Your journey with lower-watt tube amps -- Can a kit be good enough?

@hilde45 I read that a few years ago and bought one of EFB kits for one of my employees who had an SCA35.

If you can shoehorn it in I think it would be a good move.

You can use a pair of such kits and run separate cathode circuits as well.

Or a bias supply could be built in. That might be more work.

At any rate the plate to plate impedance of the output transformer is fine for the EL84s despite what is written in that article, which is otherwise pretty accurate. According to the tube’s spec sheet, 17 Watts/channel is right for that tube.

Some of the newer kit versions of the ST35 use a choke in the power supply where the older original versions use a 50Ohm resistor. This causes the B+ to sag less (IOW better regulation), which reduces some of the issues brought up in that article. There is room for that choke in the original ST35 chassis. If you google photos of it you’ll see what I mean.

People like to see how far they can push it with this amp. Take a look at this:

Customized ST35

Speaking of "how far they can push it", I wish I had photos on hand of my local 55-year tech [now retired], he let me borrow his self named "FrankenDyna" ST35 loaner amp once.

It sorta looked like its original self in some ways, but not really, Kinda Fugly actually.  Super heavy for a small footprint.  It was really fun to play. Very-very musical, beautiful midrange, not overly dynamic, or overly detailed.  Just nice.  I could have listened to that amp ongoing. His Modded AudioNote amps are over the top, look nothing like the originals.  Its fun to try amps from mad scientists who create cool stuff just for fun.   

Highs pretty clear, though perhaps some slight roll off.

@hilde45 I was curious about this so I looked at the schematic of the amp.

The input resistor is 500KOhms (or 470K) to ground. With an input impedance that high, the amp can be sensitive to interconnect cables. If the resistor were changed to 100K, the amp’s character would be unchanged and it would still work with any preamp but the cable sensitivity would be vastly reduced.

So you might consider getting that resistor changed in each channel and see if you still think there’s a slight roll off.

BTW, the older amps have an input coupling capacitor which can usually be replaced with a bit of wire as the input circuit is at zero Volts.

@atmasphere Thanks! I am passing along to my tech 

@atmasphere We tested the amp for distortion and found that the left channel had a magnitude more distortion than the right; @5 watts it was 3.5%! So, we opened the amp up and there was a mis-wired tube socket with reversed leads. Green went to blue and vice versa. Someone did not do due diligence. After fixing that, everything was fantastic -- very low distortion and balanced between channels.