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Old 22nd June 2005, 05:24 PM   #1
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Default Rh807

Hello all,

I have recently completed my RH807, a design by Alexsander Kitic which is a S.E. amp using a short feedback loop between the plates of both the driver (12AT7) and 807 output tubes. I built it as per the schematic but found the G2 voltage was higher than allowed according to the datasheets so made a voltage divider and feed G2 with 250V and bypassed with a huge 330uF cap which is all I had.
This being said, I did play it as per the schematic for a couple days with no ill effects, the 807 is a transmitting tube so should be as tough as the proverbial old army boots.

Anyway, on first build I used RCA 6l6GCs then GEC KT66s and then the 807s.
The 6L6GCs were nice but I found them a bit aggressive or rough in tone, then tried the KT66s which were much nicer, the 807s however are in another league altogether, this was good because I had to change the sockets of course and would have had to have made make some adaptors to return to the octal sockets. The 12AT7s were a new production JJ? maybe, I forget now and an old Westinghouse, not much difference I found.
This is my first taste of the 807 and have read a lot of good about them and its well deserved praise indeed, the best part is that they are cheap, I paid 15 pounds for five new tubes or valves I should say via ebay.
I had built another of Alexs designs a couple months ago, the same scheme but using the EL84 which is one my favorite tubes and this 807 is easily as good and I think better. The comparison is a bit unfair as the EL84 used 125ESEs for the outputs and the 807 is using Softone RW-20s.
I was so impressed that I bought a pair of Lundahl outputs for rebuilding the EL84 version.

Anyway, before this post gets too much longer, if it means anything I highly recommend this circuit, well worth the effort.

Andrew
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Old 23rd June 2005, 05:28 AM   #2
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Glad you found something to make you happy!

Do you have a schematic or URL for those of us that have no idea what a RH807 is?
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Old 23rd June 2005, 06:16 AM   #3
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Quote:
Do you have a schematic or URL for those of us that have no idea what a RH807 is?
That would be http://www.tubeaudio.8m.com/, the site of Alexandar Kitic. There also was a long thread (or even two) about Partial Feedback Amplifiers, just do a search on the tube forum.

Oliver
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Old 24th June 2005, 10:10 AM   #4
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Cool, thanks!

Cheers!
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Old 24th June 2005, 07:44 PM   #5
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I'm listening to an RH34 prototype right now (nice ). Saturday we are going to compare this amp, stock ZEN, a pair of Parafeed RH84 monobloks w triode EF86 driver, and a miniA.

dave
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Old 25th June 2005, 08:21 AM   #6
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Hi Dave,
With your Parafeed RH84 did you increase the value of the Partial feedback resistor. If not what value of output blocking cap did you use.
In my Parafeed 807 amp I was restricted to using 4uf parafeed caps which meant that I had to increase the value of the partial feedback restistor to 470K. In effect the partial feedback is practically gone. Still it sounds amazingly good with very tight bass (a real suprise this).
I personnally would be very interested in your comparisons between the different amps.

Shoog
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Old 25th June 2005, 01:21 PM   #7
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Hiya Dave,

I too am interested in your comparison between the amps, in particular the Mini-A.
I am thinking about building one . What speakers will you be using?


Andrew
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Old 25th June 2005, 01:59 PM   #8
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I too am thinking of turning my Zen V3 into a Mini A. The transormer I have is a 0-12V 0-12V which should do nicely. The main transistors are IRF190 and IRF210 (I think). Basically it just requires a simple re-jig of the circuit and the purchase of a few new components.
The Zen V3 was eclipsed by my Gainclones, and my Gainclones have been eclipsed by my Parafeed 807 amp. I think the Mini A has a chance of proving the validity of Nelson Passes approach. I hope that it can sound as good as the Parafeed amp. At the very least this will allow me to have a quality reserve amp.

Shoog
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Old 25th June 2005, 05:30 PM   #9
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Shoog,

Coupling cap is a Clarity 3.3 uF. I don't think we changed the feedback R (Chris built them, and they have been thru a zillion revs), but the EF86 -- even in triode -- has much more gain than anything else we tried. Parafeed choke is the primary of an identical set of OPTs (just like these) as used for OPTs.

Speakers for today's effort will be Chris' VIFA P13 dalines and 2 unkowns based on the new CSS FR125S... a set of bipoles & a set of Solo 103 inspired miniOnkens. Drivers on those will be very low hours. (i think i'll toss my $7 OBs into the back of the Suburu too). Unfortunately a pair of the drivers in the bipole FE127 met a tragic end, and the replacements are still in transit from Seattle (and a couple hundered hours from being broken in).

dave
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Old 26th June 2005, 09:18 AM   #10
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Because I am using a unity gain preamp my parafeed RH807 didn't have enough gain. I added an extra gain stage of a PCC88 to bring it up. Works but takes away a little from the purity maybe I should consider using an EF86, though this would require quite a redesign.
I increased the value of the rfb resistor early on in developement so maybe it could be returned to 100K. I used TT21's to make a CCS rather than using a choke. My amp is effectively a SEPP with a very stiff Pentode CCS.

Looking forward to your results

Shoog
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