Somewhat tweaked 5E3 layout - request for comments.

[Steve Conner]

there may be different micro-rises in potential in the ground return path(s), which bleed back into the signal path as 'backwobble'.

What is this "backwobble"? It sounds like something to do with J-Lo.

Ground loops aren't harmful by themselves: you only hear hum when the current induced in a ground loop flows in a path where the IR drop due to it can add to the signal. And this is probably the same mechanism you call backwobble.

Studio wiring is full of ground loops, but because it's balanced, the ground currents can't get into the signal.

And likewise the ground current needed to cause a "backwobble" can come from somewhere else than a ground loop. The classic case being the center tap of your power transformer.

I don't really hear the ghosting Dai mentioned, it just sounds like a stinking loud Marshall to me.

[Enzo]

About those 68k at the input. Your drawing says "to input jacks 2 of 4." Does that mean you plan on only two jacks, one per channel? SO no hi/lo jacks as on traditional Fender amps? In the stock amp, there are TWO 68k for each channel, and they form a voltage divider for the low gain jack. You won't have that. Additionally, in the stock amp, the net effect is that the two 68k for a channel will be in parallel for the high jack for a 34k resistance. Your 68k will not be 34k. I doubt it makes much difference, but there it is. Is that intentional or am I missing something?

[tubeswell]

What is this "backwobble"? It sounds like something to do with J-Lo.

Ground loops aren't harmful by themselves: you only hear hum when the current induced in a ground loop flows in a path where the IR drop due to it can add to the signal. And this is probably the same mechanism you call backwobble.

Uh-huh. To my oversimplified way of thinking, the term 'backwobble' seems to encapsulate the phenomenon whereby the ground return 'signal' 'wobbles' back into the signal path through the decoupling cap, because the decoupling cap only decouples to one point in the ground return; a point which in-turn can be affected by more powerful microrises in ground potential emanating from (an)other higher-current decoupling part(s) of the circuit that may happen to be going to the same area. I see these microrises in ground potential as being caused by the decoupled (signal) currents interacting with the minute resistances between different points in the ground return path, in the same way that different parts of the earth experience rises in ground potential. I deduce from this that the more one can eliminate the impact of these differences, the less backwobble there will be, and the quieter the amp will be. All theoretical (or other) critiques welcomed

[Wombaticus]

About those 68k at the input. Your drawing says "to input jacks 2 of 4." Does that mean you plan on only two jacks, one per channel? SO no hi/lo jacks as on traditional Fender amps? In the stock amp, there are TWO 68k for each channel, and they form a voltage divider for the low gain jack. You won't have that. Additionally, in the stock amp, the net effect is that the two 68k for a channel will be in parallel for the high jack for a 34k resistance. Your 68k will not be 34k. I doubt it makes much difference, but there it is. Is that intentional or am I missing something?

Enzo,

I'm aware of how the original was setup, but I'm debating whether I'll go that route. The old voltage divider trick on the low jack doesn't really bring anything to the table that you don't get by just turning down the guitar volume, and it trashes the input impedance of the first stage. You're also correct that the difference between a 34K and 68K grid stop is wholly inconsequential. For a 12AX7, the -3 dB high frequency roll-off point is ~24 kHz for 68K and ~48 kHz for 34k of grid resistance. Maybe my dog could hear a difference, but at those frequencies, I certainly won't be, and neither will you.

And really, who these days plugs a mic or another instrument in at the same time?

So, as an alternative, I'm considering using this clever three-jack input setup from Joeyvoltage over at AX84 that basically corresponds to Channel 1, Channel 2, and internally jumpered:

http://i52.photobucket.com/albums/g3...ge/jacks-1.jpg

Note that with this scheme, you always have a 1Meg properly setting the input impedance, even while using the internally jumpered configuration.

[Steve Conner]

The old voltage divider trick on the low jack doesn't really bring anything to the table that you don't get by just turning down the guitar volume, and it trashes the input impedance of the first stage.

You say "trashes the input impedance", I say it damps the resonant peak of your pickup when the guitar volume is full up, giving you a different tonal option.

[Wombaticus]

I built it

OK guys, just to update y'all and to dispel any impressions that I wasn't listening, I just thought I'd let you know that I incorporated (some of) your advice, and tweaked the layout some more as follows:

http://www.wombatamps.com/5E3_V2.png

It moves the cathode resistor away all by itself, and puts the voltage divider on the 2nd filter.

So I went ahead and built this, here's what it looks like wired up in a standard 5E3 size chassis:

http://www.wombatamps.com/Wombat_5E3_guts.JPG

The amp used a nominal 330-0-330 PT, but I turned out not needing the amplified Zener as the PT ended up giving me B+ of 360V rectified through a NOS CBS 5Y3GT (I did test it by the way, and the noise floor with it in the circuit is indistinguishable). It would still be useful for people with Sovtek 5Y3GTs or trannies wired assuming 110-115V mains.

Summarizing changes from stock:

1. 470R screen grid resistors;
2. Cheap insurance UF4007 diodes mounted on the rectifier socket in series with the 5Y3GT;
3. Standby switch (on the PT CT - not absolutely the best practice but worked well with this layout);
4. Extra 16uF capacitance PSU node dedicated to the input 12AY7. No sign of ghosting or weirdness;
5. 1Meg grid stopper on the cathodyne to tame hard-driven cathodyne rattiness;
6. Insulated input and output jacks. It turns out that this trick here for putting two Cliff jacks back-to-back works really well, especially in the tight confines of a 5E3: trinityamps.com • View topic - Alternate Cliff Jack Wiring;
7. Bussed-stars ground layout with only one circuit ground chassis attachment point near the input jack (and the Earth safety ground, of course).
8. Space on the layout for the optional amplified Zener B+ dropper.
9. 270R safety resistor on the output jacks.
10. Voltage divider and filter off the second PSU node used to provide an ~50V elevation to the heaters.


The amp is using the Allen Amps TO-22 PT, and the PT is what looks to be a Magnetic Components unit from CEDist:
More Info for item P-TF41316

Anyway, the layout worked well, fired up the first try, is very very quiet, and it sounds like it should. Mission accomplished, methinks. Many thanks again to all who offered guidance and advice.

[fyl]

the PT is what looks to be a Magnetic Components unit from CEDist

Yes, 166 EIA code = Magnetic Components.