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


Looking for stories about your low-watt amp journeys.

Here's the situation: I have new speakers, 97 db. Trying them with lower watt tube amps (45/211, 300b, etc) seems generally wise. I am attempting to borrow some from audiophiles in the area. 

The horizon beyond trying these things involves actually buying some. I'm looking at a budget limit of about $5k.

Curious as to folks' experience with lower-watt amp kits vs. those of good makers (e.g. Dennis Had, etc.).

If you have any thoughts about the following, I'd be interested:

Did you start out with a kit and then get dissatisfied? Why?

Did you compare kits vs. pre-made and find big differences?

Did you find you could get the equivalent level of quality in a kit for much less than the same pre-made version? How about kit vs. used?

Also: did you find there was a difference between "point to point wiring" vs. "PCB" in these various permutations?

I realize that there are good kits and bad ones, good pre-made amps and bad ones. I'm hoping you'll be comparing units which seem at comparable levels of quality and price-points.

Thanks.

hilde45

@hilde45 If you install that update I mentioned about a separate cathode resistor and cathode bypass capacitor for each channel, the cathode resistor value is 190 Ohms. I recommend a 5 Watt part so it will run cool. The bypass cap is then 50uf. They can mount inside the chassis no worries.

 

Interesting reaction I got from my tech about your idea: "That is an option and can be done. It is relatively simple and keeps the amp using only cathode bias, but if the EFB article is to be believed, the output transformers are not optimal for the EL-84 in that mode. (http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf). The EFB is kind of a hybrid, the cathodes are still connected but then tied to a regulator that allows bias adjustment for minimum distortion. The upside of individual cathode resistors is less need for a matched quad, though it's still important."

I don't have the expertise to weigh in, I just thought it would be cool if you and others on the forum saw some techie back and forth.

@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.