weber kit advice

[nateflanigan]

Hey guys,
I'm building a princeton reverb for a friend (my first commissioned job!), he insisted on going the weber route.  I picked up the kit yesterday and populated the tag board.  I was tempted to just use the tag board as a drilling template for a hoffman style turret board, it just feels really chintzy, but then I figured there must be plenty of working weber amps out there.  I really don't have any specific questions yet just wondering if anyone has any general advice.  I think I'll set up the grounding more like the hoffman style.

thanks

[tubeswell]

I built one of these (6A14) a year and a bit ago. I initially built it 'stock' - as per the kit, but wasn't really satisfied with the outcome.

The rake angle of the chassis front is too steep to accommodate a standard vintage fender bayonet lamp holder without mangling it or having it short to the chassis. What's more, the cheap plastic pilot lamp holder that ships with the kit is a dog and has teeny weeny lug holes for the filament wiring and is impossible to fit all the wires into.  What's more, because of the steep rake angle of the chassis front, it puts the lamp holder's terminal lugs devastatingly close to the 'floor' of the chassis.  So I mounted an extra stand-alone terminal tag strip next to the lamp holder and ran all the wires  that, with only one wire going to each side of the relevant lamp holder lugs.

I subsequently changed the PT to a more robust model than the one that ships with the kit. I also put in separate filter caps (not the can) and used R.G. Keen's star-grounding method. So my amp is very quiet.  I also did some other slightly more unorthodox changes, including a mosfet source follower circuit buffering the LFO to the 6V6 grids, which enhances the slam from the trem.

With a 2nd Sig12S in an extn cab plugged in alongside the on-board Sig12S, my little amp really rocks mightily.

[nateflanigan]

So I mounted an extra stand-alone terminal tag strip next to the lamp holder and ran all the wires  that, with only one wire going to each side of the relevant lamp holder lugs.

I was thinking this half way through your first paragraph Thanks for the tips.  This kit is the 6L6 version, I don't know what my friends intentions are cab/speaker wise but I bet it'll sound pretty hot with a 212.

[Fresh_Start]

I've built two Princeton Reverbs and a Super Reverb, so I've got a couple of thoughts.  Keep the leads going to and from the Intensity pot as far away from everything else as possible.  I run them over the board and have 'em drop straight down.  Shielded leads to and from the Reverb pot reduce noise a lot.  That's a sensitive, low voltage part of the circuit.  An audio taper 25K Mid pot gives you a mild "raw" boost if you want it.  Reducing the coupling caps between the PI and the power tubes may be good for reducing bass, depending on your OT, speaker & cab.  Lowering the value of the cathode bypass cap on the reverb driver (12AT7) helps keep the reverb from getting boomy.  Add 470 ohm (minimum) screen grid resistors.  1.5K grid stoppers on the power tubes also.

Adjustable bias is a really good idea in this circuit, especially with the bias vary tremolo.  There's a sweet spot where the bias is cool enough for the tremolo to work really well but hot enough to sound good.  I've got a nice circuit and layout for the bias board if you want it, but you'd need to send me your email address.

About the grounding, it's virtually impossible to use the Hoffman approach with the JJ 40-20-20-20 cap can.  I did it by adding filter caps for the preamp and not using all of the cap can.

Some guys like to reduce the 3.3meg reverb mixing resistor to 2.2meg or even lower.  It makes the "dry" signal stronger relative to the signal from the reverb return.

If you look at the schematic, there's an "unused" node on the power rail between the two 18K resistors.  Again, some like to move the plate supply for the PI to this higher voltage node with the goal of increasing clean headroom for the concertina part of the PI.  HBP and I have discussed this "Stokes Mod" at length, but I have not proven through experiments that it actually works as advertised.

That's all I can think of off the top of my head.

Cheers,

Chip

[nateflanigan]

Hey Chip!  Glad you chimed in.  You helped me out extensively with my first ab763 build.  The weber version includes some things you mentioned like adjustable bias and screen resistors.  I asked my friend/client about the mid pot (which I find pretty useful) but he's really set on keeping it "stock".  In all the video clips of this amp that he's sent me the reverb sounded too boomy to me so I really appreciate those tips.