London Power ad

Search the Forum

(Advanced Search)

London Power 3 channels amplifier project
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!
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
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

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.

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
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!

Nice job man--- It's looking good!
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
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.
Good job on the chassis! Looks very nice!
Hi all,

Vostre Roy, great project! That drilled chassis looks very good! Any recent updates on your project?
Formyx Amplifiers

Forum Jump:

Come in where it's warm!
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.
Please remember these three principles: respect, sharing, community.
Not familiar with The Ultimate Tone book series? See discussion topics, or click here to visit London Power/Power Press Publishing.

Tube Amp Forum Hosted by London Power
London Power logo