What to learn from Francesca

[Mark]

One thing that should be mentioned is the differences in construction between the newer Komet amp and the Express.

Firstly, I think replacing the standard trim pot with a ten turn pot is the way to fly. In the Komet all the caps were mounted on a PCB rather then attached to the chassis. Again, I think this an improvement.

The caps used in the amp are another interesting matter. I could be wrong here but it seems that Ken Fischer did use Mallory filter caps in both the Express amps and the Komet. However, Mallory 150 caps were used in the Komet over the Mallory Orange Drop type caps.
Mind you despite all of this, I'm not convinced that newer and small filter caps (Jamicon's for example) wouldn't perform as well.

As far as the wire used for the amp goes. I have heard stories ranging from he used solid core cable as it was easily available to he could bent it into the shape he wanted to. Still I don't think this is a big factor either.

I haven't had time to have a good look at the circuit yet, but I did suspect some time ago after looking at two versions of the Liverpool amp that K.F. had a standard circuit. Is this the case with Francesa?

I suppose an intersting thing that often gets overlooked is the extra supported needed for the transformers.

Again, thank you to the guys who took the photo's and a bigger thank you to the guy who let them that the photo's.

[Moose]

A 1K 25W resistor produces +/-100 nH of inductance at 1kHz depending on brand, I can't imagine that would have a profoud effect on sound or Ripple.

I probably wasn't clear. I was trying to explain my overthinking process and why, even though I changed nothing, I found it beneficial to analyze an enigmatic circuit. My conclusions are that:

1. The 25W is probably overrated and not strictly necessary, but a cheap overrated part. The detriments of a 25W are probably not detectable and if they are, beneficial. (those being slightly greater inductance and slightly greater size.)

2. I learned detailed information about wirewound and other power resistors, how they were made, and their characteristics. While this doesn't mean anything will be different for this circuit, it is knowledge I didn't have that now I will remember. I study crap like this to learn, afterall. That's what a hobby is all about.

I should have emphasized the part about "It doesn't matter" more. I'd bet many solutions to this power supply would produce fine results, and encourage anyone who thinks there's some magic to be had to experiment and do what their ears like the best.

[HiGain]

. I learned detailed information about wirewound and other power resistors, how they were made, and their characteristics. While this doesn't mean anything will be different for this circuit, it is knowledge I didn't have that now I will remember. I study crap like this to learn, afterall. That's what a hobby is all about.

Well said. I think you learn things with this amp that would otherwise not be a big deal in other amps that are less gainy and less self-interactive.

. I should have emphasized the part about "It doesn't matter" more. I'd bet many solutions to this power supply would produce fine results, and encourage anyone who thinks there's some magic to be had to experiment and do what their ears like the best.

I gotta say... There have been many times when I was not supposed to hear a difference in something when working on amps or effects pedals, but I did. With an amp like an express, there is so much gain, potential for positive feedback and oscillation, that maybe the feel and response (not so much tone) of the amp can be influenced by very subtle changes.

If you have an amp that is meant to be on the verge of unstable, then it makes sense that very small alterations in value or layout would tip the balance one way or the other.

If I just touch the bright switch, my amp goes wild. I can modulate the frequency and amplitude by touching the bright swith and simultaneously touching different things, especially the speaker cabinet!

Jake

[ODwan]

What overrating parts makes a difference in is reliability! This is a subject often overlooked for good amplifier design does not only concern tone but reliability, too!
I hope none of you has experienced it but is there anything more annoying than a part of your equipment breaking down in the middle of a gig? Even more so if you spend a mighty heap of bucks on it!

If I just touch the bright switch, my amp goes wild. I can modulate the frequency and amplitude by touching the bright swith and simultaneously touching different things, especially the speaker cabinet!

Cool! I experience the same in my build. Instant Theremin I say! To be rid of it try to coat the switch and caps in goop etc.

Timo

[Funkalicousgroove]

I'm not trying to start a heater here, but looking at these new pictures leads me to believe that Ken is more McGyver than "Anal Twat" (Thanks Moose, I'll be adding that one to my repetoir!) Engineer when it comes to constructing his amps, Especially with the wooden "Supports" for the transformers so they don't destroy that Hammond Chassis.

[dms]

Jake - do you have any pictures posted of these final lead dress modifications to your build (and pictures of the overall build itself)? Sounds like you have a layout well worth modeling as a baseline for additional experimentation.

Davids

[HiGain]

Jake - do you have any pictures posted of these final lead dress modifications to your build (and pictures of the overall build itself)? Sounds like you have a layout well worth modeling as a baseline for additional experimentation.

Davids

Hi Davids,

I have a pic of the amp in the "issue of express headroom" thread below. This was before I changed it. I'll try to take some pics over the weekend, but all I did was press the wires coming off the tubes to the chassis, and tie the power supply wires together with some wire ties. I had already followed Mark A's wiring layout.

I should clarify my previous post (given recent discussions). It is amazing to me that this should make a difference in the dynamics of the amp, but the change is not huge. Like I said, the tone is the same, but maybe a tad more clear (less background noise?), with more responsiveness and harmonic content.

Jake

[dobbhill]

Has anyone else noticed that the first triode gain stage of the amp goes to pin 7 of V1 and also the "live" input to the PI. Pin 2 of the PI is grounded. I was curious if there was a "known" difference in why Mr. KF did it that way??----D.

[Funkalicousgroove]

It is "rumored" that the second stage of a 12AX7 generally has higher transconductance, and is also quieter, but it varies so much from tube to tube in current production stuff that you may have to plug a few in before you actually get a quieter or more gainy one.

[dobbhill]

Funk...
Thanks for the info.
dobb

[gnugear]

Has anyone attempted to draw a "connect the dots" type diagram showing all the wiring etc,? I'm not too good at interperating this stuff and even after looking at the pictures I'm still scratching my head trying to figure out how everything hooks up.

I started doing one but stopped for fear of doing it wrong.

[Funkalicousgroove]

check the files section, you should find what you're looking for, but if you want some practise go to ampwares.com, click the link to the fender amp field guide and then go to the schematics section. Fender used to do a layout to go with all their schematics, print both out for an amp then take your hi-liter and trace the circut on both, you will have a clearer understanding of how they go together- Here's a hint: An express has the same basic archetecture as an AB165 bassman, with changes to component values of course.

[HiGain]

Here's a hint: An express has the same basic archetecture as an AB165 bassman, with changes to component values of course

Wow! That's a cool comparison. And it raises an interesting point regarding gain reduction. Place about a 1M resistor from the plate of the third stage to the plate of the second stage. Thanks for the inspiration! I'm going to give it a try as a gain reduction swtich.

Jake

[gnugear]

Thanks! That was very helpful.