New K50 build

[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

[hans-jörg]

I changed the presence control to presonance (thank you Martin!) which works great.

Pablo

Hello Pablo,
first: great build! Perfect job. To get my K50 build now as close as possible to yours is a challange now.

Therefore, could you explane the "presonance" please? Or a schem of it. I can not see it well in the layout.

Regards

Hans-Jörg

[pablogt]

Check your PM...

P.

[stephenl]

Does the Presonance control work w/o the negative feedback?

[Reeltarded]

Nope!

[hans-jörg]

Thank you Pablo,
excuse one more question: I see 2 Fets in the VVR. One for anodes and the second for screens? Or doe you regulate the Preamp too?
This is not a specific K50 question but I saw it in your layout (which b.t.w. was a huge job too, I presume).

Best

Hans-Jörg

[pablogt]

I use one of these in my k50.

http://www.londonpower.com/catalog/prod ... ucts_id=27

KOC's approach to power scaling is based on regulating independently each power node. In this case, the two mosfets regulate plate and screen voltages, while there is a small pcb by the bias test points that regulate the bias voltage with a transistor based circuit. The only tricky part of this implementation is that you really need low impedance bias supply for independent bias regulation AND scaling. It took me a while to figure it out in a previous build, but was no problem with the k50 since the PT has a 75v "half tap". Notice that, in my layout, there is a "transparent" blue resistor by the Bias supply. It was there when I tested the amp without power scale, but I removed it for the final version.

It is different from from Dana Hall's VVR were one point is scaled down and you control how far does the scaling go by branching the B+ line and isolating branches with diodes. I understand that K50's power is outside VVR's heat dissipation capabilities. Even with the SB1 you might notice that I placed a heat sink on the other side of the chassis were the mosfets are bolted to, just in case.

Glad you liked the layout. I actually have fun doing those, so I don't mind spending time doing them.

Pablo

[hans-jörg]

Hi,
thank you for your exlpenation.
I just put a VVR (similar, or close to the London PS) with 2 Fets (Anodes and Screens) in my Liverpool. It also dont affects the preamp Voltage (at least it raises 10% for shure when you go down with the voltage).
The heat sink on top of the chassisis rearly necessary

So when I want to use the VVR in a fixed Bias amp (my next build will be the K50) it would be the best to have a PT with an extra Bias tap (50V a.e.)? Not from the main HT supplay via resistor. Does I understand it right?

Best

Hans-Jörg

[pablogt]

Yes, that would be best. The standard PT for K50 (Heyboer HTS-8593) has one.

Take a look to this for possible mods for bias supply

http://www.londonpower.com/raw_bias_supply.htm

In my Plexi I went for the zener/lowered coupling resistors and full wave approach even when I had a "half tap" bias supply with that PT too.

https://tubeamparchive.com/viewtopic.php?t=17683

To give you an idea of how much more bias current you need, in the Plexi I had to convert a half wave rectification with a 100k coupling resistor to full wave rectification and 2x33k coupling resistors. Don't forget the zener if you go this route or you'll fry the bias smoothing caps!

This time I decided to give a try to the bias half tap and it worked at the first try. Anyhow, I did put the turrets in for coupling resistors to B+, just in case.


Pablo

[hans-jörg]

Hi Pablo,
I yust saw your Plexi Wreck.
How many amps you built to come to such a perfect work? Is it still a "hobby"?


Congratulations

Hans-Jörg

edit: btw, I allway try to get the "§right" PT for my builds. I mean with extra bias tap if not kathode bias. Thanks for info.

[pablogt]

The K50 is number six. The Plexi was number 5... I have to sell them to make space for the new ones, but it's still a hobby. It will probably evolve to a point in which the hobby pays for its own expenses but to turn it into a commercial en-devour is a big leap. You just cannot take days to build an amp if you want to make money with it. My approach is lots of planning, very detailed layouts and I only work on them until I stop having fun. Then I continue the next day.

cheers,
Pablo

[Zuccero]

The K50 is number six. The Plexi was number 5... I have to sell them to make space for the new ones, but it's still a hobby. It will probably evolve to a point in which the hobby pays for its own expenses but to turn it into a commercial en-devour is a big leap. You just cannot take days to build an amp if you want to make money with it. My approach is lots of planning, very detailed layouts and I only work on them until I stop having fun. Then I continue the next day.

cheers,
Pablo

Pablo,
Oh man, that is perfection in its extreme. You put so much love and planning into it, that it is a shame to be hidden and closed off in a conventional wooden case. Can't take my eye off it
I agree, this peace of art cannot be turned into a commercial en-devour, but should be kept as an perfect example of what could be possible in comparison to any commercial production.
Awesome!!!
Looking forward to see more:)

[hans-jörg]

Hi
I think the perfection of the Layout is at the same level as the build.
I like the parts you guys use too.
Here in middle Europe its not the standard. Takes much time and money to get the right stuff for one complete amp. Although the Trainwreck are not so "overloaded" like a Dumble, lets say, not so compact.
I believe in the necesserity of good quality.

So, enaugh cried

Best

Hans-Jörg

@Pablo: with tears in my eyes I had to sell my Fender 5e3 clone to suport the next Wreck and place for it. I reached right the same point like you: to sell to keep on building

[pablogt]

it is a shame to be hidden and closed off in a conventional wooden case.

Now that you mention, I'm working on a transparent front cabinet . Below is a test for my Plexi, done with 3mm plexiglass. Ideally it will be 10mm smoked.

Can you imagine it with all the tubes lit up?

[Zuccero]

it is a shame to be hidden and closed off in a conventional wooden case.

Now that you mention, I'm working on a transparent front cabinet . Below is a test for my Plexi, done with 3mm plexiglass. Ideally it will be 10mm smoked.

Can you imagine it with all the tubes lit up?

Looks very nice tho
Pablo, how have you done the front plates? I am currently struggeling to find a not too costive and good looking solution. Did one with white laser printable water slide paper , to get the white writing, but with no good results concerning the black printing. I tried to get the white more printed cream but the black turned out more blueish. See my post https://tubeamparchive.com/viewtopic.php?t=18038 here.