New Build

[stephenl]

My current project:

K50 style Preamp + Class A KT66 PP PA

I used Pablo's schemo's as a starting point.

[stephenl]

Started:
RJ's Asteroid Chassis - excellent quality and great guy to do business with.
"Glass Audio" Raw B+ (anode) supply and 12vdc heater supply on the PCB - tested/working
Zener stabilized supply (from Merlin's book) for screens and preamp - tested/working.
Apologies for the crappy Ipad picture.

[M Fowler]

Looking good.

These Asteroids are great sounding amps and will flap your bell bottoms when you hit a power chord.

Mark

[wsaraceni]

what does the diode do on the 4th gain stage?

[surfsup]

Solens...sweet!

[stephenl]

The diode on the cathode follower is for arc protection when powering up. Pablo added it to the original design - I'm not sure if I will incorporate it. It is transparent tonally speaking.

[stephenl]

I managed to make some time this past weekend to solder this thing up.
Amazingly, it powered up and worked pretty well on the first try.

There are a couple of things I need to work on though. I'm running the 12v dc heaters from the power transformer's 6.3v winding through a voltage doubler. The doubler is generating a lot of noise which is coupling back into th B+ (grrh). I was anticipating that this might happen ...so I'll need to provide a separate transformer to feed it- probably a 15 to 20 v supply and get rid of the doubler. I may just go back to AC heaters.

The second issue is that I'm getting some DC on the gain pot.

So far, I think it sounds great- nice harmonic content and bloom. The Class A KT66 output is very smooth with a nice percussive control of the bass.

I need to play around with the preamp voltages a bit. I'm also undecided on the PI tube. I tried a 12AT7 and I'm currently running a 12AV7, which is my favorite in the PI position. I need to try an AX7 as well.

[Noval_novice]

That looks awesome! Love KT66's.

[Bob S]

+++ on the KT66's.
On the schematic be careful with the diode orientation of the bridge rectifier on the 5vdc relay power supply.
Thanks for posting. Looking good.

[stephenl]

Thank you.

I haven't actually implemented Pablo's switching scheme on the split-load, I'm undecided on that function. I think I'll probably settle on a fixed position - currently I have the resistors flipped with the 20K on the plate.

The original schemo's show a .1u coupling cap after the split load, which is what I started with. Looks like Pablo dropped it to .0047. I think I might reduce that to .022u to control the bass a little better into the the next stage - I think it might be causing a little blocking distortion....but not sure. Comments/experiences on that part of the circuit would be much appreciated.

This thing is LOUD - think I might implement power scaling after I get the kinks worked out of the existing circuit.

[tictac]

Here's one way you could have it both ways.... Original or Crispy...

Plus you're not putting high B+ on a mini-switch....

TT

[tictac]

Your amp reminds me of an amp I built similar to yours. It was a JTM45 type preamp with KT66's and Solen filter caps, and power scaling with a sag control.

Had to sell it during times of unemployment, but love those KT66's...

Also, If you need more room inside the chassis you can do what I did and build a "dog-house" that will hold 3 Solens. Basically a circuit board mounted behind the preamp tubes....

Really nice looking build.... and those Glass Audio boards are super hip... I'm building an Akido stereo preamp with their All-In-One board... hadn't thought of using their stuff for guitar amps though... Great Idea!

TT

[stephenl]

Thanks Tic-Tac.

Funny you should mention the "dog house" - I was actually considering that to free up some room inside.

I like your suggestion on the fast/gradual.

[stephenl]

I dropped the preamp voltages down so I have 300v on V5, 315v on V4, 350v on the PI - dropping 100v across the 47k tail.

The PI seems to sound best with either a 5751 or a 12AV7. The 12AV7 is a clear winner hear though - I wish it were a current production tube, but there are plenty around. The JAN version of it, 5965 is a great tube as well. I have 82K's on the plates and 680 Rk

This amp is really sounding fantastic.

I also dropped the PA grid stoppers from 220k to 100k. I'm not positive, but I think the 220k might have been dulling the treble a little bit. According to Merlin's preamp book, 100K can be used without affecting treble and curbs blocking distortion.

So now, I need to sort out the DC heater circuit and I'm definitely going to add powerscaling.

[stephenl]

By the way, I don't have any intention of calling this a KF66, its just a filename I chose...to call it something.

I apologize just in case I've offended any KF friends.