The 5-Buck Amp

[single-ended, single 50EH5 pentode per channel]

07_mar_06

<A cross between a Silvertone, a ground fault isolator, some dead computer monitors,
and an old obselete scanner (+ a few parts from the bins)>

My First Tube Amp

For my first tube amp project (I have built some Dyna kits) I choose something that was very simple and had some nostalgic value.The genesis of this amplifier was a Silvertone record player my parents got when I was a kid -- one of those with the speakers on the side and the TT flips down in the middle. One of my 1st DIY in my early-teens was to convert the single unit into a TT, an amp & a pair of speakers (double walled leaky box using wood from a mandarin orange crate).


Well, the amp survived all my parent's moves and I ended up finding it. With the help of Allen Wright*, the circuit was updated. Mostly it was a case of undoing the accountants cost-cutting measures.

*(Allen also came up with the name -- "... bizarre - a pair of $50 tube coolers on a $5 amp!")

Other 50EH5 SE Amps

Giovanni's

Dad's

Jeffery's

 

5-Buck SE 50EH5 amplifier schematic

<clik images for larger pictures>

There were many parts donors for this project. Few things were actually paid for:

  • The output transformers and the heater resistor were reused from the original amp
  • A mains transformer was salvaged from a ground fault isolator
  • All caps were recycled from dead computer monitors
  • The chassis is made from an aluminum extrusion from a Centronics parallel to SCSI converter box from an old Microtek scanner
  • Tube sockets came from a parts-bin chassis
  • The 5-way posts came off an experimental set of mid-horns I built in the early 80s
  • The fuse and power cord donated by a dead Pioneer integrated amp
  • Some resistors came out of my bins, and I had to go out and buy 4
  • The RCA inputs were on-hand from a batch I got from ApexJr for some real cheap price
  • And the $50 tube coolers were rejects I picked off the floor after helping Bill get an order off to England.


The amp is 8.5" W x 4" D x4.5" H (with tubes -- the chassis is only 1.75" high) 22.6 x 10.2 x 11.4 (4.4) cm.

This diminuative little amp puts out on the order of 1 Watt.

I plan on adding wooden bottom & end plates (Arbutus is on the top of the list right now) to finish it off.

Currently the tubes are the Japanese ones that shipped with the amp -- I have a pair of NOS GEs to try once I'm ready to start tuning it.

Initial listening tests are encoraging (Linn into NAD 7020 into 5-Buck amp driving my 2-Buck speakers). This 1W amp plays these to a satisfying listening level, and gives at least a hint of what SE is all about.

Note: after about a year the mains trafo failed. Doing the math i realized that with the heaters on the trafo i was drawing WAY too much current. So i just hooked that up to the AC (and made sure it was completely isolated and self-contained -- it really should be done with a 2nd trafo) and replaced the trafo with another.

Original Amp during reverse engineering

The amp hadn't been plugged in for 20+ years. When I fired it up with variac it hummed -- big time -- but it still worked. (It sat in a seperate wooded enclosure when in use).

Original Amp's Schematic



Chassis

The chassis is a piece of Aluminum extrusion from a Centronics parallel to SCSI converter box from an old Microtek scanner.



with 120 V --> 120 V isolation transformer

extruded Aluminum

8" W x 4" D x 1.75" H

with existing amps Output Transformers and the tubes for an idea of their size in relation to the chassis and Nokia 5120 for scale (and a little color)
October 7 -- the original amp is brought up on the Variac. It works, but hums big-time.

October 8 -- "breadboard" the new circuit -- it works! No hum. Even with only FM source & a pair of modded (real) cheap 3" computer speakers it hints at the beauty inherent in SE.

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