good thing.

dave
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Old 8th December 2013, 10:25 AM   #9
45 is offline 45  Italy
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Quote:
Originally Posted by planet10 View Post
If one has a speaker that likes a current amplifier, then the "wretched" damping factor becomes a very good thing.

dave
Actually if the drivers have right specs and bass load for that kind of amp it is probably the best way to run a fullrange because HF response is (in most cases) improved in comparison to constant power or constant voltage drive.
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Old 8th December 2013, 02:53 PM   #10
mr2racer is offline mr2racer  United States
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Ok guys,

My first intention with this amp was to build, as Eli calls it, a flea amp. But with around ten watts, say, a giant flea amp.

There are some things I like about this amplifier. That I have all the parts to build it. Also, I like the automatic bias system. And I prefer the use of zeners to limit screen voltage. I have some VR150's but the zeners seem much more simple. I have read that some do not like cathode regulation. And others that do not like the use of zeners.

I want to use SV83 instead of EL84. Because I have a bunch of them and I want to hear this tube in something. Is it possible to use this as an ultralinear configuration? And if so what would the proper tap percentage be? The figures I've seen most often are 20% or around 40%.

As for the feedback? From my limited experience feedback is in the final tuning. First you build the race engine. Then you put it on the dyno and tune the cam, ignition and fuel system parameters. It would be easy enough to add a GNFB feedback circuit once I have something to listen to.

I'll have to redraw the schematic and when I do I'll post it.
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