My latest project - KT88 SE tube amp

[PWR RYD]

Re: My latest project - KT88 SE tube amp

6dj8

[[email protected]]

Re: My latest project - KT88 SE tube amp

I plan on building the same preamp. Do you have a stock of parts?

[blair]

Re: My latest project - KT88 SE tube amp

Hi Craig,

What tube for your preamp? I might be able to help with your BOM if you want some ideas.

Blair

[[email protected]]

Re: My latest project - KT88 SE tube amp

I will fallow along with great interest.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

Hi Lowell,
No. I've only run the RCA EL34's and the bigger JJ KT88's. I listen to this amp at least 4 hours/day probably 4 days/week. In my office I really prefer the EL34 tubes and I prefer to give up a few unneeded watts because I like the tone of the triode mode better than ultra linear mode. But as you heard in Iowa on Saturday night listening to CraigK's awesome speakers, this amp is what it is power wise. It was imediately clear what this amp couldn't do once Rory hooked up one of his ~ 60wpc amps!

This amp build was an extremely fun DIY project and it suits all my needs for a cool office amp. And it was fun to share the build on this forum. Maybe if I had mega effecient speakers I would use this amp at home. But I don't. Truth of the matter is my 500wpc ClassDaudio amp I built makes all of my speaker projects come to life. My Agenda's speakers sound pretty good on my tube preamp with my Rotel 60wpc amp. The ClassDaudio amp and tube preamp take it to a whole new level of enjoyment!

Hold on though. I plan to start a new thread with even better pictures and more construction details/explanations regarding that under $100 DIY tube preamp design I'm working on

[[email protected]]

Re: My latest project - KT88 SE tube amp

This is such a fun thread to read. Craig, did you ever try any other tubes as you suggested you might?

[Æ]

Re: My latest project - KT88 SE tube amp

Awesome looking amp Æ! Much nicer looking than my K-12.

Thanks. Your amp looks pretty nice too.
The problem I had with mounting the transformers the way that I did, was that the wires weren't long enough and I had to lengthen (splice) in some lengths of color matching wires.

[Pete Schumacher]

Re: My latest project - KT88 SE tube amp

If you can run KT88 in there, you might try some KT120.

[Face]

Re: My latest project - KT88 SE tube amp

IIRC, JJ EL34's don't have as much warmth as RCA.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

I've been enjoying listening to the amp with the big dawg KT88's installed. For those who like pictures of glowing glass







In my limited listening thus far I like the KT88's bass response a bit better. It seems these bigger tubes have slightly better control over the woofers in the lowest octaves. Not "knock your socks off" better, but apparent. But I actually prefer the overall sound of the old RCA EL34's. I'm thinking of buying a matched pair of JJ EL34's to try because they are on sale right now at tubedepot.com. for $22.36. They also have JJ 6L6GC's on sale for $27.96 for a matched pair. Anyone have experience with these tubes?

[PWR RYD]

Re: My latest project - KT88 SE tube amp

If I ever decide that I truly prefer Ultra Linear Mode over Triode Mode or vise-versa, I will eliminate the second switch and hard wire it one way or the other. I figure less connections, less wiring, and keeping it as simple as possible has its advantages.

[PWR RYD]

Re: My latest project - KT88 SE tube amp

I haven't posted any updates lately so here we go....

First, I added those film caps across the driver and output tubes' RC bias networks. When I fired up the amp the left channel went wacko (high pitch screaching)! I quickly powered the amp down. After some investigation I found that the film cap on the driver tube's left channel RC network was shorted. Yikes! So I replaced both the left and right caps with some 1 uF film caps I had in my stock. Problem solved.

Next, I added a series 47 ohm resistor and 0.1 uF cap across the mains power switch. This acts as a snubber for contact bounce. Probably not necessary but why not:



After several hours of listening to various material, I still don't care for the LED bias on the driver tube. So in step with the K.I.S.S. principle that is GONE!





and:

[Arius]

Re: My latest project - KT88 SE tube amp

Had to make a slight modification. After a couple of hours of burn in I measured the B+ voltage again. Might be my mains voltage here (tends to run a little high) or the power transformer, but the B+ measured 438 VDC. It's supposed to be 400 VDC. I added two 220 ohm resistors between the transformer HV and the rectifier tube. Worked perfect! Now the B+ measures 402 VDC! Here is a picture of my fix:

One of the best tube amp built threads on the net! Thanks for sharing, as you've also inspired me to get going on a project I have been contemplating for some time now.

While dropping voltage the way you did is perfectly fine, I would suggest putting the resistor after the rectifier and incorporating it into the power supply as an additional RC filter element. Of course this would require installing another capacitor to ground, however Solens are not that expensive and the improvement in SQ might be worthwhile.

[Whitneyville1]

Re: My latest project - KT88 SE tube amp

Really don't sweat the filament voltage. I ran my $1500 3Z1000 linear amp tube with a "12 volt" filament voltage at 14.1 volts DC (better linearity and less hum than AC) and it didn't shorten it's life when I had it or the guy who I sold it to who has used it for nearly 30 years! BTW, I like the blue corona glow on the plates of the 250watt amps!

[50 watt head]

Re: My latest project - KT88 SE tube amp

You can adjust your filament voltage with a resistor on each side of the filament winding just like you did your B+.

Plug the desired voltage drop and current draw of your tube set into Ohm's Law to calculate the resistor value.

There is one drawback to using resistors to drop voltage in the filament string, though, because the drop will be current-dependent and filament current requirements vary across tube types. If this is a concern, you can use rectifier diodes instead of resistors for a fixed 0.7 volt drop. Just make sure you choose diodes with a current rating in excess of your total filament string current.