New K50 build

[pablogt]

Hi all. I just finished a new build around the Concorde circuit. It is build in a Trainwreck sized chassis.

I asked Heyboer to send me a set of vertical end bells for the PT, so I could mount it on top instead of through the chassis. I thought I would need a custom sized chassis (wider) but, to my surprise, it did fit on the standard size. I did the headphone trick with the transformers and I heard no more noise in this position than in the standard Komet positions.

As you may see on the photos, it has distributed grounding (chassis connection at the input jack), power scaling, bootstrapped master volume, fully fused PT secondaries, Foot-switched fast/slow modes, etc. I also put some extra filtering on the power supply (three small value RC filters, two before, one after rectification) that also provides a little sag.

The amp is completely stable and very quiet, considering the amount of gain it has. After using it in one of the rehearsals with my band, and making sure that it is still stable at band volume levels, I changed the presence control to presonance (thank you Martin!) which works great.

In the rehearsal I noticed that the fast mode does not cut too well through the mix. So I changed the original ss relay to a traditional DPDT relay. I separated the second gain stage's 20 uf cathode bypass cap in two, a 2.2uf Solen and a 16 uf electrolytic (actually, it measured 17.5). With the other SPDT I switch off the 16 cap when in fast mode, leaving only 2.2uf. The fast mode sounds a lot more like a lead tone to me now, although I have not tried it yet at high volumes. I also upped the cathode follower's cathode resistor to 120k to make it a little less compressed, but it is hardly noticeable.

It is a really fun amp to play. I think the extra NFB provided by the 16ohm tap suits it better than the 8ohm. I made it switchable, just in case.

I do not know if it's against the forum rules to post this schematic, please let me know. Or PM me and I'll send it to you.

By the way, the awful knobs are temporary...

Pablo

[cbass]

If it sounds half as good as your build looks.

It must be a monster.

[xtian]

Wowezzz

Beautiful work, my friend.

We all love schematics. Please post!

[Colossal]

Beautiful build Pablo! Great job. Very well thought out layout. How do you find the low end, sustain, and gain as compared to the Express?

[pablogt]

Thanks everybody.... I have not played an Express (I was wondering what could I do with the 6.6k Pacific OT sitting there? ). It does have a lot of low end. It is a really fat, aggressive amp, with a great throaty distortion, specially with a Brimar in V1. For lead work it does not cut very well through the mix, but I hope to have improved that with the changes I made in the switching scheme. I think the output tubes you use have a big impact on this amp too, since a lot of distortion comes from there. Right now I'm using SED winged C, which I think are a little too much on the dark side. I have ordered a couple of RFTs, I'll let you know.

[Structo]

That looks terrific!

The layout is very nice, did you use Viso for that?'

How did you get the Sozo's?

I heard they were unobtanium.

[Colossal]

How did you get the Sozo's?

I heard they were unobtanium.

Those are Jupiters Tom, but they look just like Sozos.

[pablogt]

Yes, I used Visio.... and, yes, they are Jupiters! I am not sure if they sound better than Sozo Vintages, but they are easier to obtain. I was just not ready to wait another 4 months as for the ones in my Plexi.

Pablo

[pablogt]

I played my K50 with the band last night. Big improvement with the few changes I made. Particularly, having V1a's cathode bypass split in 2.2+16 uf (originally it was a 25uf bipolar cap) has removed the all boominess from the amp (see schematic). The lead tone (fast mode) really cuts through the mix now.

This are the tubes right now:

v1 Brimar ECC83
v2 Telefunken 12ax7
v3 Jan 5751
v4,v5 New production SED winged C EL34

A Brimar in v2 sounds good too, but makes the amp darker sounding and a litthe muddy.

I use Jan 5751 for PI in all my power scaled amps. They have less gain, but render a very detailed and warm tone, without losing sparkle. I very rarely play at full power, so I only have to turn up master vol a little to compensate for the reduced gain.

I just can't wait to try it with the RFT EL34's that are on the way...

Another good change was to up the CF's cathode to 120k. It made it a little less compressed and opened it up somehow. Heater to cathode voltage is not a problem in this case because heaters are elevated by about 70v.

Over all, a really inspiring amp to play.
Pablo

[Reeltarded]

Holy crap! Pab! Buddy.. that thing is ART! LOL you are much better at this than I am! I won't even try to approach that!

Umm.. What does fast mode do? Add filtering?

[pablogt]

Thanks!

The Fast/Gradual switch in Komet amps is a tapped plate load. In the schematic above, it takes the signal from points A or B and feed it into C which goes to the coupling cap to the next gain stage. At A (Fast) you get about 5 times the signal voltage you get in B (Gradual). Some other people at TAG have used it in other builds to increase headroom, or whatever. I really like it, just have to be careful to use components that can take plate voltage. In this case, the relay is handling close to 300vdc. Some safety issues to consider too, if you implement it with a regular switch instead of a relay.

Pablo

[cbass]

Hey Pablo, have you a tried smaller value cap For the .1uf after the fast switch.I went with a .002 and found it really helped with mushiness when the preamp volume is way up.

[tictac]

Never have understood why people just love to wire high plate voltage to an iddy-biddy mini-switch when you can do it as shown below plus you get two diffrent tones depending on what caps you use....

That unused triode could be used as well....

TT

[stephenl]

Wow really sweet build.

I'm working on a similar project right now. I'm coupling the preamp to a Class A P-P KT66 PA though.

Would you be willing to share your schematic - I'm mostly interested in what you settled on for preamp voltages.

stephenl5@verizon.net

[pablogt]

These are the voltages. The PT is wired for 240v operation. I tried 220 first, but the heater voltage went to 6.5v. It's 6v now, which I prefer.

Stephenl, I'll pm you an schematic

Cbass, no I didn't try different values for that cap. I might do when I do the final voicing for the RFT tubes.

Pablo