[Oscillate]

Sold: Promitheus Audio RCA50 monoblock push/pull Class A amplifiers


I am the original owner. I asked Nicholas Chua, an electrical engineer and
owner of Promitheus Audio, to custom build these for me in 2008. They have
had about 400 - 500 hundred hours of use. Used in a smoke, pet and child free
house. Selling these and the rest of my tube equipment and slowly transitioning
to a high-end AV pre/pro with all class D amplification for both music and home
theater ...and then calling it good.


When I asked Nicholas about the amplifiers, he stated the topology of each monoblock
push-pull amplifier is:

The driver D3A (tube A) is run in single ended with an interstage transformer to 50 tube (tube A)
The driver D3A (tube B) is run in single ended with an interstage transformer to 50 tube (tube B)
Then the two type 50 are then run into a Push Pull OPT to sum phase A and Phase B


* 38 watts output each.
* Input impedance of 47K Ohms.
* Point to point wiring.
* 7N copper wires for the signal wiring.
* Allen Bradley resistors.
* Zero global feedback.
* Promitheus Audio winds all their own transformers in-house.
   The power, interstage and output transformers are made with
   M6 audio grade cores. Cathode feedback winding topology for
   the output stage transformer. These amplifiers weigh 40 lbs each!
* Neutrik XLR input connectors.
* They are wired for a standard 120V USA outlet.
* The amplifiers are finished in Merbau hardwood with 2mm thick
   powder coated aluminum top and bottom plates.
* Dimensions each are 16" L x 11.5" W x 8" H (including tube height).


 Each amplifier uses the following tubes:

- Two Siemens D3A driver tubes (roughly $20 each to replace and readily available)

- One JAN CAHG 5R4WGY rectifier tube, Chatham Electronics (roughly $10 each to
  replace and readily available)

- Two (ST-19) RCA type 50 triodes output tubes (can also use a UX-250 or CX-350)
  [For example the RCA/Cunningham ST shape type 50 large ST-19
  glass envelope tube or the Shuguang SG-50 (RCA UX-250 replica)]

  These amplifiers have a matched quad of Cunningham Electronics
  RCA-50 in them. Cost of replacement is roughly $250 each for NOS.
  Which to me is kind of expensive, so I looked into reasonably priced
  alternatives.  With the currency value of the Euro close to 1:1 with
  the USD these are attractive options. Please see the links below...

www.tubeampdoctor.com
chinesetubes.com

Also the Psvane WR50 is stated to be a 1:1 replacement for the RCA50(?)

Psvane & Shuguang audio tubes


The price is Sold: $900 USD plus the actual shipping charges, insurance and
signature confirmation of delivery fee. Payment may be made by USPS
money order or PayPal (plus PayPal fees, if 'gift' not used). Personal Checks
are accepted, but item will not ship until the check clears at the bank. I will
ship via USPS or FedEx only. Please use the information and links below to
give an accurate estimate of the total costs ...local pick-up, delivery for the
cost of gas and meeting half way are other delivery options open to discussion.


The original factory shipping boxes were one-off cardboard boxes stuffed
with styrofoam peanuts. They were discarded. Instead all the items have
been wrapped in celophane and then rolled up in multiple layers of heavy
duty bubble wrap. Then placed (and more padding) into new, tightly sealed
Roughneck 10 Gal. Storage Totes. Each amplifier is in it's own tote and
all the tubes are in a seperate 3 Gal. Roughneck Storage Tote. I take
packaging seriously.

Parcel (x2 amplifier boxes) ships from zipcode: 43082
Parcel weight: 40 lbs. 12.6 oz.
Parcel dimensions: 9" x 16" x 24"

Parcel (x1 tube box) ships from zipcode: 43082
Parcel weight: 3 lbs. 6 oz.
Parcel dimensions: 7" x 10.5" x 16"

FedEx
USPS


If you would like to purchase or have questions, please send me a PM.
For feedback on me as a seller / buyer ...please do a search using the
terms 'For Sale', 'FS' and 'Sold' with the username 'Oscillate' on all the
AudioCircle Trading Post subforums.


Please PM me for additional high resolution photgraphs.













...Thanks for looking

[Anji12305]

Delighted new owner of these lovely amps, here.

I wanted to leave positive feedback for Bruno.
These amps and their tubes were all packed to survive rough handling.

Each tube, individually wrapped - labelled even so I wouldn't need to rebias.

He was kind enough to dig up sources for tubes, should I ever need replacements.

*******
SPOILERS - I'm thrilled.
*******

I've been experimenting with home brew, higher sensitivity loudspeakers since building Troels Gravensen's Quattro.

I have kludged together what may well be the ugliest two way ever built.  It's a DTQWT that looks like a dorm refrigerator.

(Deltalite 2512 up to 2750, Vifa soft dome in Waveguide - augmented NHT subs driven by Adcom 2535)

The sound using Bruno's amps is glorious.

My previous amps are PP Ultralinear amps emulating triodes.

The difference is immediately apparent.

Where the Promitheus amps don't go, the subs carry on.

Everything else?  Glorious.

Thanks for a lovely transaction and your patience awaiting my response.

[johnvin01]

If I would have seen this when posted I would have been the "Delighted new owner of these lovely amps, here".

Still searching 

[shooter]

Jim, Congratulations on owning these amps, 50 is my favorite output tube. You should really treasure the Cunningham that is in your amps, trying to find a match quad of 50 is a daunting task. Some time ago on ebay someone sells a tube socket converter to adapt 6L6 for the 50, he made a custom pair for me to use the 807 that I use for casual listening to save the 50 for special occasions! I have manage to hoard 7 old stock pairs, the globe sounds best in my system.
FYI the Chinese manufactured 50 don't test like the real 50, the early manufactured 50 from Shuguang have very short life either the filament would fail or emission would die, I was told tube curve trace similar to a PX4 rather than a true 50, the TJ 50 couldn't even be tested on my TV7 using 50 setting, to my surprise when I put the setting as 300B it test ok, The Shuguang sounds decent in my 50 amp when it was working. I hope they have corrected their production problems, if you buy them let us know how they turn out for you.
The  Emmission lab EML50 is the other current production from Europe about 460 Euro a pair that you can purchase from JAC music, I have heard them in an Audio show but not in my system. I wish they make a mesh plate version.
The holy grail of 50 would be the mesh plate, the Raytheon 4 pillar box plate, then the globe. A quad of mesh plate 50 would be a sight to behold!

[Anji12305]

A replacement quad of NOS type 50 tubes will cost more than the original purchase price!

When that inevitably comes to pass, I hope the maker can retrofit these to run the commonly available 6c33c, which I have never heard.

For now, these are a revelation into what all the triode fuss was about.

I can't get over how low the noise floor is in these amps.

[shooter]

Change to 6c33 will be difficult with these amps, much easier to switch  them with 300B with a series resister to the filament, if you add a switch you can use the 50 or 300B.

[Oscillate]

Anji12305 ...if you ever do wish to modify the amp, then please contact me.
There is one person I can wholly recommend to do that for you.

Shooter ...any thoughts on the Psvane WR50? Seems matched pairs are available for $410.

[shooter]

Oscillate, Since having bad luck with the new production tubes, I stayed with old stocks. bought enough to last me a long time.
Since I own a pair of 50, 300B convertible amps, I can recommended the conversion.