It didn't take long to decode the AC wiring and connections to get this baby up and running in its native mode. Everything checked out and it sounded very good. And of course, I am thinking "spud" type amp at this point. But I was disappointed that my CD player, while driving it adequately on most CD's, just not have quite enough oomph. So naturally, I reverted to form (in this case anyway LOL) and designed a low current 12AU7 input stage as in my 50EH5 amp.

First off though, the power supply had to be changed. The supply was set up to provide for other circuits (presumably a radio tuner) and when unloaded by those, provided excessive voltage to the amp stage. I knew from previous experience, that the tube sounded best when run hard and that it could easily take it and still give good life, so I settled for an output stage a little above "book" values. Fortunately, the old filter capacitor checked out very good and had significant amount of microfarads at 40-80-40. Plenty for this amp in class A. So I calculated for the voltage drops I would need including the new voltage for the input stage. Next was to get rid of certain of the old wiring. Some of it wasn't going to be needed any longer and some was old funky wire that is too hard to re-work to make it worth saving. The heater wires were of the excellent heavy stranded type and stayed. I also kept the screen power wire intact since its type and style were suitable for the job and the wire was in good condition. Everything else got changed.

The rest of the circuit is still configured much as it was. The original circuit had grid stoppers already installed, so naturally I left these in. I merely re-routed a few things to account for the new input tube.

In the process of re-wiring, I cleaned up the entire layout, but it still has the same format with the power supply on one side and all the voltage feeds crossing the chassis to their appropriate circuits. The original made heavy use of chassis ground, and except for changing a few locations, so did I. I left the original input and output connectors in place also.

I wanted to do a painted chassis for a change. I removed all the parts except the tube sockets, input connectors, and the filter cap all of which I masked off. I cleaned the chassis within an inch of its life and put down a good coat of primer. We (my wife & I) decided we'd do something retro this time. I had this vintage knob from an old Eico tuner and we happened to find a matching paint, so I painted the base in that color. The base wood is Poplar and is being used in its original dimensions. I sanded it and rounded the corners. Then I applied two coats of sandable primer and then three coats of color. The green was selected to contrast this color in a vintage way. The power tranny end-bells were painted to match as well as the waxed paper covers on the output trannys. The silver of the output trannys and the aluminum filter capacitor were cleaned and burnished. I was planning to use all stainless steel for fastening, mostly as a complementary accent color, and I think the whole thing came off pretty well.

The vintage screw-terminal speaker connectors were kept and re-attached. The original AC was a suicide-cord type, and I simply hard-wired a new AC cord in. I added an internal solder-in fuse. We added a vintage, green cut-glass lamp lens from my lens collection to complete the effect.

I get it all assembled and am ready to put in the final wires: the ac power. It is then I discovered my vintage NOS on/off volume control's switch worked backwards! Ouch! The switch was "on" when the control was "off". This was not going to work! Furthermore, I knew I could spend a long time finding a replacement. I intended the power switch to be here. Had already cut and painted the wood accordingly and drilled the chassis etc. So my concession to the modern was to add a small power switch near the filter cap. It seemed the only reasonable thing to do and it actually looked quite nice after I installed it.

How does it sound?
Well, it sounds like a 6EH5! When set up properly and run with adequate voltage, these sound very nice indeed! Typical of a pentode "spud" type amp with a smooth easy to listen to midrange. The amp covers the audio spectrum quite well and gives a good sense of localization. It is easy to listen to, and I find myself tapping my toes to the music. Slightly lower distortion than a spud type amp though, since it receives some compensation from the input tube as the characteristics run generally backward thus helping to cancel the non-linearity's. At a mere 1.3W output it is definitely not an all out rocker, but used within its range is very pleasant indeed.

Bottom line: A vintage looking amplifier with a modern sound.
And something different from us.
abraxasaudio is Jef Larson and Deborah Rutan

Thanks for looking.
Be sure to visit my other pages (see index above).

Your comments are welcome.
Critisms gracefully accepted.
All questions courteously answered.
Email to: abraxasaudio at att dot net
(this avoids the address scanners)

May I build one for YOU?
jef