Input to V1 mods in C30

[tubeswell]

Steve Ahola's mod has got a 1M resistor to ground BEFORE a 33k resistor in series with pin 2 of V1.

David Becker's mod (from the same website) has got a 1M resistor to ground after a 68k resistor in series with the input

Here are the links to the schematics of both mods FYI.

http://www.blueguitar.org/new/schem/...s/c30schmd.pdf

http://www.blueguitar.org/new/articl...ey/twdbm30.zip


Can somebody postulate (or better still tell me) what the difference (tonewise and gain-wise) is between having a 33k resistor following the 1M-to-ground resistor, as opposed to having a 68k resistor before the 1M-to-ground resistor?

Thanks

[Enzo]

Someone probably has a rationalization for one way over the other, but if you place it at the input jack, it will simply be in parallel with the guitar pickups. Considering their much lower impedance, I doubt the 1 meg will make any difference at all - you could leave it out. Furthermore when it is at the grid, again it is now in parallel with the guitar pickup and the resistor - now both in series. SO once again, those are so much lower in impedance than the 1 meg, in parallel it more or less disappears.

What the 1 meg does is provide a DC ground path for the grid. The guitar pickup also does this, but if you ever plug something into the input jack - such as an effect pedal - that has a capacitor in the output line, there would be no DC path to ground for the grid IF you omitted the 1 meg. So it has that value, so leave it in there. Personally I don't think it matters a lot which end of the grid stopper you use.

If you look at the typical low/high input jack pair like on so many Fender amps, the two 68k resistors wind up in parallel by way of the cutout contacts on the two jacks. That means there is really a 34k resistance in series with the guitar if yuo use the hot jack. The closest standard resistor value to that is 33k. SOme people use the 68k with a single jack too, while others when they make a single input amp will use the 33k to replace the virtual 34k that would have been there with two jacks. That is where the two numbers come from.

I am sure there is some small difference. WHy not wire in a small 100k pot wired as a variable resistor and tack it in series with your guitar input and dial it around and see what differences - if any - you hear.

[tubeswell]

So if I was to wire in a shielded cable between the input and pin2 of V1 (to minimise signal interference), would I be best to put the grid stopper and the 1M to ground on the V1 end of the cable ?

[tubeswell]

Yet more puzzling?

I notice there's this interleaved trace on the tube board next to V1. The trace for the grid signal is 'interleaved with the trace for the Plate of V1A. Can somebody please hypothesize for me why they designed it that way? Is it some sort of capacitor and what would its' value be?

I intend bypassing the whole signal trace with a sheilded cable from the input with a 68k resistor soldered to the grid pin of V1A, but I'm curious to know what effect the designers intedned the trace pattern to have.

See attached .pdf for reference

[Enzo]

You want the stopper between the guitar and the tube, I would put it as close to the grid as possible. If someone feels otherwise, please fill me in. Put the 1M where you like.

Yes, that is a capacitor. I would call it a gimmick capacitor, but perhaps there is a more up to date term nowdays. Long ago, and especially in HF circuits like radios and TVs and short wave, they found need of a small value of capacitance - a couple pf - might be for coupling UHF or might be for stability or RF bypass. They would take two short pieces of insulated wire maybe an inch long and twist them together, or even an inch of zip cord. Bare one end and solder the pair to a circuit. The wires are insulated from each other, but they form a small capacitor, that was called a gimmick capacitor.

If you want to know its value, unsolder the two jumpers at its end, pull the tube and lift R1. Now measure it with a cap meter. It will only be a few pf. It is there for stability and HF rejection - you don't want radio stations coming out the speaker. It is a tiny little bit of NFB within the first stage.

The guitar amp doesn't go much over 5kHz if at all, so I don't worry too much that a rolloff above audio will hurt the tone.

Are you really picking up that much noise on those couple inches of trace between jack and tube? What purpose hanging that wire? And you do realize the black paint inside the PV amp cabs is conductive paint that acts as a shield. That in place of foil or screen or something.

[tubeswell]

So I could chuck in a teency weency 10pF cap for the same purpose if I was bypassing it all with a shielded cable. Where would I put the cap? - Just straight into the signal path after the grid stopper (or before it)?

The reason I want it is since doing the MV, presence and resonance mods, I'm getting more ambient siganl from somewhere that goes away when I touches me axe, or the amp chassis. I thought if I put a shielded cable between the input and the grid of V1A, I would at least eliminate the potential for the interferenec to come from there. (I will go back and chuck in some more decent type of shielded twin cable for the MV, Resonance and Presence mods when I can source some - trouble with being in the tube-amp-modding-backblocks of the planet).

[Enzo]

This "cap" is between pins 1 and 2 of the tube socket - plate to grid. Are you leaving the tube socket on the board? Then the traces are still connected. if i had to install a little cap, I'd tack ut right across the two socket pins right there. Or leave it out and see what happens.

Noise that goes away when you touch it is not getting picked up by unshielded parts, it is a lack of solid grounding. Is the amp bolted back into its cab when this happens?

[tubeswell]

Noise that goes away when you touch it is not getting picked up by unshielded parts, it is a lack of solid grounding. Is the amp bolted back into its cab when this happens?

Yes it is. I wonder whether it is something to do with my wiring handiwork, or where I have got the shielded cables running next to the PC board etc. I might try moving them around or shortening them as much as possible. At present they are soldered from the 'outside' of the board. Is there a reliable way to solder them from the trace-side of the board? That way I could make them a whole lot shorter.

[Enzo]

Nothing prevents you from soldering a wire or component onto the foil side of the board. Wire sits in hole through foil. SOlder flows onto both. That junction has no way to know which side of the board the wire continues on from.

[tubeswell]

Nothing prevents you from soldering a wire or component onto the foil side of the board. Wire sits in hole through foil. SOlder flows onto both. That junction has no way to know which side of the board the wire continues on from.

Yep - 'realise that, but I was wondering whether the joint was going to be inherently mechanically weaker if it gets accidentally pulled (i.e. tearing the trace off the board). In which case is there some kind of techie trick way of putting a bit of 'anchoring thingy' through the board from the other side so you can solder the lead to it from the trace side and thereby minimse the risk of tearing the whole thing off.

[Enzo]

My tech thingie is using caution when working in the amps.

OK, here is one I just made up: leave some excess lead length going through the hole. On the far side, bend the wire over and add solder. Now the wire ends in a ball of solder and wire. THAT won't pull through the hole.

The age old thing to do with loose wire is anchor it. Mods on like logic boards are often done with very tiny wires, and we can route it across teh board UNDER the legs of an IC or transistor. But for more normal size stuff, a blob here and there of hot melt on top of hte wire will hold it down and keep it from flopping in the way of... well, whatever was going to RIP it free.

My goodness, just don't play rugby in your amp...

[tubeswell]

OK, here is one I just made up: leave some excess lead length going through the hole. On the far side, bend the wire over and add solder. Now the wire ends in a ball of solder and wire. THAT won't pull through the hole.

An excellent suggestion! Thank you.