Review: a Frankenstein Eico ST-70 tube amplifier

 I bought this strange modified amplifier on Ebay. It looks as if someone took a stock integrated and modified it to be a basic power amp. Gone is the normal set of controls on the front plate, replaced with a generic black panel and power indicator light.

 

A peek on top of the grille - what is inside?

Of course you know - you read the title of this post! Someone took an Eico ST-70, removed the preamplifier portion and converted it to use 5881s instead of the stock 7591s.

Since I didn't have any 5881s on hand, I modified this amplifier to use the military 6AR6. Since the 6AR6 has a completely different pinout (and bias points!) it took an entire rewire of the output stage, along with more negative bias than the stock circuit.

I also changed the power supply with new electrolytic capacitors for the main bucket, 6SN7 phase-splitters, and 12AX7 input tube. I also added a little 1.5H Dynaco choke to reduce the B+ ripple. After a little troubleshooting, and bias modifications, I finally got this amp to work; with 40mA per output tube. 

So how does this pentode output Frankenstein weird output tube sound? Even with old Russian 6N8S tubes, surprisingly good. Bass extension - with the large output transformers - is really deep. And there is the old tube midrange and smooth treble thing happening. Maybe it's been too long with an SS (Aleph J clone) amplifier, but this little push-pull tube amp is quite magical.

Chet Baker Sings, for example, really is holographic - for a mono recording - with his voice and trumpet sounding natural and forward. Compared to the Aleph J solid-state amplifier, music is more relaxed but, at the same time, more dynamic. The tube amplifier sounds way more powerful than its approximate 30 or 35Ws per channel.

I'm surprised that the KEF R500s work so well with this vintage Frankenstein; you never really know what you are going to get with modern speakers.

Once I get over the excitement of having a new amplifier, I plug in some old Zenith 6SN7s that I have on hand. Maybe things will get even better-better.