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London Power 3 channels amplifier project
#21
Greetings again!

I've been quiet as of lately, had to complete some home projects before I could pour some money in this one. As of right now, I've got a brand new steel chassis and a knockout punch kit coming my way so as soon as I get back home later next week, I'll be able to jump back on this project.

Now, quick question to Mr O'Connor: reading the PA-66 document, I just realized that to be able to have a switch to alternate between fixed and cathode bias, I'd need a DPDT switch for every single power tube, I'm I understanding the document correctly? If that's the case, I might just disregard that option and go with the Fixed-Bias option, unless you have a better recommendation.

Cheers and (late) happy new year to all!
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#22
Hi Vostre

There are three options for having individual fixed / cathode bias switching control for each tube

1 - Use a separate DPDT for each.

2 - Install the relay type listed in the PA66 notes, then use a simple SPST or SPDT per tube. The relays mounbt on the board directly.

3 - Buy PA66-S2 which provides two relays and two switches, as required for each PX66 card (the power tube portion of the PA66).

Have fun
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#23
Greetings Kevin,

Thanks for the answer, I will get myself 4 relays to be able to use a single switch for the cathode and fixed bias option. I don't really plan to mix cathode and fixed bias between the tubes (might be a cool feature but for my personal use, controlling all tubes at once is more convenient). I didn't noticed that the shape of the connection on the PCB was meant to accommodate the use of a relay but now that you mention it, it is so damn obvious ahah

Cheers!
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#24
Also, I think I've settled on what I will use as a third switch for this project. The following schematic was drawn by me but the original idea came from a website that I can no longer find, else I'd give the proper credit to it:

   

It basically change one resistor and one capacitor value in the tone stack (sorry I don't recall their precise name). I've done that mod in my first ever amp built and really dig that I can tweak the EQ depending on what I feel like playing.

I obviously won't install it right of the bat, I'll build the amp stock first and then add this mod (along with the gain and bright switch) afterward.

Cheers!
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#25
Hi Vostre

In your EQ schematic you can use the SPDT centre-OFF switches, which are not too expensive.

Note that "m" is milli and "M" is mega.

Have fun
Kevin
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#26
Greetings folks!

Not a big update, but one nonetheless. I've received my knockout punch kit (I don't recall the company's name but its not Greenlee, given I do around one amp a year they'll do the trick). Result is way closer to my expectations:

   

   

I've also populated the PSU board but didn't took any pictures, more to come at my next time off!

Cheers!
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#27
Nice job man--- It's looking good!
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#28
Thanks man!

Now that I have those punchouts, preparing a chassis is a whole lot easier, my next project will probably get started as soon as this one is done. I still have the issue of having a non-heated garage, so I try to avoid having to work in there during the winter ahah
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#29
Pretty cool what you have going.  It is going to be tight in there for sure.

I find it interesting that
  1. The send and receive knobs for effects are in the back.  If I can't see them up front, I don't use them.
  2. The parallel / series switch is unnecessary.  Default to parallel, and use the knob to select 0% (i.e. series) to 100% ( parallel) or vice versa.  See amp manufacture Shur on how they did it.  They too are using the BFX.
  3. I had to use a relay for the Z-Pre.  My switching system was years ago, things have changed since then.  My relay would picking of the high power Z-Pre and add a bunch of noise in the system.  The placement of the relay had to be away from everyone.
Good luck.  I like to see the finished project.
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#30
Good job on the chassis! Looks very nice!
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A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor's The Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
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