Is a tube amplifier worth the effort?

[kblackmore]

Gentlemen,

While my listening room is still under construction, I was pondering the thought of amp/speaker combos. I have always loved the sound of horns (EG, Klipsch Heritage series) and am seriously considering building some "Cornscalas" as discussed on Bob Crites website https://critesspeakers.com/cornscala-style-b.html . The questions for the day are, I have a Fisher KX-100 integrated amplifier (1963 vintage) that I am considering retubing/restoring. As tested out a couple of years ago, the amp ran fine and has had a relatively easy life so far, but the controls are starting to crackle and I am sure there are some updates that the amp needs (new caps, etc). Would this amp have enough power to reasonably drive the cornscalas, and is it really worth the effort and funds to have this amp gone through thoroughly? Also; as my listening room isn't large enough to support bass without lobing, how would I be able to integrate subs into the system to smooth out the bass response throughout the room? Does anybody have any experience with tube systems and would this be a good choice of an amp to use? It was given to me several years ago and I know it started its life as a kit. I also have the matching tuner, but it needs repair. My dad tells me one of the tubes in it probably just went bad. If it makes any difference, I do have all the original paperwork to include build documentation as was given to me by the previous owner who built it.

[howard]

https://www.nosvalves.com/

If you decide to have it rebuilt, I recommend Craig at this website. Yes, I believe it is enough power. Only way to tell if it is worth it is with your ears.

[djg]

I spent about $2K building a kit tube amp and preamp, along with some sensitive MLTL full range speakers. It sounds good. I don't listen to it much, but it was fun. If you think you'll use it, sure do it.

[bwaslo]

I used to have one of those, not a bad amp. As I remember, though, the output tubes in it were something weird and NLA. Maybe an EL34 could be subbed for it (it's been like 30 years....)? Dunno, maybe research it over at Audiokarma. The controls could probably be easily made non-scratchy with some contact cleaner. The filter caps (at least) probably need replacing at this point, no matter how easy a life it had. If you got it going it would probably be enough power for a Cornscala, though not what I'd choose (nor the Cornscala, really, for that matter -- try to hear one first if you haven't).

[Kornbread]

Or if you wanted the 'sound' of tubes in something more modern maybe give this new type of tube a try https://korgnutube.com/en/ with modern amps. Ready to go eval boards http://www.nutube.us/

[Thump]

.. Also; as my listening room isn't large enough to support bass without lobing, how would I be able to integrate subs into the system to smooth out the bass response throughout the room? ...

Article about using a sub with no sub out may be a useful start and the easiest approach.

Further options may include plugging up (or building) a mini DSP, perhaps even inside the thing somewhere. There are pre-built nano AVR series (variants for digital and analog, as well as actual DIY kits) which provide outstanding software. Includes full PEQ, XO, Comp and bass management at very precise levels across a large selection of bands.

It also has ready-made filters, can import biquads generated from REW and other packages and more. I love mine a lot, but I was able to put it inline between source and AVR where LFE already exists.

Is this the correct PDF service manual for your unit?

Maybe one of the guru around here can recommend a replacement or additional chip (or a simple in-line LPF) to actually add LFE output to it, then route a new plug to the back for hookup.

Page 70 has a legible schematic.

I'm no kit tinker'er or electrical engineer so I've no idea if swapping in a modern replacement (or additional) chip is feasible.

Just a thought. Seems like a fun project.

RIP Radio Shack :(

[tom_s]

Those Fishers are great amps. It probably just needs some cleaning, but like Bill said, you might want to replace the caps in the power supply. I would also replace any coupling caps while you are at it.

I fired up a Dynaco ST70 a few days ago. It had just been collecting dust for years and I was actually surprised how good it still sounds. I rebuilt it 20+ years ago.

[kblackmore]

I spoke to Craig at https://www.nosvalves.com/ and he could rebuild the amp for $340~350. He will be replacing all the capacitors in the unit and more than likely the power supply. He also believes he has the brights to replace the ones missing from the knobs. Also after inspecting the tubes, there was only 2 original 7868's that were original. One was broken. All the 12AX7 tubes were replaced sometime ago with GE Electrolux models. Things are looking promising. Question; the unit has a center speaker output. Could that high level signal be a mono combo of the L+R channels? If so, perhaps that could be sent to a sub amp high level input to solve the subwoofer issue.

[Jake]

Or if you wanted the 'sound' of tubes in something more modern maybe give this new type of tube a try https://korgnutube.com/en/ with modern amps. Ready to go eval boards http://www.nutube.us/

Looks interesting, have you heard one?

[Kornbread]

Looks interesting, have you heard one?

I think there was a big thread about it at diyaudi, and IIRC, Nelson Pass was also involved. No, I have not heard it but it is on my list of things to try. Hopefully the price will drop a little by then.

Haven't hit a yardsale in a long time, I love finding old audio stuff, especially tube gear. Bet that Fisher will sound darn good with some respectable speakers.

[skatz]

Some interesting reading the articles re valves at http://sound.whsites.net/index2.html

[Psycoacoustics]

I don’t think you should mess with! It’s way too old and dirty! Looks like just plugging it in would start a fire!
You’d be much better off buying something new and modern. Transistors sound way better than tubes anyway!

PM me and I’ll send you my address. I’ll pay for shipping and I’ll make sure it’ll get recycled in an environmently safe way! 😈

Let me know, Mark

[PWR RYD]

In my opinion, YES!

[AEIOU]

Transistors sound way better than tubes anyway!

Let me know, Mark

I'm letting you know, that your statement is untrue. Each and every amplifier has its own unique sound and not all solid state amps sound good either.

[kblackmore]

If you got it going it would probably be enough power for a Cornscala, though not what I'd choose (nor the Cornscala, really, for that matter -- try to hear one first if you haven't).

Mr Bwaslo, I am curious as far as your bias of the cornscala speaker. I have not heard one yet and was wondering if you had. I have owned 2 pairs of Klipsch La Scalas and absolutely loved both sets. I now have a pair of Klipsch Belle's and my living room is just not big enough to support them. The room I am building in the basement is about twice the size of my living room, but I believe it would also be too small to support the bass from the horn. I have heard a pair of Cornwalls before, but they were bass heavy compared to the La Scalas. I believe Bob Crites has found a workable solution with the midrange horn as it can cross lower than the Cornwall and has better efficiency. I have the schematics for several ALK Extreme Slope crossovers and will build it to Al's exact specs paying special attention to the orientation of the coils and keeping all wiring logical and neat. I plan on crossing to the tweets around 5~6 KHz (will test all crossover points with an active crossover before I choose the exact frequencies). I personally think it should be a nice project, but if you know something I don't please let me know before I sink some bucks and make an expensive mistake.