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

@hilde45 I'd go with a set of JJ EL84s before Sovteks or for that matter any Russian made version.

Have the tubes been tested? If any are bad all bets are off.

If you have level controls on your speakers it might be a good idea to run pink noise through the system and using a spectrum analyzer app on your phone, adjust the controls slightly for best results. This amp has a lower output impedance than any tube amp (such as an SET) that runs no feedback so if your controls (if you have them) were optimized for an SET they won't be right for this amp.

Your amp in the photo looks fairly new. Apparently there's a resistor option on this amp; do you know which kind your amp uses? It looks like they might be the carbon film resistors. IME the metal films sound better.

If the amp has been sitting unused for a while or is brand new, it may need some break-in time.

I doubt its actually rolled off- this amp has full power bandwidth well past 50KHz.

Thanks @atmasphere. I need to check into those things and I have a friend/tech who can help.

@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