Ingrid "Mods" to my Express Build

[prspastor]

Thanks to the great schematics, research of others, and the layout pdfs, I completed my first Express build according to the Francesca schematic.

The recent Ebay auction of Ingrid and the video Dan Boul of 65 Amps did of Ingrid has led me to prefer Ingrid's "personality" a bit better than Francesca's. So I thought I would try out the "mods" from Ingrid on my own clone.

I added the following to V1 for pin 2 and 7.
33k ohm for pin 7
630 ohm for pin 2. (I think Ingrid had 830 ohm - but I had a 630 on hand)
I didn't do the parallel .0022 ceramic cap yet on C8. I don't have one of those on hand.

To my ears, there was a marked improvement in the tone of the amp. The amp sounds bigger and rounder. The overdrive character is still Trainwreck, but it no longer has the over-the-top shattered glass/grainy distortion characteristic I was experiencing without the resistors. There are lots of harmonics going on and it turned a great amp into a fantastic amp for me.

Glad I tried this out - made a big difference for me and it was really easy to do.

[img:640:480]http://lh4.googleusercontent.com/-PFPm0 ... M_0180.JPG[/img]

[img:640:480]http://lh3.googleusercontent.com/-zBpkF ... M_0181.JPG[/img]

[img:640:480]http://lh4.googleusercontent.com/-BD9Ml ... M_0182.JPG[/img]

[img:640:480]http://lh4.googleusercontent.com/-TEM4X ... M_0183.JPG[/img]

[img:640:480]http://lh4.googleusercontent.com/-upWPk ... M_0184.JPG[/img]

[redshark]

Interesting to read that the grid resistor's change by itself made a diference, by the way the resistor in pin 2 should be a 820ohm.

[sliberty]

What is the theory behind each of these mods?

33K on pin v1-7 -- RF surpression, and slight input attenuation?

820R on pin v1-2 -- no idea here

Parallel .0022uF on C8 -- allow more mid free to pass?

[prspastor]

Redshark,
Thanks for the correction on pin2 - 820 ohms.

I too would like to know the theory behind why this worked so well. I was assuming it was some attenuation and RF suppression as well.

It made an amazing difference in the amp - for the better to my ears.

[martin manning]

I too would like to know the theory behind why this worked so well. I was assuming it was some attenuation and RF suppression as well. It made an amazing difference in the amp - for the better to my ears.

A 33k grid resistor will begin to attenuate high frequencies above about 16-24kHz depending upon how much capacitance is present in the input grid circuit. You probably can't hear that, but you may have had some ultrasonic oscillation going on that has been eliminated by adding these resistors.

[sliberty]

Today, I opened up my Express, which already has the following mods:

V3 - 12AT7
Split plate load resistor on V2 (22K/82K)

[While I do love a lot about the Express, I have always felt that the sweet spot on the amp came too soon (about 3-4 on the vol dial) and the rest of the sweep was useless for my taste. So these mods have toned things down to a level that I greatly prefer.]

It also has:

10K on V1 Pin 7

[I don't really remember installing that one ]

I am guessing that a 33K on the input would not provide a very noticible difference as compared to the 10K already on the input, so I will leave that as is for now.

Then, I alligator-clipped in a .0022uF Mallory across the .0022uF Orange Drop, ala Trudy/Ingrid. This mod is very nice - it adds just enough bottom end to be noticible as a rounder, fuller sound without significantly changing the amp otherwise. I like it, and will make it permanent (for now - is anything ever really permanent?).

I dug through my cap box, and found I actually have a small ceramic disc in the .0022uF value as well, but I have no idea about voltage handling, so I probably won't use it. I may solder in the Mallory for now, and when I can get my hands on a 400V or better disc, I'll give that a try.

I may still try the 820 on V1 Pin 2 and see what that does sonically.

[M Fowler]

The 820R is just another grid blocker and I suggest it's use.

Mark

[redshark]

I was playing my express today and yes with the 3 mods it sounds bigger but is a little bit too much, probably to balance it a little should get rid first of the 33k and leave the 820ohm V1 pin2 and leave the parallel cap.
I will report back!!

[Mark]

Have a really good look at the photo's again. I think the second resistor is a 820 ohm resistor, but I don't think the first resistor is a 33K. I think it is much smaller like a 1.2K or a 1.8K. I say this as I enlarged the photo and I thought the first band was brown and the third one was red. The middle colour was a tough one as the red wire in the background does effect ones judgement.

[redshark]

That is a posibility Mark. I emailed Dan Boul who I assume was inside the amp but he said he didn't remember there was a resistor there..??? Anyway why would you use there something less than 10k anyway? I used 33k because it is what was used in the rocket. Actually I think is a brown-blue-red which would make it 1k6.

[sliberty]

Anyway why would you use there something less than 10k anyway?

Part of the touch sensitivity of the Express is due to the fact that there is no grid stopper on the first stage - no series attenation. The bigger the resistor, the more like a Fender it becomes (well, not literally of course). If the purpose of the resistor is to tame oscilations, use the smallest resistor that works. If it is to reduce gain, then go for it.

[redshark]

prspastor, remove that 33k resistor until we confirm the value, V1pin7 and put the parallel cap(0.0022) leaving the 820 ohm resistor in V1pin2. I just did and I like it better.
Maybe I'm assuming that if the resistor in V1pin7 is a 1.2-1.8k it was there to supress an oscillation or maybe because the original owner of the amp played really hot pickups and Ken wanted some impedance matching or something. I don't know.

[M Fowler]

You can use 10k, 22k, 33k or 68k what ever works to control RF.

The same for the grid resistor to the next stage you can use whatever value gets the amp under control 680, 820, 1k2, etc. I've seen some use 3k3.

Mark

[yalesmith]

a nice before and after clip would be great.

[Mark]

Always remember it is your amp, do whatever you want with it. If 33K works then use 33K. I didn't think the resistor was 33K, and it only purpose is to minimise RF getting into the amp.

I'm sure there are other train wrecks with these resistors. The story as I heard it was, the amps originally went out with the resistor omitted, and if the amp returned with a complaint of picking up rubbish then Ken put a small resistor into the amp to stop the RF.

So I do think it is a Ken Fischer part and I think 1.2K to 1.8K would cover it nicely.