• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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 6l6GC’s then GEC KT66’s and then the 807’s.
The 6L6GC’s were nice but I found them a bit aggressive or rough in tone, then tried the KT66’s which were much nicer, the 807’s 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 12AT7’s 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 Alex’s 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 125ESE’s for the outputs and the 807 is using Softone RW-20’s.
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
 

Shoog

Member
2002-08-15 10:16 pm
Eire
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
 

Shoog

Member
2002-08-15 10:16 pm
Eire
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
 
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
 

Shoog

Member
2002-08-15 10:16 pm
Eire
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
 
Andrewbee said:
I too am interested in your comparison between the amps, in particular the Mini-A.

Things didn't turn out as planned.... the CSS speakers caused all 3 SE amps to do into coniptions (it sounded like the drivers were broken -- rubbing -- so it took a while to figure out what was happening). So the MiniA outperformed all 3, but then Chris dragged his slightly tweaked Jolida PP EL34 up from the HY system -- it had the midrange magic that the miniA didn't. The miniA was good, but sterile throu the middle.

So we are moving up our schedule for working on differential amps....

dave
 
The miniA was good, but sterile throu the middle

Well, there go my hopes and dreams of a Mini A. Seeing I find my tastes and observations seem to concur with Dave I will have to put that off.

Thanks Dave. Hopefully you will get the diff amp going soon. I guess its Allen's EL34 amp? He sent me a copy of it a while ago but I have yet to do anything with the circuit. I am into tetrodes at the moment:)

Andrew
 
Shoog said:
Are you planning on trying the experiment again with suitable speakers.

When we can get the amps all together again... i sent the RH34 up island with Scott to try out and Don took his ZEN away.

The local diyFest at the mountain now has a date -- August 27 -- so hopefully we can get everything together for that.

dave
 
Andrewbee said:
Hopefully you will get the diff amp going soon. I guess its Allen's EL34 amp? He sent me a copy of it a while ago but I have yet to do anything with the circuit. I am into tetrodes at the moment:)

We have some mules to do Allen's EL34 amp, but the siren song of the extra power from an RH style amp has us looking at a 6922/EL84 amp on the theme of the Gingertube/Yves differential evolution of the RH style amp. If we can't get enuff gain out of the 6922 we'll likely add some FETs to do a Wright-style cascode (some of this is dictated by the Scott LK48 mule we'll start with -- aluminum chassis, point-to-point, conveiniently set-up for a dual triode upfront).

Once we gain some confidence from that 2 SCA-35s should yield a pair of monobloks (1 SCA35 will donate a power tranny to the other, and i have a cool chassis from a navy transmitter with some serious trafos & chokes to accept the left over SCA 35 OPTs... then 2 Dyna ST70s & the Jolida....

dave
 

xenofonn

Member
2013-09-20 11:48 pm
GU50!!

Hi from Greece! I have the rh84 and i like it very much ,but i need more power!! I see the schematic of rh807,and i have a question. Can i use gu50 in this schematic? I have 12pc of gu50 ,found in ebay very cheap and i like to use it. I think the gu50 its like a russian 807 (almost!!). I need some help,about mods in original circuit to work properly with this tube,from the forum experts!!! Thanks a lot!!!
 

jazbo8

diyAudio Moderator Emeritus
2011-01-05 8:34 am
In Transient
I think the gu50 its like a russian 807 (almost!!). I need some help,about mods in original circuit to work properly with this tube,from the forum experts!!! Thanks a lot!!!
Never hear anyone suggest the GU50 is like the 807/6L6 before, anyway, you can contact Mr. Kitic directly via his blog to get pointers on how you can go about this project.
 
Never hear anyone suggest the GU50 is like the 807/6L6 before, anyway, you can contact Mr. Kitic directly via his blog to get pointers on how you can go about this project.

GU50 is the Russian version/copy of LS50 (Telefunken / W. Germany).
It is much more similar to 6550 or KT88. Anode dissipation is 40W. Ug2 max is 250V, which should be no problem for audio application.

I think GU50 were mentioned on AudioKarma along with the RH Universal amp - here is the link to the post: AudioKarma.org Home Audio Stereo Discussion Forums - View Single Post - GU50 Experiments...

Xenofonn, did you try contacting Mr. Kitic?